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Suspension Design Part 1

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1 Suspension Design Part 1
Rob Shanahan

2 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”

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

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

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

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

7 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

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

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

10 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

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

12 Double Wishbone Lightest weight Lowest unsprung mass
Greatest adjustability

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

14 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

15 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

16 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

17 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

18 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

19 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

20 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

21 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

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

23 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

24 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

25 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

26 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

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