Vehicle Structures - Development of the Sports Car Chassis and Stiffness Analysis of the Westfield Sports Car

7/22/2019 Vehicle Structures - Development of the Sports Car Chassis and Stiffness Analysis of the Westfield Sports Car

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VEHICLE STIFFNESS ANALYSISwith a focus on Sports Car Structures and a detailed study of the Westfield

Sports Car Spaceframe Chassis

Wayne Prangnell

November 1992


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SUMMARY

The purpose of a vehicle chassis, the different type of vehicle structures and the analysis of vehicle structures

is discussed by way of introduction to a detailed investigation of a Westfield Sports Car space frame chassis.

The bending and torsional stiffness of a spaceframe chassis was tested in the laboratory and was modelled

using finite element analysis software. Laboratory testing was carried out to establish the validity of the finite

element model. The model was then used to investigate methods of improving the torsional stiffness of the

chassis without altering the layout of the car. A number of recommendations were made to improve the

torsional stiffness of the chassis with some simple modifications.

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TABLE OF CONTENTS

..................................................................................................................................................List of F igures 4

....................................................................................................................................................List of T ables 6

..................................................................................................................................................1 Introduction 8

.........................................................................................................................1.1 Outline of Project 8..................................................................................................................................................2 Background 9

..................................................................................................................................2.1 Introduction 9

.................................................................................................................2.2 Purpose of the Chassis 9

............................................................................................2.3 Basics of Vehicle Structural Actions 10

...........................................................................................................2.4 Requirements of a Chassis 15

................................................................................................2.4.1 Strength Requirements 15

..................................................................................2.4.2 Chassis Stiffness Requirements 17

................................................................................2.4.3 Determining Torsional Stiffness 19.......................................................................2.5 Relat ionship of Suspension and Chassis Stiffness 20

...........................................................................................................2.6 Vehi cle Structure Analysis 20

..........................................................2.7 Development of the Structure of Sports and Racing Cars 22

......................................................................................................2.8 Back ground of Clubman Cars 28

............................................................................................................3 Analysis of the Westfield Sports Car 30

..................................................................................................................................3.1 Introduction 30

............................................................................................3.2 Determination of Chassis Geometry 31

............................................................................................................3.2 Chassis Bending Stiffness 32

................................................................3.2.1 Laboratory Test Description and Procedure 32

................................................................................3.1.2 Theoretical Analysis Description 34

..........................................................................................................3.3 Chassis Torsional Stiffness 36

................................................................3.3.1 Laboratory Test Description and Procedure 36

................................................................................3.3.2 Theoretical Analysis Description 38

.................................................................................................................................4 Results and Di scussion 39

............................................................................................4.1 Bend ing Test and Bending Analysis 39

..........................................................................................4.2 Torsi onal Test and Torsional Analysis 41

.......................................................................................4.3 Torsional Stiffness - Chassis Variations 45

..................................................................................................................................................5 Concl usions 54

........................................................................................................................5.1 Recommendations 57

................................................................................................................................5.2 Further Study 58

........................................................................................................................................6 Acknowledgments 59

....................................................................................................................................................7 Refere nces 60

...................................................................................................................................................8 Appendices 61

............................................................................................Appendix A - Westfield Sports Car Data 61

........................................................................Appendix B - Westfield Sports Car Chassis Drawing 63..........................................................................................Appendix C - Computer Model Data File 64

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................................................................................Appendix D - Laboratory Testing Observations 76

................................................Appendix E - Diagrams and Information for Chassis Modifications 77

...................................................................Appendix F - Components of the Westfield Sports Cars 91

................................................................................................................Appendix G - Calculations 92

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LIST OF FIGURES

..........................................Figure 2.1 - Chassis Design Considerations 10

......................................Figure 2.2 - Exploded View of Girder Chassis 12

..Figure 2.3 - Twin Tube Chassis of Triumph TR4 with Cross Bracing 12

...........................................................Figure 2.4 - Spaceframe Chassis 13..........................................................Figure 2.5 - Multitubular Chassis 13

........Figure 2.6 - Torsion Box Stressed Skin Construction, Ford GT40 14

............................Figure 2.7 - Monocoque Stressed Skin Construction 14

..........Figure 2.8 - Composite Structure of Mass Produced Renault 16 15

..........................................Figure 2.9 - 1966 McLaren Grand Prix Car 20

............................................Figure 2.10 - Lola T92/10 Rollbox Model 24

......Figure 2.11 - Live Axle, Swing Axle and Independent Suspension 26

...Figure 2.12 - Independent Suspension Attached to Vehicle Structure 27................................................................Figure 2.13 - Lotus Mark Six 27

.....................................................Figure 2.14 - Mercds-Benz 300SL 28

......................................................Figure 2.15 - Mercds-Benz W196 28

........................Figure 2.16 - Structure of the Lotus 25 Grand Prix Car 29

.........Figure 2.17 - 1989 Ferrari Grand Prix Car (bodywork cut away) 29

......Figure 2.18 - 1988 McLaren MP4/4 GP Car, Bodywork Removed 30

.....................................Figure 2.19 - Modern Cars with Space Frames 31

.......................................Figure 2.20 - Monocoque Chassis Road Cars 32

........................................................Figure 2.21 - Westfield Sports Car 33

......................................................................Figure 2.22 - Ginnetta G2 33

............................................................Figure 2.23 - Lotus Seven Body 34

...........................................................Figure 2.24 - Elfin Clubman Car 35

..................................................Figure 3.1 - Layout of Chassis Survey 38

......................................................................Figure 3.2 - Axes System 39

.........................................................Figure 3.3 - Chassis Bending Test 39

............................................Figure 3.4 - PAFEC 34000 Beam Element 41

.........................Figure 3.5 - Standard Chassis Model Member Groups 42

.....................................................Figure 3.6 - Chassis Bending Model 43

........................................................Figure 3.7 - Chassis Torsional Test 44

................................Figure 3.8 - Pattern of Loading for Torsional Test 45

............................................Figure 3.9 - Chassis Torsional Test Model 46

.................Figure 4.1 - Load Deflection Response of Chassis Bending 47

..................Figure 4.2 - Shape of Chassis for Calculated Bending Test 49....................................Figure 4.3 Torsional Load Deflection Response 51

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............................Figure 4.4 - Scatter of Measured Torsional Stiffness 52

.................................Figure 4.5 - Torsional Deflections Along Chassis 53

.....Figure 4.6 - Torsional Stiffness Plots of Changes to Member Sizes 55

..............Figure 4.7 - Torsional Stiffness Plots of Engine Bay Changes 56

..Figure 4.8 - Torsional Stiffness Plots for Chassis with Extra Bracing 58

..........Figure 4.9 - Torsional Stiffness Plots of Centre Tunnel Changes 59

.........Figure 4.10 - Torsional Stiffness Plots for Changes Using Plates 60

....................Figure 4.11 - Torsional Stiffness Plots for Other Changes 61

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LIST OF TABLES

...........................Table 2.1 - Measured Peak Accelerations of Vehicles 18

..................................................Table 2.2 - Chassis Torsional Stiffness 22

....................................................Table 4.1 - Standard Chassis Models 54................................Table 4.2 - Category I, Changes to Member Sizes 55

.............................Table 4.3 - Category II, Changes to the Engine Bay 56

.................Table 4.4 - Category III, Addition of Bracing Chassis Nose 58

........................Table 4.5 - Category IV, Changes to the Centre Tunnel 59

....................................................Table 4.6 - Category V, Use of Plates 60

...............................................Table 4.7 - Category VI, Other Changes 61

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"You have to have passion to go deep inside, where you can then experience

special feelings, very special moments which trigger some of the unique

sensations, unique touch and feelings that give you something extra when you are

right on the limit."

