Abstract

This paper acts as a design report describing the overall two seated buggy chassis designs that have been developed as a requirement for the BJF3022 Computer Aided Design 2 course.

Introduction

The goal of the project is to design the two seated chassis model of the "buggy" vehicle by means of using the selected program which supports the design. It is also necessary to perform a series of computer calculations in order to simulate the behavior of the structure under the influence of different load conditions. The frame of the vehicle model visualization is performed using Autodesk Inventor 2016. Strength calculations are made by means Stress Analysis using Autodesk Inventor 2016.

Objective The objectives for this project are:

i. To design an two seated buggy chassis ii. To analyze the stress distribution on an buggy frame

Scopes of Project

The scope for this project consists ofi. Design a two seated buggy chassis. ii. Analyzing the frame stress distribution by using Autodesk Inventor 2016 Analysis

Software.

Literature Review

A buggy car is a recreational vehicle with large wheels, and wide tires, designed for use on sand dunes or beaches. The design is usually a modified vehicle with a modified engine mounted on an open chassis. The modifications usually attempt to increase the power to weight ratio by either lightening the vehicle or increasing engine power or both. A similar, more recent generation of off road vehicle, often similar in appearance to a sand rail but designed for different use, is the "off road go kart". The difference between a dune buggy or go kart and an "off road" buggy or kart is sometimes nothing more than the type of tires fitted -sand tires or all terrain tires - but "off road" go karts and buggies are a rapidly developing category of their own. They are also often referred to as air buggies, and those with an open frame chassis are called sand rails.

Literature Review

Chassis Design Consideration

1) Modifying Production Chassis

• When considering modifying a production-based chassis to mount alternate suspension, engines or drivetrain, spend time studying the unibody (newer vehicle) or ladder-frame (older vehicle) structures. The structures formed by the manufacturer’s chassis designers have strong areas intended for loads and weak areas not intended to carry loads. Identifying the correct parts of the chassis structure to cut or modify is critical.

• Consider using scale models of the vehicle (if plastic models were made), to mockup the changes, or 3D modeling software to do the same. If the changes involve the suspension, such as lowering the vehicle, model the new suspension first. Sometimes lowering the vehicle while using the same suspension pickup points will create poor handling.

2) Build Chassis Models

• Modeling a space frame chassis with balsa wood sticks enables you to see firsthand the differences triangulation makes to the stiffness of a chassis. Herb Adams, in his book “Chassis Engineering” provides a whole chapter on chassis modeling using balsa and paper. His recommendation is for a 1/12 scale model.

• Likewise, using cardboard, paper and glue to build model monocoques can be a very rewarding and low cost learning experience as well. The great thing about these materials is that they don’t have a lot of strength and so the deformations that loads create can be easily seen when loads are applied.

• Design the chassis after the suspension.• It is much easier to design a tentative suspension according to the rules and good

geometry, and then build the chassis to conform to suspension mounting points and springs/damper mounts.

3) Consider the load paths

• A chassis is not about “absorbing” energy, but rather about support. When considering placement of tubes, visualize the “load paths”, and consider using FEA (Finite Element Analysis software) to help analyze load scenarios. Load paths are defined as the forces resulting from accelerating and decelerating, in the longitudinal and lateral directions which follow the tubing from member to member. The first forces which come to mind are suspension mounts, but things like the battery and driver place stresses on the space frame structure.

4) Maximize CG placement and vehicle balance

• Center of gravity affects the race car like a pendulum. The ideal place for the CG is absolutely between the front and rear wheels and the left and right wheels. Placing the CG fore or aft or left or right of this point means that weight transfers unevenly depending on which way the car is turning, and whether it is accelerating or decelerating. The further from this ideal point, the more one end of the car acts like a pendulum, and the more difficult it is to optimize handling. The CG is also height dependent. Placing an engine higher off the ground raises the CG, and forces larger amounts of weight to transfer when cornering, accelerating, or decelerating. The goal of vehicle design is to keep all four wheels planted if possible to maximize grip, so placing all parts in the car at their lowest possible location will help lower the CG height.

