contents 1. introduction transmission systems 8

Frederick University Cyprus AUTO 101 – Introduction to road vehicles

1

Contents

1. Introduction 2. Engines 3. Transmission Systems 4. Brake Systems 5. Steering Mechanisms and Tyres 6. Suspension Systems 7. Body Equipment and Safety systems 8. Manufacture, Dimensions, markets, environment, Legislation


2

1. Introduction

Definition of a road vehicle A road vehicle contains a number of assemblies and sub-assemblies. It uses a powertrain for propulsion, a braking system to stop, a steering to manoeuvre it, tyres to roll and so on. As times goes by more systems are integrated into a road vehicle which bring about comfort and safety not only to the occupants but to the pedestrians. Vehicle Category Descriptions

Mopeds engine displacement up to 49cc (cubic capacity)

Motorcycles engine displacement over 50cc.

3 or 4 wheeled light vehicles Motor tricycles / quadricycles, 3 or 4 wheeled vehicles with an unladen weight not exceeding Maximum Authorised Mass (MAM) 550kg

Cars Motor vehicles with a Maximum Authorised Mass (MAM) not exceeding 3500kg having not more than 8 passenger

Medium Sized Vehicles Lorries between Maximum Authorised Mass (MAM) 3500kg and 7500kg

Large Vehicles over Maximum Authorised Mass (MAM) 7500kg

Minibuses Vehicles with between 9 and 16 passenger seats

Buses Any bus with more than 16 passenger seats


3

Vehicle Overview Engine and Associated Systems Engine (type Petrol/Diesel/Electric/Hydrogen) Strokes (4/2), Rotary Engine Lubricating System

Cooling system (air/water) Induction systems (Electronic/non electronic (carburetor/mechanically driven) Exhaust systems (catalyst, feedback) Ignition Systems (electronic/non-electronic)

Transmission Driven wheels (front wheels/rear wheels/all wheels)

Gearbox type (Automatic/Manual shifting) Clutch (manual shift gearbox)

Final drive Half shafts/Axle shaft Brakes Braking system type Vacuum-hydraulic Brakes Drums/discs ABS TC (Traction contion) ASC (anti spin/slip/skid control) Suspension Front suspension (McPherson strut) Rear Suspension (De-dion Axle, Multilink Independed) Wheels (Sizes, tyre types) Steering Rack and Pinion Hydraulic assisted Electronically assisted Body equipment (General) Electrics (Batteries, charging system, starting system, climate system, Entertainment system, etc.) Integration Aesthetic Aerodynamics Legislation


4

Technical Info of a Road Vehicle Car Type: Brand, Model Engine configuration: In line 5 Cylinder, 2.0 liter 20 valve, double overhead camshsft Engine displacement: 1984cc. Bore x Stroke: 81/77mm Max output (hp/rpm): 143/5500 Max torque (nm/rpm): 176/6500 Fuel system: Microprocessor-controlled adaptive system and diagnostic unit (LH-Jetronic) Exhaust system: Catalytic converter with sensor Fuel Tank Volume (l/gallon) 73/16 Transmission (4sp Automatic or 5 speed manual) Steering system Turning circle 10.2m Tyre size: 185/65/r15 Performance: Top speed: 200km/h (124 mph) Acceleration 0-100km/h: 10s Weights Kerb weight: 1455 kg. Towing weight: 1600kg MAX 1250kg Recomended


5

VIN (vehicle Identification Number)

Since 1954, American automobile manufacturers have used a vehicle identification number (V.I.N.) to describe and identify motor vehicles. The early VINs came in a wide array of configurations and variations, depending on the individual manufacturer.

Beginning with model year (MY) 1981, the National Highway Traffic Safety Administration required that all over-the-road-vehicles sold must contain a 17-character VIN. This standard established a fixed VIN format.

The Department of Transportation issued the Federal Motor Vehicle Theft Prevention Standard to reduce the number of motor vehicle thefts by assisting law enforcement authorities in tracing and recovering parts from stolen motor vehicles. This standard became effective beginning with model year (MY) 1987 and required that designated high-theft car lines 12 or14 (two-door/four door models) of its major component parts be marked with the vehicle identification number (VIN). In Europe VIN number is found on dashboards, printed on the frame of vehicle and on an aluminium plate in the engine bay.

Example of a VIN number: XLB 345 21 1 B C 545573 Manufacturer code Vehicle type Engine type Transmission type Model year Market code Chassis number


6

2. Engines

The petrol and diesel engine which is the source of power with which we are immediately concerned, is a form of internal combustion “heat engine”. The function of which is to convert potential heat energy contained in the fuel into mechanical work. Efficiency is varied between the two engines and it was found that petrol engines are less efficient (20-32%) than diesel engines (37-40%). On the whole however what it must be noted is that internal combustion (IC) engines have a low efficiency as machineries mainly due to vast amount of losses in heat from the cooling and braking system. The most practical form of working vessel is a straight cylinder closed at the one end which is provided with a closely fitting moveable plug or “piston” on which the work is done by pressure of the fuel. This design has been found to be the most practical and satisfactory due to mechanical and manufacturing reasons. The first type of engines is the reciprocating engines. They are well established into the market and are characterised by the direct acting engine mechanism with connecting rod and crankshaft. This is so well established that it is estimated not to be replaced in the near future. Thus in most applications the reciprocating motion of the piston must be converted to rotation of the crankshaft by a suitable mechanism. The most important mechanism is the well established crankshaft and connecting rod. It is universal in most internal combustion engines owning to its simplicity and high mechanical efficiency. So the most fundamental parts which are common to all reciprocating engines are the cylinder, piston, connecting rod and crankshaft shown in the figure below.

Piston

Cylinder

Connecting Rod

Crankshaft


7

Diagram of a conventional engine In the figure in the previous page the crank is of a single web or overhung on the left engine (illustrated by a single circle). This type was used in many steam engines and certain motor cycle engines but the double web type with a bearing on each side of the crank, is practically universal for internal combustion engines. This is illustrated on the figure in the previous page (right engine) with the two circles. The description of these parts will be given here: Cylinder The ideal form consists of a plain cylindrical barrel in which the piston slides, the movement of the piston or Stroke being in some cases somewhat longer that the bore (diameter of the piston), but tending to equality or even less. This is known as the stroke: bore ratio.

Engine displacement = Volume of cylinder X number of cylinders = π d2 h / 4 X number of cylinders = π Bore2 Stroke / 4 X number of cylinders = Vswept X number of cylinders The upper end of a combustion or clearance space (usually found in the cylinder head for petrol engines) in which ignition and combustion of the mixture takes place. In practice it is necessary to depart from the ideal hemispherical shape in order to accommodate the valves, sparking plug, etc., and to control the process of combustion.

Stroke

Bore


8

Piston The usual form of piston for internal combustion engines is an inverted bucket shape, machined to a close (but free sliding) fit in the cylinder barrel. Gas tightness is secured by means of flexible “piston rings” fitting closely in grooves turned in the upper part of the piston. The pressure of the gasses is transmitted to the upper end of the connecting rod through the “gudgeon pin” on which the “small end” of the connecting rod is free to swing.

A four stroke engine piston Connecting Rod The connecting rod transmits the piston load to the crankshaft causing it to turn, thus converting the reciprocating movement of the piston into a rotating motion of the crankshaft. The lower end or “big end” of the connecting rod turns on the crankshaft pin.


9

Connecting Rod

Crankshaft In the great majority of IC engines this is of a double web type as previously mentioned. The crankshaft is made of a shaft which is bolted onto the engine block through the webs and the connection rods are attached to the pins. The shaft turns two or more main bearings (at the webs) depending on the number and arrangement of cylinders.

The crankshaft

Small End Connected to piston

Bid End connected to crankshaft

Pin Web


10

Flywheel At the one end of the crankshaft (the one attached to a transmission system) a heavy flywheel is attached. This flywheel absorbs the variations in impulse transmitted to the shaft by the gas and inertia loads and also drives the pistons over dead points and idle strokes. In motor vehicles the flywheel usually forms one member of the clutch (for manual transmission systems) through which the power is transmitted to the road wheels. The flywheel has a tooth ring which is used to crank (start) the engine

The Flywheel Method of Working It is now necessary to describe the sequence of operations by which the combustible mixture is introduced, ignited, burned and finally discharged after it has completed its work. There are two important methods or cycles in practical uses namely the four strokes or “Otto cycle” and the two strokes or “Clerk” cycle. The cycles take their names from the number of single piston sequence of operations which is repeated continuously so long as the engine works. However there is another method of working which is the rotary or “Wankel” type.

Tooth ring Connection with crankshaft Balancing marks


11

Four Stroke Cycle The figure below shows in a diagrammatic manner a four stroke engine cylinder provided with two valves of the poppet type. The cylinder is shown horizontal for convenience. The inlet valve (IV) communicates through a throttle valve with the carburettor or vaporiser, from which a combustible mixture of fuel and air is drawn. The exhaust valve (EV) communicates with the silencer through which the burnt gases are discharged to the atmosphere. These valves opened and closed at suitable intervals by mechanisms, which will be described in another course. The four strokes of the complete cycle are shown at (a), (b), (c) and (d). Below the diagrams of the cylinder are shown the corresponding portions of what is known as the indicator diagram, that is to say a diagram which shows the variation in pressure of the gases in the cylinder throughout the cycle. In practice such diagrams can be automatically recorded when the engine is running.

The four strokes are as follows: (a) Induction stroke The exhaust valve is closed and the intake valve is opened. The momentum imparted to flywheel during the previous cycles causes the connecting rod to draw the piston outwards, setting up a partial vacuum which sucks in a new charge of combustible mixture. The pressure will be below atmospheric pressure by an amount which depends upon the speed of the engine and the throttle opening.


12

(b) Compression or working stroke Here both valves are closed and the pistons returns still being driven by the momentum of the flywheel. It compresses the charge into the combustion head of the cylinder. The pressure rises to an amount which depends on the compression ratio that is the ratio of the full volume of the cylinder when the piston is at the outer end of its stroke to the volume of clearance space when the piston is at the inner or upper end. In ordinary petrol engines the ratio is between 6 and 9, for direct diesel engines 12-13 and for indirect diesel engines up to 22. The pressure at the end of the compression is about 620.5 to 827.4 KN/m2 with full throttle open. Compression ratio = (Vswept volume + Vclearence volume)/ Vclearence volume

(c) Combustion or working stroke Both valves are still closed and just before the end of the compression stroke ignition of the charge takes place by means of an electric spark. A rapid rise of temperature and pressure occurs inside the cylinder. Combustion is completed while the piston is practically at rest, and is followed by the expansion of the hot gases as the piston moves outwards. The pressure of the gasses drives the piston forward and turns the crankshaft thus propelling the car against the external resistances and restoring to the flywheel the momentum lost during the idle strokes. The pressure falls as the volume increases. (d) Exhaust stroke The inlet valve is remained closed but the exhaust valve is opened. The piston returns again driven by the momentum of the flywheel, and discharges the spent gases through the exhaust valve. The pressure will be slightly above atmospheric pressure by an amount depending an the resistance to flow offered by the exhaust valve and silencer.

Two stroke Engine Due to the fact that four strokes were required for one power stroke engineers searched for a cycle which would reduce the proportion of idle strokes. This resulted in the invention of a two stroke engine. Two stroke engines are characterised from both the specific output and the potential for smoothness of torque at any given speed which are restricted in the four stroke engines. The objective was to complete both induction and exhaust within the period that the piston was swinging over Bottom Dead Centre (BDC) and thus detract very little from either the exhaust or compression stroke.


13

Inlet and exhaust valves are eliminated together with their actuating gear leading to an extreme simplicity in design and there lower cost than a four stroke engine. This leads however to one of its principle disadvantage which is the increased fuel consumption as some of the charge inevitably is lost through the exhaust port during the overlap period. Although both efficiency and specific power output can be improved by measures such as injection of the fuel after the exhaust ports are closed, incorporating popper type exhaust valves into the head, scavenging the exhaust gases more effectively by supercharging, or even incorporating extra cylinders for scavenging by providing extra air, all involve increasing the complexity to a four-stroke unit. Even all the advantages (mechanical simplicity, low cost, greater mechanical, smooth torque owing to the shortness of the intervals between combustion impulses, and consequently the small flywheel and therefore light weight) were valid, they would still have to be set against the apparently inescapable disadvantages. These are: greater noise due to the sudden uncovering of the ports by the pistons, high specific fuel consumption, excessive hydrocarbon content of the exhaust gas, and some more, including difficultly of starting an irregular firing at idling load with some types of two-stroke engine. Together, these disadvantages have, in fact, led to the abandonment of this type of engine for cars. Moreover, although in diesel engines, injection after the inlet ports have closed obviates the fuel consumption problem, two-stroke diesels are still widely regarded as too noisy for commercial vehicles. Noise is of course a major disadvantage for an engine that may have to be offered for use in busses as well as trucks. Three-port two-stroke engine The figure illustrated in the next page shows in a simple diagram the Day three-port two stroke engine. The exhaust port is shown at E, this being uncovered by the piston after completion of about 80% of its stroke. The transfer port T, though which the charge is pumped from the crankcase, opens slightly later than the exhaust port, as shown in 1, to reduce the risk of hot exhaust gas passing into the crankcase and igniting the new charge. It follows than the transfer port is closed by the rising piston slightly before the exhaust port, so that the final pressure in the cylinder, and therefore the total quantity of charge (consisting of a mixture of burnt gases, air and fuel vapour) is determined not by the pump delivery pressure but only by the extent to which the throttling and pulse effects of the exhaust pipe, silencer, etc., raise the cylinder pressure above that of the atmosphere. The piston head is specially shaped to deflect the entering gases to the top of the cylinder. This is known as cross-flow scavenge. The piston rises and compresses the charge, after which it is ignited and expands in the usual way. The indicator diagram takes the form shown at (a) in figure, which differs from that as the exhaust ports are uncovered and the elimination of the “bottom loop” showing the exhaust and suction strokes. This bottom loop is replaced, of course, by the indicator diagram, shown at (b), obtained from the crank case or scavenge pump cylinder. There is no possibility of eliminating this pump work from either the four-stroke or the two-stroke cycle in one case it is done in alternate


14

revolutions in the main working cylinder, and in the other in every revolution in the scavenge pump cylinder.

