Advanced Vehicle TechnologyTo my long-suffering wife, who has provided sup-port and understanding throughout the preparationof this book.AdvancedVehicle TechnologySecond editionHeinz Heisler MSc., BSc., F.I.M.I., M.S.O.E., M.I.R.T.E., M.C.I.T., M.I.L.T.Formerly Principal Lecturer and Head of Transport Studies,College of North West London, Willesden Centre, London, UKOXFORD AMSTERDAM BOSTON LONDON NEW YORK PARISSAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYOButterworth-HeinemannAn imprint of Elsevier ScienceLinacre House, Jordan Hill, Oxford OX2 8DP225 Wildwood Avenue, Woburn, MA 01801-2041First published by Edward Arnold 1989Reprinted by Reed Educational and Professional Publishing Ltd 2001Second edition 2002Copyright#1989, 2002 Heinz Heisler. All rights reservedThe right of Heinz Heisler to be identified as the author of this work has beenasserted in accordance with the Copyright, Designs and Patents Act 1988No part of this publication may be reproduced in any material form (includingphotocopying or storing in any medium by electronic means and whetheror not transiently or incidentally to some other use of this publication) withoutthe written permission of the copyright holder except in accordance with theprovisions of the Copyright, Designs and Patents Act 1988 or under the terms ofa license issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road,London, England W1T 4LP. Applications for the copyright holder's writtenpermission to reproduce any part of this publication should be addressedto the publishersWhilst the advice and information in this book are believed to be true andaccurate at the date of going to press, neither the authors nor the publishercan accept any legal responsibility or liability for anyerrors or omissions that may be made.Library of Congress Cataloguing in Publication DataA catalogue record for this book is available from the Library of CongressISBN 0 7506 5131 8For information on all Butterworth-Heinemann publicationsvisit our website at www.bh.comTypeset by Integra Software Services Pvt. Ltd, Pondicherry, Indiawww.integra-india.comPrinted and bound in Great Britain1 Vehicle structure ..........................................................................................1.1 Integral body construction ................................................................................1.2 Engine, transmission and body structures .......................................................1.3 Fifth wheel coupling assembly .........................................................................1.4 Trailer and caravan drawbar couplings ............................................................1.5 Semi-trailer landing gear ..................................................................................1.6 Automatic chassis lubrication system ..............................................................2 Friction clutch ..............................................................................................2.1 Clutch fundamentals ........................................................................................2.2 Angular driven plate cushioning and torsional damping ..................................2.3 Clutch friction materials ....................................................................................2.4 Clutch drive and driven member inspection .....................................................2.5 Clutch misalignment .........................................................................................2.6 Pull type diaphragm clutch ...............................................................................2.7 Multiplate diaphragm type clutch .....................................................................2.8 Lipe rollway twin driven plate clutch .................................................................2.9 Spicer twin driven plate angle spring pull type clutch ......................................2.10 Clutch (upshift) brake .....................................................................................2.11 Multiplate hydraulically operated automatic transmission clutches ................2.12 Semicentrifugal clutch ....................................................................................2.13 Fully automatic centrifugal clutch ...................................................................2.14 Clutch pedal actuating mechanisms ..............................................................2.15 Composite flywheel and integral single plate diaphragm clutch ....................3 Manual gearboxes and overdrives .............................................................3.1 The necessity for a gearbox .............................................................................3.2 Five speed and reverse synchromesh gearboxes ...........................................3.3 Gear synchronization and engagement ...........................................................3.4 Remote controlled gear selection and engagement m ....................................3.5 Splitter and range change gearboxes ..............................................................3.6 Transfer box power take-off .............................................................................3.7 Overdrive considerations .................................................................................3.8 Setting gear ratios ............................................................................................4 Hydrokinetic fluid couplings and torque converters ...............................4.1 Hydrokinetic fluid couplings .............................................................................4.2 Hydrokinetic fluid coupling efficiency and torque capacity ...............................4.3 Fluid friction coupling .......................................................................................4.4 Hydrokinetic three element torque converter ...................................................4.5 Torque converter performance terminology .....................................................4.6 Overrun clutches ..............................................................................................4.7 Three stage hydrokinetic converter ..................................................................4.8 Polyphase hydrokinetic torque converter .........................................................4.9 Torque converter with lock-up and gear change friction clutches ....................5 Semi- and fully automatic transmission ....................................................5.1 Automatic transmission consideration .............................................................5.2 Four speed and reverse longitudinally mounted automatic transmissionmechanical power flow ...........................................................................................5.3 The fundamentals of a hydraulic control system ..............................................5.4 Basic principle of a hydraulically controlled gearshift .......................................5.5 Basic four speed hydraulic control system .......................................................5.6 Three speed and reverse transaxle automatic transmission mechanicalpower flow ..............................................................................................................5.7 Hydraulic gear selection control components ..................................................5.8 Hydraulic gear selection control operation .......................................................5.9 The continuously variable belt and pulley transmission ...................................5.10 Five speed automatic transmission with electronic-hydraulic control ............5.11 Semi-automatic (manual gear change two pedal control) transmissionsystem ....................................................................................................................6 Transmission bearings and constant velocity joints ...............................6.1 Rolling contact bearings ...................................................................................6.2 The need for constant velocity joints ................................................................7 Final drive transmission .............................................................................7.1 Crownwheel and pinion axle adjustments ........................................................7.2 Differential locks ...............................................................................................7.3 Skid reducing differentials ................................................................................7.4 Double reduction axles ....................................................................................7.5 Two speed axles ..............................................................................................7.6 The third (central) differential ...........................................................................7.7 Four wheel drive arrangements .......................................................................7.8 Electro-hydralic limited slip differential .............................................................7.9 Tyre grip when braking and accelerating with good and poor roadsurfaces .................................................................................................................7.10 Traction control system ..................................................................................8 Tyres .............................................................................................................8.1 Tractive and braking properties of tyres ...........................................................8.2 Tyre materials ..................................................................................................8.3 Tyre tread design .............................................................................................8.4 Cornering properties of tyres ...........................................................................8.5 Vehicle steady state directional stability ..........................................................8.6 Tyre marking identification ...............................................................................8.7 Wheel