Ayrton Senna, December 1991

The analysis of a vehicle structure takes the designer deep inside, looking forsomething extra to give the driver when he is right on the limit.

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1 INTRODUCTION

The motor car is an important part of our lives that most of us use every day. Usually considered as a

mechanical product because of all the mechanisms attached to it, the car is also an important structure. The

motor car is subject to such a variation of loads and a severe fatigue life. For efficiency and performance

reasons the car must weigh as little as possible, thus the design and analysis of this structure is essential. Theanalysis of vehicle structures is also very important because the public who use cars will tolerate the

occasional mechanical breakdown, but they expect never to have any problems with the structure of the

vehicle regardless of the severity of conditions the vehicle has been subject through its life.

The analysis of vehicle structures is an area where Civil Engineers, or more specifically Structural Engineers

are well equipped to tackle.

The analysis of vehicle structures was researched and a fairly broad overview provided. A detailed stiffness

analysis of the Westfield Sports Car chassis was then carried out using a finite element computer model and

validated with laboratory testing. Potential modifications to the chassis and their effect of vehicle stiffness

using the computer model.

1.1 Outline of Project

Many ideas were pursued with this project and the aims often shifted with new information that was learned

and new ideas, but the basic goal of this project has remained the same: To learn about the structure of

vehicles. The subject of this project was narrowed by the authors interest in motor sport and sports cars which

led to acquaintances with one of two vehicle manufacturers in Western Australia, Westfield Sports Cars

Australia. Stephen Fox from Westfield Sports Cars showed enthusiasm at learning more about the structure of

the sports car that his company produces and he agreed to lend a completed chassis for testing.

Early plans for the project were ambitious and some of the activities planned were: Track testing of the

Westfield Sports car to determine the loads on a vehicle, analysis of the chassis for stiffness, analysis of the

vehicle for stresses and laboratory testing of chassis stiffness and stresses.

Unfortunately track testing was not viable due to the cost of the equipment that would be required such as

strain gauges and high speed multi channel data loggers. With improvements in data logger technology and

availability, measuring the loads on a vehicle may make an interesting project in the future.

On a simpler level, it was attempted to measure vehicle loads with brake meters. Brake meters work on a

principal of lateral accelerations causing an angular deflection of a pendulum in a damping fluid. These

devices which were used by British authorities for testing the brakes of commercial vehicles were found to be

inaccurate for measuring car accelerations as the pitch and roll of a car about its horizontal axes and the slope

of a road visibly affected the angle of the pendulum.

The laboratory program was limited to stiffness testing because strain gauges for measuring stresses were not

able to be supplied and fitted at the University for financial reasons. In hindsight it was sensible to carry outlaboratory testing for stiffness only for reasons of simplicity and the limited time available to the project.

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To establish computer models for stress and stiffness analysis of the chassis, various data was collected. The

geometry of the chassis was measured using optical surveying techniques in the first instance and then using a

tape measure. Around thirty of the major components of the Westfield Sports car were weighed, measured and

drawn for use with a lumped mass finite element stress model. However the stress analysis did not proceed

because laboratory stress measurements would not have been available to confirm any model results and the

time available would not have allowed the use of a detailed stress model. A model for stiffness analysis was

created and analysed for bending and torsion load cases. A number of variations to the standard Westfield

Sports Car were also investigated.

All models were created for and analysed using the PAFEC finite element software, running on an Apollo

workstation at Curtin University of Technology.

Some of the results of testing and analysis have been interesting, others were what was expected, but the

overall result was learning a lot about vehicle structures and learning of the potential of computer analysis as a

tool for the development of motor vehicle structures.

The author has found this subject very interesting and hopes that this report conveys its information in a way

that will pass on this interest to the reader.

2 BACKGROUND

2.1 Introduction

Information is presented here as background on vehicle structures. The purpose of a vehicle chassis, its effect

on the performance of a vehicle, the different types of vehicle structures and how analysis of the vehicle

structure is approached is explained. The importance of stiffness of a vehicle structure is also discussed in this

section.

The chassis of a vehicle is frequently referred to throughout this project. The intended meaning is the main

structural parts of the vehicle. This does not include suspension components or non structural bodywork, eg

fibreglass cladding.

A background on the structural developments of racing and sports cars is given as racing and sports cars are

usually at the forefront of chassis development. Background on Clubman cars has been included to help

understanding of the analysis of the Westfield Sports Car chassis which is a Clubman car. Clubman is the

name given to a particular style car and this is explained in the background on Clubman cars.

2.2 Purpose of the Chassis

A car chassis may be thought of as a large bracket. This bracket must keep all the parts of the car rigidly in

place for the normal loads to which a car is subjected. Additionally this bracket must protect the driver in

situations of abnormal loading such as crash loading. A summary of considerations for chassis design is given

in Figure 2.1.

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Figure 2.1 - Chassis Design Considerations

Strength is required for safety and long life. Rigidity or stiffness is required for servicability reasons to

eliminate low frequency shaking, fatigue problems, door closure problems on uneven ground. For

performance reasons adequate chassis stiffness ensures that the full road holding and handling potential of the

suspension system and tyres is reached.

2.3 Basics of Vehicle Structural Actions

The vehicle structure is required to be strong and stiff in bending between the front and rear wheels and strong

and stiff in torsion between the front and rear wheels. In addition the vehicle structure must have sufficient

strength and stiffness in local areas where loads are applied by components mounted to the structure. Theseinclude loads from the pedals, steering wheel, seats, engine, fuel tank, differential, aerodynamic devices etc.

In dealing with vehicle loads there are a number of structural systems employed by the different types of

chassis. Looking at the predominant structural action, the four main types of structural actions for vehicle

structures are discussed in the following order: i) Beam structures, ii) Framed structures, iii) Stressed

skin construction and iv) Compound structures.

i) Beam structures

Bending and torsional, are carried by relatively thick walled beams. There are usually two beamslongitudinally along the base of the car. Essentially there have been two types of beam structures used

for vehicles. Historically the first type was the conventional girder chassis which consisted of two

longitudinal steel girders of channel section spaced by transverse members of similar construction. A

girder chassis is shown in Figure 2.2.

Vehicles which commonly employ this structural system are trucks. It is unusual to find this structural

system in a new car today.

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Figure 2.2 - Exploded View of Girder Chassis

The second type of beam structure is the twin tube or ladder chassis. This chassis has two large sectionhollow members joined by lateral or diagonal bracing or a combination of both which increases the

torsional stiffness of the structure. The torsional stiffness of a twin tube chassis is far superior to a

girder chassis of similar weight. A twin tube chassis with diagonal bracing is shown in Figure 2.3.

Figure 2.3 - Twin Tube Chassis of Triumph TR4 with Cross Bracing

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iii) Stressed skin construction

With stressed skin construction loads are carried by a series of thin walled panels. The panels are

usually stabilised with stiffeners and reinforced locally in regions of high stress such as near suspension

mountings. The panels are most commonly sheet steel or aluminium, moulded glass fibre composites,

carbon fibre composites.

Stressed skin construction can be categorised into two main forms. Firstly those chassis consisting of

two closed boxed sections down either side of the car, essentially a very large diameter, twin tube

chassis. Figure 2.6 illustrates this type of construction.