Operation of a 3 port 2 stroke engine

It is necessary now to describe how the charge is drawn into the crankcase from the carburettor. As the piston rises a partial vacuum is formed in the crankcase, the pressure becoming steadily lower until, near the top of its stroke, the rising piston uncovers the induction port I, which communicates with the carburettor as shown in 2. Air rashes in to fill the vacuum and carries with it the petrol from the jet necessary to form a combustible mixture. It will be realised that the suction impulse on the jet is a violent one of short duration – the vary worst from the point of view of obtaining a correct and homogeneous mixture - while the timing interval during which the induction port is opened is also unduly short from the point of view of the inspiration of the full charge of air. In the figure above (c) shows a typical timing diagram of the various port openings and closings expressed in degrees of crank angle.


15

NSU Wankel Rotary Engine Although the Wankel engine represented a major advance in the search for a rotary mechanism it was not based on any new principle or thermodynamic cycle. The four events of the four stroke engine take place in one rotation of the driving member. The general profile of the straight working chamber is of epitrochoid form, a group of curves of the cycloid family. The general design of a single rotor type with a threelobed rotor is shown in the figure below.

Rotary engine

The rotor provides three equal working spaces and clearly an exhaust release will occur each time an apex seal overruns the leading edge of the exhaust port E, which is three times per revolution of the rotor and this exhaust will continue until the following seal reaches the trailing edge of the port. Induction will have commenced in the same space about 60o of rotor movement earlier. There as thus three complete 4 stroke cycles per revolution of the rotor in different working spaces, but all fired by the same sparking plug as maximum compression is reached.


16

Most of the developing problems have been associated with reducing the wear rate of the apex seals and bore and improving the efficiency of combustion. Disadvantages of the Wankel engine include the fact that at low speeds the rate of leakage past its seals is five times that past the piston rings in an equivalent piston engine. For this reason torque falls off steeply at low speeds. However a twin rotor engine can replace a reciprocating engine with tremendous differences in torque and with lower weight. Diesel versions were also seen in the market by Rolls-Royce. Diesel Injection Engines Here quickly we must stress out that both 2-stroke and 4-stroke engine can use diesel as fuel as well as the Rotary engine. However the main differences are the self igniting fuel and thus the elimination of the spark plug necessity and the high compression ratio. High compression pressure ignites the fuel mixture which is injected in a small volume at a high velocity so as to be atomised. The efficiency of a diesel is approximately 5% higher than that of a petrol engine. Turbocharging and Supercharging Basically the power output of an engine depends on the amount of energy in the form of a fuel which its cylinders can be charged, but the quantity of fuel that can be burned in the cylinder is limited by the mass of air that can introduced. In a naturally aspirated (NA) atmospheric pressure forces air in but because of viscous drag in the induction system and throttling by components such as venturis, bends in pipes and valve throats, the pressure of air in the cylinder is less than atmospheric. In order to increase the density of air supercharging and turbocharing or any other type of cooling the charge can be used, thus increasing the power output per unit of size and weight of the engine. Supercharging can used to modify the torque characteristic, generally to help to increase the torque over broad speed range as the throttle is closed, so that fewer gear changes are needed. However the most widely method used is the turbocharging. It is a compressor driven by a turbine powered by exhaust gas energy. The energy would off-course otherwise be wasted. Similarly, a supercharger is a compressor but mechanically driven and therefore consuming energy taken from the crankshaft. Both can be referred to as pressure charging and 10% increase in output is claimed.


17

Even though turbochargers were found in 1925, it was only since about the 1950s that compact, high efficiency, reliable units have been available at costs low enough for automotive applications.

Turbocharger

The major disadvantage of turbochargers is what is termed as turbocharger lag. This arises because of the need to accelerate and decelerate the rotor at extremely high speeds to satisfy the demand of torque. It can take several seconds to respond to such changes. Modern designs are able to provide much shorter response times.


18

3. Transmission Systems Why a transmission system is required? A transmission system is required a number of reason that are going to be analysed step by step. First of all an engine requires a starting system and to do so provision must be made for disconnecting the engine from the drive line during starting operation. Connecting it again to drive line, for propelling the vehicle, must be effected as smoothly as possible both for the sake of passengers and to prevent damage to vehicle mechanisms. In passing down the drive line, the torque of the engines is modified stage by stage until it becomes the propulsive force, or tractive force, at the interface between the tyres and the road. If rapid acceleration is required, either when starting from rest or in any other circumstances – for overtaking for example – that tractive effort must be increased. This is done partly by increasing the torque output of the engine but since this alone may not be enough the gear ratios will generally have to be changed too. In this context, gear changing can be linked to altering the leverage between the engine and the road wheels, so that the relatively small torque available can be translated into a large tractive force. A large leverage may be required for climbing hills or traversing very soft or rough ground. Since a large leverage implies a correspondingly reduced movement at the output end, this implies a big reduction of rotational speed between the engine and the road wheels. Consequently the leverage must be reduced as the speed of vehicle increases, otherwise the engine speed would become too high and maximum potential speed of the vehicle would be unattainable. Mover over torque falls off as speed increases so for this reason some simple mean of varying the leverage – speed changing – is necessary. Consider a car having road wheels 0.66m diameter and cruising at 100 km/h. Under these conditions the engine speed would have about 3500 Rev/min and the road wheels would be rotating at about 800 rev/min. Consequently the overall ratio of the gearing between the engine and the road wheels would have to be about 4.5:1. In practice this ratio would differ to some extent, depending on the weight of the car and the size of the engine. For example until recently a car with a large engine might have an overall ratio of 3:1, a medium weight commercial vehicle 5.5:1 and a heavy truck 10:1 or even higher. Now the demand for fuel economy is tending to encourage gearboxes and overall gear ratios as high as 6:1 could become common on, for instance, cars with five-speed gearboxes. While the basic principles of transmission remain the same for virtually all classes of vehicles, the actual arrangements vary – for instance some have four-wheel drive and other either front- or rear- wheel drive. Where the engine is installed at the front and the axis of crankshaft is parallel to, or coincident with, the longitudinal axis of the


19

vehicle, ultimately the drive must be turned through 90o in order that it maybe transmitted to the wheels. Such turn however is not necessary if the engine is mounted transversely (i.e. front wheel drive car). Another requirement for the transmission systems arises from the fact that when the vehicle is cornering the outer wheels must roll faster than the inner ones which will be traversing circles of smaller radii, yet their mean speed and therefore both the rotational speed of the engine and the translational speed of the vehicle may be required to remain constant. Then again, to reduce the transmission of vibrations to the chassis frame, the engine is universally mounted on it, while the driving wheels attached to the frame by road springs also have a degree of freedom of movement relative to it. Both these movements must be accommodated by the transmission. Clutch, gearbox, and live axle transmission – general arrangement (Rear and front wheel drive) This system is shown diagrammatically in the figure below and is a general set-up for rear wheel drive vehicles (RWD). The engine is at the front with its crankshaft parallel to the axis of the vehicle. From the engine the drive is transmitted through a clutch and a short shaft (c) to the gearbox. In car this shaft is almost invariably integral with the primary gear in the gearbox but for most commercial vehicles it is a separate component. From the gearbox a “propeller shaft” or “cardan shaft” takes the drive to a live back axle. This shaft is fitting with two universal joints (UV joints). A live axle is the one through which the drive is transmitted to the wheels while a dead axle is the one that does not transmit any drive (i.e. the rear axle at front wheel cars). The gearing (g) contains the differential within the axle divides the drive equally to the two wheels and turns it 90o.


20

General set-up of a rear wheel drive vehicle

The bell housing found on the gearbox usually covers the clutch and the shaft (c) or some of it which is secured to the engine crankcase. This has the advantages of cleanliness and neatness of appearance and a chief disadvantage associated with the inaccessibility of the clutch for repairs. The rear engine and live axle arrangement has the advantage for buses and coaches primary because it allows the floor to be set at a low level and to be flat through out the length of the chassis. Front wheel drive is shown in the figure below. The engine is mounted longitudinally forward of the front wheels and the gearbox to the rear of them. The final drive is being interposed between the two. Consequently its output shafts are two and are directly mounted to the front wheels. Since these wheels are also turning the car and there is a relative movement due to the suspension system universal joints are used. Such set ups are common in Audis.

Front wheel drive set-up with longitudinal engine mounting

However most front wheel drive vehicles use a transverse mounting of the engine as shown in the figure below. Disadvantages of such engine set up include that of final drive torque is reach by the engine mountings. So front wheel drive cars with high torque outputs, need heavy duty engine mountings. Also shock and harshness is easily transmitted to the steering in front wheel drive cars whereas rear wheel drive cars provide a more smooth feeling. This is why luxury cars are rarely found with a front wheel drive systems.

Front wheel drive set-up with transverse engine mounting

The rear engine layout for cars (engine behind the rear axle) has fallen from favour. This is because of the associated instability due to poor weight distribution coupled with a rear suspension that is almost inevitably unfavourable owning to the small space available for it on each side of the engine. Off-course Porsche still uses this setup and provides electronic systems to keep the car stable.


21

A Porsche 911 with rear engine layout Mid-engine installation is popular for sport cars because it tends to give a reasonably uniform weight distribution between all four wheels. Major disadvantages are noise inside the vehicle, difficulty of access for maintenance, complexity of transferring the coolant between the radiator and engine (the same occurs for air-condition compressor and condenser) and finally a two-seater set up as the space available for passengers at the rear is taken by the engine. Four-wheel-drive Transmission sytems Typically in a four-wheel-drive (4WD) transmission layout (figure below), a transfer box is interposed between the gearbox and back axle unit. The function of this transfer box is to transfer the drive from the main gearbox to both front and rear wheel. In this box a pinion A, is driven by the gearbox output shaft. The pinion through an intermediate gear B drives a third gear C. From the differential gears one shaft is taken forward to the front axle and the other rearwards to the back axle. Both axles have their own differentials and final drive gears but the front has universal joints D to allow the front wheels to be steered.

General arrangement of a four-wheel driven chassis

The differential C is necessary to distribute the drive equally between the front and rear axles and to allow for the fact that when the vehicle is driven in a circle the mean speeds of the front wheels is different from that of the rear wheels and therefore the speeds of the two propeller shafts must differ too. Other factors include different


22

rolling radii of the tyres owning to, for example manufacturing tolerances, different degrees of wear and perhaps different tyre pressures. Four-wheel-drive offers two main advantages. First there is the increased traction obtainable from the driven wheels, which is especially useful on soft or slippery ground. Secondly if the front wheels drop in a ditch they tend to climb out, whereas with rear-wheel drive they tend to be forced downwards, except when the vehicle is driven in reverse, in which case, of course the disadvantage of the lower traction of two-wheel drive remains. The principal disadvantages are increased weight, bulk and cost. Clutches A clutch is a releasable coupling connecting the adjacent ends of two coaxial shafts. It is said to be engaged or in when the shafts are coupled and disengaged or out when are released. The simplest friction type clutch comprises two discs, the more substantial of which is usually the engine flywheel and the other generally termed the presser or pressure plate, is lighter. The flywheel is bolted on the end of the crankshaft, while the other plate slides axially on the output shaft, except in as a spring or springs tending to press it against the flywheel. Such a clutch is engaged by its spring or springs and disengaged by a pedal-actuated linkage under control of the driver. If a clutch is stripped of all complications such as friction linings and actuation mechanism then the result is something shown in the figure below. The two plates are keyed on the ends of the two shafts and are carried by two bearings. Because the engine torque has to be transmitted from the engine flywheel to the pressure plate by friction between the two faces, a specially formulated material having a high coefficient of friction and good wearing properties is required.

A simple arrangement of a clutch system However a much better arrangement is to introduce a third disc which is positioned between the driving discs. This disc is much lighter and is lined on both sides (faces) with a high friction material. It is termed as friction disc or centre plate.

In a car, you need a clutch because the engine spins all the time and the car wheels don't. In order for a car to stop without killing the engine, the wheels need to be


23

disconnected from the engine somehow. The clutch allows us to smoothly engage a spinning engine to a non-spinning transmission by controlling the slippage between them. To understand how a clutch works, it helps to know a little bit about friction.

In the figure below, you can see that the flywheel is connected to the engine, and the clutch plate is connected to the transmission.

Arrangement of a clutch on a vehicle

When your foot is off the pedal, the springs push the pressure plate against the pressure disc, which in turn presses against the flywheel. This locks the engine to the transmission input shaft, causing them to spin at the same speed.

The pressure plate assembly

When the clutch pedal is pressed, a cable or hydraulic piston pushes on the release fork, which presses the throw-out bearing against the middle of the diaphragm spring. As the middle of the diaphragm spring is pushed in, a series of pins near the outside of the spring causes the spring to pull the pressure plate away from the clutch plate (friction disc). This releases the clutch from the spinning engine. This is illustrated on the figures below.

Diaphragm Spring

Pressure disc


24

Engagement and disengagement of clutch

The amount of force the clutch can hold depends on the friction between the pressure disc and the flywheel, and how much force the spring puts on the friction plate. The figure in the next page illustrates a friction plate. Note the springs in the friction plate. These springs help to isolate the transmission from the shock of the clutch engaging. The most common problem with clutches is that the friction material on the disc wears out. The friction material on a friction disc is very similar to the friction material on the pads of a disc brake, or the shoes of a drum brake - after a while, it wears away. When most or all of the friction material is gone, the clutch will start to slip, and eventually it will not transmit any power from the engine to the wheels.

