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Suspension Design Part 1

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

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  1. Suspension Design Part 1 Rob Shanahan 11-15-05

  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” HowStuffWorks.com

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

  4. 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

  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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