Suspension Design Part 1 Rob Shanahan 11-15-05

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

Basic Suspension Terminology Ride Height Bump / Droop Camber Caster Toe In / Out

Ride Height, Bump & Droop The neutral / middle position of the Suspension Bump When the wheel moves upwards Droop When the wheel moves downwards

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

Caster Shopping cart action Causes self-centering action in the steering More caster results in more camber as front wheels are turned

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

Common Suspension Designs Beam Axle Swing Axle De Dion Double Wishbone / Unequal Length A-arm

Beam Axle Around since horse and chariot days Always keeps wheels parallel Often used in rear Rarely used in front OK on smooth tracks

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

De Dion Essentially a beam axle with the diff now sprung weight Keeps wheels parallel Relatively light weight Better on smooth tracks

Double Wishbone Lightest weight Lowest unsprung mass Greatest adjustability

Basic Vehicle Dynamics Part 2 What is Vehicle Dynamics? The understanding and study of how a vehicle and its components move and react

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

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

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

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

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

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

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

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

Tire Slip Angle Angle between the centerline of the wheel and the actual path Tires generate highest cornering forces at a certain slip angle

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

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

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

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