Engaged

Disengaged


25

Friction disc of a clutch system

The friction disc only wears while the pressure disc and the flywheel are spinning at different speeds. When they are locked together, the friction material is held tightly against the flywheel, and they spin in synchronization. It is only when the friction disc is slipping against the flywheel that wearing occurs. So if you are the type of driver who slips the clutch a lot, you will wear out your clutch a lot faster.

Another problem sometimes associated with clutches is a worn throw-out (release) bearing. This problem is often characterized by a rumbling noise whenever the clutch engages.

Gearboxes

Control over power output, by means of the throttle pedal, simply regulates the rate at which the engine is doing work: at very high speeds, the power output will be correspondingly high but, the torque output can at the same time be significantly less than at considerably lower speeds. In other words, maximum torque may be available over only a very limited speed range. Consequently, one needs to be able to regulate both the power output and the speed range of the engine relative to the range of speeds over which the vehicle is at any given time likely to be required to operate. Only in this way can the torque at the wheels be balanced against demands for either a steady speed uphill or downhill, or on the level, or for acceleration or deceleration. Α gearbox is necessary, therefore, so that the driver can regulate torque by selecting the appropriate speed range or, in other words, the vehicle speed at which maximum torque is obtainable. This is generally achieved by a gear ratio, meaning the altering of rotation between the engine and the rear axle at a suitable ratio that is determined by a number of factors, such as speed of vehicle, gradient (uphill, downhill), etc. Gearboxes can be classified into three main categories:

Lining to wear out

Spring to isolate shock transmitted to gearbox


26

• Manual gearbox • Automatic gearbox • Semi-automatic gearbox

Manual Gearbox

The manual transmission provides a means of varying the relationship between the speed of the engine and the speed of the wheels. Varying these gear ratios allows the right amount of engine power at many different speeds.

Manual transmissions require use of a clutch to apply and remove engine torque to the transmission input shaft. The clutch allows this to happen gradually that so that the car can be started from a complete stop.

Typical 5 speed manual gearbox

Most cars have five forward gears, and one reverse gear. The transmission changes the ratio of the engine speed and the wheels by connecting gears in various combinations. If a gear with 10 teeth is driving a gear with 20 teeth, the drive would be said to have a 2:1 ratio.

First gear connects the engine power to the driven wheels via a pair of reduction gear sets, which gives increased power and reduced wheel-speed when the car is beginning to move. This means the engine is turning much faster than the output shaft, typically around a 4:1 ratio. Intermediate speeds are delivered by changing the gear ratio closer to 1:1. Final drive is usually accomplished by directly linking the input and output shafts, giving a 1:1 gear ratio. Using a moveable set of different sized gears, it's possible to get several degrees of torque output.


27

The table below provides typical gear ratios and also provide the rotation of the output shaft of a gearbox at 3000 revolutions per minute (RPM).

Gear Ratio RPM at Transmission Output Shaft with Engine at 3,000 rpm

1st 2.315:1 1,295

2nd 1.568:1 1,913

3rd 1.195:1 2,510

4th 1.000:1 3,000

5th 0.915:1 3,278

Typical gear ratios

To understand the basic idea behind a standard transmission, the diagram below shows a very simple two-speed transmission in neutral.

A simple two speed gearbox

Shaft 1 provides the input from the engine through the clutch. Shaft 1 and gear A are connected as a single unit. The shaft 2 and gears B, D and F are called the layshaft. These are also connected as a single piece, so all of the gears on the layshaft and the layshaft itself spin as one unit. The shaft 1 and the shaft 2 are directly connected through their meshed gears so that if the shaft 1 is spinning, so is the shaft 2. In this

A

B

C

D

E

F

Shaft 1

Shaft 2

Shaft 3


28

way, the layshaft receives its power directly from the engine whenever the clutch is engaged.

The shaft 3 is a splined shaft that connects directly to the drive shaft through the differential to the drive wheels of the car. If the wheels are spinning, the shaft 3 is spinning. The gears C and E ride on bearings, so they spin on the shaft 3. If the engine is off but the car is coasting, the shaft 3 can turn inside the gears C and E while these gears and the layshaft are motionless. The purpose of the collar is to connect one of the gears C and E to the shaft 3. The collar is connected, through the splines, directly to the shaft 3 and spins with it. However, the collar can slide left or right along the shaft 3 to engage either of the gear C and E. Teeth on the collar, called dog teeth, fit into holes on the sides of the gears C and E to engage them.

In order to estimate the gear ratio of the second gear we use the following formula:

Speed of shaft 1 No. of teeth B No. of teeth C ------------------- = ------------------- X ------------------ Speed of shaft 3 No. of teeth A No. of teeth D The same will apply for the first gear. Generally as the gear on the output shaft get bigger and on the layshaft smaller the lower that gear will be.

When you make a mistake while shifting and hear a horrible grinding sound, you are not hearing the sound of gear teeth miss-meshing. As you can see in these diagrams, all gear teeth are all fully meshed at all times. The grinding is the sound of the dog teeth trying unsuccessfully to engage the holes in the side of a gear.

The transmission shown here does not have "synchros", so if you were using this transmission you would have to double-clutch it. Double-clutching was common in older cars and is still common in some modern race cars. In double-clutching, you first push the clutch pedal in once to disengage the engine from the transmission. This takes the pressure off the dog teeth so you can move the collar into neutral. Then you release the clutch pedal and rev the engine to the "right speed." The right speed is the rpm value at which the engine should be running in the next gear. The idea is to get the blue gear of the next gear and the collar rotating at the same speed so that the dog teeth can engage. Then you push the clutch pedal in again and lock the collar into the new gear. At every gear change you have to press and release the clutch twice, hence the name "double-clutching."

Manual transmissions in modern passenger cars use synchronizers to eliminate the need for double-clutching. A synchro's purpose is to allow the collar and the gear to make frictional contact before the dog teeth make contact. This lets the collar and the gear synchronize their speeds before the teeth need to engage. The figure in the next page illustrates the use of a synchronizer. With reference to the figure the cone on the gear fits into the cone-shaped area in the collar, and friction between the cone and the collar synchronize the collar and the gear. The outer portion of the collar then slides so that the dog teeth can engage the gear. Every manufacturer implements transmissions and synchros in different ways, but this is the general idea.


29

Synchronizer on a gear

Reverse gear is handled by a small idler gear as shown in the figure below. At all times, the blue reverse gear in this diagram is turning in a direction opposite to all of the other blue gears. Therefore, it would be impossible to throw the transmission into reverse while the car is moving forward - the dog teeth would never engage. However, they will make a lot of noise.

Reverse gear set up

Automatic Transmissions

Output shaft gear

Laysfaft shaft gear


30

Just like that of a manual transmission, the automatic transmission's primary job is to allow the engine to operate in its narrow range of speeds while providing a wide range of output speeds.

A typical Automatic transmission

In the USA, the first automatic transmission with a torque converter and epicyclic gearing was introduced in the mid-1930s and, in Europe, in 1950. Simple automatic systems may be refined by the inclusion of facilities for changing the gear range to cope with difficult conditions, such as on rough terrain or in heavy traffic, and for inhibiting upward changes, for example when ascending steep-gradients. The latter facility can be used to avoid repeated up and down changes on such gradients. To provide the different ratios required, all these automatic transmissions feature a mechanical gearbox, mostly epicyclic, and usually with a torque converter, through which the drive is transmitted to the mechanical gearbox. Since the introduction of electronics for road vehicles, the trend has been towards ever increasing sophistication of control.

In general, the control must bring about changes from low to high ratios as the vehicle speed rises, and from high to low as it falls. However, it is frequently possible to employ the higher gears even at low vehicle speeds, for example on level roads and with following winds, when the resistances to be overcome are low.

The control system must therefore take account of the engine load and, in general, produce changes up when the load is light and changes down when the load is heavy. There are, however, occasions, such as on descending hills, when it is desirable to employ a low gear although the load on the engine may be nil or the engine may be acting as a brake. It is under these diverse conditions that the human element has to be retained in the control.

All automatic transmission systems are controlled with reference to vehicle speed and


31

engine load. With electronic control, however, additional factors may be introduced, such as engine temperature, ambient temperature, icy road conditions, and rate of change of accelerator position. These data are obtained by the use of sensors which, in the earlier systems, were mechanical, electrical, pneumatic (manifold depression). Now, however, electronic sensors predominate.

The fuel consumption of an automatic transmission embodying a torque converter is inherently higher than that of the equivalent manually controlled transmission. This is attributable to factors such as friction losses in the multi-plate clutches and brakes used to change gear ratios, losses in their hydraulic control systems, converter losses, and friction losses in the gears and preloaded rolling element bearings.

The main components that make up an automatic transmission include:

• The Torque Converter which acts like a clutch to allow the vehicle to come to a stop in gear while the engine is still running.

• Planetary Gear Sets which are the mechanical systems that provide the various forward gear ratios as well as reverse.

• The Hydraulic System which uses a special transmission fluid sent under pressure by an Oil Pump through the Valve Body to control the Clutches and the Bands in order to control the planetary gear sets. .

• The Governor and the Modulator or Throttle Cable that monitor speed and throttle position in order to determine when to shift.

• On newer vehicles, shift points are controlled by Computer which directs electrical solenoids to shift oil flow to the appropriate component at the right instant

Torque converter

The torque converter is a type of fluid coupling between the engine and the gearbox to even out speed changes and to multiply the engine torque. The torque converter is used as a clutch to send the power (torque) from the engine to the transmission input shaft. It provides a continuous variation of ratio from the lowest to the highest.

It has three parts; an impeller (pump) connected to the engine's crankshaft, a turbine to turn the turbine shaft which is connected to the gears, and a stator between the two. The torque converter is filled with transmission fluid that is moved by the impeller blades. The stator's vanes catch the oil thrown off from the impeller, and use it to move the turbine's blades. When the impeller spins above a certain speed, the turbine spins, driven by the impeller.

In some designs, the torque converter locks the impeller and the turbine together when at highway speeds, which increases efficiency. This is called torque converter lock-up and since no torque is wasted it is more efficient.


32

The torque converter

Planetary Gear Sets

Automatic transmissions contain many gears in various combinations. In a manual transmission, gears slide along shafts as you move the shift lever from one position to another, engaging various sized gears as required in order to provide the correct gear ratio. In an automatic transmission, however, the gears are never physically moved and are always engaged to the same gears. This is accomplished through the use of planetary gear sets.

The basic planetary gear set consists of a sun gear, a ring gear and two or more planet gears, all remaining in constant mesh. The planet gears are connected to each other through a common carrier which allows the gears to spin on shafts called "pinions" which are attached to the carrier .

One example of a way that this system can be used is by connecting the ring gear to the input shaft coming from the engine, connecting the planet carrier to the output shaft, and locking the sun gear so that it can't move. In this scenario, when we turn the ring gear, the planets will "walk" along the sun gear (which is held stationary) causing the planet carrier to turn the output shaft in the same direction as the input shaft but at a slower speed causing gear reduction (similar to a car in first gear).


33

If we unlock the sun gear and lock any two elements together, this will cause all three elements to turn at the same speed so that the output shaft will turn at the same rate of speed as the input shaft. This is like a car that is in third or high gear. Another way that we can use a Planetary gear set is by locking the planet carrier from moving, then applying power to the ring gear which will cause the sun gear to turn in the opposite direction giving us reverse gear.

The illustration on the figure in the next page shows how the simple system described above would look in an actual transmission. The input shaft is connected to the ring gear (1), The Output shaft is connected to the planet carrier (2) which is also connected to a "Multi-disk" clutch pack. The sun gear is connected to a drum (3) which is also connected to the other half of the clutch pack. Surrounding the outside of the drum is a band (4) that can be tightened around the drum when required to prevent the drum with the attached sun gear from turning.

The clutch pack is used, in this instance, to lock the planet carrier with the sun gear forcing both to turn at the same speed. If both the clutch pack and the band were released, the system would be in neutral. Turning the input shaft would turn the planet gears against the sun gear, but since nothing is holding the sun gear, it will just spin free and have no effect on the output shaft. To place the unit in first gear, the band is applied to hold the sun gear from moving. To shift from first to high gear, the band is released and the clutch is applied causing the output shaft to turn at the same speed as the input shaft.

Planetary gear system

Many more combinations are possible using two or more planetary sets connected in various ways to provide the different forward speeds and reverse that are found in modern automatic transmissions.

1

2

3

4


34

Some of the clever gear arrangements found in four and now, five, six and even seven-speed automatics are complex enough to make a technically astute lay person's head spin trying to understand the flow of power through the transmission as it shifts from first gear through top gear while the vehicle accelerates to highway speed. On newer vehicles, the vehicle's computer monitors and controls these shifts so that they are almost imperceptible.

Clutch packs

A clutch pack consists of alternating disks that fit inside a clutch drum. Half of the disks are steel and have splines that fit into groves on the inside of the drum. This is illustrated in the figure in the next page. The other half have a friction material bonded to their surface and have splines on the inside edge that fit groves on the outer surface of the adjoining hub. There is a piston inside the drum that is activated by oil pressure at the appropriate time to squeeze the clutch pack together so that the two components become locked and turn as one.

Clutch packs

One way clutch

A one-way clutch (also known as a "sprag" clutch) is a device that will allow a component such as ring gear to turn freely in one direction but not in the other. This effect is just like that of a bicycle, where the pedals will turn the wheel when pedaling forward, but will spin free when pedaling backward.

A common place where a one-way clutch is used is in first gear when the shifter is in the drive position. When you begin to accelerate from a stop, the transmission starts out in first gear. But have you ever noticed what happens if you release the gas while it is still in first gear? The vehicle continues to coast as if you were in neutral. Now, shift into Low gear instead of Drive. When you let go of the gas in this case, you will


35

feel the engine slow you down just like a standard shift car. The reason for this is that in Drive, a one-way clutch is used whereas in Low, a clutch pack or a band is used.

