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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?

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suspension design part 1

Suspension Design Part 1

Rob Shanahan 11-15-05

introduction
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
Basic Suspension Terminology
  • Ride Height
  • Bump / Droop
  • Camber
  • Caster
  • Toe In / Out
ride height bump droop
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
camber
Camber
  • Tires generate more cornering force with a small amount of negative camber
  • Camber changes as suspension moves up (bump) and down (droop)
caster
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 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
Common Suspension Designs
  • Beam Axle
  • Swing Axle
  • De Dion
  • Double Wishbone /

Unequal Length A-arm

beam axle
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
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
De Dion
  • Essentially a beam axle with the diff now sprung weight
  • Keeps wheels parallel
  • Relatively light weight
  • Better on smooth tracks
double wishbone
Double Wishbone
  • Lightest weight
  • Lowest unsprung mass
  • Greatest adjustability
basic vehicle dynamics part 2
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
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
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
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
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
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
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
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
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
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
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
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
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
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