Understanding Steering And Wheel Alignment Angles

UNDERSTANDING STEERING AND WHEEL ALIGNMENT ANGLES
 

Steering angles affecting a vehicle's alignment

• UNDERSTANDING STEERING AND WHEEL ALIGNMENT ANGLES
   
  Steering angles affecting a vehicle's alignment
   
  Caster
  Camber
  Steering Axis Inclination (SAI)
  Included Angle
  Thrust Angle
  Toe-In and Toe-Out
  Other factors related to tracking, stability and tyre life
  Set Back
  Toe-Out on Turns
  Bump Steer
  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 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  Neutral  +  Positive
  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
  
   
  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.
   
  
   
  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
  
   
  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.
   
  
   
  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:
   
  
   
  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".
   
  
   
  8. Toe-Out On Turns
  The Ackerman Angle provides the correct toe-out on turns:
  
   
  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
  
   
  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
  
   
  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 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 Lines Meet Above Road Surface
  Used On Front Wheel Drive With Front Wheel Toe Out Or Zero Toe
  SAI Usually Greater Than 10 Degrees