Steel bands

A band is a steel strap with friction material bonded to the inside surface. One end of the band is anchored against the transmission case while the other end is connected to a servo. At the appropriate time hydraulic oil is sent to the servo under pressure to tighten the band around the drum to stop the drum from turning.

Steel band Oil pump

The transmission oil pump (not to be confused with the pump element inside the torque converter) is responsible for producing all the oil pressure that is required in the transmission. The oil pump is mounted to the front of the transmission case and is directly connected to a flange on the torque converter housing. Since the torque converter housing is directly connected to the engine crankshaft, the pump will produce pressure whenever the engine is running as long as there is a sufficient amount of transmission fluid available. The oil enters the pump through a filter that is located at the bottom of the transmission oil pan and travels up a pickup tube directly to the oil pump. The oil is then sent, under pressure to the pressure regulator, the valve body and the rest of the components, as required.

The oil pump


36

Valve body

The valve body is the control center of the automatic transmission. It contains a maze of channels and passages that direct hydraulic fluid to the numerous valves which then activate the appropriate clutch pack or band servo to smoothly shift to the appropriate gear for each driving situation. Each of the many valves in the valve body has a specific purpose and is named for that function. For example the 2-3 shift valve activates the 2nd gear to 3rd gear up-shift or the 3-2 shift timing valve which determines when a downshift should occur.

The most important valve, and the one that you have direct control over is the manual valve. The manual valve is directly connected to the gear shift handle and covers and uncovers various passages depending on what position the gear shift is placed in. When you place the gear shift in Drive, for instance, the manual valve directs fluid to the clutch pack(s) that activates 1st gear. it also sets up to monitor vehicle speed and throttle position so that it can determine the optimal time and the force for the 1 - 2 shift. On computer controlled transmissions, you will also have electrical solenoids that are mounted in the valve body to direct fluid to the appropriate clutch packs or bands under computer control to more precisely control shift points.

The Valve body

Controls of the Automatic transmission

These three components are important in the non-computerized transmissions. They provide the inputs that tell the transmission when to shift. The Governor is connected to the output shaft and regulates hydraulic pressure based on vehicle speed. It accomplishes this using centrifugal force to spin a pair of hinged weights against pull-back springs. As the weights pull further out against the springs, more oil pressure is allowed past the governor to act on the shift valves that are in the valve body which then signal the appropriate shifts.


37

Of course, vehicle speed is not the only thing that controls when a transmission should shift, the load that the engine is under is also important. The more load you place on the engine, the longer the transmission will hold a gear before shifting to the next one.

There are two types of devices that serve the purpose of monitoring the engine load: the Throttle Cable and the Vacuum Modulator. A transmission will use one or the other but generally not both of these devices. Each works in a different way to monitor engine load.

The Throttle Cable simply monitors the position of the gas pedal through a cable that runs from the gas pedal to the throttle valve in the valve body. The Vacuum Modulator monitors engine vacuum by a rubber vacuum hose which is connected to the engine. Engine vacuum reacts very accurately to engine load with high vacuum produced when the engine is under light load and diminishing down to zero vacuum when the engine is under a heavy load. The modulator is attached to the outside of the transmission case and has a shaft which passes through the case and attaches to the throttle valve in the valve body. When an engine is under a light load or no load, high vacuum acts on the modulator which moves the throttle valve in one direction to allow the transmission to shift early and soft. As the engine load increases, vacuum is diminished which moves the valve in the other direction causing the transmission to shift later and more firmly.

The governor

Semi-Automatic Transmissions

Ideally, the transmission would be so flexible in its ratios that the engine could always run at its single, best-performance rpm value. That is the idea behind the continuously variable transmission (CVT). It uses a steel belt and pulleys to vary the ratio from 4-25:1.So a CVT has a nearly infinite range of gear ratios. In the past, CVTs could not compete with four-speed and five-speed transmissions in terms of cost, size and


38

reliability, so you didn't see them in production automobiles. These days, improvements in design have made CVTs more common. The Toyota Prius is a hybrid car that uses a CVT.

The differential The differential is the device that divides the torque input from the propeller shaft equally between the two output shafts to the wheels, regardless of the fact that they may be rotating at different speeds, for instance on rounding a corner.

The differential

The crown wheel is fixed to the differential cage and is driven by a pinion which is connected to the axle shaft. The side or output gears are connected through shafts to the driven wheels. These gears mesh inside the differential with the two pinion gears. The pinions are free to turn on their pin fixed in the differential cage.

When the car moves in a circle the road wheels are constrained to move at different speeds and do so by one wheel going faster than the differential cage while the other goes an equal amount slower than the differential cage. Thus the speed of the differential cage is the mean of the road wheel speeds. When the car moves in a straight line, the road wheels turn at the same speed as the differential cage, and the differential pinions do not have to turn an their pins at all.

Another function that the differential has is ratio. This ratio is estimated using the

Crown wheel

Pinion

Pinion gears

Output/side gears ours

Differential cage

Pinion gear pin


39

gear teeth of the crown wheel and pinion and is typically close to 4:1. That is one revolution of the crown wheel is equal to 4 of the pinion. Also it will rotate the drive (if required) at angle necessary to reach the driven wheels.

In order to get better traction out of car different types of differentials can be used and include:

o Clutch LSD (limited Slip Differential) o Viscous Coupling o Locking and Torsen


40

4. Brake Systems

Definition and Purpose The braking system is the most important system in your car. If your brakes fail, the result can be disastrous. Brakes are actually energy conversion devices, which convert the kinetic energy (momentum) of your vehicle into thermal energy (heat). When you step on the brakes, you command a stopping force ten times as powerful as the force that puts the car in motion. All newer cars have dual systems, with two wheels' brakes operated by each subsystem. That way, if one subsystem fails, the other can provide reasonably adequate braking power. Safety systems like this make modern brakes more complex, but also much safer than earlier braking systems. Generally the brake systems discharge the following functions:

• Reduce the vehicle speed • Bring a moveable vehicle to a stop • Keeping it stopped when stationery

This means that brake systems playa vital role in making motor vehicles suitable for practical application. They are essential for ensuring highway safety, which is why brake systems are subject to strict official regulations. A vehicle's approval for homologation and highway operation is contingent upon compliance with a number of national and international regulations. Dynamics The stopping distance is the distance covered in the interval between the time when a hazard or obstacle is recognized and the point where the vehicle comes to a stop. It is the sum of the distance travelled during the reaction time tr, the brake system's initial response delay ta (at constant vehicle speed v) and the distance covered during the effective braking time tw Maximum retardation a is obtained during the pressure build-up period ts. Alternatively, half of the pressure build-up period can be viewed as representing full deceleration.

Graphical representation of deceleration


41

The upper limits on retardation are determined by the static coefficient of friction between tyres and road surface which will be discussed later. The reaction time is the period that elapses between recognition of hazard or obstacle, the driver’s decision to apply the brakes, and the time it takes for the foot to contact the brake petal. This no fixed constant and depends upon individual driver and various environmental variables and can range from 0.3 to 1.7 seconds. This is illustrated in the table below. Tyre and road contact is important and this is emphasised by the tread depth. Low tread depth is accompanied by a commensurate reduction in the size of the protective layer covering belts and carcass. Tyre rotation between axles is recommended when the tyres at various axles are exposed to different rates of tread wear. Adequate tread depth is especially important for operating safety on passenger cars and high-speed commercial vehicles, as worn tread leads to reduced traction on wet roads. Reduced tread depth also leads to an over-proportional increase in braking distances.

Tread depth and braking distance at 100 km/h


42

Main Components The brake system is composed of the following basic components: The "master cylinder" which is located under the hood, and is directly connected to the brake pedal, converts your foot's mechanical pressure into hydraulic pressure. Steel "brake lines" and flexible "brake hoses" connect the master cylinder to the "slave cylinders" located at each wheel. Brake fluid, specially designed to work in extreme conditions, fills the system. "Shoes" and "pads" are pushed by the slave cylinders to contact the "drums" and "rotors" thus causing drag, which (hopefully) slows the car.

Example of a passenger-car power-assisted brake system In recent years, brakes have changed greatly in design. Disc brakes, used for years for front wheel applications, are fast replacing drum brakes on the rear wheels of modern cars. This is generally due to their simpler design, lighter weight and better braking performance. The greatest advantage of disc brakes is that they provide significantly better resistance to "brake fade" compared to drum type braking systems. Brake fade is a temporary condition caused by high temperatures generated by repeated hard braking. It occurs when the pads or shoes "glaze" due to the great pressure and heat of hard use. Once they cool, the condition subsides. Disc brakes allow greater air ventilation (cooling) compared to drum brakes. Drum brakes are not internally ventilated because if they were, water could accumulate in them. Disc brakes can rapidly fling off any water that they are exposed to, and so they can be well ventilated. "Boosters" are present in "power brake" systems, and use the engine's energy to add pressure to the master cylinder. "Anti-lock" (ABS) systems, originally developed for aircraft braking systems, use computer controlled valves to limit the pressure delivered to each slave cylinder. If a wheel locks up, steering input cannot affect the car's direction. With ABS, no matter how hard the pedal is pressed, each wheel is prevented from locking up. This prevents skidding (and allows the driver to steer while panic-braking).


43

As impressive as these advances are, the basic process of converting a vehicle's momentum into (wasted) heat energy has not changed since the days of the horse and buggy. To stop a horse drawn carriage, the driver would pull on a lever which would rub on the wheel. But today, with the advent of regenerating brakes on electric vehicles, new ways of recapturing this lost energy are being developed. In these types of electric cars, when you step on the brakes, the motor switches into "generator mode", and stores the car's momentum as chemical energy in the battery, to be used again when the light turns green! Disc Brakes

Disc brakes use a clamping action to produce friction between the "rotor" and the "pads" mounted in the "caliper" attached to the suspension members. Inside the calipers, pistons press against the pads due to pressure generated in the master cylinder. The pads then rub against the rotor, slowing the vehicle. Disc brakes work using much the same basic principle as the brakes on a bicycle; as the caliper pinches the wheel with pads on both sides, it slows the bicycle. Disc brakes offer higher performance braking, simpler design, lighter weight, and better resistance to water interference than drum brakes.

Disc brakes, like many automotive innovations, were originally developed for auto racing, but are now standard equipment on virtually every car made. On most cars, the front brakes are of the disc type, and the rear brakes are of the "drum" type. Drum brakes use two semi-circular shoes to press outward against the inner surfaces of a steel drum. Older cars often had drum brakes on all four wheels, and many new cars now have 4-wheel disc brakes.

Floating-caliper disc brake

"Floating caliper" disc brakes, the most common variety, allow the caliper to move from side to side slightly when the brakes are applied. This is because only one pad moves (in relation to the caliper). Some calipers contain two or four seperate pistons.


44

These calipers are fixed in place; i.e., there is no lateral movement like the floating caliper, the pistons take up the slack on each side of the rotor. These are called "dual cylinder" or "dual piston" calipers, and are standard equipment on many performance cars.

Because disc brakes can fling off water more easily than drum brakes, they work much better in wet conditions. This is not to say that water does not affect them, it definitely does. If you splash through a puddle and then try to apply the brakes, your brakes may not work at all for a few seconds! Disc brakes also allow better airflow cooling, which also increases their effectiveness. Some high performance disc brakes have drilled or slotted holes through the face of the rotor, which helps to prevent the pads from "glazing" (becoming hardened due to heat). Disc brakes were introduced as standard equipment on most cars in the early seventies.

Drum brakes

The brake drum is a heavy flat-topped cylinder, which is sandwiched between the wheel rim and the wheel hub. The inside surface of the drum is acted upon by the linings of the brake shoes. When the brakes are applied, the brake shoes are forced into contact with the inside surface of the brake drums to slow the rotation of the wheels.

The drums are usually covered with fins on their outer surfaces to increase cooling. They are not cooled internally, because water could enter through the air vent cooling holes and braking would then be greatly impaired.

Drum Brake

Drum brakes are found on the rear wheels of most older cars, but they are increasingly being fazed out in favor of rear disc brakes. Drum brakes were standard equipment on all four wheels of most cars until the early 70's.


45

Brake cylinders, also called the "slave" cylinders, are cylinders in which movable piston(s) convert hydraulic brake fluid pressure into mechanical force. Hydraulic pressure against the piston(s) within the wheel cylinder forces the brake shoes or pads against the machined surfaces of the drum or rotor. There is one cylinder (or more in some systems) for each wheel. Drum brake wheel cylinders are usually made up of a cylindrical casting, an internal compression spring, two pistons, two rubber cups or seals, and two rubber boots to prevent entry of dirt and water. This type of wheel cylinder is fitted with push rods that extend from the outer side of each piston through a rubber boot, where they bear against the brake shoes. In disc brakes, the wheel cylinder is built into the caliper. All wheel cylinders have bleeder screws (or bleeder valves) to allow the system to be purged of air bubbles.

As the brake pedal is depressed, it moves pistons within the master cylinder, pressurizing the brake fluid in the brake lines and slave cylinders at each wheel. The fluid pressure causes the wheel cylinders' pistons to move, which forces the shoes or pads against the brake drums or rotors. Drum brakes use return springs to pull the pistons back away from the drum when the pressure is released. On disc brakes, the calipers' piston seals are designed to retract the piston slightly, thus allowing the pads to clear the rotor and thereby reduce rolling friction.