Figure 2.6 - Torsion Box Stressed Skin Construction, Ford GT40

Secondly chassis which are like a closed top bath tub; a nearly closed single shell with apertures for

driver and engine. This is illustrated in Figure 2.7 by the Lotus 25 structure.

Figure 2.7 - Monocoque Stressed Skin Construction

These forms of chassis have both been called monocoque, unitary, bath tub, torsion box and stressed

skin bodies. Torsion box probably best describes the former, while the latter fits the definition of a

monocoque. Monocoque comes from French: mono- + coque, shell, from Latin.

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iv) Compound structures

Many vehicles employ a combination of these principal structural actions. Almost all modern

production cars are a composite structure of frame members such as the roof and door pillars and

stressed skins such as the roof, floor and other panels in the engine bay and boot. Figure 2.8 illustrates

with an exploded view of a Renault production car. Commercial vehicles such as buses and coachesoften use a basic frame, very flexible on its own which is stiffened by the addition of exterior body

panels. The structures of many light buses and four wheel drive wagons are similar in principal to this.

Tray backed vehicles usually have two longitudinal beams along their length and a stressed skin cabin,

often with a frame inside the skin. Some of the more limited volume production sports cars are

composite structures with a braced frame, further stiffened and strengthened with stressed panels.

Figure 2.8 - Composite Structure of Mass Produced Renault 16

The loads in a beam structure are carried by flexure of the main beams, in a braced frame system loads arecarried primarily by tension and compression in the members as in a truss. A partially braced frame carries

load by bending moment and tension and compression in the members and with stressed skin construction,

loads are carried by in plane stresses in the skin.

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2.4 Requirements of a Chassis

2.4.1 Strength Requirements

The strength requirements of any vehicle structure depend upon :

i) The magnitude of the loads to which it is subject

ii) Whether the loads are dynamic or static.

iii) The method of transmission of the loads into the structure.

iv) The variability of the loading.

v) The factor of safety which is required against failure.

Vehicle structure loadings are generally specified in terms of peak accelerations to which the vehicle is subject.

This is independent of the weight of the vehicle thus allowing uniform comparison between loads on cars of

different weight.

The magnitude of peak accelerations to which a vehicle is subject and the use of these accelerations to

determine the loads on a vehicle structure is described by Garrett (1953). He suggests that the worst

combinations of loading which could affect a vehicle structure are represented by four design cases. These

four load cases do not include any consideration for crash loading, a separate area of vehicle design which is

not considered in this report.

The four load cases are based on peak accelerations of 1g for forward acceleration or braking, 1g for lateral

acceleration due to cornering and 3g for vertical acceleration. These accelerations should be multiplied by 1.5

as a safety factor in arriving at maximum loads for design. A safety factor of 1.5 which is relatively high for

steel structures is used due to uncertainty as to the actual magnitude of the loads.

The four loading cases which should be considered are:

i) Hitting a bump / kerbing while braking in a straight line.

ii) Cornering.

iii) Hitting a bump while accelerating straight ahead.

iv) Hitting a bump while cornering.

Costin and Phipps (1965) used a similar approach in an example of the design of a racing sports car chassis.

The peak accelerations and the safety factor used in arriving at loads they used were identical to those

suggested by Garrett.

Other methods of analysing loads include determining serviceability loads from spring and damper actions and

and analysis of the tyre / road interface. Vertical loading for normal operation would be through determining

the relationship between compression of the spring damper unit and loading which is a combination of simple

elastic deformation of the spring plus dynamic force from the damper unit, whereas strength or ultimate load

cases are invariably outside of the normal spring and suspension movement range and may be more

dependent on other factors including tyre, bump stop and bushing deformation. Analysing the tyre contactPage 15


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patch and tyre deflection may provide an envelope of the longitudnal, lateral and vertical forces potentially

transmitted from the road into the vehicle.

With steady improvements in suspension design, tyre properties, aerodynamics and downforce, vehicle weight,

torsional stiffness and engine outputs, the peak accelerations that a modern production car, sports car or racing

car can generate has increased. Examples of the peak accelerations that can be generated by several differentcars are given in Table 2.1.

Table 2.1 - Measured Peak Accelerations of Vehicles

Peak AcceleLongitudinal

ations, gLateral .

Wheels, October 1992 - Road and race Nissan GTR's

Standard Nissan GTR .87 1.17

Australian Group A Nissan GTR 1.22 1.46

Wheels, May 1992Ferrari 1992 Formula 1 Car 3.4 4.31

Ferrari F40 1.17 1.29

Ferrari 348 1.14 1.0

Wheels, October 1991 - Australian Group A Racing Cars

Holden Commodore 1.0 1.4

Nissan GTR 1.0 1.7

Motor, July 1991 - Tyre Testing Feature Nissan 300zx, Dunlop D40 M2 225/50 ZR16 Tyres

Dry, low friction, smooth concrete track 1.09 1.01

Wheels, May 1991 - Handling Test

Tested with Valentine Research Inc. G-analyst

BMW M5 N/A 1.05

Ferrari Mondial t348 N/A 1.03

Honda NSX N/A 1.07

Nissan GTR N/A 1.10

Porsche Carrera 4 N/A 1.06

L J K Setright1968

1953 Grand Prix Car N/A 0.7

1965 Grand Prix Car N/A 1.4

Note that comparisons between these results will not be accurate as facto

of road, temperature, test circuit and driver are variables.

rs such as coeffi cient of fricti n

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Modern cars, in particular sports and racing cars commonly achieve higher peak accelerations than those given

in references by Garrett (1953) and Costin and Phipps (1965). Consequently the loads on a modern car will be

larger, however may be slightly offset by reduction of the safety factor due to:

i) More accurate determination of loads using modern measuring equipment

ii) Better understanding of the vehicle structure through computer structural analysis techniques

iii) Better quality control of materials and manufacturing processes used in vehicle construction

This project is concerned with chassis stiffness rather than strength. This discussion about vehicle loads is

intended to provide background for vehicle strength. Strength analysis of vehicle structures requires further

more detailed information of the loads that apply to a particular vehicle and a range of different load cases

apply ranging from serviceability where fatigue stress is a primary consideration to impact loads where

controlled failure and permanent plastic deformation occurs.

Further discussion focuses on chassis stiffness which can be analysed independently of loads and stresses,

thus detailed load cases are not further developed.

2.4.2 Chassis Stiffness Requirements

Chassis stiffness is important in any vehicle for reasons such as door aperture tolerance, durability of fitments,

occupant comfort and impression of safety, but most importantly for a performance car, chassis stiffness is

fundamental to cornering performance.

Bending stiffness of a chassis is typically expressed as a maximum vertical deflection of a chassis resulting

from a certain mid span load. Fenton (1980) suggests that the maximum deflection for a 680 kg mid span load

should be 1.27 mm. Fenton has not discussed the type or weight range of the vehicles that this would be

applicable to and this would be necessary where deflection is the design criteria. For instance if two cars meet

the requirement of a maximum mid span deflection of 1.27 mm for a 680 kg load, yet one of these cars is very

heavy and the other is much lighter, the in service deflections of the heavier car will be larger approximately in

proportion to the difference in weight.

Chassis torsional stiffness is expressed in vehicle publications and by automotive engineers as the amount of

torque required to twist the chassis one degree over the length of its wheelbase. Metric units are Nm/degree

and imperial units are ft.lb/degree. This expression of chassis stiffness is independent of the wheelbase of the

car, allowing direct comparison between cars of different length.