balancing ..............................................................................................9 Steering ........................................................................................................9.1 Steering gearbox fundamental design .............................................................9.2 The need for power assisted steering ..............................................................9.3 Steering linkage ball and socket joints .............................................................9.4 Steering geometry and wheel alignment ..........................................................9.5 Variable-ratio rack and pinion ..........................................................................9.6 Speed sensitive rack and pinion power assisted steering ...............................9.7 Rack and pinion electric power assisted steering ............................................10 Suspension ................................................................................................10.1 Suspension geometry ....................................................................................10.2 Suspension roll centres ..................................................................................10.3 Body roll stability analysis ..............................................................................10.4 Anti-roll bars and roll stiffness ........................................................................10.5 rubber spring bump or limiting stops ..............................................................10.6 Axle location ...................................................................................................10.7 Rear suspension arrangements .....................................................................10.8 Suspension design consideration ..................................................................10.9 Hydrogen suspension ....................................................................................10.10 Hydropneumatic automatic height correction suspension ...........................10.11 Commercial vehicle axle beam location .......................................................10.12 Variable rate leaf suspension springs.........................................................10.13 Tandem and tri-axle bogies .........................................................................10.14 Rubber spring suspension ...........................................................................10.15 Air suspensions for commercial vehicles .....................................................10.16 Lift axle tandem or tri-axle suspension ........................................................10.17 Active suspension ........................................................................................10.18 Electronic controlled pneumatic (air) suspension for on and off road use ...11 Brake system .............................................................................................11.1 Braking fun .....................................................................................................11.2 Brake shoe and pad fundamentals ................................................................11.3 Brake shoe expanders and adjusters ............................................................11.4 Disc brake pad support arrangements ...........................................................11.5 Dual- or split-line braking systems .................................................................11.6 Apportional braking ........................................................................................11.7 Antilocking brake system (ABS) .....................................................................11.8 Brake servos ..................................................................................................11.9 Pneumatic operated disk brakes (for trucks and trailers) ...............................12 Air operated power brake equipment and vehicle retarders .................12.1 Introductions to air powered brakes ...............................................................12.2 Air operated power brake systems ................................................................12.3 Air operated power brake equipment .............................................................12.4 Vehicle retarders ............................................................................................12.5 Electronic-pneumatic brakes ..........................................................................13 Vehicle refrigeration ..................................................................................13.1 Refrigeration terms ........................................................................................13.2 Principles of a vapour-compression cycle refrigeration system .....................13.3 Refrigeration system components .................................................................13.4 Vapour-compression cycle refrigeration system with reverse cycledefrosting ...............................................................................................................14 Vehicle body aerodynamics .....................................................................14.1 Viscous air flow fundamentals .......................................................................14.2 Aerodynamic drag ..........................................................................................14.3 Aerodynamic lift .............................................................................................14.4 Car body drag reduction ................................................................................14.5 Aerodynamic lift control ..................................................................................14.6 Afterbody drag ...............................................................................................14.7 Commercial vehicle aeordynamic fundamentals ...........................................14.8 Commercial vehicle drag reducing devices ...................................................Index ................................................................................................................1 Vehicle Structure1.1 Integral body constructionThe integral or unitary body structure of a car canbe considered to be made in the form of three boxcompartments; the middle and largest compart-ment stretching between the front and rear roadwheel axles provides the passenger space, theextended front box built over and ahead of the frontroad wheels enclosing the engine and transmissionunits and the rear box behind the back axleproviding boot space for luggage.These box compartments are constructed in theform of a framework of ties (tensile) and struts(compressive), pieces (Fig. 1.1(a & b)) made fromrolled sheet steel pressed into various shapes suchas rectangular, triangular, trapezium, top-hat or acombination of these to form closed box thin gaugesections. These sections are designed to resist directtensile and compressive or bending and torsionalloads, depending upon the positioning of the mem-bers within the structure.Fig. 1.1 (a and b) Structural tensile and compressive loading of car body11.1.1 Description and function of bodycomponents (Fig. 1.2)The major individual components comprising thebody shell will now be described separately underthe following subheadings:1 Window and door pillars2 Windscreen and rear window rails3 Cantrails4 Roof structure5 Upper quarter panel or window6 Floor seat and boot pans7 Central tunnel8 Sills9 Bulkhead10 Scuttle11 Front longitudinals12 Front valance13 Rear valance14 Toe board15 Heel boardWindow and door pillars (Fig. 1.2(3, 5, 6, and 8))Windowscreen and door pillars are identified by aletter coding; the front windscreen to door pillarsare referred to as A post, the centre side door pillarsas BC post and the rear door to quarter panel asD post. These are illustrated in Fig. 1.2.These pillars form the part of the body structurewhichsupports the roof. The short formApillar andrear D pillar enclose the windscreen and quarterwindows and provide the glazing side channels,whilst the centre BC pillar extends the full height ofthe passenger compartment from roof to floor andsupports the rear side door hinges. The front andrear pillars act as struts (compressive members)which transfer a proportion of the bending effect,due to underbody sag of the wheelbase, to each endof the cantrails which thereby become reactivestruts, opposing horizontal bending of the pas-senger compartment at floor level. The central BCpillar however acts as ties (tensile members), trans-ferring some degree of support fromthe mid-spanofthe cantrails to the floor structure.Windscreen and rear window rails (Fig. 1.2(2))These box-section rails span the front windowpillars and rear pillars or quarter panels dependingupon design, so that they contribute to the resist-ance opposing transverse sag between the wheeltrack by acting as compressive members. Theother function is to support the front and rearends of the roof panel. The undersides of the railsalso include the glazing channels.Cantrails (Fig. 1.2(4)) Cantrails are the horizon-tal members which interconnect the top ends of thevertical A and BC or BC and D door pillars (posts).These rails form the side members which make upthe rectangular roof framework and as such aresubjected to compressive loads. Therefore, theyare formed in various box-sections which offer thegreatest compressive resistance with the minimumof weight and blend in with the roofing. A drip rail(Fig. 1.2(4)) is positioned in between the overlap-ping roof panel and the cantrails, the joins beingsecured by spot welds.Roof structure (Fig. 1.2) The roof is constructedbasically from four channel sections which formthe outer rim of the slightly dished roof panel.The rectangular outer roof frame acts as the com-pressive load bearing members. Torsional rigidityto resist twist is maximized by welding the fourcorners of the channel-sections together. The slightcurvature of the roof panel stiffens it, thus prevent-ing winkling and the collapse of the unsupportedcentre region of the roof panel. With large cars,additional cross-rail members may be used toprovide more