Parking (Emergency) Brakes

The parking brake (sometimes called the emergency brake) is a cable-activated system used to hold the brakes continuously in the applied position. The parking brake activates the brakes on the rear wheels. Instead of hydraulic pressure, a cable (mechanical) linkage is used to engage the brake shoes or discs. When the parking-brake pedal is pressed (or, in many cars, a hand lever is pulled), a steel cable draws the brake shoes or pads firmly against the drums or rotors. The release lever or button slackens the cables and disengages the brake shoes. The parking brake is self adjusting on most systems. An automatic adjuster compensates for lining (brake shoe) wear. On many cars, the parking brake is used to re-adjust the brake shoes as they wear in, or when the shoes are replaced. In these systems, the adjustment is made by repeatedly applying the parking brake while backing up.

The parking brake can be useful while driving up hills: If you're driving a manual transmission car, and you pull up to a stop on an incline, you might notice that you don't have enough feet to operate the clutch, brake, and gas at the same time. In other words, you will likely roll backwards slightly while getting started again. If a someone pulls up right behind you, this can be a problem. Your parking brake is useful in this situation: Apply the parking brake after you stop. When you want to go, release the clutch while pressing the gas, and release the parking brake. This keeps you from having to quickly switch your left foot from the brake to the clutch, or your right foot from the brake to the gas pedal. A little practice, and you'll be able to do it smoothly. Also, remember if you pull up behind someone who is stopped on a hill, give them extra room to roll back a little. Especially if it's a truck.

Some cars have no parking brake release! They automatically release the parking brake when the car is placed in drive or reverse.


46

Master Cylinder

The master cylinder displaces hydraulic pressure to the rest of the brake system. It holds the most important fluid in your car, the brake fluid. It actually controls two seperate subsystems which are jointly activated by the brake pedal. This is done so that in case a major leak occurs in one system, the other will still function. The two systems may be supplied by seperate fluid reservoirs, or they may be supplied by a common reservoir. Some brake subsystems are divided front/rear and some are diagonally separated. When you press the brake pedal, a push rod connected to the pedal moves the "primary piston" forward inside the master cylinder. The primary piston activates one of the two subsystems. The hydraulic pressure created, and the force of the primary piston spring, moves the secondary piston forward. When the forward movement of the pistons causes their primary cups to cover the bypass holes, hydraulic pressure builds up and is transmitted to the wheel cylinders. When the brake pedal retracts, the pistons allow fluid from the reservoir(s) to refill the chamber if needed.

Brakes master Cylinder.

Power Brakes

Power brakes (also called "power assisted" brakes) are designed to use the power of the engine and/or battery to enhance braking power. The four most common types of power brakes are: vacuum suspended; air suspended; hydraulic booster, and electro-hydraulic booster. Most cars use vacuum suspended units (vacuum boosters), which employ a vacuum-powered booster device to provide added thrust to the foot pressure applied.

In a vacuum booster type system, pressure on the brake pedal pushes forward a pushrod connected to the pistons within the master cylinder. At the same time, the pushrod opens the vacuum-control valve so that it closes the vacuum port and seals off the forward half of the booster unit. The engine vacuum line then creates a low-


47

pressure vacuum chamber. Atmospheric pressure in the control chamber then pushes against the diaphragm. The pressure on the diaphragm forces it forward, supplying pressure on the master cylinder pistons.

Hydraulic booster systems usually tap into the power steering pump's pressure, and use this power to augment pressure to the master cylinder. Electro-hydraulic booster systems use an electric motor to pressurize a hydraulic system which augments pressure to the master cylinder. This allows the vehicle to have power assisted brakes even if the engine quits.

Brake Fluid

Brake fluid is a special liquid for use in hydraulic brake systems, which must meet highly exact performance specifications. It is designed to be impervious to wide temperature changes and to not suffer any significant changes in important physical characteristics such as compressibility over the operating temperature range. The fluid is designed to not boil, even when exposed to the extreme temperatures of the brakes.

Different types of brake fluid are used in different systems, and should NEVER be mixed. Most cars use "DOT 3" or "DOT 4" brake fluid. Some newer cars use silicone brake fluids. These should NEVER be mixed together, because the seals in each car are designed to work with only their specific fluid types. For example, the mixing of "Silicone" brake fluid and conventional glycol based DOT 3 or DOT 4 fluids should be avoided, as the two fluid types are not miscible (they will not mix together). DOT 3 brake fluids and DOT 4 brake fluids can be mixed.

The table below gives some information on boiling points and viscosity of DOT fluids. Low temperature viscosity is important in brake systems equipped with ABS. Dry boiling point is the normal boiling point of the brake fluid and wet boiling point is the equilibrium boiling point of the fluid after it absorbed some moisture.

DOT 3

DOT4 DOT 5

Dry boiling point at least oC

205 230 260

Wet boiling point at least oC

140 155 180

Cold Viscosity at -40oC mm2/s

1500 1800 900

Brake Fluids


48

ABS Systems

Originally developed for aircraft, ABS basically works by limiting the pressure to any wheel which decelerates too rapidly. This allows maximum stopping force to be applied without brake lockup (skidding). If standard brakes are applied too hard, the wheels "lock" or skid, which prevents them from giving directional control. If directional control (steering) is lost, the vehicle skids in a straight line wherever it is going. ABS allows the driver to steer during hard braking, which allows you to control the car much better. In the old days, drivers had to know how to "pump" the brakes or sense the lockup and release foot pressure in order to prevent skidding. This meant that if only one wheel lost traction and started to skid, the driver would have to reduce braking force to prevent a skid. The advantage of ABS is that the brakes on the wheels with good traction can be used to the fullest possible amount, even if other wheels lose traction.

In operation, the wheelspeed sensors at each wheel send electronic pulse signals to the control unit. If wheel lockup (rapid deceleration) is detected during brake application, the computer signals the valve unit to limit the hydraulic pressure to the wheel cylinder. This is usually accomplished by diverting the fluid into a small reservoir. The fluid is later pumped out of the reservoir and returned to the main fluid reservoir when the brakes are not being applied.

There are four main components to an ABS system:

• Speed sensors • Valves • pump • Controller

The anti-lock braking system needs some way of knowing when a wheel is about to lock up, and this comes from the speed sensors. The speed sensors, which are located at each wheel, or in some cases in the differential, provide this information.

There is a valve in the brake line of each brake controlled by the ABS. On some systems, the valve has three positions:

• In position one, the valve is open; pressure from the master cylinder is passed right through to the brake.

• In position two, the valve blocks the line, isolating that brake from the master cylinder. This prevents the pressure from rising further should the driver push the brake pedal harder.

• In position three, the valve releases some of the pressure from the brake.

Since the valve is able to release pressure from the brakes, there has to be some way to put that pressure back. That is what the pump does; when a valve reduces the pressure in a line, the pump is there to get the pressure back up.


49

The controller is a computer in the car. It watches the speed sensors and controls the valves.

There are many different variations and control algorithms for ABS systems. One of the simpler systems works will be discussed.

The controller monitors the speed sensors at all times. It is looking for decelerations in the wheel that are out of the ordinary. Right before a wheel locks up, it will experience a rapid deceleration. If left unchecked, the wheel would stop much more quickly than any car could. It might take a car five seconds to stop from 60 mph (96.6 kph) under ideal conditions, but a wheel that locks up could stop spinning in less than a second.

ABS system

The ABS controller knows that such a rapid deceleration is impossible, so it reduces the pressure to that brake until it sees acceleration, then it increases the pressure until it sees the deceleration again. It can do this very quickly, before the tyre can actually significantly change speed. The result is that the tyre slows down at the same rate as the car, with the brakes keeping the tyres very near the point at which they will start to lock up. This gives the system maximum braking power.

When the ABS system is in operation you will feel a pulsing in the brake pedal; this comes from the rapid opening and closing of the valves. Some ABS systems can cycle up to 15 times per second.

Anti-lock brakes really do help you stop better. They prevent wheels from locking up and provide the shortest stopping distance on slippery surfaces. But do they really prevent accidents? This is the true measure of the effectiveness of ABS systems.

The Insurance Institute for Highway Safety (IIHS) has conducted several studies trying to determine if cars equipped with ABS are involved in more or fewer fatal accidents. It turns out that in a 1996 study, vehicles equipped with ABS were overall no less likely to be involved in fatal accidents than vehicles without. The study actually stated that although cars with ABS were less likely to be involved in

1. Brakes master cylinder 2. Hydraulic pressure modulator 3. Damper chamber 4. Return pump 5. Engine 6. Accumulator 7. Inlet valves 8. Exhaust Valves


50

accidents fatal to the occupants of other cars, they are more likely to be involved in accidents fatal to the occupants of the ABS car, especially single-vehicle accidents.

There is much speculation about the reason for this. Some people think that drivers of ABS-equipped cars use the ABS incorrectly, either by pumping the brakes or by releasing the brakes when they feel the system pulsing. Some people think that since ABS allows you to steer during a panic stop, more people run off the road and crash.

Some more recent information may indicate that the accident rate for ABS cars is improving, but there is still no evidence to show that ABS improves overall safety.


51

5. Steering mechanism and Tyres

Definition and Purpose The function of Steering system is thought of simply as that of providing means whereby the driver can place a vehicle as accurately as practicable where is desired to be on the road, so as to avoid other road users and obstructions. It must also keep the vehicle stable on course regardless of irregularities in the surface over which the vehicle is travelling. When a vehicle is turning, front wheels are not pointing in the same direction as shown in the figure below.

Illustration of different angles when the wheels turn

For a car to turn smoothly, each wheel must follow a different circle. Since the inside wheel is following a circle with a smaller radius, it is actually making a tighter turn than the outside wheel. If you draw a line perpendicular to each wheel, the lines will intersect at the center point of the turn. The geometry of the steering linkage makes the inside wheel turn more than the outside wheel.

Main Types of steering wheels Rack and Pinion Rack-and-pinion steering is quickly becoming the most common type of steering on many vehicles. It is actually a pretty simple mechanism. A rack-and-pinion gearset is enclosed in a metal tube, with each end of the rack protruding from the tube. A rod, called a tie rod, connects to each end of the rack.

The pinion gear is attached to the steering shaft. When you turn the steering wheel, the gear spins, moving the rack. The tie rod at each end of the rack connects to the steering arm on the spindle. This illustrated on the figure in next page.


52

The rack-and-pinion gearset does two things:

• It converts the rotational motion of the steering wheel into the linear motion needed to turn the wheels.

• It provides a gear reduction, making it easier to turn the wheels.

Rack and Pinion steering type

On most cars, it takes three to four complete revolutions of the steering wheel to make the wheels turn from lock to lock (from far left to far right).

The steering ratio is the ratio of how far you turn the steering wheel to how far the wheels turn. For instance, if one complete revolution (360 degrees) of the steering wheel results in the wheels of the car turning 20 degrees, then the steering ratio is 360 divided by 20, or 18:1. A higher ratio means that you have to turn the steering wheel more to get the wheels to turn a given distance. However, less effort is required because of the higher gear ratio.

Generally, lighter, sportier cars have lower steering ratios than larger cars and trucks. The lower ratio gives the steering a quicker response - you don't have to turn the steering wheel as much to get the wheels to turn a given distance - which is a desirable trait in sports cars. These smaller cars are light enough that even with the lower ratio, the effort required to turn the steering wheel is not excessive.

Some cars have variable-ratio steering, which uses a rack-and-pinion gearset that has a different tooth pitch (number of teeth per cm) in the center than it has on the outside. This makes the car respond quickly when starting a turn (the rack is near the center), and also reduces effort near the wheel's turning limits.


53

Recirculating-ball steering Recirculating-ball steering is used on many trucks and SUVs today. The linkage that turns the wheels is slightly different than on a rack-and-pinion system.

Recirculating-ball Steering System

The recirculating-ball steering gear contains a worm gear. You can image the gear in two parts. The first part is a block of metal with a threaded hole in it. This block has gear teeth cut into the outside of it, which engage a gear that moves the pitman arm (see diagram above). The steering wheel connects to a threaded rod, similar to a bolt, that sticks into the hole in the block. When the steering wheel turns, it turns the bolt. Instead of twisting further into the block the way a regular bolt would, this bolt is held fixed so that when it spins, it moves the block, which moves the gear that turns the wheels.

Recirculating-ball gearbox


54

Instead of the bolt directly engaging the threads in the block, all of the threads are filled with ball bearings that recirculate through the gear as it turns. The balls actually serve two purposes: First, they reduce friction and wear in the gear; second, they reduce slop in the gear. Slop would be felt when you change the direction of the steering wheel - without the balls in the steering gear, the teeth would come out of contact with each other for a moment, making the steering wheel feel loose.

Power Assisted Steering

There are a couple of key components in power steering in addition to the rack-and-pinion or recirculating-ball mechanism which are the pump and the rotary valve. When the rack-and-pinion is in a power-steering system, the rack has a slightly different design.

Rack and pinion power assisted steering

A power-steering system should assist the driver only when he is exerting force on the steering wheel (such as when starting a turn). When the driver is not exerting force (such as when driving in a straight line), the system shouldn't provide any assist. The device that senses the force on the steering wheel is called the rotary valve.

The key to the rotary valve is a torsion bar. The torsion bar is a thin rod of metal that twists when torque is applied to it. The top of the bar is connected to the steering wheel, and the bottom of the bar is connected to the pinion or worm gear (which turns the wheels), so the amount of torque in the torsion bar is equal to the amount of torque the driver is using to turn the wheels. The more torque the driver uses to turn the wheels, the more the bar twists.

The input from the steering shaft forms the inner part of a spool-valve assembly. It also connects to the top end of the torsion bar. The bottom of the torsion bar connects to the outer part of the spool valve. The torsion bar also turns the output of the steering gear, connecting to either the pinion gear or the worm gear depending on which type of steering the car has.


55

As the bar twists, it rotates the inside of the spool valve relative to the outside. Since the inner part of the spool valve is also connected to the steering shaft (and therefore to the steering wheel), the amount of rotation between the inner and outer parts of the spool valve depends on how much torque the driver applies to the steering wheel.

