wheel alignment fundamentals

© 2012 Delmar, Cengage Learning

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

wheel alignment fundamentals

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