5) Layout the tube members for easy access and maintenance

• Maintaining a race car comes after construction. Placing tubes across openings is a natural way of ensuring a rigid chassis. However, in practical terms you may be making it difficult or impossible to reach the maintenance demanding components. A good chassis design will allow quick and easy access to all components and will not hamper removal or replacement of any part.

6) Check out vehicles which are competitive in your class

• Vehicles which are competitive are usually built well, and with appropriate materials and methods. Observe them at the track and in the pits, and you can infer a great deal about what makes them winners.

7) Optimize the tubing shape for the job

• Square tubing is the easiest structural shape to build a chassis from. However, there are circumstances where round tubing can be useful, albeit at a penalty in the complexity of construction. Oval tubing is useful in open wheel race cars for wishbones.

8) Optimize the tubing size and gauge for the job

• Tubing which is used in tension, can be of a lighter gauge than that used in compression. Keeping this in mind can save considerable weight, although it requires additional joining work and variety of tubing.

Materials factor that been consider in building a buggy car

1) Lightweight

As there is a high emphasis on greenhouse gas reductions, reduction of emission and improving fuel efficiency this criterion is most important one for an automotive company. Lightweight materials can improve fuel efficiency more than other factors. Experiments reveal that 10 percent of weight reduction can lead to 6 to 8 percent improvement in fuel usage. Weight reduction can be obtained by three ways:

Replacing materials of high specific weight with lower density materials without reducing rigidity and durability. For example replacement of steel with aluminium, magnesium, composites and foams.

Optimizing the design of load-carrying elements and exterior attachments so as to reduce their weight without any loss in rigidity or functionality.

Optimizing the production process, such as reducing spot welding and replacing new joining techniques.

2) Economic effectiveness

One of the most important consumer driven factors in automotive industry is the cost that determines whether any new material has an opportunity to be selected for a vehicle component. Cost includes three components: actual cost of raw materials, manufacturing value added, and the cost to design and test the product.

Aluminium and magnesium alloys are certainly more costly than the currently used steel and cast irons. Since cost may be higher, decisions to select light metals must be justified on the basis of improved functionality. Meanwhile the high cost is one of the major obstacles in use of the composite materials.

3) Safety

The ability to absorb impact energy and be survivable for the passengers is called “crashworthiness” of the structure in vehicle. At first two concepts in automotive industry should be considered: crashworthiness and penetration resistance. In the more accurate definition of crashworthiness, it is the potential of absorption of energy through controlled failure modes and mechanisms. However penetration resistance is concerned with the total absorption without allowing projectile or fragment penetration.

Materials normally used in making buggy car

Steel

• The main factors of selecting material especially for body is wide variety of characteristics such as thermal, chemical or mechanical resistance, ease of manufacture and durability. So if we want to choose a material with these characteristics, Steel is their first choice. There was many developments in irons and steels over the past couple decades that made the steel more light-weight, stronger, stiffer and improving other performance characteristics. Applications include not only vehicle bodies, but also engine, chassis, wheels and many other parts. Iron and steel form the critical elements of structure for the vast majority of vehicles, and are low-cost materials.

• The past several years have seen steady increases in the use of high-strength steels that are referred to as high-strength, low-alloy steels. These materials formed the basis of Ultra light Steel Auto Body (ULSAB). The ULSAB car body demonstrated a 19% mass reduction in a body structure that had superior strength and structural performance. Comparable mass reductions and other benefits were achieved for doors, hoods, deck lids, and the hatchbacks.

• The prime reason for using steel in the body structure is its inherent capability to absorb impact energy in a crash situation.

Aluminium

There are a wide variety of aluminium usages in automotive powertrain, chassis and body structure. Use of aluminium can potentially reduce the weight of the vehicle body. Its low density and high specific energy absorption performance and good specific strength are its most important properties.

Aluminium is also resistance to corrosion. But according to its low modulus of elasticity, it cannot substitute steel parts and therefore those parts need to be re-engineered to achieve the same mechanical strength, but still aluminium offers weight reduction.