When the steering wheel is not being turned, both hydraulic lines provide the same amount of pressure to the steering gear. But if the spool valve is turned one way or the other, ports open up to provide high-pressure fluid to the appropriate line.

The hydraulic power for the steering is provided by a rotary-vane pump (see diagram below). This pump is driven by the car's engine via a belt and pulley. It contains a set of retractable vanes that spin inside an oval chamber.

Hydraulic pump

As the vanes spin, they pull hydraulic fluid from the return line at low pressure and force it into the outlet at high pressure. The amount of flow provided by the pump depends on the car's engine speed. The pump must be designed to provide adequate flow when the engine is idling. As a result, the pump moves much more fluid than necessary when the engine is running at faster speeds.

The pump contains a pressure-relief valve to make sure that the pressure does not get too high, especially at high engine speeds when so much fluid is being pumped.

Power steering in a recirculating-ball system works similarly to a rack-and-pinion system. Assist is provided by supplying higher-pressure fluid to one side of the block.


56

Tyres

Their primary function is to provide a comfortable ride by absorbing the high frequency and low amplitude disturbances generated by its rolling along the road. Also tyres are subject to great forces during braking, acceleration, steering and so on.

Some design requirements of tyres are as follows:

• Adequate capacity to support both static and dynamic loads • Ability to withstand centrifugal loading • Structural Stability • Reasonable protection against abuse. (side wall strength) • Cooling to avoid overheating • Good grip on the road, in both wet and dry conditions • Light weight to provide good ride, ease of control and fuel economy • Long life • Low cost

Tyre construction varies and the main categories are the cross-ply and radial-ply tyres. Plies are layers of wires and fabric reinforcement. In cross-ply tyres they are set at an angle of 45o with each alternate layer being orientated in the opposite sense to those above and below it. Radial-ply tyres tend to be stiffer in resisting loading than cross-ply tyre. This reduces the under- and over-steering.

Now let say that a tyre provides the following information P235/75 R15 105S. Each letter and number has a specific meaning which is given below:

o The P designates that the tyre is a passenger vehicle tyre. Some other designations are LT for light truck, and T for temporary, or spare tyres.

o The 235 is the width of the tyre in millimeters (mm), measured from sidewall to sidewall. Since this measure is affected by the width of the rim, the measurement is for the tyre when it is on its intended rim size.

o This number tells you the height of the tyre, from the bead to the top of the tread. This is described as a percentage of the tyre width. In our example, the aspect ratio is 75, so the tyre's height is 75 percent of its width, or 176.25 mm ( .75 x 235 = 176.25 mm). The smaller the aspect ratio, the wider the tyre in relation to its height.

o The R designates that the tyre was made using radial construction. This is the most common type of tyre construction. Older tyres were made using diagonal bias (D) or bias belted (B) construction. A separate note indicates how many plies make up the sidewall of the tyre and the tread.

o This number specifies, in inches, the wheel rim diameter the tyre is designed for.

o The load rating is a number that correlates to the maximum rated load for that tyre. A higher number indicates that the tyre has a higher load capacity. The rating "105," for example, corresponds to a load capacity of 925kg. A separate note on the tyre indicates the load rating at a given inflation pressure.See table below


57

o The letter that follows the load rating indicates the maximum speed allowable for this tyre (as long as the weight is at or below the rated load). For instance, S indicates that the tyre can handle speeds up to 180km/h. See table below

Maximum Load-carrying per tyre

Speed Symbols Additional information can be found on tyres which include:

• Tread Wear: This number comes from testing the tyre in controlled conditions on a government test track. The higher the number, the longer you can expect the tread to last. Since no one will drive his or her car on exactly the same surfaces and at the same speeds as the government test track, the number is not an accurate indicator of how long your tread will actually last. It's a good relative measure, however: You can expect a tyre with a larger number to last longer than one with a smaller number.


58

• Traction: Tyre traction is rated AA, A, B or C, with AA at the top of the scale. This rating is based on the tyre's ability to stop a car on wet concrete and asphalt. It does not indicate the tyre's cornering ability.

• Temperature: The tyre temperature ratings are A, B or C. The rating is a measure of how well the tyre dissipates heat and how well it handles the build-up of heat. The temperature grade applies to a properly inflated tyre that is not overloaded. Under-inflation, overloading or excessive speed can lead to more heat build-up. Excessive heat build-up can cause tyres to wear out faster, or could even lead to tyre failure.

• If a tyre has MS, M+S, M/S or M&S on it, then it meets the Rubber Manufacturers Association (RMA) guidelines for a mud and snow tyre. For a tyre to receive the Mud and Snow designation, it must meet these geometric requirements

High performance tyres usually have a lower aspect ratio than other tires. This is because tyres with a lower aspect ratio provide better lateral stability. When a car goes around a turn lateral forces are generated and the tire must resist these forces. Tyres with a lower profile have shorter, stiffer sidewalls so they resist cornering forces better.

Aspect Ratio (low profile to normal tyres)

So low profile tyres provide improved handling and grip and more traction and braking power. However they provide harsher ride and increased road noise. Another important aspect of tyres is the tyre pressure which is usually set to 2.1 bar or 30 psi (pounds per square inch). Over-inflation will cause rapid wear of the centre part of tyre tread, coupled with reduce grip, harsher ride and the danger of shock damage occurring in the tyre casing. This is known a centre wear. Shoulder wear (wear on both sides) can occur in tyres if is under-inflated. This will cause loss of grip and excessive wear, not to mention the danger of sudden tyre failure due to heat build-up. If wear is on the one side then the camber angle should be checked or the speed of cornering should be reduced.


59

Uneven wear can occur in tyres due to various reasons mainly concerned with suspension settings such as camber and castor, toe setting, unbalanced wheels and malfunction in suspensions.

Wear patterns of tyres with different conditions


60

6. Suspension Systems Obviously, if the loads applied to the rolling wheels of a vehicle were transmitted directly to the chassis, not only would its occupants suffer severely but also its structure would be subjected to an excessive degree of fatigue loading. The primary function of the suspension system, therefore, is to isolate the structure, so far as is practicable, from shock loading and vibration due to irregularities of the road surface. Secondly, it must do this without impairing the stability, steering or general handling qualities of the vehicle. The primary requirement is met by the use of flexible elements and dampers, while the second is achieved by controlling, by the use of mechanical linkages, the relative motions between the unsprung masses - wheel-and-axle assemblies - and the sprung mass. These linkages may be either as simple as a semielliptic spring and shackle or as complex as a double transverse link and anti-roll bar or some other such combination of mechanisms. Spring Types

Springs generally reduce the upward acceleration that a vehicle is subjected to. The force on the spring acts on a small unsprung mass. Since the variations in the spring force are relatively small, the downward acceleration of the carriage unit supported by the spring is correspondingly reasonable as compared with that which under the influence of gravity it would be if there was no spring.

The most common spring types are coil springs, torsion bars and leaf springs. Coil springs are what most people are familiar with, and are actually coiled torsion bars.

Leaf springs are what you would find on almost all heavy duty vehicles. They look like layers of metal connected to the axle. The layers are called leaves, hence leaf-spring.

The torsion bar on its own is a bizarre little mechanism which gives coiled-spring-like performance based on the twisting properties of a steel bar. It's used in the suspension of VW Beetles, air-cooled Porsches (356 and 911 until 1989 when they went to springs), and the rear suspension of small European vehicles amongst other cars. Torsion bar suspension uses the flexibility of a steel bar or tube, twisting lengthwise to provide spring action. Instead of the flexing action of a leaf spring, or the compressing-and-extending action of a coil spring, the torsion bar twists to exert resistance against up-and-down movement. Two rods of spring steel are used in this type of suspension. One end of the bar is fixed solidly to a part of the frame behind the wheel; the other is attached to the lower control arm. As the arm rises and falls with wheel movement, the bar twists and absorbs more of the road shocks before they can reach the body of the car. The bar untwists when the pressure is released, just like a spring rebounding after being compressed.


61

Torsion bars and coil springs

Shock Absorbers (dampers)

In the past, a wide variety of direct and indirect shock absorbing devices were used to control spring action of passenger cars. Today, direct, double-acting hydraulic shock absorbers and shock absorber struts have almost universal application.

Typical dampers

The operating principle of direct-acting hydraulic shock absorbers is in forcing fluid through restricting openings in the valves. This restricted flow serves to slow down and control rapid movement in the car springs as they react to road irregularities. Usually, fluid flow through the pistons is controlled by spring-loaded valves. Hydraulic shock absorber automatically adapt to the severity of the shock. If the axle moves slowly, resistance to the flow of fluid will be light. If the axle movement is rapid or violent, the resistance is stronger, since more time is required to force fluid

This has the effect of giving the spring increasing resistance, the more it is compressed


62

through the openings. By these actions and reactions, the shock absorbers permit a soft ride over small bumps and provide firm control over spring action for cushioning large bumps. The double-acting units must be effective in both directions because spring rebound can be almost as violent as the original action that compressed the shock absorber.

Suspensions types

There are number of different types of suspension available for both front and rear axles. The main groupings are dependant and independent suspension types.

PASSIVE SUSPENSION SYSTEMS

Front suspension - dependent systems

So-called because the front wheel's suspension systems are physically linked. For everyday use. There is only one type of dependant system you need to know about. It is basically a solid bar under the front of the car, kept in place by leaf springs and shock absorbers. It's still common to find these on trucks. They haven't been used on mainstream cars for years for three main reasons:

• Because the wheels are physically linked, the beam can be set into oscillation if one wheel hits a bump and the other does not. It sets up a gyroscopic torque about the steering axis which starts to turn the axle left-to-right. Because of the axle's inertia, this in turn feeds back to amplify the original motion.

• Weight or more specifically unsprung weight. Solid front axles weigh a lot and need huge springs to keep their wheels on the road.

• You cannot adjust the alignment of wheels on a rigid axis. From the factory, they are perfectly set, but if the beam gets even slightly distorted, you cannot adjust the wheels to compensate.

It must be admitted that for off-road use, it probably is pretty good.

Front suspension - independent systems

So-named because the front wheel's suspension systems are independent of each other (except where joined by an antiroll bar) These came into existance around 1930 and have been in use in one form or another pretty much ever since then.

MacPherson strut is currently, without doubt, the most widely used front suspension system in cars of European origin. It is simple. The system basically comprises of a strut-type spring and shock absorber combo, which pivots on a ball joint on the single, lower arm. At the top end there is a needle roller bearing. The strut itself is the load-bearing member in this assembly, with the spring and shock absorber merely performing their duty as oppose to actually holding the car up. In the rendered image


63

here, you can't see the shock absorber because it is encased in the strut tower, inside the spring.

MacPherson strut

The steering gear is either connected directly to the lower shock absorber housing, or to an arm from the front or back of the spindle (in this case). When you steer, it physically twists the shock absorber housing (and consequently the spring) to turn the wheel. The spring is seated in a special plate at the top of the assembly which allows this twisting to take place.

There are many variations to the Mac-Pherson strut and the pictures below illustrate some. Generally they include greater complexity and the number of control arms is doubled. The control arms are called wishbones because of their shape. The strut assembly is separated and there is a damper and a coil spring. The spindle is much more complex to contract as well.

Bearing

Coil Spring

Shock Absorber

Strut assembly

Axis of Rotation

Control Arm Ball joint

Spindle

Steering Ball joint

Pivoting Points

Control Arm


64

Coil spring types

Double wishbone systems (left picture below) rapidly become one of the most favoured suspension types for new cars as it gives excellent roadholding capabilities whilst taking up very little room under the car. This allows for smoother lines on the bodywork, and less intrusion in to the engine bay. Multi-link is the latest incarnation of the double wishbone system described above. It's currently being used in the Audi A8 and A4 amongst other cars. The basic principle of it is the same, but instead of solid upper and lower wishbones, each 'arm' of the wishbone is a separate item. These are joined at the top and bottom of the spindle thus forming the wishbone shape. The advantage is that as the spindle turns for steering, it alters the geometry of the suspension by torquing all four suspension arms. They have complex pivot systems designed to allow this to happen. Car manufacturers claim that this system gives even better road-holding properties, because all the various joints make the suspension almost infinitely adjustable. There are a few variations on this theme appearing at the moment, with differences in the numbers of joints, numbers of arms, positioning of the parts etc. But they are all fundamentally the same.

Double wishbone and multi-link front suspension systems

Rear suspension - dependant systems

Contrary to the front version of this system, many many cars are still designed and built with dependant (linked) rear suspension systems.


65

Solid-axle, leaf-spring

This system was favoured by the Americans for years because it was dead simple and cheap to build. The ride quality is decidedly questionable though. The drive axle (purple in this image) is clamped (green) to the leaf springs (red). The shock absorbers (yellow) are also attached to the clamps. The ends of the leaf springs are attached directly to the chassis, as are the shock absorbers. Simple, not particularly elegant, but cheap. The main drawback with this arrangement is the lack of lateral location for the axle.

Solid rear axle with leaf springs

Solid-axle, coil-spring

This is a variation and update on the system described above. The basic idea is the same, but the leaf springs have been removed in favour of coil springs and shock absorbers. Because the leaf springs have been removed, the axle now needs to have lateral support from a pair control arms. The front ends of these are attached to the chassis, the rear ends to the axle. A variation on this has the shock absorbers separate from the springs, allowing much smaller springs. This in turn allows the system to fit in a smaller area under the car.

Leaf-springs

De-dion Axle

Clamp

Shock Absorbers


66

Solid rear axle with coil springs and control arms There are many other variations to dead axles and are illustrated in the figures below.

