Tyre Wear Patterns Tell The Technician A Story

Understanding Steering and Wheel Alignment Angles

Steering angles affecting a vehicle's alignment

  1. Caster
  2. Camber
  3. Steering Axis Inclination (SAI)
  4. Included Angle
  5. Thrust Angle
  6. Toe-In and Toe-Out

Other factors related to tracking, stability and tyre life

  1. Set Back
  2. Toe-Out on Turns
  3. Bump Steer
  4. Scrub Radius and Wheel Offset

1. Caster

Caster Angles

  • Caster is not a tyre wearing angle
  • Caster assists in the self-centering action of the steering
  • Caster is the angle formed by a line through the pivot points and a vertical line through the stub axle
  • Caster can be positive or negative
  • Excessive variation side to side means the vehicle will pull to the side of least positive caster
  • Caster provides vehicle stability

Caster Trail Correction for High-Caster Angles

Caster Trail Correction for High-Caster Angles

  • Ford Falcon AU and Commodore VT run high-caster angles
  • Reducing caster trail prevents low-speed shimmy
  • Maintains high-speed stability

2. Camber

  • Camber is a tyre wearing angle
  • Camber can be negative or positive
  • Positive camber tyre and wheel assembly leans outwards at the top
  • Negative camber leans inwards at the top
  • Negative camber provides full footprint when cornering
  • Camber settings a compromise between good handling and tyre wear

Negative and Positive Camber Angles

Affects of Camber

  • Too much positive wears outside of tyre tread
  • Too much negative wears inside of tyre tread
  • Excessive variation side to side will pull to the side with the most positive camber
  • Excessive negative camber stresses outer wheel bearing
  • Excessive positive camber stresses inner wheel bearing

Influences of Camber

  • Uneven loading of vehicle
  • Body roll in turns
  • Road camber
  • Conditions of suspension
  • Ride height of each spring
  • Caster angle

Changes to vehicle ride height will affect Camber Angles

Camber Angles are affected by changes to vehicle ride height. If you raise or lower your vehicle, you should always check/adjust wheel alignment.

3. Steering Axis Inclination (SAI)

Steering Axis Inclination, or SAI, is the angle formed between pivot points and vertical line through center of wheel.

Steering Axis Inclination

  • SAI is not adjustable
  • SAI is a diagnostic angle
  • Assists steering returnability

4. Included Angle

The Included Angle is a diagnostic angle that's obtained by:

  • Adding positive camber to SAI
  • Subtracting negative camber from SAI

Included Angle

5. Thrust Angle

The Thrust Angle is measured off the centerline of the vehicle:

Geometric centerline of the vehicle

Thrust Angle must be on the centreline:

Thrust Angle

Rear Toe adjustments will correct the Thrust Angle.

6. Toe-In and Toe-Out

Various Toe Settings:

Various Toe Settings

7. Set Back

Set Back is the name given to the variation in the wheel base of a vehicle. It can be measured by modern wheel alignment machines and can be altered by caster adjustment. Excessive Set Back can cause the vehicle to "Run Off".

Set Back Angles

8. Toe-Out On Turns

The Ackerman Angle provides the correct toe-out on turns:

Ackerman Angle

The line is drawn through:

  • Centre of the footprint
  • Tie Road attaching point on steering arm
  • Meet in centre of line drawn through rear axle
  • Angle of turn of front wheels will be correct

Ackerman Effect In Corners

  • Inside wheel in turn requires greater angle of turn
  • Outside wheel requires less turning angle
  • Lines projected will meet at intersection point outside vehicle

Ackerman Effect In Corners

Effect On Ackerman Angle When Wheel Base Increased Or Extra Axle Added

  • Line now drawn midway between rear axles
  • Intersection points changed
  • Front turn angle not correct
  • Tyres will scrub and squeal

Effect On Ackerman Angle When Wheel Base Increased Or Extra Axle Added

9. Bump Steer

Bump Steer is the variation in toe that occurs as the front suspension moves up and down without body roll and is caused by:

  • Incorrect Steering Linkage Design Or Tie Rod Location
  • Bent Steering Linkage
  • Toe Adjusted Without Centralising Steering Gear
  • Soft Suspension Bushes and Worn Components
  • Misalignment of Rack or Steering Linkage in Frame

10. Scrub Radius and Wheel-Offset

Positive Scrub Radius

Positive Scrub Radius

  • Positive Scrub Radius Lines Meet Below Road Surface
    • Usually Rear Wheel Drive With Front Wheel Toe In
    • SAI Usually Less Than 10 Degrees

Negative Scrub Radius

Negative Scrub Radius

  • Negative Scrub Radius Lines Meet Above Road Surface
    • Used On Front Wheel Drive With Front Wheel Toe Out Or Zero Toe
    • SAI Usually Greater Than 10 Degrees
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