roof support and to prevent the roofcrushing in should the car roll over.Upper quarter panel or window (Fig. 1.2(6)) Thisis the vertical side panel or window which occupiesthe space between the rear side door and the rearwindow. Originally the quarter panel formed animportant part of the roof support, but improvedpillar design and the desire to maximize visibilityhas either replaced them with quarter windows orreduced their width, and in some car models theyhave been completely eliminated.Floor seat and boot pans (Fig. 1.3) These consti-tute the pressed rolled steel sheeting shape toenclose the bottom of both the passenger and lug-gage compartments. The horizontal spread-outpressing between the bulkhead and the heel boardis called the floor pan, whilst the raised platformover the rear suspension and wheel arches is knownas the seat or arch pan. This in turn joins onto alower steel pressing which supports luggage and isreferred to as the boot pan.To increase the local stiffness of these platformpanels or pans and their resistance to transmittedvibrations such as drumming and droning, manynarrow channels are swaged (pressed) into the steelsheet, because a sectional end-view would show a2semi-corrugated profile (or ribs). These channelsprovide rows of shallow walls which are both bentand stretched perpendicular to the original flatsheet. In turn they are spaced and held togetherby the semicircular drawn out channel bottoms.Provided these swages are designed to lay thecorrect way and are not too long, and the metal isnot excessively stretched, they will raise the rigidityFig. 1.2 Load bearing body box-section members3Fig. 1.3 (ac) Platform chassis4of these panels so that they are equivalent to a sheetwhich may be several times thicker.Central tunnel (Fig. 1.3(a and b)) This is thecurved or rectangular hump positioned longitudin-ally along the middle of the floor pan. Originally itwas a necessary evil to provide transmission spacefor the gearbox and propeller shaft for rear wheeldrive, front-mounted engine cars, but since thechassis has been replaced by the integral box-section shell, it has been retained with front wheeldrive, front-mounted engines as it contributesconsiderably to the bending rigidity of the floorstructure. Its secondary function is now to housethe exhaust pipe system and the hand brake cableassembly.Sills (Figs 1.2(9) and1.3(a, bandc)) These membersform the lower horizontal sides of the car bodywhich spans between the front and rear road-wheelwings or arches. To prevent body sag between thewheelbase of the car and lateral bending of thestructure, the outer edges of the floor pan are givensupport by the side sills. These sills are made in theform of either single or double box-sections(Fig. 1.2(9)). To resist the heavier vertical bendingloads they are of relatively deep section.Open-top cars, such as convertibles, which do notreceive structural support from the roof members,usually have extra deep sills to compensate for theincreased burden imposed on the underframe.Bulkhead (Figs 1.2(1) and 1.3(a and b)) This is theupright partition separating the passenger andengine compartments. Its upper half may formpart of the dash panel which was originally used todisplay the driver's instruments. Some body manu-facturers refer tothe whole partitionbetweenengineand passenger compartments as the dash panel. Ifthere is a double partition, the panel next to theengine is generally known as the bulkhead and thaton the passenger side the dash board or panel. Thescuttle and valance on each side are usually joinedonto the box-section of the bulkhead. This bracesthe vertical structure to withstand torsional distor-tion and to provide platform bending resistancesupport. Sometimes a bulkhead is constructedbetween the rear wheel arches or towers to reinforcethe seat pan over the rear axle (Fig. 1.3(c)).Scuttle (Fig. 1.3(a and b)) This can be consideredas the panel formed under the front wings whichspans between the rear end of the valance, where itmeets the bulkhead, and the door pillar and wing.The lower edge of the scuttle will merge with thefloor pan so that in some cases it may form part ofthe toe board on the passenger compartment side.Usually these panels forminclined sides to the bulk-head, and with the horizontal ledge which spans thefull width of the bulkhead, brace the bulkhead wallso that it offers increased rigidity to the structure.The combined bulkhead dash panel and scuttle willthereby have both upright and torsional rigidity.Front longitudinals (Figs 1.2(10) and 1.3(a and b))These members are usually upswept box-sectionmembers, extending parallel and forward from thebulkhead at floor level. Their purpose is to with-stand the engine mount reaction and to support thefront suspension or subframe. A common featureof these members is their ability to support verticalloads in conjunction with the valances. However, inthe event of a head-on collision, they are designedto collapse and crumble within the engine compart-ment so that the passenger shell is safeguarded andis not pushed rearwards by any great extent.Front valance (Figs 1.2 and 1.3(a and b)) Thesepanels project upwards from the front longitudinalmembers and at the rear join onto the wall of thebulkhead. The purpose of these panels is to transferthe upward reaction of the longitudinal memberswhich support the front suspensionto the bulkhead.Simultaneously, the longitudinals are preventedfrom bending sideways because the valance panelsare shaped to slope up and outwards towards thetop. The panelling is usually bent over near theedges to form a horizontal flanged upper, thuspresenting considerable lateral resistance. Further-more, the valances are sometimes stepped andwrapped around towards the rear where they meetand are joined to the bulkhead so that additionallengthwise and transverse stiffness is obtained.If coil spring suspension is incorporated, thevalance forms part of a semi-circular tower whichhouses and provides the load reaction of the springso that the merging of these shapes compounds therigidity for both horizontal lengthwise and lateralbending of the forward engine and transmissioncompartment body structure. Where necessary,double layers of sheet are used in parts of the springhousing and at the rear of the valance where theyare attached to the bulkhead to relieve some of theconcentrated loads.5Rear valance (Fig. 1.2(7)) This is generally con-sidered as part of the box-section, forming the fronthalf of the rear wheel arch frame and the panelimmediately behind which merges with the heelboard and seatpan panels. These side inner-sidepanels position the edges of the seat pan to itsdesigned side profile and thus stiffen the underfloorstructure above the rear axle and suspension. Whenrear independent coil spring suspension is adopted,the valance or wheel arch extends upwards to forma spring tower housing and, because it forms asemi-vertical structure, greatly contributes to thestiffness of the underbody shell between the floorand boot pans.Toe board The toe board is considered to formthe lower regions of the scuttle and dash panel nearwhere they merge with the floor pan. It is thispanelling on the passenger compartment sidewhere occupants can place their feet when the caris rapidly retarded.Heel board (Fig. 1.3(b and c)) The heel board isthe upright, but normally shallow, panel spanningbeneath and across the front of the rear seats. Itspurpose is to provide leg height for the passengersand to form a raised step for the seat pan so thatthe rear axle has sufficient relative movementclearance.1.1.2 Platform chassis (Fig. 1.3(ac))Most modern car bodies are designed to obtaintheir rigidity mainly from the platform chassis andto rely less on the upper framework of windowand door pillars, quarter panels, windscreen railsand contrails which are becoming progressivelyslender as the desirefor better visibilityis encouraged.The majority of the lengthwise (wheelbase) bend-ing stiffness to resist sagging is derived from boththe central tunnel and the side sill box-sections(Fig. 1.3(a and b)). If further strengthening isnecessary, longitudinal box-section members maybe positioned parallel to, but slightly inwards from,the sills (Fig. 1.3(c)). These lengthwise membersmay span only part of the wheelbase, or the fulllength, which is greatly influenced by the design ofroad wheel suspension chosen for the car, the depthof both central tunnel and side sills, which are builtinto the platform, and if there are subframesattached fore and aft of the wheelbase (Fig. 1.6(a and b)).Torsional rigidity of the platform is usuallyderived at the front by the bulkhead, dash panand scuttle (Fig. 1.3(a and b)) at the rear by theheel board, seat pan, wheel arches (Fig. 1.3(a, b andc)), and if independent rear suspension is adopted,by the coil spring towers (Fig. 1.3(a and c)).Between the wheelbase, the floor pan is normallyprovided with box-section cross-members to stiffenand prevent the platform sagging where thepassenger seats are positioned.1.1.3 Stiffening of platform chassis(Figs 1.4 and 1.5)To appreciate the stresses imposed on and theresisting stiffness offered by sheet steel when it issubjected to bending, a small segment of a beamgreatly magnified will now be considered (Fig.1.4(a)). As the beam deforms, the top fibres con-tract and the bottom fibres elongate. The neutralplane or axis of the beam is defined as the planewhose length remains unchanged during deforma-tion and is normally situated in the centre of auniform section (Fig. 1.4(a and b)).The stress distribution from top to bottom withinthe beam varies from zero along the neutral axis(NA), where there is no change in the length of thefibres, toa maximumcompressive stress onthe outertop layer and a maximum tensile stress on the outerbottom layer, the distortion of the fibres beinggreatest at their extremes as shown in Fig. 1.4(b).It has been found that bending resistanceincreases roughly with the cube of its distancefrom the neutral axis (Fig. 1.5(a)). Therefore, bend-ing resistance of a given section can be greatlyimproved for a given weight of metal by takingmetal away from the neutral axis where the metalfibres do not contribute very much to resistingdistortion and placing it as far out as possiblewhere