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To obtain good road holding and handling, the suspension geometry of a car is carefully designed and often

refined to sub millimetre accuracy. For the suspension system to be most effective, the mounting points for

the suspension on the chassis must be held rigidly in place by a stiff chassis. For this reason torsional stiffness

is most important. Torsional stiffness is almost always more important than bending stiffness for performance

reasons because in bending there is very little deflection at the supports, which in the case of a vehicle are the

suspension mountings. However with torsional deformation the maximum deflections are likely to occur at

the suspension mounting points.

A good illustration of the effect of chassis torsional stiffness is documented by Setright (1968). The 1966

McLaren Grand Prix car was uncompetitive with the leading teams of that season because the engine was

large, heavy and underpowered. However the chassis which was designed by young Aerospace Engineer,

Robin Herd, which had an aluminium skin over a balsa wood core was of exceptional torsional stiffness. The

McLaren had a torsional stiffness of about 13500 Nm/deg compared to about 3300 Nm/deg for a competitive

Lotus 33. Setright timed the McLaren of Bruce McLaren shown in Figure 2.9 through Hunzberg Corner at theZandervoort Grand Prix circuit faster than that achieved by Jim Clark in his Lotus although Clark's lap times

were significantly faster than McLaren's.

Figure 2.9 - 1966 McLaren Grand Prix Car

Another illustration of the importance of chassis torsional stiffness was the Porsche 904 Bergspyder developed

for the 1965 European Hillclimb Championship. Its structure was very poor for torsional loads and as a result

the handling was erratic and the car was called 'Kangaroo'. Heavy modifications were needed to make the carcompetitive (Cotton 1988).

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2.4.3 Determining Torsional Stiffness

There are two common strategies for measuring the torsional stiffness of a chassis which are:

i) Fit solid bars in place of spring and shock absorber units and mount either the front or rear suspension

uprights to a rigid datum. Measure the torque required to twist the unrestrained end of the car one

degree, or a similar measured amount. Thus the chassis stiffness is deduced from the rotation of the

unrestrained suspension uprights for the particular torque applied.

ii) Reasonably restrain one end of the chassis from rotation about the centreline at its suspension mounting

points at that end. Apply a known torque to the unrestrained end of the chassis through the chassis

mounting points, measuring the rotations on the chassis at the front and the rear. The chassis stiffness

may be deduced from the relative rotation of the unrestrained end of the chassis to the restrained end for

the torque applied.

The measured torsional stiffness of the chassis may vary if the torsional stiffness is calculated from rotations atthe ends of suspension members as in i) or if the stiffness is calculated from rotations measured at the front and

rear of the actual chassis as in ii).

Table 2.2 summarises literature review of recommendations and observations for chassis torsional stiffness.

Table 2.2 - Chassis Torsional Stiffness

Source Vehicle Recommendation orObservation

Setright 1968 1962 Lotus 25/33 GP car 3300 Nm/deg(basic structure weighed 32kg complete)

Setright 1968 1966 Brabham GP car about 1400 Nm/deg

multitubular chassis

Setright 1968 1966 McLaren GP car over 13 500 Nm/deg

chassis of thin aluminium alloy, chemically bondedto end grain balsa core.

Fenton 1980 typical family saloon, Minimum: 6100 Nm/degRecommended: 6500 - 7500 Nm/deg

Webb 1984 family size saloon most cars range 4000 -9000 Nm/deg

Gard 1992 Ford Falcon EBII 1992 8200 Nm/deg

Campbell 1978 Lotus Elan (about 1963) backbone chassis only 6870 Nm/deg

Fenton 1980 Ford GT40 13560 Nm/deg

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Fothergill 1984 Open sports car 4000 Nm/deg is designaim

light road racing car 4070 Nm/deg suitable

Gard 1992 any performance car ideally 10 times thesuspension roll stiffness

Gard 1992 early 1990's F1 estimated 35 000 to 45000 Nm/deg

Qld Govt. Low volume cars: 4 cyl 4000 Nm/deg

(Road registration) 6 cyl 6000 Nm/deg

8 cyl 8000 Nm/deg

Gard 1992 English kit Cobra 300 Nm/deg

RMC racing Cobra 8300 Nm/deg

Jaguar 2013 2014 F Type aluminium body Jaguar

2.5 Relationship of Suspension and Chassis Stiffness

An improvement in roadholding and cornering performance may be possible by increasing the stiffness of the

suspension, but often increasing the spring stiffness gives no improvement or even worse overall performance.

The reason for this may be that the torsional stiffness of the chassis has not been considered. For instance

where springs are already quite stiff, or the chassis is quite flexible much of the suspension movement may be

as a result of flexure of the chassis and in such a case stiffer springs are unlikely to increase cornering capacity.

The other problem of fitting stiffer springs, also associated with the torsional stiffness of the car is that stiffer

springs transfer bigger loads into the chassis resulting in larger chassis deflections. When these deflections

become large enough to affect a carefully designed suspension geometry, cornering performance will be lost.

In order to achieve the full potential of the suspension system and the tyres, the torsional stiffness of the

chassis should be ten times the roll stiffness of the suspension. The roll stiffness of the suspension is the

torsional stiffness of the car minus the flexibility of the chassis, measured at the wheel positions with springs

in place and the suspension movement unrestricted.

2.6 Vehicle Structure Analysis

Traditional engineering statics and mechanics formula can be applied relatively easily to early beam and

tubular chassis however manual methods become more difficult with complex three dimensional geometry of

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ii) Perform the solution

iii) Communicate the results of the analysis. Interactive graphics could be considered essential for this

purpose.

Factors which make the analysis of vehicle structures complex have been discussed, but a factor which can

greatly simplify the structural design and analysis of many vehicles is that the design of a vehicle structure is

usually controlled by deflections rather than stresses. That is, if the car is designed to achieve a suitable

stiffness, the stresses will be below safe limits. This simplifies design because interpreting the results of a

stiffness analysis is generally much simpler and less time consuming than with the analysis of stresses. Hence

most analysis work during the design stage is for stiffness and a stress analysis is carried to check the final

structural details.

2.7 Development of the Structure of Sports and Racing CarsThe historical course of the development of the car chassis has been led in the past by racing and sports car

designers who have either failed or achieved glory in applying new technology and new ways of thinking to

their car designs. In the past it was thought that the car engine embodied the main technology in the car, but

in this era of motor car development and with the benefit of hindsight, the importance of the role that the

vehicle chassis has played in successful cars can be seen. The following brief history pays particular attention

to developments in Grand Prix racing, as this is seen as the show case for automotive technology.

The earliest cars were built on a steel girder frame which supported a timber body. It didn't matter whether the

car was a Grand Prix racing car or a family saloon, the structural action of chassis was the same. This

technology had come straight from the coach building industry and it was generally believed that a degree of

flexing of the chassis was a necessary part of the suspension. If built along substantial lines, the girder

chassis possessed adequate bending stiffness, but its torsional stiffness was very poor. The conventional girder

chassis consisted of two longitudinal steel girders of channel section spaced by transverse members of similar

construction. This was used almost exclusively in sports and racing cars up until the 1930's.

Even racing cars are subject to Newton's laws of motion, and so it is that a heavy racing car requires more

power to accelerate and brake and has a greater desire to continue in a straight line when the driver is trying to

turn a corner. In pursuit of better performance from their racing cars, designers recognised the need to reduce

weight. As these early chassis were particularly heavy for their strength and stiffness, the chassis was an ideal

place to reduce weight.

The move to tubular ladder chassis was led by racing car designers when in 1934 the German Auto Union

team introduced a Grand Prix racing car with a twin tube chassis, Mercds-Benz also introducing a chassis of

similar layout that year. This considerably increased the torsional stiffness of the chassis with minimal change

in the bending stiffness.

