© 2012 delmar, cengage learning wheel alignment fundamentals chapter 67
TRANSCRIPT
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Wheel Alignment Fundamentals
Chapter 67
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Objectives• Describe each wheel alignment angle• Tell which alignment angles cause wear or pull
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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
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Wheel Alignment Angles• Five wheel alignment angles
– Toe
– Camber
– Caster
– Steering axis inclination (SAI)
– Turning radius
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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