Wheel Alignment Fundamentals Chapter 67

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

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

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

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

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

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

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

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

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

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

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

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

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

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