Aluminium usage in automotive industry has grown within past years. In automotive powertrain, aluminim castings have been used for almost 100% of pistons, about 75% of cylinder heads, 85% of intake manifolds and transmission. For chassis applications, aluminium castings are used for about 40% of wheels, and for brackets, brake components, suspension, steering components and instrument panels. Aluminium is used for body structures, closures and exterior attachments such as crossbeams, doors or bonnets.

Recent developments have shown that up to 50% weight saving for the body in white (BIW) can be achieved by the substitution of steel by aluminium. This can result in a 20-30% total vehicle weight reduction.

The cost of aluminium and price stability is its biggest obstacle for its application.

Magnesium

• Magnesium is another light metal that is becoming increasingly common in automotive engineering. It is 33% lighter than aluminium and 75% lighter than steel/cast iron components. Magnesium components have many mechanical/physical property disadvantages that require unique design for application to automotive products. Although its tensile yield strength is about the same, magnesium has lower ultimate tensile strength fatigue strength, and creep strength compared to Aluminium. The modulus and hardness of magnesium alloys is lower than aluminium and the thermal expansion coefficient is greater.

• Magnesium alloys have distinct advantages over aluminium that include better manufacturability, longer die life and faster solidification. Also magnesium components have higher machinability.

• Because of its too low mechanical strength, pure magnesium must be alloyed with other elements. The most common alloying elements for room temperature applications is Mg-Al-Zn group that contains aluminium, manganese, and zinc.

Advanced composite materials

• Fibre reinforced composites offer a wide range of advantages to the automotive industry. It has the potential for saving weight offered by their low density. Component designs can be such that the fibres lie in the direction of the principal stresses, and amount of fibre used is sufficient to withstand the stress, thus optimizing materials usage.

Carbon-fibre epoxy composite

• Most recently, the most of the racing car companies much more rely on composites form whether it would be plastic composites, Kevlar and most importantly carbon-fibre epoxy composition. It is because the composite structure is the high strength/low weight ratio. The most common materials used for racing cars are carbon (graphite), Kevlar and glass fibres. Epoxy composites have been the first choice in Formula 1 car industries and other race cars.

Glass-fibre composites

• Glass fibre is being used mostly for the sports car which includes Formula 1 cars. It is lighter than steel and aluminium, easy to be shaped and rust-proof. And more important factor is that it is cheap to be produced in small quantity.

Aerodynamics Basics and Design Consideration

Aerodynamics is the science of how air flows around and inside objects. More generally, it can be labeled “Fluid Dynamics” because air is really just a very thin type of fluid. Above slow speeds, the air flow around and through a race vehicle begins to have a more pronounced effect on the acceleration, top speed and handling. Therefore, in race car design we need to understand and optimize how the air flows around and through the body, its openings and its aerodynamic devices.

Aerodynamics Consideration

1) Cover Open wheels

• Open wheels create a great deal of drag and air flow turbulence, similar to the diagram of the mirror in the “Turbulence” section above. Full covering bodywork is probably the best solution, if legal by regulations, but if partial bodywork is permitted, placing a converging fairing behind the wheel provides maximum benefit.

2) Minimize Frontal Area

• The smaller the hole your race car punches through the air, the better it will accelerate the higher the top speed it will have. It is usually much easier to reduce FA (frontal area) than the Cd (Drag coefficient).

3) Converge Bodywork Slowly

• Bodywork which quickly converges or is simply truncated, forces the air flow into turbulence, and generates a great deal of drag. As mentioned above, it also can affect aerodynamic devices and bodywork further behind on the vehicle body.

4) Use Spoilers

• Spoilers are widely used on sedan type cars such as NASCAR stock cars. These aerodynamic aids produce down force by creating a “dam” at the rear lip of the trunk, raising the air pressure over the trunk. Where a notch left by the rear window exists a spoiler can help restore pressure to the void behind the window.

5) Use Wings

• Wings are the inverted version of what you find on aircraft. They work very efficiently, and in less aggressive forms generate more downforce than drag, so they are loved in many racing circles. Wings are best placed in areas that have clear airflow to them. Placing a wing behind an obstruction reduces the downforce the wing can produce.

