Faculty of engineering

School of mechanical engineering

M.Sc. Automotive engineering

Automotive Chassis Engineering

Jos Antonio Hernndez Castillo

Date: October 30th 2012

Report: 2D analysis of Double wishbone system

Index

1. - Introduction...3

2. - Goal.......3

3. - Acronym.3

4.1. - First system analysis....4

4.2. - Second system analysis10

5. - Conclusion..17

6. - Bibliography.......18

Introduction:

The suspension is the system and mechanism in charge of absorbing the

irregularities of the road, making the travel more comfortable or improving the

performance on curves for a racing car. Depending of the design of this system, the

car will have a specific behaviour [1]

Goal:

Simulate a 2D system on CAD software in order to get the data of the variables

distance implied on a double wishbone configuration, the compare charts and

graphics with another 2D system with similar components but different configurations

in order to get final conclusions.

Acronym used on this analysis

[2]

First system to analyse

The next picture shows the main components and fixed points that will define the will

position

Main diagram of the components

The concept shows a negative camber with a positive scrub ratio [3]

On the next picture, it is possible to see the main distances of the components and

the distances that define the wheel position. The grey colour dimensions indicate the

dynamics lengths; this means these distances will change as a result of increasing

the lower wishbone angle. Black dimensions will not change with exception of lower

wishbone angle; the designer will change this value adding one degree per step,

increasing by 10 degrees at the end, in order to simulate the mechanism and get the

data of the grey dimensions.

It is important to declare that this is the initial position; it means the middle plane of

the tire patch that is on contact with the ground (VWC with and LWA of 90.03

degrees against the vertical), on the next pages the LWA will have one degree more

per step.

Main distances and starting position

Chart of results

3D kinematics analysis for SR and VWC [4]

3D kinematics analysis for camber and VWC [4]

0, 51.37

1.22, 51.37

2.44, 51.37

3.67, 51.36

4.89, 51.36

6.1, 51.34

7.32, 51.33

8.54, 51.319.75, 51.28

10.96, 51.25

12.16, 51.2251.2

51.22

51.24

51.26

51.28

51.3

51.32

51.34

51.36

51.38

0 2 4 6 8 10 12 14

SR d

ista

nce

Vertical position

SR Behavior

0, 2.34

1.22, 2.312.44, 2.28

3.67, 2.26

4.89, 2.24

6.1, 2.217.32, 2.19

8.54, 2.18

9.75, 2.16

10.96, 2.1412.16, 2.13

2.1

2.15

2.2

2.25

2.3

2.35

2.4

0 2 4 6 8 10 12 14

Cam

be

ran

gle

Vertical Position

Camber Behavior

Second system to analyse

For the second system, the same components are used but with some differences

on the length. Positions and configurations were changed in order to get new values

for the table previously developed. The result is a system technically opposite to the

first mechanism, this has a positive camber with a negative scrub ratio [3].

The next picture shows the initial dimensions with the middle centre of the tire path

at 0 positions with the ground and an angle at the lower wishbone of 88.40 degrees

against the vertical axis.

Chart of results

3D kinematics analysis for SR and VWC [4]

3D kinematics analysis for camber and VWC [4]

0, 11.76

1.41, 12.11

2.82, 12.46

4.24, 12.79

5.66, 13.1

7.07, 13.4

8.49, 13.69

9.91, 13.95

11.33, 14.2

12.75, 14.43

14.17, 14.64

0

2

4

6

8

10

12

14

16

0 2 4 6 8 10 12 14 16

SR d

ista

nce

Vertical position

SR Behavior

0, 1.81

1.41, 1.83

2.82, 1.85

4.24, 1.885.66, 1.91

7.07, 1.948.49, 1.97

9.91, 2.0111.33, 2.05

12.75, 2.09 14.17, 2.13

1.75

1.8

1.85

1.9

1.95

2

2.05

2.1

2.15

0 2 4 6 8 10 12 14 16

Cam

be

r A

ngl

e

Vertical position

Camber Behavior

Conclusions

Comparing data and charts it is possible to see the main difference of behaviour and

displacements. The first system has a negative camber and a positive scrub radius;

this configuration gave a decreasing camber and Scrub radius while the vertical

position increased.

The opposite happened on the second configuration with a camber positive and a

negative scrub ratio, both quantities were increasing while the vertical position

increased too.

The reason for the scrub radius behaviour is the kingpin inclination position. The first

configuration has an intersection with the ground at the inner side of the tyre, this

means the positive value mentioned above, so, the smaller the camber angle, the

smaller the scrub radius is. The opposite occurs on the second configuration.

For the Camber behaviour this is explained by the change induced by the lower

wishbone angle on the kingpin inclination, while the lower wishbone angle increased

the kingpin inclination decreased, this incites the self-centring effect on the wheel,

and at the same time changes the camber angle, decreasing in the first system and

increasing on the second.

Pros First system

Negative camber gives better handling performance

Positive scrub radius will improve returnability

Smaller components give more packaging space

Second system

Positive camber reduce tire wear Negative scrub radius reduce torque steer The travel will be more comfortable

Cons First system

The amount of scrub radius is bigger than the second system; this will increase the feedback of the road to the driver by increasing steering effort.

Tyre wear is increased because of the negative camber

Second system

Configurations is difficult to achieve, bigger components are required, reducing the packaging space

The positive camber will give a poor handling performance

Bibliography

MECH5185 Automotive Chassis Engineering, Towers David. University of Leeds.

Leeds UK [1] page. 1-1 [2] page 1-4, 1-5, 1-6, 1-16 [3] page 1-4, 1-5 [4] page1-30 ,

1-31

http://en.wikipedia.org/wiki/Scrub_radius

http://www.hrsprings.com/technical/scrub_radius