Download - Suspension Design: Case Study
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Suspension Design Case
Study
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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
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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.
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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 designermust allow adequate droop travel.
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Introduction
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Components
Upper A-arm The upper A-arm serves to
carry some of the loadgenerated on thesuspension by the tire.
This force is considerablyless then the load carriedby the lower A-arm in apush rod set-up
The arm only has to
provide a restoring force tothe moment generated bythe tire on the lower ball
joint
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Components
Lower A-arm
The lower A-arm serves
the same purpose as the
upper arm, except that in a
pushrod configuration it isresponsible for carrying the
vertical load
In this case study the lower
A-arm will carry a larger
rod end to compensate forthe larger forces seen by
this component.
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Components
Upright
The upright serves several
purposes in the suspension
Connects the upper A-
arm, lower A-arm, steeringarm, 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
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Components
Spindle Spindle can come in two
basic configurations Live spindle
Fixed spindle
In the live spindleconfiguration the wholespindle assembly rotatesand carries the tire andwheel
The fixed spindle
configuration carries a hubassembly which rotatesabout the spindle
Both configurations carrythe brake rotor
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Live Vs. Fixed Spindle
Advantages and Disadvantages Live Spindle :
Less parts
Lighter weight if designedcorrectly
More wheel offset
Bearing concerns Retention inside of the
upright assembly
Fixed spindle Simple construction
Hub sub-assembly
Spindle put in considerablebending
More components, andheavier
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Components
Push rod
The push rod carries
the load from the lower
A-arm to the inboardcoil over shock
The major concern
with this component is
the buckling force
induced in the tube
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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 ofthis like are extremely
critical to bump steer and
Ackermann of the steering
system
This link is also used toadjust the amount of toe-
out of the wheels
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Components
Bellcrank This is a common racing
description of the leverpivot that translates tomotion of the push rod into
the coil over shock The geometry of this pivot
can be designed to enablethe suspension to have aprogressive or digressivenature
This component also offersthe designer the ability toinclude a motion ratio in thesuspension
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Components
Coil-over ShockAbsorber
This componentcarries the vehicle
corner weight It is composed of a coil
spring and the damper
This component can
be used to adjust rideheight, dampening,spring rate, and wheelrate
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Components
Anti-Roll bar This component is an
additional spring in thesuspension
Purpose: resist body roll
It accomplishes this bycoupling the left and rightcorners of the vehicle
When the vehicle rolls theroll bar forces the vehicle tocompress the spring on
that specific corner as wellas some portion of theopposite corners spring
This proportion is adjustedby changing the springrate of the bar itself*Unclear in this pic ture the
An t i-Rol l bar tu be actual ly
passes inside the chassis
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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
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Camber
The main consideration at this step is the
camber change throughout the
suspension travel.
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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 maneuveri.e.: cornering,
launching, braking
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Contact Patch
Tread area in contact
with the road at anyinstant in time
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Camber
Camber is used to offsetlateral tire deflection and
maximize the tire contact
patch area while cornering.
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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
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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
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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
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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
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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
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Instant Center
The instant center is a dynamic point which thewheel will pivot about and any instant during thesuspension travel For a double wishbone configuration this point moves
as the suspension travels
CHASSIS
Instant Center
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Mild Camber Change Design
-Suspension arms are close to parallel
-Wide instant center locations
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Mild Camber Change Design
0.4 of Neg. Camber Gain Per inch of Bump
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Aggressive Camber Change Design
-Suspension arms are far from parallel
-Instant center locations are inside the track width
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More Aggressive Camber Change Design
1.4 of Neg. Camber Gain Per inch of Bump
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Jacking forces
It is important to consider the Instant
Center Posit ion, because when it moves
vertically off the ground plane Jacking
forces are introduced
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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
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Jacking Forces
CHASSIS
I. C.
Lateral Force
I.C. Height
Caused by geometrical binding of the upper andlower A-arms
These forces are transferred from the tire to thechassis by the A-arms, and reduce the amount offorce seen by the spring
Jacking
Forces
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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
VehicleCen
ter
Line
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Roll Center
For a parallel-Iink Situation the Roll Center is
found on the ground plane
Roll Center
Vehicle
Center
Lin
e
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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.
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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
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Roll Axis
To consider the total vehicle you must
look at the roll axis
Rear Roll CenterFront Roll Center
Sprung Mass C.G.
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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
GroundFront Rear
CHASSIS
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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
GroundFront Rear
CHASSIS
Anti-DiveAnti-Lift
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Anti-Features
Racecars rely heavily on wings andaerodynamics for performance.
Aerodynamically efficient, high-down forcecars are very sensitive to pitch changes.
A pitch change can drastically affect theamount of down force being produced.
Much less important for lower speed cars
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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
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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
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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