the distortion is greatest. Bending resistancemay be improved by using longitudinal or cross-member deep box-sections (Fig. 1.5(b)) and tunnelsections (Fig. 1.5(c)) to restrain the platform chas-sis from buckling and to stiffen the flat horizontalfloor seat and boot pans. So that vibration anddrumming may be reduced, many swaged ribs arepressed into these sheets (Fig. 1.5(d)).1.1.4 Body subframes (Fig. 1.6)Front or rear subframes may be provided to bracethe longitudinal side members so that independentsuspension on each side of the car receives adequatesupport for the lower transverse swing arms (wish-bone members). Subframes restrain the two halvesof the suspension from splaying outwards or the6longitudinal side members from lozenging as alter-native road wheels experience impacts when travel-ling over the irregularities of a normal road surface.It is usual to make the top side of the subframethe cradle for the engine or engine and transmissionmounting points so that the main body structureitself does not have to be reinforced. This particu-larly applies where the engine, gearbox and finaldrive form an integral unit because any torquereaction at the mounting points will be transferredto the subframe and will multiply in proportion tothe overall gear reduction. This may be approxi-mately four times as great as that for the frontmounted engine with rear wheel drive and willbecome prominent in the lower gears.One advantage claimed by using separate sub-frames attached to the body underframe throughthe media of rubber mounts is that transmittedvibrations and noise originating from the tyresand road are isolated from the main body shelland therefore do not damage the body structureand are not relayed to the occupants sittinginside.Cars which have longitudinally positionedengines mounted in the front driven by the rearwheels commonly adopt beam cross-membersubframes at the front to stiffen and support thehinged transverse suspension arms (Fig. 1.6(a)).Saloon cars employing independent rear suspen-sion sometimes prefer to use a similar subframe atthe rear which provides the pivot points for thesemi-trailing arms because this type of suspensionrequires greater support than most other arrange-ments (Fig. 1.6(a)).Fig. 1.4 Stress and strain imposed on beam when subjected to bending7When the engine, gearbox and final drive arecombined into a single unit, as with the front longi-tudinally positioned engine driving the front wheelswhere there is a large weight concentration, a sub-frame gives extra support to the body longitudinalside members by utilising a horseshoe shaped frame(Fig. 1.6(b)). This layout provides a platform forthe entire mounting points for both the swing armand anti-roll bar which between them make up thelower part of the suspension.Fig. 1.5 Bending resistance for various sheet sections8Fig. 1.6 (ac) Body subframe and underfloor structure9Front wheel drive transversely positionedengines with their large mounting point reactionsoften use a rectangular subframe to spread outboth the power and transmission unit's weightand their dynamic reaction forces (Fig. 1.6(c)).This configuration provides substantial torsionalrigidity between both halves of the independentsuspension without relying too much on the mainbody structure for support.Soundproofing the interior of the passengercompartment (Fig. 1.7)Interior noise originating outside the passengercompartment can be greatly reduced by applyinglayers of materials having suitable acoustic proper-ties over floor, seat and boot pans, central tunnel,bulkhead, dash panel, toeboard, side panels, insideof doors, and the underside of both roof andbonnet etc. (Fig. 1.7).Acoustic materials are generally designed for oneof three functions:a) Insulation from noise This may be created byforming a non-conducting noise barrierbetween the source of the noises (which maycome from the engine, transmission, suspensiontyres etc.) and the passenger compartment.b) Absorption of vibrations This is the transfer-ence of excited vibrations in the body shell toa media which will dissipate their resultantenergies and so eliminate or at least greatlyreduce the noise.c) Damping of vibrations When certain vibra-tions cannot be eliminated, they may be exposedto some form of material which in some waymodifies the magnitude of frequencies of thevibrations so that they are less audible to thepassengers.The installation of acoustic materials cannotcompletely eliminate boom, drumming, droningand other noises caused by resonance, but merelyreduces the overall noise level.Insulation Because engines are generally mountedclose to the passenger compartment of cars or thecabs of trucks, effective insulation is important. Inthis case, the function of the material is to reducethe magnitude of vibrations transmitted throughthe panel and floor walls. To reduce the transmis-sion of noise, a thin steel body panel should becombined with a flexible material of large mass,based on PVC, bitumen or mineral wool. If theinsulation material is held some distance from thestructural panel, the transmissibility at frequenciesabove 400 Hz is further reduced. For this type ofapplication the loaded PVC material is bonded to aspacing layer of polyurethane foam or felt, usuallyabout 7 mm thick. At frequencies below 400 Hz, theuse of thicker spacing layers or heavier materialscan also improve insulation.Absorption For absorption, urethane foam orlightweight bonded fibre materials can be used.In some cases a vinyl sheet is bonded to the foamto form a roof lining. The required thickness of theabsorbent material is determined by the frequenciesinvolved. The minimum useful thickness ofpolyurethane foam is 13 mm which is effectivewith vibration frequencies above 1000 Hz.Damping To damp resonance, pads are bondedto certain panels of many cars and truck cabs. Theyare particularly suitable for external panels whoseresonance cannot be eliminated by structuralalterations. Bituminous sheets designed for thispurpose are fused to the panels when the paint isbaked on the car. Where extremely high dampingor light weight is necessary, a PVC base material,which has three times the damping capacity ofbituminous pads, can be used but this material israther difficult to attach to the panelling.1.1.5 Collision safety (Fig. 1.8)Car safety may broadly be divided into two kinds:Firstly the active safety, which is concerned withthe car's road-holding stability while being driven,steered or braked and secondly the passive safety, Fig. 1.7 Car body sound generation and its dissipation10which depends upon body style and design struc-ture to protect the occupants of the car from seriousinjury in the event of a collision.Car bodies can be considered to be made in threeparts (Fig. 1.8); a central cell for the passengersof the welded bodywork integral with a rigidplatform, acting as a floor pan, and chassis withvarious box-section cross- and side-members. Thistype of structure provides a reinforced rigid crush-proof construction to resist deformation on impactand to give the interior a high degree of protection.The extension of the engine and boot compart-ments at the front and rear of the central passengercell are designed to form zones which collapse andcrumble progressively over the short duration of acollision impact. Therefore, the kinetic energy dueto the car's initial speed will be absorbed fore andaft primarily by strain and plastic energy within thecrumble zones with very little impact energy actu-ally being dissipated by the central body cell.1.1.6 Body and chassis alignment checks(Fig. 1.9)Body and chassis alignment checks will be neces-sary if the vehicle has been involved in a majorcollision, but overall alignment may also be neces-sary if the vehicle's steering and ride characteristicsdo not respond to the expected standard of a simi-lar vehicle when being driven.Structural misalignment may be caused by allsorts of reasons, for example, if the vehicle hasbeen continuously driven over rough ground athigh speed, hitting an obstacle in the road, mount-ing steep pavements or kerbs, sliding off the roadinto a ditch or receiving a glancing blow from someother vehicle or obstacle etc. Suspicion that some-thing is wrong with the body or chassis alignment isfocused if there is excessively uneven or high tyrewear, the vehicle tends to wander or pull over toone side and yet the track and suspension geometryappears to be correct.Alignment checks should be made on a level,clear floor with the vehicle's tyres correctly inflatedto normal pressure. A plumb bob is required in theform of a stubby cylindrical bar conical shaped atone end, the other end being attached to a length ofthin cord. Datum reference points are chosen suchas the centre of a spring eye on the chassis mount-ing point, transverse wishbone and trailing armpivot centres, which are attachment points to theunderframe or chassis, and body cross-member toside-member attachment centres and subframebolt-on points (Fig. 1.9).Initially the cord with the plumb bob hangingfrom its end is lowered from the centre of eachreference point to the floor and the plumb bob con-tact point with the ground is marked with a chalkedcross. Transverse and diagonal lines between refer-ence points can be made by chalking the full lengthof a piece of cord, holding it taut between referencecentres on the floor and getting somebody to pluckthe centre of the line so that it rebounds and leavesa chalked line on the floor.A reference longitudinal centre line may be madewith a strip of wood baton of length just greaterthan the width between adjacent reference markson the floor. A nail is punched through one endand this is placed over one of the reference marks.A piece of chalk is then held at the tip of the freeend and the whole wood strip is rotated aboutthe nailed end. The chalk will then scribe an arcbetween adjacent reference points. This is repeatedfrom the other side. At the points where these twoarcs intersect a straight line is made with a plucked,chalked cord running down the middle of the vehi-cle. This procedure should be followed at each endof the vehicle as shown in Fig. 1.9.Once all the reference points and transverse anddiagonal joining lines have been drawn on theTable 1.1 Summary of function and application ofsoundproofing materialsFunction Acoustic materials ApplicationInsulation Loaded PVC,bitumen, with orwithout foam orfibres base,mineral woolFloor, bulkheaddash panelDamping Bitumen ormineralwoolDoors, sidepanels,underside of roofAbsorption Polyurethane foam,mineral wool, orbonded fibresSide panels,underside ofroof, enginecompartment,bonnetFig. 