The types of suspension in use at the time, namely live axles and later swing axles, were not dependent on a

stiff chassis to preserve the suspension geometry. These suspension types have the wheels connected to axles

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and the wheel and axle assembly moves a single unit. Torsional deformation of the car structure has little

effect on the wheel angles whereas the mechanism of wishbone independent suspensions rely on the relative

positions of suspension member pivots to determine the angular positions of the wheels. Figure 2.11 shows

typical independent wishbone, swing axle and live axle suspension systems.

Figure 2.11 - Live Axle, Swing Axle and Independent Suspension

Around 1934 came the application of independent suspension to racing cars. Whereas before this the angular

relationship of the wheels was determined by a live axle acting as a beam joining the wheels, now the car itself

was part of the structure required to preserve the angular relationship of the wheels. Figure 2.12 is a simplified

diagram of the connection of independent suspension to the vehicle structure.

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Figure 2.15 - Mercds-Benz W196

The spaceframe chassis and multitubular chassis were used exclusively in Grand Prix racing until in 1962.

Fuelled by the desire to win races, the search for further chassis stiffness and light weight brought about the

introduction of the stressed skin construction Lotus 25 Grand Prix car. The Lotus 25, later becoming the Lotus

33 with its stressed skin structure achieved a torsional stiffness of around 2 to 2 ! times that of the a

conventional Grand Prix. It also achieved a typical weight saving of around 10 kg. The benefits of weight

saving, excellent torsional stiffness and improved driver safety offered by this form of construction were soon

recognised and followed by the majority of Grand Prix teams. The basic structure of the Lotus 25 Grand Prix

car is shown in Figure 2.16.

Figure 2.16 - Structure of the Lotus 25 Grand Prix Car

The excellent stiffness and strength to weight ratio achievable with stressed skin construction currently sees all

Grand Prix teams building their racing cars this way. It has also proven ideal for construction with new

materials that have since become available such as aluminium honeycomb and currently carbon fibre. Figures

2.17 and 2.18 show modern Grand Prix car chassis.

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Figure 2.17 - 1989 Ferrari Grand Prix Car (bodywork cut away on top)

Figure 2.18 - 1988 McLaren MP4/4 Grand Prix Car, Bodywork Removed

In Australia today, ladder chassis for cars are common only in go-karts, vintage cars and some drag racing

cars. Space frame chassis are popular for many types of racing cars, for example; Formula Ford and Formula

Vee are restricted to tubular steel construction, Clubman racing cars must be of the "space frame" type and

many original sports cars and sports sedans use space frame chassis. Many of the kit cars that are available in

Australia are of space frame type construction such as Westfield Sports Car and the PRB Clubman and the AT

Riciardi. In contrast to these budget sports cars is the Lamborghini Diablo, currently one of the fastest road

cars it employs a space frame chassis (Sports Car World, 1990/91). Two cars with aluminium space frame

chassis currently undergoing development for mass production are the Pininfarina Ethos (Motor, 1992) and the

Audi Avus (Chiton's Automotive Industries, 1992).

Audi Avus

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2.8 Background of Clubman Cars

As part of this project theoretical and laboratory analysis of the Westfield Sports Car chassis was carried out.

As this is a clubman style car, background on clubman cars has been included in this report.

The basic formula that defines a clubman car would be: A front longitudinally mounted engine, two seats in

an open cockpit with no doors, live rear axle, multitubular space frame chassis and front wheels separate from

the main body of the car. A typical car is shown in Figure 2.21.

Figure 2.21 - Westfield Sports Car

Two of the earliest clubman cars were the Lotus Mark 6, which was being produced in 1954 and the Ginnetta

G2 which was put into production in 1958. Based around multitubular space frames with aluminium body

panels, these cars were designed to provide an unprecedented level of performance at a price affordable to the

average motoring enthusiast. Their appointments were sparse, with little concession to comfort. They were

suitable for transport during the week and could perform well on the racing track or in trials at the weekend.

Figure 2.22 - Ginnetta G2

Many specials' constructors and limited production manufacturers have since produced similar clubman cars,

some copies of the more recognised designs, others of more original design, but the principals of the clubman

have led to these cars often looking similar and usually performing well.

The structural design of these cars is often very similar, many being based on a Lotus design for the Lotus

Seven which first appeared in 1957. Since this time engine power outputs have risen, the price of steel has

dropped, spring rates of the suspension have risen and there have been significant advances in tyre technology.

Hence there is the desire to improve the chassis to gain the most advantage from these changes.

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Figure 2.23 - Lotus Seven Body

Elfin Sports Cars first produced the clubman car shown in Figure 2.20 in Australia in 1962. Currently

clubman cars are available in Australia in kit form from Westfield Sports Cars (WA), PRB Motors (NSW),

Tilke Engineering (NSW) and Fraser Cars Ltd (New Zealand). Specifications and general information

concerning the Westfield Sports Car is included in Appendix A.

Figure 2.24 - Elfin Clubman Car

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3 ANALYSIS OF THE WESTFIELD SPORTS CAR

3.1 Introduction

To investigate vehicle chassis analysis, a Westfield SEi chassis supplied by Westfield Sports Cars Australia

was used. The stiffness of this chassis was investigated using a finite element computer model. The computer

model was validated using the results of laboratory testing. Two tests were carried out for evaluation of the

computer model.

i) Bending stiffness of the chassis.

ii) Torsional stiffness of the chassis.

The effect of variations on the torsional stiffness of the chassis was investigated using a computer model. The

model was created for, and analysed with PAFEC finite element software on an Apollo workstation. Variations

that were tested were aimed at either improving the torsional stiffness of the chassis or reducing construction

costs.

Only the stiffness of the chassis was investigated because of the following reasons:

i) The strength of the Westfield Sports Car has been well proven

ii) Measurement of stresses is expensive and was beyond the finances available to this project

iii) Computer stress analysis of a vehicle requires a much more complicated model than does stiffness

analysis. The number of load cases that must be considered for stress analysis also extends the time

required to set up and analyse a model.

iv) Stresses predicted by a model can only be as accurate as the loads that are used. To determine loads with

reasonable accuracy would require special measuring equipment, unavailable to this project. Alternatively

loads may be used as determined from other peoples work, however it appeared that the references that

were available (Garrett 1953 and Costin and Phipps 1965) were somewhat dated as are the analysis

methods that were used when these load cases were first suggested.

v) Developments in suspension and tyre technology mean that the cornering performance of the car is likely

to benefit from improved chassis stiffness.

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A quick overview of the activities that were involved in testing and analysis follows:

Laboratory Testing

Construction of sub frames for attachment of chassis to testing frame.

Setting up the chassis, loading devices and measuring equipment for testing.

Carrying out the test.Recording observations and the results of the test for later analysis and scrutiny.

Theoretical Model Analysis

Determining the geometry of the model

Creating a data file that describes the geometry of the chassis, member and section properties, loads and

restraints.

Checking the data for errors

Analysis of the model

Interpretation of the results of analysis.

3.2 Determination of Chassis Geometry

The physical characteristics of the chassis were required for a theoretical model of the chassis to be generated.

Information such as section types and plate thicknesses were available from the management of Westfield's,

however no plans or drawings of the chassis were available. Two methods of determining chassis geometry

were considered:

i) Survey using optical surveying instruments.

ii) Tape measure, measuring from reference beams.

At the time it was thought that an optical survey would provide the most accurate measurement of the chassis

geometry, so with the assistance of Associate Professor L. A. White a survey of the chassis was commenced.

