suspension design: case study

7/28/2019 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

suspension design: case study

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