1. Suspension Design Case Study

2. Purpose
Suspension to be used on a small (lightweight) formula style racecar.
Car is intended to navigate tight road courses
Surface conditions are expected to be relatively smooth

3. Performance Design Parameters
For this case the main objective is to optimize mechanical grip from the tire.
This is achieved by considering as much tire information as possible while designing the suspension
Specific vehicle characteristics will be considered.

4. Considerations
Initially the amount of suspension travel that will be necessary for this application must be considered.
One thing that is often overlooked in a four wheeled vehicle suspension design is droop travel.
Depending on the expected body roll the designer must allow adequate droop travel.

5. Introduction

6. Components Upper A-arm
The upper A-arm serves to carry some of the load generated on the suspension by the tire.
This force is considerably less then the load carried by the lower A-arm in a push rod set-up
The arm only has to provide a restoring force to the moment generated by the tire on the lower ball joint

7. Components Lower A-arm
The lower A-arm serves the same purpose as the upper arm, except that in a pushrod configuration it is responsible for carrying the vertical load
In this case study the lower A-arm will carry a larger rod end to compensate for the larger forces seen by this component.

8. Components
Upright
The upright serves several purposes in the suspension
Connects the upper A-arm, lower A-arm, steering arm, and the tire
Carries the spindle and bearing assembly
Holds the brake caliper in correct orientation with the rotor
Provides a means for camber and castor adjustment

9. Components Spindle Spindle can come in two basic configurations
Live spindle
Fixed spindle
In the live spindle configuration the whole spindle assembly rotates and carries the tire and wheel
The fixed spindle configuration carries a hub assembly which rotates about the spindle
Both configurations carry the brake rotor

10. Live Vs. Fixed Spindle Advantages and Disadvantages
Live Spindle :
Less parts
Lighter weight if designed correctly
More wheel offset
Bearing concerns
Retention inside of the upright assembly
Fixed spindle
Simple construction
Hub sub-assembly
Spindle put in considerable bending
More components, and heavier

11. Components
Push rod
The push rod carries the load from the lower A-arm to the inboard coil over shock
The major concern with this component is the buckling force induced in the tube

12. Components Toe rod (steering link)
The toe rod serves as a like between the steering rack inboard on the vehicle
The location of the ends of this like are extremely critical to bump steer and Ackermann of the steering system
This link is also used to adjust the amount of toe-out of the wheels

13. Components
Bellcrank
This is a common racing description of the lever pivot that translates to motion of the push rod into the coil over shock
The geometry of this pivot can be designed to enable the suspension to have a progressive or digressive nature
This component also offers the designer the ability to include a motion ratio in the suspension

14. Components Coil-over Shock Absorber
This component carries the vehicle corner weight
It is composed of a coil spring and the damper
This component can be used to adjust ride height, dampening, spring rate, and wheel rate

15. Components Anti-Roll bar
This component is an additional spring in the suspension
Purpose: resist body roll
It accomplishes this by coupling the left and right corners of the vehicle
When the vehicle rolls the roll bar forces the vehicle to compress the spring on that specific corner as well as some portion of the opposite corners spring
This proportion is adjusted by changing the spring rate of the bar itself
*Unclear in this picture the Anti-Roll bar tube actually passes inside the chassis

16. Beginning the Design Process
Initially the suspension should be laid out from a 2-D front view
Static and dynamic camber should be defined during this step

17. Camber
The main consideration at this step is the camber change throughout the suspension travel.

18. Camber Static Camber Dynamic Camber
Describes the camber angle with loaded vehicle not in motion
Dynamic Camber
Describes the camber angle of a corner at any instant during a maneuver i.e.: cornering, launching, braking

19. Contact Patch
Tread area in contact with the road at any instant in time

20. Camber
Camber is used to offset lateral tire deflection and maximize the tire contact patch area while cornering.

21. Camber Negative Camber angles good for lateral acceleration, cornering
bad for longitudinal acceleration, launching/braking
This is because the direction of the tire deflection is obviously not the same for these two situations