1.8 Collision body safety11floor, a rule or tape is used to measure the distancesbetween centres both transversely and diagonally.These values are then chalked along their respectivelines. Misalignment or error is observed when apair of transverse or diagonal dimensions differand further investigation will thus be necessary.Note that transverse and longitudinal dimen-sions are normally available from the manufac-turer's manual and differences between paireddiagonals indicates lozenging of the frameworkdue to some form of abnormal impact which haspreviously occurred.1.2 Engine, transmission and body structuremountings1.2.1 Inherent engine vibrationsThe vibrations originating within the engine arecaused by both the cyclic acceleration of the reci-procating components and the rapidly changingcylinder gas pressure which occurs throughouteach cycle of operation.Both the variations of inertia and gas pressureforces generate three kinds of vibrations which aretransferred to the cylinder block:1 Vertical and/or horizontal shake and rock2 Fluctuating torque reaction3 Torsional oscillation of the crankshaft1.2.2 Reasons for flexible mountingsIt is the objective of flexible mounting design tocope with the many requirements, some havingconflicting constraints on each other. A list of theduties of these mounts is as follows:1 To prevent the fatigue failure of the engine andgearbox support points which would occur ifthey were rigidly attached to the chassis orbody structure.2 To reduce the amplitude of any engine vibrationwhich is being transmitted to the body structure.3 To reduce noise amplification which would occurif engine vibration were allowed to be transferreddirectly to the body structure.Fig. 1.9 Body underframe alignment checks124 To reduce human discomfort and fatigue bypartially isolating the engine vibrations fromthe body by means of an elastic media.5 To accommodate engine block misalignmentand to reduce residual stresses imposed on theengine block and mounting brackets due tochassis or body frame distortion.6 To prevent road wheel shocks when drivingover rough ground imparting excessive reboundmovement to the engine.7 To prevent large engine to body relative move-ment due to torque reaction forces, particularlyin low gear, which would cause excessive mis-alignment and strain on such components asthe exhaust pipe and silencer system.8 To restrict engine movement in the fore and aftdirection of the vehicle due to the inertia of theengine acting in opposition to the acceleratingand braking forces.1.2.3 Rubber flexible mountings (Figs 1.10, 1.11and 1.12)A rectangular block bonded between two metalplates may be loaded in compression by squeezingthe plates together or by applying parallel butopposing forces to each metal plate. On compres-sion, the rubber tends to bulge out centrally fromthe sides and in shear to form a parallelogram(Fig. 1.10(a)).To increase the compressive stiffness of therubber without greatly altering the shear stiffness,an interleaf spacer plate may be bonded in betweenthe top and bottom plate (Fig. 1.10(b)). This inter-leaf plate prevents the internal outward collapse ofthe rubber, shown by the large bulge around thesides of the block, when no support is provided,whereas with the interleaf a pair of much smallerbulges are observed.Whentworubber blocks are inclinedtoeachotherto forma `V' mounting, see Fig. 1.11, the rubber willbe loaded in both compression and shear shown bythe triangle of forces. The magnitude of compressiveforce will be given by Wcand the much smaller shearforce by WS. This produces a resultant reactionforceWR. The larger the wedge angle , the greater theproportion of compressive load relative to the shearload the rubber block absorbs.The distorted rubber provides support underlight vertical static loads approximately equal inboth compression and shear modes, but withheavier loads the proportion of compressive stiffnessFig. 1.10 (a and b) Modes of loading rubber blocksFig. 1.11 `V' rubber block mounting13to that of shear stiffness increases at a much fasterrate (Fig. 1.12). It should also be observed that thecombined compressive and shear loading of therubber increases in direct proportion to the staticdeflection and hence produces a straight line graph.1.2.4 Axis of oscillation (Fig. 1.13)The engine and gearbox must be suspended so thatit permits the greatest degree of freedom whenoscillating around an imaginary centre of rotationknown as the principal axis. This principal axisproduces the least resistance to engine and gearboxsway due to their masses being uniformly distrib-uted about this axis. The engine can be consideredto oscillate around an axis which passes throughthe centre of gravity of both the engine and gearbox(Figs 1.13(a, b and c)). This normally produces anaxis of oscillation inclined at about 1020 to thecrankshaft axis. To obtain the greatest degree offreedom, the mounts must be arranged so that theyoffer the least resistance to shear within the rubbermounting.1.2.5 Six modes of freedom of a suspended body(Fig. 1.14)If the movement of a flexible mounted engine iscompletely unrestricted it may have six modes ofvibration. Any motion may be resolved into threelinear movements parallel to the axes which passthrough the centre of gravity of the engine but atright angles to each other and three rotations aboutthese axes (Fig. 1.14).These modes of movement may be summarizedas follows:Linear motions Rotational motions1 Horizontal 4 Rolllongitudinal 5 Pitch2 Horizontal lateral 6 Yaw3 Vertical1.2.6 Positioning of engine and gearboxmountings (Fig. 1.15)If the mountings are placed underneath the com-bined engine and gearbox unit, the centre of gravityis well above the supports so that a lateral (side)force acting through its centre of gravity, such asexperienced when driving round a corner, will causethe mass to roll (Fig. 1.15(a)). This condition isundesirable and can be avoided by placing themounts on brackets so that they are in thesame plane as the centre of gravity (Fig. 1.15(b)).Thus the mounts provide flexible opposition toany side force which might exist without creating aroll couple. This is known as a decoupled condition.An alternative method of making the naturalmodes of oscillation independent or uncoupled isachieved by arranging the supports in an inclined`V' position (Fig. 1.15(c)). Ideally the aim is tomake the compressive axes of the mountings meetat the centre of gravity, but due to the weight of thepower unit distorting the rubber springing theinter-section lines would meet slightly below thispoint. Therefore, the mountings are tilted so thatthe compressive axes converge at some focal pointabove the centre of gravity so that the actual linesof action of the mountings, that is, the directionof the resultant forces they exert, converge on thecentre of gravity (Fig. 1.15(d)).The compressive stiffness of the inclined mountscan be increased by inserting interleafs betweenthe rubber blocks and, as can be seen inFig. 1.15(e), the line of action of the mounts con-verges at a lower point than mounts which do nothave interleaf support.Engine and gearbox mounting supports arenormally of the three or four point configuration.Petrol engines generally adopt the three pointsupport layout which has two forward mounts(Fig. 1.13(a and c)), one inclined on either side ofthe engine so that their line of action converges onthe principal axis, while the rear mount is supportedcentrally at the rear of the gearbox in approximatelythe same plane as the principal axis. Large dieselengines tend to prefer the four point supportFig. 1.12 Loaddeflection curves for rubber block14arrangement where there are two mounts either sideof the engine (Fig. 1.13(b)). The two front mountsare inclined so that their lines of action pass throughthe principal axis, but the rear mounts which arelocated either side of the clutch bell housing are notinclined since they are already at principal axis level.1.2.7 Engine and transmission vibrationsNatural frequency of vibration (Fig. 1.16) Asprungbody when deflected and released will bounce up anddown at a uniform rate. The amplitude of this cyclicmovement will progressively decrease and the num-ber of oscillations per minute of the rubber mountingis known as its natural frequency of vibration.There is a relationship between the static deflec-tion imposed on the rubber mount springing by thesuspended mass and the rubber's natural frequencyof vibration, which may be given byn0 = 30