Two theodolites, two subtense bars and the chassis were layed out as shown in Figure 3.1. A subtense bar is a

bar with markings accurately calibrated to two metres.

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Figure 3.1 - Layout of Chassis Survey

Vertical and horizontal angles to each end of the subtense bars and several of the nodes were observed and the

distance between stations A and B was measured. This data enabled calculation of positions of the nodes in

three dimensional space. However the chassis survey technique of measurement was found unsuitable for thefollowing reasons:

i) Making the observations was very time consuming.

ii) A check on measurements determined by the survey with a tape showed errors of 4 to 7mm.

The time consuming nature of the theodolite observations and the large errors were partly to the level of skill

of the operator.

The geometry of the chassis was subsequently measured using a tape. Beams were clamped to the chassis to

act as a reference for measurements. A one fifth scale orthogonal drawing was produced as a reference for

further work. A copy of this drawing has been included in Appendix B.

The geometry of the theoretical model was compiled into standard file format for the PAFEC finite element

software by typing the node coordinates, member connectivities and other information defining loads,

restraints and member properties. A graphics interface was used for checking that information was correct.

The following diagram, Figure 3.2 shows the global axes of the model. This is the axes system used

consistently in this report.

Figure 3.2 - Axes System

3.2 Chassis Bending Stiffness

3.2.1 Laboratory Test Description and Procedure

The chassis bending test of the Westfield Sports Car involved simply supporting the chassis on its front and

rear extremities as shown in Figure 3.3 and applying loads near the middle of the chassis while the deflections

at known positions were measured.

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Figure 3.3 - Chassis Bending Test

The chassis was placed on timber blocks in the four restraint positions shown in Figure 3.3. The blocks were

supported on a smooth concrete floor. Dial gauges were set up to measure deflections at nodes 12 and 24

relative to the concrete floor. A load hanger was placed midway between nodes 151 and 154 on which dead

weights were placed.

The following steps were carried out during testing:

i) The chassis was first proof loaded to with 50 kg to bed in the chassis at the supports and to ensure the

chassis was sitting evenly on its supports.

ii) The proof load was removed and dial gauge readings were observed at nodes 12 and 24.

iii) A load of 10 kg was applied and dial gauge readings at nodes 12 and 24 were observed. This was

repeated for loads of 10 kg, 20 kg, 40 kg, 50 kg and 60 kg. Observations were made as the load was

increased to 60 kg and then reduced in the same increments back to zero.

iv) Dial gauge deflections were then observed for a loading pattern of 0 kg, 50 kg, 0 kg, 50 kg, 0 kg.

v) The average deflections of the gauges at nodes 12 and 24 were calculated for the load increments.

These are plotted in Figure 4.1 in the results section.

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Figure 3.5 - Standard Chassis Model Member Groups

The restraints and loading used for the chassis bending analysis as a model of the chassis bending test are

shown in Figure 3.6.

Figure 3.6 - Chassis Bending Model

A load of 1000 N was applied mid way between nodes 151 and 154. The chosen value of the load was not

important, just that the value was known because the theory used to analyse the model assumed linear elastic

response.

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ii) After incrementing the load, the chassis was tapped with a small block of wood until the dial gauge

readings became steady, before deflections were observed.

The testing procedure was carried out as follows:

i) The chassis and subframes were set up in the test frame as shown in Figure 3.7.

ii) Dial gauges were set up at selected nodes on the chassis. Two dial gauges at the front measured rotation

at the front while two dial gauges at the rear measured rotation of the rear of the chassis. Deflections

were measured at the rear of the chassis because although the rear was prevented from mechanistic

rotation, elastic deformations of the sub frame and wishbones allowed some rotation.

iii) Distances between dial gauges and the load lever arm were measured with a tape.

iv) A 60 kg proof load was applied to settle in the chassis at its supports. Loads were applied at the front

wishbone as shown in Figure 3.7.

v) Dial gauge readings were observed before loading was commenced. Loads were then applied and

removed in the pattern shown in Figure 3.8. Before observing dial gauge readings and after the load

was applied, the chassis was tapped with a small block of wood until dial gauge readings stabilised.

vi) Measurements of distance between dial gauges and load lever arm distance were checked.

A full set of observations from the laboratory tests is given in Appendix D.

Figure 3.8 - Pattern of Loading for Torsional Test

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4 RESULTS AND DISCUSSION

4.1 Bending Test and Bending Analysis

The chassis bending test was a simple test which was carried out to help establish the accuracy of the

theoretical model of the chassis.

The chassis was loaded between nodes 151 and 154 with deflections at nodes 12 and 24 being observed.

(These nodes have been identified previously in Figure 3.6). The average vertical deflections of nodes 12 and

24 are plotted for different loads in Figure 4.1.

Figure 4.1 - Load Deflection Response of Chassis Bending

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Figure 4.2 - Shape of Chassis for Calculated Bending Test

The average deflections of nodes 12 and 24 are shown in Figure 4.2 by a dashed line. The maximum vertical

deflection of the chassis between front and rear wheel centres of 0.26mm is shown on Figure 4.2.

The observed vertical deflection of 0.26 mm per 1000 N can be linearly extrapolated to 1.73 mm per 680 kg

mid span load. This compares to a recommended bending stiffness of not more than 1.27mm deflection for a

mid span load of 680 kg by Fenton (1980). The bending stiffness achieved by the Westfield Sports Car chassis

is obviously less than that recommended by Fenton, however Fenton's recommendations include no discussion

on the weight of the vehicle for which his recommendation is made. It would be logical to include the weight

of a car in a recommendation for bending stiffness as the bending deflections are likely to increase

proportionally to the weight of the car.

The sharp change in stiffness graph of Figure 4.2 at point A is as a consequence of the presentation of the data

for this graph. Point A is an external node on the bottom plane of the chassis, point B is a node on the same

member as point A, but it is closer to the longitudinal centreline of the chassis and directly under the rear

support. Also there is no reason for concern over a sharp decrease in the stiffness of the chassis in this position

because this part of the structure is outside of the wheelbase of the car and only subject to small loads.

4.2 Torsional Test and Torsional Analysis

The chassis torsional stiffness test was carried out to establish the accuracy of the theoretical model of the

chassis.

As mentioned earlier the torsional stiffness test was carried out on two occasions. In the first instance there

was large scatter of the results and virtually no consistency. This was thought to be the result of applying the

load through a pulley in the first test. The pulley was not of good quality and although it appeared to runPage 41


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smoothly and freely when unloaded, it was likely that the pulley was binding against the pulley shaft when

load was applied. Thus the pulley was discarded for the second test.

Another precaution that was taken for the second test was to tap several times checking that dial gauge

positions did not fluctuate before dial gauge readings were recorded.

The results of the first torsional test are not included in this report because due to their inconsistent nature,

they are of little use.

The torsional deflection response for the second torsion test of the Westfield Sports Car chassis is shown in

Figure 4.3. The load deflection response calculated from the chassis model is also shown on this graph for

comparison with the measured response.

Figure 4.3 Torsional Load Deflection Response

The results of the second torsional stiffness test show very little scatter. A response which is clearly linear may

be observed.

For the torsional stiffness test, deflections were measured at the front and rear of the chassis on each side of

the chassis at nodes 102 and 123 at the front and nodes 105 and 111 at the rear. Figure 3.9 previously defined

these node numbers.

To calculate the torsional stiffness, the rotation at the rear of the chassis was subtracted from the rotation at the

front of the chassis. The torque applied at the front of the chassis was calculated from the magnitude and lever

arm of the load. Thus the torsional stiffness was the Torque applied divided by the rotation between the front

and rear of the chassis.

