Suspension DesignPart 1

Rob Shanahan 11-15-05

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Introduction What is an Automotive Suspension?

An Automotive Suspension is the system of parts that give a vehicle the ability to maneuver.

It is a 3 Dimensional Four Bar Linkage

What does a suspension do?“The job of a car suspension is to maximize the

friction between the tires and the road surface, to provide steering stability with good handling” HowStuffWorks.com

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Basic Suspension Terminology

Ride Height Bump / Droop Camber Caster Toe In / Out

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Ride Height, Bump & Droop

Ride Height The neutral /

middle position of the Suspension

Bump When the wheel

moves upwards Droop

When the wheel moves downwards

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Camber Tires generate

more cornering force with a small amount of negative camber

Camber changes as suspension moves up (bump) and down (droop)

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Caster Shopping cart

action Causes self-

centering action in the steering

More caster results in more camber as front wheels are turned

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Toe-in or Toe-out Toe-in results is

inherently stable Toe-out is

inherently unstable

Race cars often use front toe-out, & rear toe-in

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Common Suspension Designs

Beam Axle Swing Axle De Dion Double Wishbone /

Unequal Length A-arm

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Beam Axle Around since horse

and chariot days Always keeps

wheels parallel Often used in rear Rarely used in front OK on smooth

tracks

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Swing Axle Often used on VW

based off road cars Simple and rugged Camber curve too

steep Only adjustment

you can make is ride height

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De Dion Essentially a

beam axle with the diff now sprung weight

Keeps wheels parallel

Relatively light weight

Better on smooth tracks

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Double Wishbone Lightest weight Lowest unsprung

mass Greatest

adjustability

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Basic Vehicle DynamicsPart 2

What is Vehicle Dynamics?

The understanding and study of how a vehicle and its components move and react

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Yaw, Pitch, and Roll Same terminology

as aircraft X is the

longitudinal axis Yawing refers to

normal change of direction

Pitching is dive or squat

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Understeer Front end of the car

“washes out” or doesn’t “turn in”

NASCAR boys call it “push” or “tight”

Safe, because lifting off throttle reduces it

Most road cars have a ton of it

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Oversteer Rear end of car slides out NASCAR boys call it

“loose” Excessive application of

power can cause oversteer Throttle induced oversteer

is never the fast way around a corner

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Weight Transfer Occurs anything the

vehicle accelerates or decelerates

Cornering force Fc will cause weight to transfer from the inside to outside tires

Braking and accelerating forces cause a similar front and rear weight transfer

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Roll Center A geometric construct Represents the instantaneous

point about which the sprung mass will rotate due to cornering forces

Roll center moves as suspension travels

Goal of any suspension designer is to minimize Roll Center Migration

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Roll Couple Distance from roll

center to CG is key Low roll center results

in more roll for a given lateral acceleration

Most designs use a low roll center to reduce jacking forces

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Anti-dive Purely geometric

method to reduce pitch movement

Reduces suspension compliance over bumps

No longer in favor with formula car and sports racers

Might work well for Baja

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Bump Steer Caused when toe

changes as suspension moves up and down

Causes car to react unexpectedly over bumps and in roll

Sometimes used intentionally, but be careful

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Tire Slip Angle Angle between

the centerline of the wheel and the actual path

Tires generate highest cornering forces at a certain slip angle

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Slip Angle vs. Grip Grip is highest a set

angle, then falls off as the slip angle increases

Sharper peak will give a less predictable breakaway

Radial tires typically have a steeper slope than bias ply

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Friction Circle Plots the

theoretical limits of adhesion in 2 axes

Great tool for analyzing driver to driver variation

G-analyst is a cheap tool for this

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Friction Circle, cont. Illustrates the trade off

between cornering and braking/accelerating

The driver that follows the path closest to the outside of the circle wins

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Car Balance A well balanced car will

exhibit both understeer and oversteer at different points on the course and at corner entry and exit

A good driver can change his technique to change the basic oversteer/understeer balance