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
– 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. 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
Lateral Force Ground
I.C. Height

33. Jacking Forces
CHASSIS
I. C.
Lateral Force
I.C. Height
– 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

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
I. C.
Roll Center
VehicleCenter
Line

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

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
Roll Center
Sprung Mass C.G.

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)
R. C.
Sprung Mass
C.G.
B

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

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
Ground
Front Rear
CHASSIS

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
Ground
Front Rear
CHASSIS
Anti-Dive
Anti-Lift

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
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

43. Pitch Center
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

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