6) Use Front Air Dams

• Air dams at the front of the car restrict the flow of air reaching the underside of the car. This creates a lower pressure area under the car, effectively providing downforce. In many cases, the air dam also reduces the Cd of the vehicle.

7) Use Aerodynamics to Assist Vehicle Operation

• Using vehicle bodywork to direct airflow into openings, for instance, permits more efficient, smaller openings that reduce drag penalties. Quite often, with some forethought, you can gain an advantage over a competitor by these small dual purpose techniques.

• Another useful technique is to use the natural high and low pressure areas created by the bodywork to perform functions. For instance, Mercedes, back in the 1950s placed radiator outlets in the low pressure zone behind the driver. The air inlet pressure which fed the radiator became less critical, as the low pressure outlet area literally sucked air through the radiator.

• A useful high pressure area is in front of the car, and to make full use of this area, the nose of the car is often slanted downward. This allows the higher air pressure to push down on the nose of the car, increasing grip. It also has the advantage of permitting greater driver visibility.

8) Keep Protrusions Away From The Bodywork

• The smooth airflow achieved by proper bodywork design can be destroyed quite easily if a protrusion such as a mirror is too close to it. While it is important to design an aerodynamic mount for a mirror, the mirror itself needs to be placed far enough away from the bodywork to avoid adverse effects.

9) Rake the chassis

• The chassis, as mentioned in the aerodynamics theory section above, is capable of being slightly lower to the ground in the front than in the rear. The lower “Nose” of the car reduces the volume of air able to pass under the car, and the higher “Tail” of the car creates an expanding space where a vacuum effect can form. This lowers the air pressure beneath the car, creating down force.

10) Cover or streamline Exposed Wishbones

• Exposed wishbones (on open wheel cars) are often made from circular steel tube to save cost. However, these circular tubes generate turbulence. It may be worth considering the use of oval tubing, or a tube fairing that creates an oval shape over top of the round tubing.

Figure 2 : Streamlined wishbone tubing improves the smoothness of the air flow to parts of the car behind and reduces drag.

Chassis Types

There are multiple types of chassis but all of them can be classified into one of two approaches:1) Use lengths of round or square tubing, or other structural metal shapes to form the chassis

structure (Space frame, multi-tube, ladder frame)2) Use joined panels to form the chassis structure (Monocoque, Unibody)

Both approaches can provide a structure capable of mounting other race vehicle components, but each has its own advantages and disadvantages.

Space frame Chassis

The Space frame chassis uses numerous cut and shaped pieces structural metal tubing (usually steel) joined together to form a strong framework.

Diagram Space frame chassis for a “Low cost” car

Monocoque Chassis

The monocoque chassis is technically an improvement over the space frame chassis. Diagram 1 below shows a simple example of the difference between space frame and monocoque design.

Diagram 1. Comparing the behavior of a monocoque versus a space frame under tension load

The monocoque “Box” on the left uses a panel of material to structurally “complete” the box. When the hand pushes against it in the direction shown by the green arrow, it creates a shear force across the panel. This force is effectively handled the same way a tension load is by the space frame triangulated box on the right. However, if the hand were to push from the other side of the box, the space frame tube could potentially collapse in compression, whereas the monocoque box would behave the same way it did before.

Diagram 2 Comparing the behavior of a monocoque versus a space frame under compression load.

Both types of chassis can be made just as strong as each other. However, to make an equivalent strength space frame generally requires more material and therefore more weight. The materials used make a big difference as well.

In Diagram 3 below, both the monocoque “box” on the left and the fully triangulated space frame “box” on the right would handle loads in the same manner (the rear of the space frame “box” to avoid visually complicating the diagram)

Diagram 3 Monocoque box and “equivalent” triangulated space frame.