4-bay Systems

Beam Axle

Rear suspension - independent systems It follows, that what can be fitted to the front of a car, can be fitted to the rear to without the complexities of the steering gear. Simplified versions of all the independent systems described above can be found on the rear axles of cars. The multi-link system is currently becoming more and more popular. In advertising, it's put across as '4-wheel independent suspension'. This means all the wheels are independently mounted and sprung. There are two schools of thought as to whether


67

this system is better or worse for handling than, for example, Macpherson struts and a twist axle. The drive towards 4-wheel independent suspension is primarily to improve ride quality without degrading handling. Other Types of Suspension Systems Hydrolastic Suspension The front and rear suspension units have Hydrolastic displacers, one per side. These are interconnected by a small bore pipe. Each displacer incorporates a rubber spring (as in the Moulton rubber suspension system), and damping of the system is achieved by rubber valves. So when a front wheel is deflected, fluid is displaced to the corresponding suspension unit. That pressurises the interconnecting pipe which in turn stiffens the rear wheel damping and lowers it. The rubber springs are only slightly brought into play and the car is effectively kept level and freed from any tendency to pitch. Hydragas Suspension Hydragas is an evolution of Hydrolastic, and essentially, the design and installation of the system is the same. The difference is in the displacer unit itself. In the older systems, fluid was used in the displacer units with a rubber spring cushion built-in. With Hydragas, the rubber spring is removed completely. The fluid still exists but above the fluid there is now a separating membrane or diaphragm, and above that is a cylinder or sphere which is charged with nitrogen gas. The nitrogen section is what has become the spring and damping unit whilst the fluid is still free to run from the front to the rear units and back.

Displacer units Hydropneumatic Suspension


68

Since the early fifties Citroen have been running a fundamentally different system to the rest of the auto industry. They call it hydropneumatic and it encompasses features as diverse as brakes, suspension & steering. As its name may suggest, its core technology and mainstay of its functionality is hydraulics. Superbly smooth suspension is provided by the fluid's interaction with a presurised gas. The system is powered by a large hydraulic pump operated directly by the engine in much the same way as an alternator or an air conditioner is, and provides fluid to an "accumulator" at pressure, where it is stored ready to be delivered to servo a system.

Hydropneumatic Suspension ACTIVE SUSPENSION Linear Electromagnetic Suspension The idea is that instead of springs and shock absorbers on each corner of the car, a single liner electromagnetic motor and power amplifier can be used instead. One of the big advantages of an electromagnetic approach is speed. The linear electromagnetic motor responds quickly enough to counter the effects of bumps and potholes, thus allowing it to perform the actions previously reserved for shock absorbers.

Linear Electromagnetic Suspension


69

In its second mode of operation, the system can be used to counter body roll by stiffening the suspension in corners. As well as these functions, it can also be used to raise and lower ride height dynamically. These types of suspension are called active suspension systems as the act to road anomalies to provide a smoother ride. The main drawback is that the system cannot compensate for situations beyond its design parameters.

Anti-roll Bars

Stabilizer bars are part of a car's suspension system. They are sometimes also called anti-sway bars or anti-roll bars. Their purpose is to try to keep the car's body from "rolling" in a sharp turn. Think about what happens to a car in a sharp turn. If you are inside the car, you know that your body gets pulled toward the outside of the turn. The same thing is happening to all the parts of the car. So the part of the car on the outside of the turn gets pushed down toward the road and the part of the car on the inside of the turn rises up. In other words, the body of the car "rolls" 10 or 20 or 30 degrees toward the outside of the turn. If you take a turn fast enough, the tires on the inside of the turn actually rise off the road and the car flips over. Roll is bad. It tends to put more weight on the outside tires and less weigh on the inside tires, reducing traction. It also messes up steering. What you would like is for the body of the car to remain flat through a turn so that the weight stays distributed evenly on all four tires.

Anti-roll bar A stabilizer bar tries to keep the car's body flat by moving force from one side of the body to another. To picture how a stabilizer bar works, imagine a metal rod that is an


70

2 to 5 cm in diameter. If your front tires are 1.6 meters apart, make the rod about 1.2 meters long. Attach the rod to the frame of the car in front of the front tires, but attach it with bushings in such a way that it can rotate. Now attach arms from the rod to the front suspension member on both sides. When you go into a turn now, the front suspension member of the outside of the turn gets pushed upward. The arm of the sway bar gets pushed upward, and this applies torsion to the rod. The torsion them moves the arm at the other end of the rod, and this causes the suspension on the other side of the car to compress as well. The car's body tends to stay flat in the turn. If you don't have a stabilizer bar, you tend to have a lot of trouble with body roll in a turn. If you have too much stabilizer bar, you tend to lose independence between the suspension members on both sides of the car. When one wheel hits a bump, the stabilizer bar transmits the bump to the other side of the car as well, which is not what you want. The ideal is to find a setting that reduces body roll but does not hurt the independence of the tires.

Sprung vs. unsprung weight.

Sprung weight is everything from the springs up, and unsprung weight is everything from the springs down. Wheels, shock absorbers, springs, and tyres contribute to the unsprung weight. The car, engine, fluids, passengers, luggage all contribute to the sprung weight. Reducing unsprung weight is the key to increasing performance of the car. If you can make the wheels, tyres and swingarms lighter, then the suspension will spend more time compensating for bumps in the road, and less time compensating for the mass of the wheels etc. The greater the unsprung weight, the greater the inertia of the suspension, which will be unable to respond as quickly to rapid changes in the road surface. As an added benefit, putting lighter wheels on the car can increase your engine's apparent power. Why? Well the engine has to turn the gearbox and driveshafts, and at the end of that, the wheels and tyres. Heavier wheels and tyres require more torque to get turning, which saps engine power. Lighter wheels and tyres allow more of the engine's torque to go into getting you going than spinning the wheels. That's why sports cars have carbon fibre driveshafts and ultralight alloy wheels.


71

7. Body Equipment and Safety systems

Modern vehicles are loaded with extras and accessories so as to provide the occupants with greater degree of comfort, pleasure and safety. This chapter will list the main component terminology and will provide some information of how the safety systems work.

Vehicle body components

1. Body 2. Front fender 3. Front section 4. Bonnet 5. Front floor section 6. Rear floor section 7. Body side including rear fender 8. Firewall 9. Roof section 10. Wheel housing 11. Doors 12. Trunk lid (Boot lid) 13. Bumpers (not shown in the picture)

Interior Equipment The major interior equipment found in all vehicles is the seats, door panels, dashboard assembly, instruments, switches, etc. Some of these are illustrated below:

2

8

10


72

Front Seat Door panel

Dashboard assembly Steering Switches


73

It is obvious that each component is further divided into many smaller parts. For example the dashboard has many components such as the air outlets (2, 11, 12, 13), the glove compartment (17), covers and screws and so many other parts. Off-course the complexity is increased since the interior dashboard’s and seats’ colour differs so as to suit the customer choice, preference and individual taste. This increases the number of parts dramatically. Safety Systems Seat Belts According to a recent research report from seatbelts save 13,000 lives in the United States each year. Meanwhile, estimates reveal that 7,000 U.S. car accident fatalities would have been avoided if the victims had been wearing belts. While seatbelts do occasionally contribute to serious injury or death, nearly all safety experts agree that buckling up dramatically increases your chances of surviving an accident. Seatbelts reduce the risk of death for a front seat car occupant by about 50 percent. The basic idea of a seatbelt is very simple: It keeps you from flying through the windshield or hurdling toward the dashboard when your car comes to an abrupt stop. Inertia is an object's tendency to keep moving until something else works against this motion. To put it another way, inertia is every object's resistance to changing its speed and direction of travel. Things naturally want to keep going. If a car is speeding along at 50 miles per hour, inertia wants to keep it going 50 mph in one direction. Air resistance and friction with the road are constantly slowing it down, but the engine's power compensates for this energy loss.

Anything that is in the car, including the driver and passengers, has its own inertia, which is separate from the car's inertia. The car accelerates riders to its speed. Imagine that you're coasting at a steady 50 miles per hour. Your speed and the car's speed are pretty much equal, so you feel like you and the car are moving as a single unit.

But if the car were to crash into a telephone pole, it would be obvious that your inertia and the car's were absolutely independent. The force of the pole would bring the car to an abrupt stop, but your speed would remain the same. Without a seatbelt, you would either slam into the steering wheel at 50 miles per hour or go flying through the windshield at 50 miles per hour. Just as the pole slowed the car down, the dashboard, windshield or the road would slow you down by exerting a tremendous amount of force.

It is a given that no matter what happens in a crash, something would have to exert force on you to slow you down. But depending on where and how the force is applied, you might be killed instantly or you might walk away from the damage unscathed. If you hit the windshield with your head, the stopping power is concentrated on one of the most vulnerable parts of your body. It also stops you very quickly, since the glass is a hard surface. This can easily kill or severely injure a person.


74

A seatbelt applies the stopping force to more durable parts of the body over a longer period of time. A seatbelt's job is to spread the stopping force across sturdier parts of your body in order to minimize damage.

A typical seatbelt consists of a lap belt, which rests over your pelvis, and a shoulder belt, which extends across your chest. The two belt sections are tightly secured to the frame of the car in order to hold passengers in their seats.

When the belt is worn correctly, it will apply most of the stopping force to the rib cage and the pelvis, which are relatively sturdy parts of the body. Since the belts extend across a wide section of your body, the force isn't concentrated in a small area, so it can't do as much damage. Additionally, the seatbelt webbing is made of more flexible material than the dashboard or windshield. It stretches a little bit, which means the stop isn't quite so abrupt. The seatbelt shouldn't give more than a little, however, or you might bang into the steering wheel or side window. Safe seatbelts will only let you shift forward slightly. A car's crumple zones do the real work of softening the blow. Crumple zones are areas in the front and rear of a car that collapse relatively easily. Instead of the entire car coming to an abrupt stop when it hits an obstacle, it absorbs some of the impact force by flattening, like an empty soda can. The car's cabin is much sturdier, so it does not crumple around the passengers. It continues moving briefly, crushing the front of the car against the obstacle. Of course, crumple zones will only protect you if you move with the cab of the car - that is, if you are secured to the seat by your seatbelt. The simplest sort of seatbelt, found in some roller coasters, consists of a length of webbing bolted to the body of the vehicle. These belts hold you tightly against the seat at all times, which is very safe but not particularly comfortable. This is shown in the figure below.

Seat belt mechanism


75

Car seatbelts have the ability to extend and retract so that you can lean forward easily while the belt stays fairly taut. But in a collision, the belt will suddenly tighten up and hold you in place. In a typical seatbelt system, the belt webbing is connected to a retractor mechanism. The central element in the retractor is a spool, which is attached to one end of the webbing. Inside the retractor, a spring applies a rotation force, or torque, to the spool. This works to rotate the spool so it winds up any loose webbing. When you pull the webbing out, the spool rotates counter-clockwise, which turns the attached spring in the same direction. Effectively, the rotating spool works to untwist the spring. The spring wants to return to its original shape, so it resists this twisting motion. If you release the webbing, the spring will tighten up, rotating the spool clockwise until there is no more slack in the belt. The retractor has a locking mechanism that stops the spool from rotating when the car is involved in a collision. There are two sorts of locking systems in common use today:

• systems triggered by the car's movement • systems triggered by the belt's movement

The first sort of system locks the spool when the car rapidly decelerates (when it hits something, for example). The diagram below shows the simplest version of this design.

Locking System in Seat belts (Relative to car’s movement)

The second kind of system locks the spool when something jerks the belt webbing. The activating force in most designs is the speed of the spool rotation. The diagram shows a common configuration.


76

Locking System in Seat belts (Relative to belt’s movement) In some newer seatbelt systems, a pretensioner also works to tighten the belt webbing. The idea of a pretensioner is to tighten up any slack in the belt webbing in the event of a crash. Whereas the conventional locking mechanism in a retractor keeps the belt from extending any farther, the pretensioner actually pulls in on the belt. This force helps move the passenger into the optimum crash position in his or her seat. Pretensioners normally work together with conventional locking mechanisms, not in place of them. There are a number of different pretensioner systems on the market. Some pretensioners pull the entire retractor mechanism backward and some rotate the spool itself. Generally, pretensioners are wired to the same central control processor that activates the car's air bags. The processor monitors mechanical or electronic motion sensors that respond to the sudden deceleration of an impact. When an impact is detected, the processor activates the pretensioner and then the air bag. Some pretensioners are built around electric motors or solenoids, but the most popular designs today use pyrotechnics to pull in the belt webbing. The diagram below shows a representative model. The central element in this pretensioner is a chamber of combustible gas. Inside the chamber, there is a smaller chamber with explosive igniter material. This smaller chamber is outfitted with two electrodes, which are wired to the central processor.


77

When the processor detects a collision, it immediately applies an electrical current across the electrodes. The spark from the electrodes ignites the igniter material, which combusts to ignite the gas in the chamber. The burning gas generates a great deal of outward pressure. The pressure pushes on a piston resting in the chamber, driving it upward at high speed.

Pretensioner - When the gas is ignited, the pressure pushes the piston up to rotate the retractor.

