Suspension DesignPart 1Rob Shanahan 11-15-05

IntroductionWhat 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

Basic Suspension Terminology

Ride HeightBump / DroopCamberCasterToe In / Out

Ride Height, Bump & DroopRide HeightThe neutral / middle position of the SuspensionBumpWhen the wheel moves upwards DroopWhen the wheel moves downwards

CamberTires generate more cornering force with a small amount of negative camberCamber changes as suspension moves up (bump) and down (droop)

CasterShopping cart actionCauses self-centering action in the steeringMore caster results in more camber as front wheels are turned

Toe-in or Toe-outToe-in results is inherently stableToe-out is inherently unstableRace cars often use front toe-out, & rear toe-in

Common Suspension Designs

Beam AxleSwing Axle De DionDouble Wishbone / Unequal Length A-arm

Beam AxleAround since horse and chariot daysAlways keeps wheels parallelOften used in rearRarely used in frontOK on smooth tracks

Swing AxleOften used on VW based off road carsSimple and ruggedCamber curve too steepOnly adjustment you can make is ride height

De DionEssentially a beam axle with the diff now sprung weightKeeps wheels parallelRelatively light weightBetter on smooth tracks

Double WishboneLightest weightLowest unsprung massGreatest adjustability

Basic Vehicle DynamicsPart 2

What is Vehicle Dynamics?

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

Yaw, Pitch, and RollSame terminology as aircraftX is the longitudinal axisYawing refers to normal change of directionPitching is dive or squat

UndersteerFront end of the car washes out or doesnt turn in NASCAR boys call it push or tightSafe, because lifting off throttle reduces itMost road cars have a ton of it

OversteerRear end of car slides outNASCAR boys call it looseExcessive application of power can cause oversteerThrottle induced oversteer is never the fast way around a corner

Weight TransferOccurs anything the vehicle accelerates or deceleratesCornering force Fc will cause weight to transfer from the inside to outside tiresBraking and accelerating forces cause a similar front and rear weight transfer

Roll CenterA geometric constructRepresents the instantaneous point about which the sprung mass will rotate due to cornering forcesRoll center moves as suspension travelsGoal of any suspension designer is to minimize Roll Center Migration

Roll CoupleDistance from roll center to CG is keyLow roll center results in more roll for a given lateral accelerationMost designs use a low roll center to reduce jacking forces

Anti-divePurely geometric method to reduce pitch movementReduces suspension compliance over bumpsNo longer in favor with formula car and sports racersMight work well for Baja

Bump SteerCaused when toe changes as suspension moves up and downCauses car to react unexpectedly over bumps and in roll Sometimes used intentionally, but be careful

Tire Slip AngleAngle between the centerline of the wheel and the actual pathTires generate highest cornering forces at a certain slip angle

Slip Angle vs. GripGrip is highest a set angle, then falls off as the slip angle increasesSharper peak will give a less predictable breakawayRadial tires typically have a steeper slope than bias ply

Friction CirclePlots the theoretical limits of adhesion in 2 axesGreat tool for analyzing driver to driver variationG-analyst is a cheap tool for this

Friction Circle, cont.Illustrates the trade off between cornering and braking/acceleratingThe driver that follows the path closest to the outside of the circle wins

Car BalanceA well balanced car will exhibit both understeer and oversteer at different points on the course and at corner entry and exitA good driver can change his technique to change the basic oversteer/understeer balance