Although the monocoque can usually be made lighter and stronger than a space frame, it does have some downsides that make it more complicated to design, build and operate.First, the monocoque requires the structure formed by the panels to be “complete”. If you observe the “box” in diagram 3 that we used to demonstrate the monocoque, imagine one side of it is missing as shown in diagram 4 below:

Diagram 4. Incomplete load handling by a monocoque will cause it to deform and buckle.

We can push on the corner of the box where three panels meet (shown on the left) and it won’t warp (much), but push on a corner next to where the missing side should be and the box will

buckle (as shown on the right). Where an opening exists, the chassis must handle loads through a supporting sub-structure.A primary goal in monocoque design is to ensure that there are no unhandled load paths that can cause the monocoque structure to buckle. A buckled monocoque is no better than a buckled space frame tube.In the case of poorly handled load paths, the space frame can be more forgiving as the tubing diameter and steel material usually provide a more gradual failure than a monocoque. However, it is better to design the chassis correctly in the first place then to rely upon noticing gradual failures.

This brings us to another key point about the monocoque If it is damaged, it is difficult to repair compared to space frame tubes. It is also difficult to detect damage on a monocoque whereas bent or broken tubing is quite easy to spot.

Torsional Rigidity

Torsional rigidity is a property of every race vehicle chassis that determines how much twist

the chassis will experience when loads are applied through the wheels and suspension. Diagram

5 below shows the principle.

Diagram 5 Torsional Rigidity. The less the chassis twists, the more torsionally rigid it is considered.

A chassis that has a lot of twist won’t handle as predictably as one which has very little because by twisting, the chassis begins to act like an extension of the suspension. The suspension is designed to allow the wheels/tires to follow the road’s bumps and dips. If the chassis twists when a tire hits a bump, it acts like part of the suspension, meaning that tuning the suspension is difficult or impossible. Ideally, the chassis should be ultra-rigid, and

the suspension compliant. Torsional rigidity is measured in lbs-ft/degree or kg-m/degree. One end of the chassis (front or rear) is held stationary and the other end is balanced on a point and twist is applied via a beam. Diagram 6 below shows the basic idea

Diagram 6 Method to measure torsional rigidity.

Design of Two Seated Buggy

     

Stress Analysis Report

Analyzed File: Part1.iptAutodesk Inventor Version:

2016 (Build 200138000, 138)

Creation Date: 12/13/2015, 10:40 PMSimulation Author: BMSSummary:

  Project Info (iProperties)

  SummaryAuthor

USER1

  ProjectPart Number Part1

Designer USER1Cost $0.00Date Created

12/1/2015

  StatusDesign Status

WorkInProgress

  Physical

Material Aluminum 6061

Density 2.7 g/cm^3Mass 150.693 kg

Area 5682900 mm^2

Volume 55812300

mm^3

Center of Gravity

x=1.11546 mmy=345.013 mmz=252.95 mm

Note: Physical values could be different from Physical values used by FEA reported below.

  Simulation:1General objective and settings:Design Objective Single PointSimulation Type Static Analysis

Last Modification Date 12/13/2015, 10:39 PM

Detect and Eliminate Rigid Body Modes No

Mesh settings:Avg. Element Size (fraction of model diameter) 0.1

Min. Element Size (fraction of avg. size) 0.2Grading Factor 1.5

Max. Turn Angle 60 deg

Create Curved Mesh Elements Yes

  Material(s)Name Aluminum 6061

General

Mass Density 2.7 g/cm^3

Yield Strength 275 MPaUltimate Tensile Strength 310 MPa

Stress

Young's Modulus 68.9 GPaPoisson's Ratio 0.33 ul

Shear Modulus 25.9023 GPa

Part Name(s) Part1.ipt

  Operating conditions

  Force:1Load Type Force

Magnitude

1000000.000 N

Vector X 0.000 NVector Y 0.000 N

Vector Z 1000000.000 N

Selected Face(s)

  Fixed Constraint:1Constraint Type

Fixed Constraint

 Selected Face(s)

  Results

   Reaction Force and Moment on Constraints

Constraint Name

Reaction Force Reaction MomentMagnitude

Component (X,Y,Z)

Magnitude

Component (X,Y,Z)