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The torsional stiffness of the chassis in the test calculated in this way was 1134 Nm/deg on average (see Figure

4.4) and the stiffness of the model was 1121 Nm/deg.

Figure 4.4 - Scatter of Measured Torsional Stiffness

For the torsional test errors such as error of measurement of the chassis geometry, approximations in the model

by ignoring some eccentricities and error in reading dial gauges should be the same as those for the bending

stiffness test.

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The difference between the measured torsional stiffness of 1134 Nm/deg and the model torsional stiffness of

1121 Nm/deg of 1.2% was extremely good and suggests that the model was a good representation of the

chassis. The range of torsional stiffness observed during testing was from +5.4% to -10.4% of the model

stiffness. The biggest difference of 10.4% between model and test is still within the average difference

observed for the bending test..

When the torsional stiffness of the model was calculated from the rotation of the front suspension wishbones

with the rear wishbone restrained from movement by the supports, the torsional stiffness of the chassis was

found to be 1050 Nm/degree. The difference between the two calculated stiffnesses is due to the position of

the load relative to where the stiffness was measured. Measuring the torsional stiffness from the wishbones

resulted in an apparently more flexible structure because the loads and supports were attached directly to the

wishbones. Nodes 102, 123, 111 and 115 were away from the loads and supports which were the most highly

stressed regions, thus the measured stiffness was higher.

The graph of angular deflections along the chassis in Figure 4.5 highlights the most flexible areas of the

chassis. The most flexible areas, which are where the curve is steepest are the first 70mm from the front of the

chassis and 200mm to 500mm from the front of the chassis which is in the engine bay area. The stiffest parts

of the chassis is the 250mm directly behind the hoop on which the steering wheel is mounted and the very

front of the chassis, after the first 70mm and where there is corner bracing in the front, top plane of the engine

bay.

If the entire chassis was able to be increased to the same stiffness as directly behind the steering hoop, the

chassis would have a torsional stiffness of over 2000 Nm/deg.

Figure 4.5 - Torsional Deflections Along Chassis

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4.3 Torsional Stiffness - Chassis Variations

A number of variations to the standard chassis have been considered. Mostly these variations are intended to

be suitable for production at some time in the future, however some less practical variations have been

considered on the basis that they may help understanding of the structural actions of the chassis. Table 4.1 to

Table 4.7 describe the various changes made and the effect of these changes on the torsional stiffness andweight of the chassis. Following each table is a graph with the torsional stiffness along the length of the

chassis plotted. In each case Car1, the standard chassis configuration is included as a reference. The

variations to the chassis are shown graphically in Appendix E along with information about masses, centres of

mass and moments of inertia of the chassis variations.

The types of variations to the basic chassis structure are grouped together according to the type of change

which was made. In general terms the changes which resulted in a worthwhile increase in chassis stiffness

were extra centre tunnel bracing, increased member section sizes with same or even reduced wall thicknesses,

extra engine bay bracing and extra bracing in the nose. The changes which were least desirable were the

removal of the existing main engine bay brace and attaching steel plates to various areas such as the front of

the drive train tunnel and the sides of the engine bay.

Table 4.1 - Standard Chassis Models

File Description TorsionalStiffness(Nm/deg)

Weight(kg)

% ChanCa

Stiffness

ge from1Weight

Stiffnessto Weight

RatioCar1 Standard chassis with minimum three

point restraintTorsional stiffness calculated as perlaboratory test

1121 63.3

Torsional stiffness calculated fromwishbones deflections

1050 63.3 0.0 0.0 16.6

Car20 As Car1, but using PIGS generateddata file (as a check)

1050 63.3 0.0 0.0 16.6

Car23 This file models the bending stiffnesstest

63.3 0.0

Hereafter all files are the same as the standard Westfield Sports Car chassis except for those variations

specified. Minimum three point restraint and torsional stiffness calculated from deflections at the wishbones is

used consistently.

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Table 4.2 - Category I, Changes to Member Sizes


Weight(kg)

% ChanCa

Stiffness

ge from1Weight

Stiffnessto Weight

RatioCar2 Top plane members changed to 31.8 x

1.6 SHS1152 64.4 9.7 1.7 17.9

Car3 Bottom plane members changed to31.8 x 1.6 SHS

1112 65.2 5.9 3.0 17.1

Car 16 Top and bottom plane memberschanged to 40 x 1.2 SHS

2051 71.0 95.3 12.2 28.9

Car26 Bottom side members changed to 40 x1.6 SHS

1185 67.0 12.9 5.8 17.7

Car27 Bottom and top side members changedto 40 x 1.6 SHS

1412 69.4 34.5 9.6 20.3

Car29 All member changed to 40x40x1.0SHS

2845 74.6 171 17.9 38.1

Figure 4.6 - Torsional Stiffness Plots of Changes to Member Sizes

Changes to member sizes produced the biggest increases in torsional stiffness to weight ratio when the section

sizes of the members were increased significantly and the wall thicknesses of the hollow members decreased.

Comparing changes of the top longitudinal members to changes to the bottom longitudinal members showed

that changes to the top longitudinal members produced a more pronounced effect on torsional stiffness.

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Table 4.3 - Category II, Changes to the Engine Bay


Weight(kg)

% ChanCa

Stiffness

ge from1Weight

Stiffnessto Weight

RatioCar4 Extra brace in top plane of engine bay 1261 63.8 20.1 0.8 19.8Car5 Side bracing in engine bay changed 1040 63.5 -1.0 0.3 16.4Car6 Extra lateral member across top of

engine bay1105 64.0 5.2 1.1 17.3

Car9 Extra top, right hand engine bay brace 1060 63.8 1.0 0.8 16.6Car10 Normal engine bay brace removed 683 62.5 -35.0 -1.3 10.9Car11 Normal engine bay brace replaced by

LH and RH braces1148 63.5 9.3 0.3 18.1

Car15 Engine bay side braces replaced by1mm steel panels

1061 66.9 1.0 5.7 15.9

Car 18 Extra cross members in engine bay 1475 64.9 40.5 2.5 22.7

Figure 4.7 - Torsional Stiffness Plots of Engine Bay Changes

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The changes around the engine bay consisted of changes to the top plane bracing, changes to the bracing in the

side of the engine bay and use of plates instead of bracing in the sides.

Additional bracing in the top plane, correctly positioned achieved an excellent increase in torsional stiffness

for a simple change. When the added bracing was not well positioned only insignificant increases to torsional

stiffness were observed. The removal of the main top plane engine bay brace caused a dramatic reduction inthe torsional stiffness of the chassis.

Changes to the the bracing in the side of the engine bay was carried out so that the degree of triangulation was

not reduced. Consequently there was no large changes to the torsional stiffness for the variations analysed.

Using plates instead of bracing was a solution which increased the weight of the chassis with no significant

gain in torsional stiffness.

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Car15 Engine bay side braces replaced by1mm steel panels

1061 66.9 1.0 5.7 15.9

Figure 4.10 - Torsional Stiffness Plots for Changes Using Plates

The increases in torsional stiffness that were achieved by adding plates to the chassis were generally offset bythe increases in weight and reduction in accessibility that resulted from using plates. The plated engine

support beams gave a torsional stiffness increase of 12% but the engine support beams were also modified in

this case. The modified engine support beams increased torsional stiffness by 11% without the use of plates.

These results should not be construed to suggest that plate solutions will not be viable, rather that the

variations which were tested were not particularly viable.

