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
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 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