1. Wheel Alignment Fundamentals
Chapter 67

2. Objectives Describe each wheel alignment angle
Tell which alignment angles cause wear or pull

3. Introduction Correct wheel alignment
Allows vehicle to run straight on the highway
Little steering effort
Minimal tire wear
This chapter deals with principles of different wheel alignment angles

4. Wheel Alignment Angles
Five wheel alignment angles
Toe
Camber
Caster
Steering axis inclination (SAI)
Turning radius

5. Toe
Comparison of distances between fronts and rears of a pair of tires
Alignment angle most responsible for tire wear
Toe-in: tires closer together at the front
Every 1/16" of toe-in results in 11 feet per mile scuff
Tires move sideways for 11 feet out of every mile
Toe-out: tires further apart at the front

6. Toe (cont'd.) Causes of incorrect toe
Improper adjustment
Bent steering linkage
Change in caster or camber adjustment
Looseness in steering linkage due to wear
Change in toe on one side of the vehicle
Will be split through steering linkage with the wheel on the other side
Front toe: adjustable on all vehicles
Rear toe adjustable on some

8. Camber Camber: inward or outward tilt of tire at top
Adjustable on most vehicles
Positive camber: tire tilts out
Negative camber: tire tilts in
Inside and outside edges of tread on cambered tire have different radii
Rotate at different speeds
Camber angle
Controlled by position of control arms or struts
Camber roll: tire tends to roll in a circle

10. Caster Caster: forward or rearward tilt of the spindle support arm
Positive caster: top tilted to the rear
Lead point in front of true vertical
Negative caster: steering axis tilts forward
Moving the point of load behind the wheel
Sometimes adjustable on front wheels
Front wheels have different caster settings
Vehicle will pull toward the side with the most negative caster

12. Steering Axis Inclination
Amount the spindle support arm leans in at top
Not a tire wearing angle
Three functions
After a turn, SAI helps vehicle return to straight
SAI keeps vehicle going straight down the road
Allows car to have less positive caster
Included angle
Combination of SAI and camber
Some cars with large SAI wear outsides of tires

14. Scrub Radius Factor of steering axis inclination
Pivot point for front tire’s footprint
Distance at the road surface between centerline of true vertical and steering axis pivot centerline
More scrub radius makes it harder to steer
Positive camber reduces scrub radius

16. Scrub Radius (cont'd.) Causes of incorrect scrub radius
Lower profile tires and offset wheel rims
Tires and wheels too tall installed on RWD vehicle
Bent front suspension member
Damage to frame at crossmember

17. Turning Radius
When turning: outside wheel must travel in wider arc than inside wheel
Turning radius: alignment angle that controls arc traveled
Also called Ackermann angle
Tires toe out during turn
Steering arms are angled inward or outward

19. Tracking Wheel base: distance between front and rear tires
Track: side-to-side distance between axle’s tires
All four wheels should form an exact rectangle
Tracking is off: car tries to steer to the side
Front wheels try to follow direction of rear wheels
Dog tracking: rear axle out of line to the right
Causes steering to be aimed to the right

21. Set-Back
Amount that one front wheel is behind front wheel on other side
Measured in degrees
Negative angle
Wheel on left side is set back
Cause vehicle to steer to left
Cause brake pull

22. Special Handling Characteristics
Slip angle: during a turn for a tire to continue turning in the same direction
Amount of slip angle depends on:
Weight exerted vertically on the tire
Tire pressure and wheel alignment setting
Positive camber
Understeer: vehicle does not respond to movement of steering wheel during hard turn
Oversteer: vehicle turns too far in response to steering wheel movement