Fixed Constraint:1

1000000 N

0 N344636 N m

344636 N m0 N 0 N m-1000000 N 0 N m

  Result SummaryName Minimum MaximumVolume 55812600 mm^3Mass 150.694 kgVon Mises Stress 0.00864369 MPa 3846.89 MPa

1st Principal Stress -1766.4 MPa 2631.6 MPa

3rd Principal Stress -5442.86 MPa 360.628 MPa

Displacement 0 mm 73.5085 mmSafety Factor 0.0714863 ul 15 ulStress XX -2613.8 MPa 2006.47 MPaStress XY -595.196 MPa 643.325 MPaStress XZ -1254.66 MPa 809.018 MPaStress YY -2820.45 MPa 2567.59 MPaStress YZ -1685.78 MPa 1110.07 MPaStress ZZ -4013.53 MPa 897.355 MPaX Displacement -16.3807 mm 15.5728 mmY Displacement -67.7719 mm 0.0472493 mmZ Displacement -2.82059 mm 36.8581 mmEquivalent Strain

0.00000011671 ul 0.0530019 ul

1st Principal Strain

0.0000000564021 ul 0.0343036 ul

3rd Principal Strain -0.0628001 ul

-0.0000000216866 ul

Strain XX -0.0319246 ul 0.0290405 ulStrain XY -0.0114893 ul 0.0124183 ulStrain XZ -0.0242192 ul 0.0156167 ulStrain YY -0.0329888 ul 0.0330681 ulStrain YZ -0.0325411 ul 0.021428 ulStrain ZZ -0.0351072 ul 0.0123625 ul

  Figures

  Von Mises Stress

  1st Principal Stress

  3rd Principal Stress

  Displacement

  Safety Factor

  Stress XX

  Stress XY

  Stress XZ

  Stress YY

  Stress YZ

 Stress ZZ

  X Displacement

  Y Displacement

 Z Displacement

  Equivalent Strain

  1st Principal Strain

  3rd Principal Strain

  Strain XX

  Strain XY

  Strain XZ

  Strain YY

  Strain YZ

  Strain ZZ

Discussion

This design data and analysis of is only a start in the long and complex way of designing and manufacturing a two seated Racing Buggy, which could compete with other Buggys. But in my opinion no one can really understand and add on the more complex mechanism without pass through the starting point and understanding thoroughly the basic functionality, and the theory behind the basic design. So this starting point is the most essential factor in any designing process. The next step in designing the complete a two seated Racing Buggy is to the design the complete rear and front axles, then the back axe and engine mounting, which also includes the linkages between them. The selection of a motor will help in both designing the gear transmission and the back axe. This selection will include also a generator set, a battery set, an exhaust system, gasoline tank and car accessories. Then will come the selection for the right fasteners. After that will be the auditing process on all the designing processes which include a through checking of the design data and measurements. Finally will be the manufacturing and assembling of the parts to form the complete buggy (product).

Conclusion

Frame design, as most design problems are, comes down to a series of tradeoffs between various competing aspects. With a chassis, the main aspects are stiffness, weight and cost. There are several ways to maximize these trade-offs as discussed earlier and this is essentially what the design process consists of.

The chassis design incorperated the concepts of triangulation and polar moment of inertia into a coherent chassis design that is representative of a first cut. The actual design process, is an iterative effort and once the analysis results are in, the design can be tweaked and updated to accommodate the discovered weaknesses. This is where the design created currently sits. It has been created and analyzed and the next step would be to update the design to address the issues found in the analysis.

References

Books

1) Baja Bugs and Buggies by Jeff Hibbard and Ron Sessions2) Building a Dune Buggy – The Essential Manual: Everything You Need to Know Build

Any VW-based Dune Buggy Yourself by Paul Shakespeare

Websites

1) http://www.vw-store.com/Buggy%20Frames-%20Chassis.htm 2) http://www.buggyworld.com/parts/products.php?id=2 3) http://www.v-dubstore.com/Articles.asp?ID=138 4) http://trentfabrication.com/chassis/