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The changes that have been considered are only a sample of the changes that could be considered in a serious

effort to improve the torsional stiffness of this chassis. For instance no additional bracing was considered for

the rear part of the chassis. The changes that were considered targeted the more flexible areas of the chassis,

as indicated by the torsional stiffness diagram, Figure 4.5 where changes could be made without disrupting the

layout of the chassis. Where positive improvements to the chassis have been determined, these changes could

be refined by further analysis with the computer model.

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There were a number of changes that Westfield Sports Cars may test further on a full scale chassis and there

were a number of other changes that may be beneficial. The changes fell into five broad categories:

i) Changes to member sizes

ii) Addition or removal of bracing in the engine bay

iii) Addition of bracing to the nose of the chassis

iv) Addition of members to the centre tunnel

v) Use of plates instead of bracing

vi) Other changes

The first category which was changes to member sizes showed excellent improvements to torsional stiffness

for a minimal weight penalty with increased section sizes. Reducing the wall thicknesses of the hollow

members when the section sizes were increased minimised increases of weight in the chassis. The most pronounced effects of changing member sizes were observed where members in areas with a lack of bracing

were changed such as the top and bottom plane members in the engine bay and cockpit. The largest increase

of stiffness of all the changes analysed, changing all members to 40x40x1.0 SHS, was in this category.

Although this change could not be directly incorporated into manufacture of new Westfields because there is

physically not enough space in some places for these larger members, it demonstrates the efficiency of larger

section sizes and smaller wall thicknesses for this type of chassis.

The second category which was the addition and removal of bracing in the engine bay showed that the bracing

in the top plane of the engine bay was very significant. The removal of the existing main engine bay brace inthe top plane reduced torsional stiffness by 35% while adding a second main engine bay brace in the top plane

increased the torsional stiffness by 20%. No major changes were made to the side bracing of the engine bay

but presumably there is little potential for increased torsional stiffness by adding bracing to the sides because

the sides are already well braced. Significant decreases in torsional stiffness would be likely if the side

bracing of the engine bay is partly or wholly removed.

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5.1 Recommendations

From analysis of the computer model, there are a number of changes to the chassis that should be investigated

for immediate inclusion into the production of the Westfield Sports Car chassis. The most practical and

effective changes were:

i) Additional top plane engine bay brace

ii) Additional bracing of the centre tunnel

iii) Increased top plane member section size with same or reduced wall thickness.

iv) Geometry of the engine support beams altered

v) Extra nose bracing. Suitable bracing geometry may be determined by investigation of complete car

and further analysis with the computer model.

In general for a structure of this type the stiffness will be increased for any given weight when section sizes areincreased and wall thicknesses decreased. Such changes should be subject to further investigation to

determine if welding thinner walls will cause a problem and if particular wall thicknesses are required for

withstanding rust, abrasion and local stresses around mounting brackets such as suspension mounting brackets.

A recommendation not associated with the analysis of the Westfield Sports Car, comes from applying an

engineering knowledge to the background information given in this report. It very beneficial to consider the

weight of vehicle as well as the vehicle's purpose or engine size when recommending or legislating for

stiffness of the vehicle. Whether torsional stiffness or bending stiffness is considered, the reason stiffness is

required is to limit deflections. The deflections of a structure are just as dependent on the applied loads as thestiffness of the structure. In the case of a vehicle, the loads are as a direct result of the weight of the vehicle,

thus any sensible recommendations or legislation for vehicle stiffness should include consideration for vehicle

weight.

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Carey J (1991) "The G Force", Wheels Magazine, May

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Setright, LJK (1968) The Grand Prix Car 1954-1966, George Allen and Unwin Ltd, London.

Setright, LJK (1976) The Designers, Weidenfeld and Nicolson, London.

Sturz, W D, (1990/91) "Hell Fire", Sports Car World , Summer pp 14-21

Timishenko and Gere (1968) Elements of Strength of Materials,

Webb, G G (1984) "Torsional Stiffness of Passenger Cars", Proceedings of Institution of Mechanical

Engineers

Westerman, A (1991) "Tyre Supertest", Motor Magazine , July

Westfield Sports Cars Ltd, Westfield SE - SEi Instruction Manual, Westfield Sports Cars Ltd

Williams, G (1991) McLaren. A Racing History , The Crowood Press Ltd, Wiltshire.

8 APPENDICES

Appendix A - Westfield Sports Car Data

Motor magazine recently conducted tests of four clubman cars available and able to be licensed in Australia

(September 1992). The following information about the Westfield Sports Car is sourced from tests conducted

by Motor.

Kits sold in Australia 60

Cars registered in Australia 14

Engine Front, longitudinally mounted

1.6L, 88kW (Toyota Corolla)

Suspension front - independent double wishbones

rear - double wishbones or live axle

Tyres Yokohama A-008R, 205/60 R13 85H

Wheelbase 2270mm

Front Track 1310mm

Rear Track 1330mm

Overall Length 3515mm

Overall Width 1580mm

Height 1040mm

Ground Clearance 105mmPage 61


63/96

Kerb Weight 580kg

Weight/Power 6.6 kg/kW

Acceleration 0 - 100m 6.53s

Standing 400m 14.85s

Member Properties of Westfield Sports Car Chassis

Tubemakers B.T.M. Square Hollow Sections

Section Size mm Wall

Thickness

mm

Area

mm "

kg/m Ixx mm 4 J mm 4

2020 20 1.6 111 0.873 6080 10300

2525 25 1.6 143 1.12 12800 21200

Page 62


64/96


65/96

Appendix C - Computer Model Data File

(diagrams showing nodes and elements at end of data file listing)

CC Standard Chassis

C Generally units are in Newtons, N and metres, mCONTROLCONTROL.ENDCC .......1.........2.........3.........4.........5.........6.........7.........8BEAMS MATERIAL=1C (NOTE THAT SECTION.NUM IS THAT REFERRED TO BY PROPERTY NO. IN ELEMENTSMODULE)SECTI IYY IZZ TORSION AREA KY KZ ZY ZZ 1 12.8E-9 12.8E-9 21.2E-9 143E-6 .9 .9 816E-9 816E-9 C BOTTOMPLANE 2 6.08E-9 6.08E-9 10.3E-9 111E-6 .9 .9 608E-9 608E-9 C CENTRETUNNEL 3 12.8E-9 12.8E-9 21.2E-9 143E-6 .9 .9 816E-9 816E-9 C TOP PLANE 4 5.36E-9 5.36E-9 10.7E-9 103E-6 .9 .9 487E-9 487E-9 C 5 6.08E-9 6.08E-9 10.3E-9 111E-6 .9 .9 608E-9 608E-9 C 6 5.36E-9 5.36E-9 10.7E-9 103E-6 .9 .9 487E-9 487E-9 C 7 6.08E-9 6.08E-9 10.3E-9 111E-6 .9 .9 608E-9 608E-9 C SUSP'NMEMBERS 8 12.8E-9 12.8E-9 21.2E-9 143E-6 .9 .9 816E-9 816E-9 C UPRIGHTS 9 16.1E-9 16.1E-9 1.0E-9 111E-6 .9 .9 800E-9 800E-9 C BRACKETS 10 16E-9 810E-9 20E-9 492E-6 .3 .7 100E-9 12.1E-6 C FLOORPANSCC

MATERIAL MATE.NUM E NU RO

1 200E9 0.3 7850CCC *** THE NODES MODU

Documents

Vehicle Structures - Development of the Sports Car Chassis and Stiffness Analysis of the Westfield Sports Car