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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
Chapter discusses different alignment angles
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Alignment Angles
Five wheel alignment angles Toe
Camber
Caster Steering axis inclination (SAI)
Turning radius
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Toe (continued)
Causes of incorrect toe Improper adjustment
Looseness in steering linkage due to wear
Collision with a curb Change in caster or camber adjustment
Front toe adjustable on all vehicles Rear toe adjustable on some
Change in toe affects position of the steeringwheel
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Camber
Camber is inward or outward tilt of tire at topAdjustable on most vehicles
Positive camber tire tilts out
Negative camber tire tilts in Inside and outside edges of the tread on
cambered tire have different radii Rotate at different speeds
Camber angle controlled by position of controlarms or struts
Camber roll tire tends to roll in a circle
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Caster
Caster is the angle that describes the forward or
rearward tilt of the spindle support arm
Positive caster top tilted to the rear
Lead point in front of true vertical
Car pulls to side with most negative caster
Too much positive caster results in front wheel
shimmy High amount of wear if car driven on winding
roads, or in city
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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 roadAllows car to have less positive caster
Included angle combination of SAI and camber
Some cars with large SAI wear outsides of tires SAI can be used as diagnostic angle to
determine if parts are bent
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Scrub Radius
Factor of steering axis inclination
Pivot point for front tires 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 (continued)
Causes of incorrect scrub radius Lower profile tires and offset wheel rims
Tires and wheels too tall installed on rear-wheel-
drive vehicle Bent front suspension part
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 traveledAlso called Ackermann angle
Tires toe out during turn
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Tracking
Wheel base distance between front and rear
tires
Track side-to-side distance between axles
tires
All four wheels should form an exact rectangle
When tracking is off, car tries to steer to the side
Increases tire wear Dog tracking rear axle out of line to the right,
causes steering to be aimed to the right
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Set-Back
Set-back amount that one front wheel is
behind front wheel on other side Measured in degrees
Negative set-back wheel on left side is setback Cause vehicle to steer to left
Cause brake pull
Collision causes incorrect set-back
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Special Handling Characteristics
Slip angle tendency for a tire to continue to goin the direction it was going
Amount of slip angle depends on Tire pressure
Weight exerted vertically on the tire
Wheel alignment setting
Positive camber
Understeer car does not respond to movementof steering wheel during hard turn
Oversteer car turns too far in response to
steering wheel
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