xFig. 1.13 Axis of oscillation and the positioning of the power unit flexible mounts15where n0 = natural frequency of vibration(vib/min)x = static deflection of the rubber (m)This relationship between static deflection andnatural frequency may be seen in Fig. 1.16.Resonance Resonance is the unwanted synchron-ization of the disturbing force frequency imposed bythe engine out of balance forces and the fluctuatingcylinder gas pressure and the natural frequency ofoscillation of the elastic rubber support mounting,i.e. resonance occurs whennn0= 1where n = disturbing frequencyn0 = natural frequencyTransmissibility (Fig. 1.17) When the designerselects the type of flexible mounting the Theory ofTransmissibility can be used to estimate criticalresonance conditions so that they can be eitherprevented or at least avoided.Transmissibility (T) may be defined as the ratioof the transmitted force or amplitude which passesthrough the rubber mount to the chassis to that ofthe externally imposed force or amplitude generatedby the engine:T = FtFd= 11 nn0 2where Ft = transmitted force or amplitudeFd = imposed disturbing force oramplitudeThis relationship between transmissibility andthe ratio of disturbing frequency and naturalfrequency may be seen in Fig. 1.17.Fig. 1.14 Six modes of freedom for a suspended blockFig. 1.16 Relationship of static deflection and naturalfrequency16Fig. 1.15(ae) Coupled and uncoupled mounting points17The transmissibility to frequency ratio graph(Fig. 1.17) can be considered in three parts as follows:Range(I) This istheresonancerangeandshouldbeavoided. It occurs when the disturbing frequencyis very near to the natural frequency. If steel mountsare used, a critical vibration at resonance would goto infinity, but natural rubber limits the trans-missibility to around 10. If Butyl synthetic rubber isadopted, its damping properties reduce the peaktransmissibility to about 212. Unfortunately, highdamping rubber compounds such as Butyl rubberare temperature sensitive to both damping anddynamic stiffness so that during cold weather anoticeably harsher suspension of the engine results.Damping of the engine suspension mounting isnecessary to reduce the excessive movement of aflexible mounting when passing through resonance,but at speeds above resonance more vibration istransmitted to the chassis or body structure thanwould occur if no damping was provided.Range (II) This is the recommended workingrange where the ratio of the disturbing frequencyto that of the natural frequency of vibration of therubber mountings is greater than 112 and the trans-missibility is less than one. Under these conditionsoff-peak partial resonance vibrations passing to thebody structure will be minimized.Range (III) This is known as the shock reductionrange and only occurs when the disturbingfrequency is lower than the natural frequency.Generally it is only experienced with very softrubber mounts and when the engine is initiallycranked for starting purposes and so quickly passesthrough this frequency ratio region.Example An engine oscillates vertically on itsflexible rubber mountings with a frequency of 800vibrations per minute (vpm). With the informationprovided answer the following questions:a) From the static deflectionfrequency graph,Fig. 1.16, or byformula, determine the natural fre-quency of vibration when the static deflection ofthe engine is 2 mm and then find the disturbing tonatural frequencyratio. Comment ontheseresults.b) If the disturbing to natural frequency ratio isincreased to 2.5 determine the natural frequencyFig. 1.17 Relationship of transmissibility andthe ratio of disturbing and natural frequenciesfor natural rubber, Butyl rubber and steel18of vibration and the new static deflection of theengine. Comment of these conditions.a) n0 = 30

x= 30

0.002= 300.04472= 670.84 vib/min; nn0= 800670.84= 1.193The ratio 1.193 is very near to the resonancecondition and should be avoided by using softermounts.b) nn0=800n0= 2.5; n0 =8002.5 = 320 vib/minNow n0 = 30

xthus x =30n0; x = 30n0 2= 30320 2= 0.008789 m or 8.789 mmA low natural frequency of 320 vib/min is wellwithin the insulation range, therefore from eitherthe deflectionfrequency graph or by formulathe corresponding rubber deflection necessary is8.789 mm when the engine's static weight bearsdown on the mounts.1.2.8 Engine to body/chassis mountingsEngine mountings are normally arranged toprovide a degree of flexibility in the horizontallongitudinal, horizontal lateral and vertical axis ofrotation. At the same time they must have suffi-cient stiffness to provide stability under shockloads which may come from the vehicle travellingover rough roads. Rubber sprung mountingssuitably positioned fulfil the following functions:1 Rotational flexibility around the horizontallongitudinal axis which is necessary to allow theimpulsive inertia and gas pressure componentsof the engine torque to be absorbed by rolling ofthe engine about the centre of gravity.2 Rotational flexibility around both the horizontallateral and the vertical axis to accommodate anyhorizontal and vertical shake and rock caused byunbalanced reciprocating forces and couples.1.2.9 Subframe to body mountings(Figs 1.6 and 1.19)One of many problems with integral body design isthe prevention of vibrations induced by the engine,transmissionandroadwheels frombeingtransmittedthrough the structure. Some manufacturers adopt asubframe (Fig. 1.6(a, b and c)) attached by resilientmountings (Fig. 1.19(a and b)) to the body to whichthe suspension assemblies, and in some instances theengine and transmission, are attached. The massof the subframes alone helps to damp vibrations.It also simplifies production on the assembly line,and facilitates subsequent overhaul or repairs.In general, the mountings are positioned so thatthey allow strictly limited movement of thesubframe in some directions but provide greaterfreedom in others. For instance, too much lateralfreedom of a subframe for a front suspensionassembly would introduce a degree of instabilityinto the steering, whereas some freedom in verticaland longitudinal directions would improve thequality of a ride.1.2.10 Types of rubber flexible mountingsA survey of typical rubber mountings used forpower units, transmissions, cabs and subframesare described and illustrated as follows:Double shear paired sandwich mounting (Fig.1.18(a)) Rubber blocks are bonded between thejaws of a `U' shaped steel plate and a flat interleafplate so that a double shear elastic reaction takesplace when the mount is subjected to vertical load-ing. This type of shear mounting provides a largedegree of flexibility in the upright direction andthus rotational freedom for the engine unit aboutits principal axis. It has been adopted for bothengine and transmission suspension mountingpoints for medium-sized diesel engines.Double inclined wedge mounting (Fig. 1.18(b)) Theinclined wedge angle pushes the bonded rubberblocks downwards and outwards against thebent-up sides of the lower steel plate when loadedin the vertical plane. The rubber blocks are subjectedto both shear and compressive loads and the propor-tion of compressive to shear load becomes greaterwith vertical deflection. This form of mounting issuitable for single point gearbox supports.Inclined interleaf rectangular sandwich mounting(Fig. 1.18(c)) These rectangular blocks are19Fig. 1.18(ah) Types of rubber flexible mountings20Fig. 1.18 contd21designed to be used with convergent `V' formationengine suspension system where the blocks areinclined on either side of the engine. This configura-tion enables the rubber to be loaded in both shearand compression with the majority of engine rota-tional flexibility being carried out in shear. Verticaldeflection due to body pitch when accelerating orbraking is absorbed mostly in compression. Verticalelastic stiffness may be increased without greatlyeffecting engine roll flexibility by having metalspacer interleafs bonded into the rubber.Double inclined wedge with longitudinal controlmounting (Fig. 1.18(d)) Where heavy verticalloads and large rotational reactions are to beabsorbed, double inclined wedge mounts positionedon either side of the power unit's bell housingat principal axis level may be used. Longitudinalmovement is restricted by the double `V' formedbetween the inner and two outer members seen ina plan view. This `V' and wedge configuration pro-vides a combined shear and compressive strain tothe rubber when there is a relative fore and aft move-ment between the engine and chassis, in addition tothat created by the vertical loading of the mount.This mounting's major application is for the rearmountings forming part of a four point suspensionfor heavy diesel engines.Metaxentric bush mounting (Fig. 1.18(e)) Whenthe bush is in the unloaded state, the steel innersleeve is eccentric relative to the outer one so thatFig. 1.18 contd22there is more rubber on one side of it than on theother. Precompression is applied to the rubberexpanding the inner sleeve. The bush is set so thatthe greatest thickness of rubber is in compressionin the laden condition. A slot is incorporated inthe rubber on either side where the rubber is at itsminimum in such a position as to avoid stressingany part of it in tension.When installed, its stiffness in the fore and aftdirection is greater than in the vertical direction, theratio being about 2.5 : 1. This type of bush providesa large amount of vertical deflection with very littlefore and aft movement which makes it suitable forrear gearbox mounts using three point power unitsuspension and leaf spring eye shackle pin bushes.Metacone sleeve mountings (Fig. 1.18(f and g))These mounts are formed from male and femaleconical sleeves, the inner male member beingcentrally positioned by rubber occupying thespace between both surfaces (Fig. 1.18(f)). Duringvertical vibrational deflection, the rubber betweenthe sleeves is subjected to a combined shear andcompression which progressively increases the stiff-ness of the rubber as it moves towards full distor-tion. The exposed rubber at either end overlaps theflanged outer sleeve and there is an upper andlower plate bolted rigidly to the ends of the innersleeve. These plates act as both overload (bump)and rebound stops, so that when the inner memberdeflects up or down towards the end of its move-ment it rapidly stiffens due to the surplus rubberbeing squeezed in between. Mounts of this kind areused where stiffness is needed in the horizontaldirection with comparative freedom of movementfor vertical deflection.An alternative version of the Metacone mountuses a solid aluminium central cone with a flangedpedestal conical outer steel sleeve which can bebolted directly onto the chassis side member, seeFig. 