Suspension Design Part 1

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Suspension Design Part 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 Dynamics Part 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 doesnt
  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