22. Camber Cornering Situation
Maximum lateral grip is needed during cornering situations.
In a cornering situation the car will be rolled to some degree
Meaning the suspension will not be a static position
For this reason static suspension position is much less relevant than the dynamic

23. Camber Launch/Braking Situation
Maximum longitudinal grip is needed during launch/brake situations.
In a launch/brake situation the car will be pitched to some degree
Suspension will not be in a static position

24. Compromise
It is apparent that the suspension is likely to be at the same position for some cornering maneuvers as it is during launching/braking maneuvers
For this reason we must compromise between too little and too much negative camber
This can be approximated with tire data and often refined during testing

25. Defining Camber
Once we set our static camber we must adjust our dynamic camber curves
This is done by adjusting the lengths of the upper and lower A-arms and the position of the inboard and out board pivots
These lengths and locations are often driven by packaging constraints

26. Instant Center
The instant center is a dynamic point which the wheel will pivot about and any instant during the suspension travel
For a double wishbone configuration this point moves as the suspension travels
CHASSIS
Instant Center

27. Mild Camber Change Design -Suspension arms are close to parallel -Wide instant center locations

28. Mild Camber Change Design 0.4° of Neg. Camber Gain Per inch of Bump

29. Aggressive Camber Change Design -Suspension arms are far from parallel -Instant center locations are inside the track width

30. More Aggressive Camber Change Design 1. 4° of Neg
More Aggressive Camber Change Design 1.4° of Neg. Camber Gain Per inch of Bump

31. Jacking forces
It is important to consider the Instant Center Position, because when it moves vertically off the ground plane Jacking forces are introduced

32. Jacking forces Caused during cornering by a moment
Force: lateral traction force of tire
Moment arm: Instant Center height
Moment pivot: Instant center
CHASSIS
Instant Center
I.C. Height
Lateral Force
Ground

33. Jacking Forces
Caused by geometrical binding of the upper and lower A-arms
These forces are transferred from the tire to the chassis by the A-arms, and reduce the amount of force seen by the spring
Jacking Forces
CHASSIS
I. C.
I.C. Height
Lateral Force

34. Roll Center The roll center can be identified from this 2-D front view
Found at the intersection lines drawn for the Instant center to the contact patch center point, and the vehicle center line
Vehicle Center
Line
I. C.
Roll Center

35. Roll Center
For a parallel-Iink Situation the Roll Center is found on the ground plane
Vehicle Center
Line
Roll Center

36. Significance of the Roll Center
Required Roll stiffness of the suspension is determine by the roll moment. Which is dependant on Roll center height
Sprung Mass C.G.
Roll Center

37. Roll Moment
Present during lateral acceleration (the cause of body roll)
Moment Arm:
B = Sprung mass C.G. height – Roll center height
Force:
F = (Sprung Mass) x (Lateral Acceleration)
Sprung Mass C.G. B
R. C.

38. Roll Axis To consider the total vehicle you must look at the roll axis
Sprung Mass C.G.
Roll Axis
Rear Roll Center
Front Roll Center

39. Side View
The next step will be to consider the response of the suspension geometry to pitch situation
For this we will move to a 2-D side-view
Inboard A-arm pivot points
CHASSIS
Ground
Front
Rear

40. Anti-Features
By angling the A-arms from the side jacking forces are created
These forces can be used in the design to provide pitch resistance
Anti-Lift
Anti-Dive
CHASSIS
Ground
Rear
Front

41. Anti-Features
Racecars rely heavily on wings and aerodynamics for performance.
Aerodynamically efficient, high-down force cars are very sensitive to pitch changes.
A pitch change can drastically affect the amount of down force being produced.
Much less important for lower speed cars

42. Pitch Center
The pitch center can be identified from this 2-D side view
Found at the intersection lines drawn for the Instant center to the contact patch center point
Pitch Center

43. Pitch Center
The pitch center can be identified from this 2-D side view
Found at the intersection lines drawn for the Instant center to the contact patch center point
Pitch Center

44. Pitch Moment Present during longitudinal acceleration Moment Arm:
B = Sprung mass C.G. height – Roll center height
Force:
F = (Sprung Mass) x (Longitudinal Acceleration) F B
Pitch Center