Suspension Design Case Study

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. Static 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 Camber

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 Design0.4° 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 Design1.4° 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