1.18(g). An overload plate is clamped betweenthe inner cone and mount support arm, but norebound plate is considered necessary.These mountings are used for suspension appli-cations such as engine to chassis, cab to chassis,bus body and tanker tanks to chassis.Double inclined rectangular sandwich mounting(Fig. 1.18(h)) A pair of rectangular sandwichrubber blocks are supported on the slopes of atriangular pedestal. A bridging plate merges theresilience of the inclined rubber blocks so thatthey provide a combined shear and compressivedistortion within the rubber. Under small deflec-tion conditions the shear and compression isalmost equal, but as the load and thus deflectionincreases, the proportion of compression over theshear loading predominates.These mounts provide very good lateral stabilitywithout impairing vertical deflection flexibility andprogressive stiffness control. When used for roadwheel axle suspension mountings, they offer goodinsulation against road and other noises.Flanged sleeve bobbin mounting with reboundcontrol (Fig. 1.19(a and b)) These mountingshave the rubber moulded partially around the outerflange sleeve and in between this sleeve and an innertube. A central bolt attaches the inner tube to thebody structure while the outer member is bolted ontwo sides to the subframe.When loaded in the vertical downward direction,the rubber between the sleeve and tube walls will bein shear and the rubber on the outside of theflanged sleeve will be in compression.There is very little relative sideway movementbetween the flanged sleeve and inner tube due torubber distortion. Anoverloadplate limits the down-ward deflection and rebound is controlled by thelower plate and the amount and shape of rubbertrapped between it and the underside of the flangedsleeve. A reduction of rubber between the flangedsleeve and lower plate (Fig. 1.19(a)) reduces therebound, but an increase indepth of rubber increasesrebound (Fig. 1.19(b)). The load deflection charac-teristics are given for both mounts in Fig. 1.19c.These mountings are used extensively for body tosubframe and cab to chassis mounting points.Hydroelastic engine mountings (Figs 1.20(ac) and1.21) A flanged steel pressing houses and sup-ports an upper and lower rubber spring diaphragm.The space between both diaphragms is filled andsealed with fluid and is divided in two by a separatorplate and small transfer holes interlink the fluidoccupying these chambers (Fig. 1.20(a and b)).Under vertical vibratory conditions the fluid willbe displaced from one chamber to the otherthrough transfer holes. During downward deflec-tion (Fig. 1.20(b)), both rubber diaphragms aresubjected to a combined shear and compressiveaction and some of the fluid in the upper chamberwill be pushed into the lower and back again byway of the transfer holes when the rubber rebounds(Fig. 1.20(a)). For low vertical vibratory frequencies,23the movement of fluid between the chambers isunrestricted, but as the vibratory frequenciesincrease, the transfer holes offer increasing resist-ance to the flow of fluid and so slow down the upand down motion of the engine support arm. Thisdamps and reduces the amplitude of mountingsvertical vibratory movement over a number ofcycles. A comparison of conventional rubber andhydroelastic damping resistance over the normaloperating frequency range for engine mountings isshown in Fig. 1.20(c).Instead of adopting a combined rubber mountwith integral hydraulic damping, separate diagon-ally mounted telescopic dampers may be used inconjunction with inclined rubber mounts to reduceboth vertical and horizontal vibration (Fig. 1.21).1.3 Fifth wheel coupling assembly(Fig. 1.22(a and b))The fifth wheel coupling attaches the semi-trailer tothe tractor unit. This coupling consists of a semi-circular table plate with a central hole and a veesection cut-out towards the rear (Fig. 1.22(b)).Attached underneath this plate are a pair of pivot-ing coupling jaws (Fig. 1.22(a)). The semi-trailerhas an upper fifth wheel plate welded or bolted tothe underside of its chassis at the front and in thecentre of this plate is bolted a kingpin which facesdownwards (Fig. 1.22(a)).When the trailer is coupled to the tractor unit,this upper plate rests and is supported on top of thetractor fifth wheel table plate with the two halves ofthe coupling jaws engaging the kingpin. To permitFig. 1.19(ac) Flanged sleeve bobbin mounting withrebound control24relative swivelling between the kingpin and jaws,the two interfaces of the tractor fifth wheeltables and trailer upper plate should be heavilygreased. Thus, although the trailer articulatesabout the kingpin, its load is carried by the tractortable.Flexible articulation between the tractor andsemi-trailer in the horizontal plane is achieved bypermitting the fifth wheel table to pivot on hori-zontal trunnion bearings that lie in the same verticalplane as the kingpin, but with their axes at rightangles to that of the tractor's wheel base (Fig.1.22(b)). Rubber trunnion rubber bushes normallyprovide longitudinal oscillations of about 10.The fifth wheel table assembly is made fromeither a machined cast or forged steel sections, orfrom heavy section rolled steel fabrications, and theupper fifth wheel plate is generally hot rolled steelwelded to the trailer chassis. The coupling lockingsystem consisting of the jaws, pawl, pivot pins andkingpin is produced from forged high carbon man-ganese steels and the pressure areas of these com-ponents are induction hardened to withstand shockloading and wear.1.3.1 Operation of twin jaw coupling(Fig. 1.23(ad))With the trailer kingpin uncoupled, the jaws will bein their closed position with the plunger withdrawnfrom the lock gap between the rear of the jaws,which are maintained in this position by the pawlcontacting the hold-off stop (Fig. 1.23(a)). Whencoupling the tractor to the trailer, the jaws of the Fig. 1.20(ac) Hydroelastic engine mount25fifth wheel strike the kingpin of the trailer. Thejaws are then forced open and the kingpin entersthe space between the jaws (Fig. 1.23(b)). The king-pin contacts the rear of the jaws which thenautomatically pushes them together. At the sametime, one of the coupler jaws causes the trip pin tostrike the pawl. The pawl turns on its pivot againstthe force of the spring, releasing the plunger, allow-ing it to be forced into the jaws' lock gap by itsspring (Fig. 1.23(c)). When the tractor is moving,the drag of the kingpin increases the lateral force ofthe jaws on the plunger.To disconnect the coupling, the release handlever is pulled fully back (Fig. 1.23(d)). Thisdraws the plunger clear of the rear of the jawsand, at the same time, allows the pawl to swinground so that it engages a projection hold-off stopsituated at the upper end of the plunger, thus jam-ming the plunger in the fully out position in readi-ness for uncoupling.1.3.2 Operation of single jaw and pawl coupling(Fig. 1.24(ad))With the trailer kingpin uncoupled, the jaw will beheld open by the pawl in readiness for coupling(Fig. 1.24(a)). When coupling the tractor to thetrailer, the jaw of the fifth wheel strikes the kingpinof the trailer and swivels the jaw about its pivot pinagainst the return spring, slightly pushing out thepawl (Fig. 1.24(b)). Further rearward movement ofthe tractor towards the trailer will swing the jawround until it traps and encloses the kingpin. Thespring load notched pawl will then snap over thejaw projection to lock the kingpin in the couplingposition (Fig. 1.24(c)). The securing pin shouldthen be inserted through the pull lever and tableeye holes. When the tractor is driving forward, thereaction on the kingpin increases the lockingforce between the jaw projection and the notchedpawl.To disconnect the coupling, lift out the securingpin and pull the release hand lever fully out(Fig. 1.24(d)). With both the tractor and trailerstationary, the majority of the locking forceapplied to notched pawl will be removed so thatwith very little effort, the pawl is able to swing clearof the jaw in readiness for uncoupling, that is, byjust driving the tractor away from the trailer. Thusthe jaw will simply swivel allowing the kingpin topull out and away from the jaw.1.4 Trailer and caravan drawbar couplings1.4.1 Eye and bolt drawbar coupling for heavygoods trailers (Figs 1.25 and 1.26)Drawbar trailers are normally hitched to the truckby means of an `A' frame drawbar which is coupledby means of a towing eye formed on the end of thedrawbar (Fig. 1.25). When coupled, the towing eyehole is aligned with the vertical holes in the upperand lower jaws of the truck coupling and an eyebolt passes through both coupling jaws and draw-bar eye to complete the attachment (Fig. 1.26).Lateral drawbar swing is permitted owing to theeye bolt pivoting action and the slots between theFig. 1.21 Diagonally mounted hydraulic dampers suppress both vertical and horizontal vibrations26jaws on either side. Aligning the towing eye to thejaws is made easier by the converging upper andlower lips of the jaws which guide the towing eye asthe truck is reversed and the jaws approach thedrawbar. Isolating the coupling jaws from thetruck draw beam are two rubber blocks which actas a damping media between the towing vehicle andtrailer. These rubber blocks also permit additionaldeflection of the coupling jaw shaft relative to thedraw beam under rough abnormal operating con-ditions, thus preventing over-straining the drawbarand chassis system.Fig. 1.22(a and b) Fifth wheel coupling assembly27Fig. 1.23(ad) Fifth wheel coupling with twin jaws plunger and pawl28Fig. 1.24(ad) Fifth wheel coupling with single jaw and pawl29The coupling jaws, eye bolt and towing eye aregenerally made from forged manganese steel withinduction hardened pressure areas to increase thewear resistance.Operation of the automatic drawbar coupling(Fig. 1.26) In the uncoupled position the eyeboltis held in the open position ready for coupling(Fig. 1.26(a)). When the truck is reversed, the jawsof the coupling slip over the towing eye and in theprocess strike the conical lower end of the eye bolt(Fig. 1.26(b)). Subsequently, the eye bolt will lift. Thistrips the spring-loadedwedge lever whichnowrotatesclockwise so that it bears down on the eye bolt.Further inward movement of the eye bolt betweenthe coupling jaws aligns the towing eye with the eyebolt. The spring pressure nowacts through the wedgelever to push the eye bolt through the towing eye andthe lower coupling jaw (Fig. 1.26(c)). When the eyebolt stop-plate has been fully lowered by the springtension, the wedge lever will slot into its grooveformed in the centre of the eye bolt so that it locksthe eye bolt in the coupled position.To uncouple the drawbar, the handle is pulledupwards against the tension of the coil springmounted on the wedge level operating shaft(Fig. 1.26(d)). This unlocks the wedge, freeing theeyebolt and then raises the eye bolt to theuncoupled position where the wedge lever jams itin the open position (Fig. 1.26(a)).1.4.2 Ball and socket towing bar coupling forlight caravan/trailers (Fig. 1.27)Light trailers or caravans are usually attached tothe rear of the towing car by means of a ball andsocket type coupling. The ball part of the attach-ment is bolted onto a bracing bracket