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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 is the angle formed by a line through the pivot points and a vertical line through the stub axle. It can be negative or positive.

Caster Angles

  • Caster assists in the self-centering action of the steering
  • Caster is not a tyre wearing angle
  • Excessive variation side to side means the vehicle will pull to the side of least positive caster
  • Reducing caster trail prevents low-speed shimmy
  • Increased caster improves vehicle stability and steering ‘feel’

Caster Trail Correction for High-Caster Angles

Caster Trail Correction for High-Caster Angles

2. Camber

Positive camber means the tyre and wheel assembly leans outwards at the top and negative camber means it leans inwards at the top.

  • Camber is a tyre wearing angle
  • Camber can be negative or positive
  • Negative camber provides a full footprint when cornering
  • Camber settings can be a compromise between good handling and tyre wear
  • Excessive positive camber wears outside of tyre tread and stresses the inner wheel bearing
  • Excessive negative camber wears inside of tyre tread and stresses the outer wheel bearing
  • Excessive variations will cause the car to pull to the side with the most positive camber

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

Camber Angles are affected by the following:

  • Changes to vehicle ride height
  • Uneven loading of vehicle
  • Body roll during cornering
  • Road camber
  • Condition of the suspension
  • Ride height of each spring
  • Caster angle

Changes to vehicle ride height will affect Camber Angles

When you change your camber angle, wheel alignment should be checked and adjusted accordingly.

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
  • SAI 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 must be on the centreline of the vehicle, from where the Thrust Angle must be measured. Rear Toe adjustments will correct the Thrust Angle.

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

When a pair of wheels is set so their leading edges are pointed slightly towards each other, this is referred to as toe-in. If the leading edges point away from each other, then it’s toe-out.

  • Toe settings affect three major areas of performance: tyre wear, straight-line stability and corner entry handling characteristics
  • For minimum tyre wear and power loss, the wheels on a given axle of a car should point directly ahead when it is travelling in a straight line
  • Excessive toe-in or toe-out causes the tyres to scrub, since they are always turned relative to the direction of travel
  • Too much toe-in causes accelerated wear at the outside edges of the tyres, while too much toe-out causes wear on the inside edges

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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