In severe crashes, when a car collides with an obstacle at extremely high speed, a seatbelt can inflict serious damage. As a passenger's inertial speed increases, it takes a greater force to bring the passenger to a stop. In other words, the faster you're going on impact, the harder the seatbelt will push on you. Some seatbelt systems use load limiters to minimize belt-inflicted injury. The basic idea of a load limiter is to release a little more excess belt webbing when a great deal of force is applied to the belt. The simplest load limiter is a fold sewn into the belt webbing. The stitches holding the fold in place are designed to break when a certain amount of force is applied to the belt. When the stitches come apart, the webbing unfolds, allowing the belt to extend a little bit more. More advanced load limiters rely on a torsion bar in the retractor mechanism. A torsion bar is just a length of metal material that will twist when enough force is applied to it. In a load limiter, the torsion bar is secured to the locking mechanism on one end and the rotating spool on the other. In a less severe accident, the torsion bar will hold its shape, and the spool will lock along with the locking mechanism. But


78

when a great deal of force is applied to the webbing (and therefore the spool), the torsion bar will twist slightly. This allows the webbing to extend a little bit farther. Airbags or Supplemental Restraint System (SRS) Air bags have been under development for many years. The attraction of a soft pillow to land against in a crash must be very strong -- the first patent on an inflatable crash-landing device for airplanes was filed during World War II! In the 1980s, the first commercial air bags appeared in automobiles. Since model year 1998, most new cars have been required to have air bags on both driver and passenger sides. To date, statistics show that air bags reduce the risk of dying in a direct frontal crash by about 30 percent. Newer than steering-wheel-mounted or dashboard-mounted bags, are seat-mounted and door-mounted side air bags. Having evoked some of the same controversy that surrounded seat-belt use in its early years, air bags are the subject of serious government and industry research and tests. So according to the laws of motion: Moving objects have momentum (the product of the mass and the velocity of an object). Unless an outside force acts on an object, the object will continue to move at its present speed and direction. Cars consist of several objects, including the vehicle itself, loose objects in the car and, of course, passengers. If these objects are not restrained, they will continue moving at whatever speed the car is traveling at, even if the car is stopped by a collision. Stopping an object's momentum requires force acting over a period of time. When a car crashes, the force required to stop an object is very great because the car's momentum has changed instantly while the passengers' has not - there is not much time to work with. The goal of any supplemental restraint system (SRS) is to help stop the passenger while doing as little damage to him or her as possible. What an air bag wants to do is to slow the passenger's speed to zero with little or no damage. The constraints that it has to work within are huge. The air bag has the space between the passenger and the steering wheel or dash board and a fraction of a second to work with. Even that tiny amount of space and time is valuable, however, if the system can slow the passenger evenly rather than forcing an abrupt halt to his or her motion. There are three parts to an air bag that help to accomplish this feat:

• The bag itself is made of a thin, nylon fabric, which is folded into the steering wheel or dashboard or, more recently, the seat or door.

• The sensor is the device that tells the bag to inflate. Inflation happens when there is a collision force equal to running into a brick wall at 10 to 15 miles per hour (16 to 24 km per hour). A mechanical switch is flipped when there is a mass shift that closes an electrical contact, telling the sensors that a crash has occurred. The sensors receive information from an accelerometer built into a microchip.


79

• The air bag's inflation system reacts sodium azide (NaN3) with potassium nitrate (KNO3) to produce nitrogen gas. Hot blasts of the nitrogen inflate the air bag.

The inflation system is not unlike a solid rocket booster (see How Rocket Engines Work for details). The air bag system ignites a solid propellant, which burns extremely rapidly to create a large volume of gas to inflate the bag. The bag then literally bursts from its storage site at up to 200 mph (322 kph) - faster than the blink of an eye! A second later, the gas quickly dissipates through tiny holes in the bag, thus deflating the bag so you can move.

The steering Air-bag

Inflator

Even though the whole process happens in only one-twenty-fifth of a second, the additional time is enough to help prevent serious injury. The powdery substance released from the air bag is regular cornstarch or talcum powder, which is used by the air bag manufacturers to keep the bags pliable and lubricated while they're in storage.


80

It didn't take long to learn that the force of an air bag can hurt those who are too close to it. Researchers have determined that the risk zone for driver air bags is the first 2 to 3 inches (5 to 8 cm) of inflation. Until recently, most of the strides made in auto safety were in front and rear impacts, even though 40 percent of all serious injuries from accidents are the result of side impacts, and 30 percent of all accidents are side-impact collisions. Many carmakers have responded to these statistics (and the resulting new standards) by beefing up doors, door frames and floor and roof sections. But cars that currently offer side air bags represent the new wave of occupant protection. Engineers say that designing effective side air bags is much more difficult than designing front air bags. This is because much of the energy from a front-impact collision is absorbed by the bumper, hood and engine, and it takes almost 30 to 40 milliseconds before it reaches the car's occupant. In a side impact, only a relatively thin door and a few inches separate the occupant from another vehicle. This means that door-mounted side air bags must begin deploying in a mere five or six milliseconds. Volvo engineers experimented with different ways of mounting side air bags and chose seat-back installation because that protects passengers of all sizes regardless of how the seat is positioned. This arrangement allows them to place a triggering mechanical sensor on the sides of the seat cushions under the driver and front passenger. This prevents the air bag on the undamaged side of the car from inflating. Installing the entire air bag package in the seat-back also offers the advantage of preventing unnecessary deployments that might be caused by collisions with pedestrians or bicycles. It takes a collision of about 12 mph (19 kph) to trigger side air bags. BMW engineers have chosen door-mounted air bags. The door has more space, allowing for a bigger bag that provides more coverage. The head air bag, or Inflatable Tubular Structure (ITS), was featured in all of BMW's 1999 models (except convertibles). The head bags look a little like big sausages and, unlike other air bags, are designed to stay inflated for about five seconds to offer protection against second or third impacts. Working with the side air bag, the ITS is supposed to offer better protection in some side collisions.

BMW’s ITS


81

All of this makes it pretty clear that the science of air bags is still new and under rapid development. You can expect many advances in this field as designers come up with new ideas and learn from real-world crash data. Inflatable Curtain (IC) covers the entire upper part of a vehicle's side, cushioning the heads of the driver and all passengers (both front and rear seat occupants) seated next to the side.

Volvo’s IC Stored in the head liner above the doors, the cells of the IC are inflated in less than 25 thousands of a second in a triggering accident (four times faster than the blink of an eye). To avoid stitches from sewing the bag, its cells are woven on the loom directly from the yarn using one-piece-weaving technology. Therefore the bag can remain inflated for several seconds, which is imperative in roll-over accidents. Laboratory tests have shown that the so-called Head Injury Criterion (HIC) can be reduced by approximately 80%. The Inflatable Curtain was developed in cooperation with Mercedes and Volvo, who began to introduce it in their cars in 1998.


82

Anti-Whiplash Seat (AWS)

AWS is based on a yieldable backrest that will be tilted in a controlled way in a rear-end collision to absorb energy and reduce the forward rebound of the occupant. Rear-end collisions are rarely fatal, but they give rise to fully one quarter of all personal injuries - often with permanent impairment - and to extended sick-leave and inability to work. In addition to the human suffering, these injuries account in many countries for more than 50% of all insurance claims and costs for societies for personal injuries sustained by car occupants. For front-seat occupants, anti-whiplash system AWS was developed, which has proven to be very effective in rear-end impact test. In a study by the U.S. Insurance Institute for Highway Safety, for instance, the Neck-Injury-Criterion (NIC) levels were reduced by approximately 50%, to levels that are deemed to be safe. These rear-end crash tests were performed at 15 mph or 24 km/h. Anti-Whiplash Seat (AWS) was introduced in 1998 by Volvo.

Volvo’s AWS


83

CASE STUDY: The Volvo XC90 sport utility vehicle, is an example of a new vehicle with many state-of-the-art safety products.

1. On-Call-System The airbags' electronic control unit automatically calls a Volvo On-Call Emergency Center after a severe crash and provides the rescue team with the location of the vehicle from the vehicle's GPS navigation system. This post-crash system can also be used to trace a stolen vehicle. Introduced in 2000.

2. Steering Wheel Driver airbags are increasingly being delivered integrated with the steering wheels. This was started with this concept in 1995.

3. Driver Airbag Estimated to reduce driver fatalities in frontal crashes by approximately 25% (for belted drivers). The vehicle's frontal airbags have two stages to adjust the deployment to the crash severity.

4. Thorax Bags Estimated to reduce the risk of serious chest injuries in side-impact crashes by approximately 20%. Introduced by Volvo in 1994, and now available in most cars.

5. Automatic Height Adjuster (for the front seat belts) Assures that the shoulder belt is correctly positioned to provide the best possible restraint characteristics for different-sized occupants.

6. Seat Structures Produced since 1996 in order to develop and promote stronger seat structures.

7. Seat Belt Systems Estimated to reduce the risk of serious injuries in frontal crashes by 40-50%.


84

Produced since 1965. The seat belts in the Volvo XC90 have: a) Pretensioners that tighten the belt at the onset of a crash, using a small pyrotechnic charge, so that the restraining of the occupant starts as early as possible. Introduced in 1989. b) Load Limiters which pay out some seat belt webbing before the load on the occupant's chest becomes too high. In the front seats where there is a risk of hitting the steering wheel or the dash board, the excessive energy is instead absorbed more uniformly by the frontal airbags. The load limiters in the Volvo XC90 are of a new design with two stages to provide an even load on the occupant's body from the combined seat belt and airbag system.

8. Passenger Airbag Estimated to reduce fatalities in frontal crashes by approximately 20% (for belted occupants).

9. Inflatable Curtain (IC) Estimated to reduce the risk of life threatening head injuries in side-impact collisions by more than 50%. It is also very efficient for rollover protection. This was introduced in 1998.

10. Anti-Whiplash Seat (AWS) Estimated to reduce the risk of neck injuries in rear-end collisions by more than 50%. An innovation introduced in 1998 and available in all Volvo cars. 11. Integrated Child Seat A foldable seat, which makes it possible for children to use the vehicle's seat belt system, which is more efficient than a separately attached belt. 12. Belt-In-Seat (BIS) Volvo has developed a unique recliner to allow the shoulder belt to be attached to the backrest of the seat (instead of the car structure). BIS will be especially effective in maintaining clearance between the head and the roof in rollovers.


85

8. Vehicle manufacture, materials, dimensioning, markets, environment and legislation

Until the end of the second World War the majority of cars had separate frames similar to, though of course much smaller than, those now associated with most modern commercial vehicles. The function of the frame is to carryall the major components or sub-assemblies making up the complete vehicle - engine, transmission, suspension, body, etc. In Figure below a fairly early ladder-type frame is illustrated, but it has been drawn with dissimilar side members, to show two different types of layout. These side, or main longitudinal, members normally would be virtually identical, though opposite handed.

Cross section of both the longitudinal and transverse members

• A= straight longitudinal member (easy to manufacture) • B= cranked inwards member • C= transverse members • D & E= suspension linkage points • F= carry the running boards (i.e. Body sides, engine, etc.)

After the Second World War manufacturer found more practical ways in manufacturing the vehicle body. They produced a vehicle body that was called monocoque. This implies that the body of vehicle will include the lower frame, but it would also include some outer panels which are welded together to provide strength. It is estimated that 95% of the vehicle manufacturers use this technique today.


86

Steel monocoque construction However manufacturers seek for newer techniques and nowadays some vehicles are built based on a technique called spaceframe. It a structural frame with non loading panels where the body panels are attached on extruded metal structure. This provides great flexibility in production assembly and choice of materials.

Aluminium spaceframe construction Automotive manufacturers have tried and are trying to save weight in vehicle construction and thus they have looked for new materials other than steel. These newer materials include aluminium and polymers. Such materials have been used through the years but they were found to be costly and therefore were used only on a limited number of vehicles such as luxury and sport. Today it is estimated that 76% of total vehicle production use steel for their body and only 9% aluminium. The rest are manufactured through polymers. Below are some of the advantages of each material category.

Steel: Relatively cheap Good sheet formability Strength, stiffness and ductility

Aluminium:

Light weight Strong and durable

Excellent corrosion resistance

Polymer composites: Strong, stiff and tough Light weight


87

The vehicle is measured by length, width and height, by taking the maximum values. The wheel base is the distance between the centre of front and rear wheel. The track is the distance between the centre of right and left wheel. Some vehicles have a different track value for front and rear axles but the majority have the same. Another important aspect is the ground clearance which is the distance from the lower point at the front or rear of the vehicle with respect to the lower point of the tyres.

Vehicle dimensions

Now since we have seen the manufacture and materials used on vehicle it is important to state some of the main factors that influence a vehicles manufacture which are environment, legislation and markets (or customers).

Environment is widely influenced by motor vehicle. Motor vehicles are the single largest contributor to urban air pollution. As such air quality improvement mechanisms necessarily involve control and reduction of emissions from motor vehicles. In the past 20 odd years, controls on these emissions have been progressively tightened. Over the last 10 years in particular there have been improvements in a number of air quality indicators, and it is generally accepted that the increasing proportion of vehicles meeting tighter emission standards has played a major part in these air quality improvements. Nevertheless, there is still community concern over urban air quality and the contribution that motor vehicles make to urban air pollution. Vehicle noise is identified by the community as a significant environmental issues associated with vehicle use.

Legislation therefore imposes acts on manufacturers to control vehicles’ emissions and in some countries limits the importation of large engine vehicles through high taxation. Another problem that is associated with vehicles is the noise pollution. This cannot be controlled through manufacturers though.


88

Legislation does not stop into emissions but goes more deeply into the manufacture of the vehicles and imposes the use of safe materials. For example lead was forbidden in vehicle manufacture other than batteries. Paints used in the automotive industry are water-based nowadays. Such acts are not uncommon in the automotive industry and new acts are coming all the time. Another important act is the end-of-life derivative, where the manufacturer is responsible for the vehicles produced at their end of their life. Manufacturers are imposed to set recycling stations so as to recycle the vehicles they produce. This means that newer vehicle must be designed for recycling. So legislation and environment are related in the same ways customers and legislation are related. Vehicles must be produced so as to provide safety not only to passengers but to the other road users as well. Pedestrians have to be taken into consideration, so the body work of a vehicle must not have sharp edges by legislation. Bonnet emblems are cancelled; engine to bonnet clearance is increased and so on. Another aspect is the signaling and lighting which differs for example in the US and in Europe and should be designed according to legislation that each country imposes. However this does not stop just at lighting but other features must be taken into account which include acoustic, radio transmitting or any other sort of detail that differs from country to country. More to that, customers are giving continually feedback to the manufacturers so as to produce vehicles will features that suit the customers in each country.

contents 1. introduction transmission systems 8

Documents