fitted directlyto the boot pan or the towing load may be sharedout between two side brackets attached to the rearlongitudinal box-section members of the body.A single channel section or pair of triangularlyarranged angle-section arms may be used to formthe towbar which both supports and draws thetrailer.Attached to the end of the towbar is the sockethousing with an internally formed spherical cavity.This fits over the ball member of the coupling sothat it forms a pivot joint which can operate in boththe horizontal and vertical plane (Fig. 1.27).To secure the socket over the ball, a lock devicemust be incorporated which enables the coupling tobe readily connected or disconnected. This lockmay take the form of a spring-loaded horizontallypositioned wedge with a groove formed across itstop face which slips underneath and against theball. The wedge is held in the closed engaged pos-ition by a spring-loaded vertical plunger which hasa horizontal groove cut on one side. An uncouplinglever engages the plunger's groove so that when thecoupling is disconnected the lever is squeezed to liftand release the plunger from the wedge. At thesame time the whole towbar is raised by the handleto clear the socket and from the ball member.Coupling the tow bar to the car simply reversesthe process, the uncoupling lever is again squeezedagainst the handle to withdraw the plunger and thesocket housing is pushed down over the ball mem-ber. The wedge moves outwards and allows the ballto enter the socket and immediately the wedgesprings back into the engaged position. Releasingthe lever and handle completes the coupling bypermitting the plunger to enter the wedge lockgroove.Sometimes a strong compression spring is inter-posed between the socket housing member and thetowing (draw) bar to cushion the shock load whenthe car/trailer combination is initially driven awayfrom a standstill.1.5 Semi-trailer landing gear (Fig. 1.28)Landing legs are used to support the front of thesemi-trailer when the tractor unit is uncoupled.Extendable landing legs are bolted vertically toeach chassis side-member behind the rear wheels ofFig. 1.25 Drawbar trailer30Fig. 1.26(ae) Automatic drawbar coupling31the tractor unit, just sufficiently back to clear therear tractor road wheels when the trailer is coupledand the combination is being manoeuvred(Fig. 1.28(a)). To provide additional support forthe legs, bracing stays are attached between the legsand from the legs diagonally to the chassis cross-member (Fig. 1.28(b)).The legs consist of inner and outer high tensilesteel tubes of square section. A jackscrew with abevel wheel attached at its top end supported by theouter leg horizontal plate in a bronze bush bearing.The jawscrew fits into a nut which is mounted atthe top of the inner leg and a taper roller bearingrace is placed underneath the outer leg horizontalsupport plate and the upper part of the jackscrewto minimize friction when the screw is rotated (Fig.1.28(b)). The bottom ends of the inner legs maysupport either twin wheels, which enable the trailerto be manoeuvred, or simply flat feet. The latter areable to spread the load and so permit greater loadcapacity.To extend or retract the inner legs, a windinghandle is attached to either the low or high speedshaft protruding from the side of the gearbox. Theupper high speed shaft supports a bevel pinionwhich meshes with a vertically mounted bevelwheel forming part of the jackscrew.Rotating the upper shaft imparts motion directlyto the jackscrew through the bevel gears. If greaterleverage is required to raise or lower the front of thetrailer, the lower shaft is engaged and rotated.This provides a gear reduction through a com-pound gear train to the upper shaft which thendrives the bevel pinion and wheel and hence thejackscrew.1.6 Automatic chassis lubrication system1.6.1 The need for automatic lubrication system(Fig. 1.29)Owing to the heavy loads they carry commercialvehicles still prefer touse metal tometal joints whichare externally lubricated. Such joints are kingpinsand bushes, shackle pins and bushes, steering balljoints, fifth wheel coupling, parking brake linkageetc. (Fig. 1.29). These joints require lubricating inproportion to the amount of relative movement andthe loads exerted. If lubrication is to be effective inreducing wear between the moving parts, fresh oilmust be pumped between the joints frequently. Thiscan best be achieved by incorporating an automaticlubrication system which pumps oil to the bearing'ssurfaces in accordance to the distance travelled bythe vehicle.1.6.2 Description of airdromic automatic chassislubrication system (Fig. 1.30)This lubrication system comprises four major com-ponents; a combined pump assembly, a power unit,an oil unloader valve and an air control unit.Pump assembly (Fig. 1.30) The pump assemblyconsists of a circular housing containing a ratchetoperated drive (cam) shaft upon which aremounted one, two or three single lobe cams (onlyone cam shown). Each cam operates a row of 20pumping units disposed radially around the pumpcasing, the units being connected to the chassisbearings by nylon tubing.Power unit (Fig. 1.30) This unit comprises acylinder and spring-loaded air operated pistonwhich is mounted on the front face of the pumpassembly housing, the piston rod being connectedindirectly to the drive shaft ratchet wheel by way ofa ratchet housing and pawl.Oil unloader valve (Fig. 1.30) This consists of ashuttle valve mounted on the front of the pumpassembly housing. The oil unloader valve allows airpressure to flow to the power unit for the powerstroke. During the exhaust stroke, however, whenair flow is reversed and the shuttle valve is liftedfrom its seat, any oil in the line between the powerunit and the oil unloader valve is then discharged toatmosphere.Fig. 1.27 Ball and socket caravan/trailer towingattachment32Fig. 1.28(a and b) Semi-trailer landing gear33Air control unit (Fig. 1.30) This unit is mountedon the gearbox and is driven via the speedometertake-off point. It consists of a wormand wheel drivewhich operates an air proportioning controlunit. This air proportioning unit is operated by asingle lift face cam which actuates two poppetvalves, one controlling air supply to the powerunit, the other controlling the exhaust air from thepower unit.1.6.3 Operation of airdromic automatic chassislubrication system (Fig. 1.30)Air from the air brake auxiliary reservoir passes byway of the safety valve to the air control (propor-tioning) unit inlet valve. Whilst the inlet valve isheld open by the continuously rotating face camlobe, air pressure is supplied via the oil unloadervalve to the power unit attached to the multipumpassembly housing. The power unit cylinder is sup-ported by a pivot to the pump assembly casing,whilst the piston is linked to the ratchet and pawlhousing. Because the pawl meshes with one of theratchet teeth and the ratchet wheel forms part ofthe camshaft, air pressure in the power cylinder willpartially rotate both the ratchet and pawl housingand the camshaft clockwise. The cam (or cams) arein contact with one or more pump unit, and so eachpartial rotation contributes to a proportion of thejerk plunger and barrel pumping cycle of each unit(Fig. 1.30).As the control unit face cam continues to rotate,the inlet poppet inlet valve is closed and the exhaustpoppet valve opens. Compressed air in the air con-trol unit and above the oil control shuttle valve willnow escape through the air control unit exhaustport to the atmosphere. Consequently the com-pressed air underneath the oil unloader shuttlevalve will be able to lift it and any trapped air andoil in the power cylinder will now be released viathe hole under the exhaust port. The power unitpiston will be returned to its innermost position bythe spring and in doing so will rotate the ratchetand pawl housing anti-clockwise. The pawl is thusFig. 1.29 Tractor unit automatic lubrication system34Fig. 1.30 Airdromic automatic chassis lubrication system35able to slip over one or more of the ratchet teeth totake up a new position. The net result of the powercylinder being charged and discharged with com-pressed air is a slow but progressive rotation of thecamshaft (Fig. 1.30).A typical worm drive shaft to distance travelledrelationship is 500 revolutions per 1 km. For 900worm drive shaft revolutions the pumping camrevolves once. Therefore, every chassis lubricationpoint will receive one shot of lubricant in thisdistance.When the individual lubrication pump unit'sprimary plunger is in its outermost position, oilsurrounding the barrel will enter the inlet port,filling the space between the two plungers. As thecam rotates and the lobe lifts the primary plunger,it cuts off the inlet port. Further plunger rise willpartially push out the secondary plunger and soopen the check valve. Pressurised oil will thenpass between the loose fitting secondary plungerand barrel to lubricate the chassis moving part itservices (Fig. 1.30).362 Friction clutch2.1 Clutch fundamentalsClutches are designed to engage and disengage thetransmission system from the engine when a vehicleis being driven away from a standstill and when thegearbox gear changes are necessary. The gradualincrease in the transfer of engine torque to thetransmission must be smooth. Once the vehicle isin motion, separation and take-up of the drive forgear selection must be carried out rapidly withoutany fierceness, snatch or shock.2.1.1 Driven plate inertiaTo enable the clutch to be operated effectively, thedriven plate must be as light as possible so thatwhen the clutch is disengaged, it will have the mini-mum of spin, i.e. very little flywheel effect. Spinprevention is of the utmost importance if the vari-ous pairs of dog teeth of the gearbox gears, be theyconstant mesh or synchromesh, are to align in theshortest time without causing excessive pressure,wear and noise between the initial chamfer of thedog teeth during the engagement phase.Smoothness of clutch engagement may beachieved by building into the driven plate somesort of cushioning device, which will be discussedlater in the chapter, whilst rapid slowing down ofthe driven plate is obtained by keeping the diameter,centre of gravity and weight of the driven plate tothe minimum for a given torque carryin