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.
â 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.
6. Components
⢠Upper A-arm
â The upper A-arm serves to
carry some of the load
generated on the
suspension by the tire.
â This force is considerably
less then the load carried
by the lower A-arm in a
push rod set-up
â 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
â 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
â 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
â 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
â Spindle can come in two
basic configurations
⢠Live spindle
⢠Fixed spindle
â In the live spindle
configuration the whole
spindle assembly rotates
and carries the tire and
wheel
â The fixed spindle
configuration carries a hub
assembly which rotates
about the spindle
â Both configurations carry
the brake rotor
10. Live Vs. Fixed Spindle
Advantages and Disadvantages
⢠Live Spindle :
â Less parts
â Lighter weight if designed
correctly
â More wheel offset
â Bearing concerns
â Retention inside of the
upright assembly
⢠Fixed spindle
â Simple construction
â Hub sub-assembly
â Spindle put in considerable
bending
â More components, and
heavier
11. Components
⢠Push rod
â The push rod carries
the load from the lower
A-arm to the inboard
coil over shock
â The major concern
with this component is
the buckling force
induced in the tube
12. Components
⢠Toe rod (steering link)
â The toe rod serves as a
like between the steering
rack inboard on the vehicle
â The location of the ends of
this like are extremely
critical to bump steer and
Ackermann of the steering
system
â This link is also used to
adjust the amount of toe-
out of the wheels
13. Components
⢠Bellcrank
â This is a common racing
description of the lever
pivot that translates to
motion of the push rod into
the coil over shock
â The geometry of this pivot
can be designed to enable
the suspension to have a
progressive or digressive
nature
â This component also offers
the designer the ability to
include a motion ratio in the
suspension
14. Components
⢠Coil-over Shock
Absorber
â This component
carries the vehicle
corner weight
â It is composed of a coil
spring and the damper
â This component can
be used to adjust ride
height, dampening,
spring rate, and wheel
rate
15. Components
⢠Anti-Roll bar
â This component is an
additional spring in the
suspension
â Purpose: resist body roll
â It accomplishes this by
coupling the left and right
corners of the vehicle
â 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
â Describes the camber angle with loaded vehicle not
in motion
⢠Dynamic Camber
â Describes the camber angle of a corner at any
instant during a maneuver i.e.: cornering,
launching, braking
20. Camber
⢠Camber is used to offset
lateral tire deflection and
maximize the tire contact
patch area while cornering.
21. Camber
⢠Negative Camber angles
â good for lateral acceleration,
cornering
â 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
â 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
â 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
â For this reason we must compromise between too
little and too much negative camber
â 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
â This is done by adjusting the lengths of the
upper and lower A-arms and the position of
the inboard and out board pivots
â 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
â 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
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
â Force: lateral traction force of tire
â Moment arm: Instant Center height
â Moment pivot: Instant center
CHASSIS
Instant Center
Lateral Force Ground
I.C. Height
33. Jacking Forces
CHASSIS
I. C.
Lateral Force
I.C. Height
â Caused by geometrical binding of the upper and
lower A-arms
â 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
â 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)
â Moment Arm:
B = Sprung mass C.G. height â Roll center height
â 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
â 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
â 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.
â Aerodynamically efficient, high-down force
cars are very sensitive to pitch changes.
â 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
â 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
â 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
â Moment Arm:
B = Sprung mass C.G. height â Roll center height
â Force:
F = (Sprung Mass) x (Longitudinal Acceleration)
B
F