chap92 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 92, the reader should
be able to:

• Prepare for ASE Suspension and Steering (A4)
•
•

certification test content area “D” (Wheel
Alignment Diagnosis, Adjustment, and Repair).
Discuss which vehicle handling problems can

and cannot be corrected by an alignment.
Define camber, toe, caster, SAI, included
angle, scrub radius, turning radius, setback,
and thrust line.
Continued

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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OBJECTIVES:
After studying Chapter 92, the reader should
be able to:

•

Explain how camber, caster, and toe affect the
handling and tire wear of the vehicle.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
BJI • camber • camber roll • caster
dog tracking • drift
four-wheel alignment • included angle • KPI • lead
MSI • pull • returnability • road crown
SAI • scrub radius • setback • shimmy • steering
dampener • steering offset

thrust line • toe • toot • tramp
wander
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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A wheel alignment is the adjustment of the suspension
and steering to ensure proper vehicle handling with
minimum tire wear.
When a vehicle is new, the alignment angles are set at the
factory. After many miles and/or months of driving, the
alignment angles can change slightly.
By adjusting the suspension and steering components,
proper alignment angles can be restored. An alignment
includes checking and adjusting, if necessary, both

front and rear wheels.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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ALIGNMENT-RELATED PROBLEMS
Most alignment diagnosis is symptom­based diagnosis. The 
problem with the alignment is determined from symptoms such as 
excessive tire wear or a pull to one side of the road. 
Pull  Defined as a definite tug on the steering wheel or pull  
toward the left or the right while driving straight on a level road. 
Figure 92–1 A pull is usually defined as a tug on the steering wheel
toward one side or the other.

Bent, damaged,  

worn suspension 
and/or steering 
components or 
tire problems
can cause this.
Continued
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Road Crown Effects  Most roads are constructed with a slight 
angle to permit water to drain from the road surface. On a two­lane 
road, the center of the road is often higher than the berms, resulting 
in a road crown. 

Figure 92–2 The crown of the road refers to the angle or slope of the roadway needed to drain
water off the pavement. (Courtesy of Hunter Engineering Company)

Continued
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Lead or Drift  A lead or drift is a mild pull that does not cause a 
force on the steering wheel the driver must counteract. A lead or 
drift is observed by momentarily removing your hands from the 
steering wheel while driving on a straight, level road. If the vehicle 
moves toward one side or the other, this is lead or drift.
On a four­lane expressway (freeway), the crown is often between 
the two sets of lanes. Because of this slight angle to the road, 

some vehicles may lead or drift away from the road crown.
CAUTION: When test­driving a vehicle for a lead or a drift, make sure 
that the road is free of traffic and that your hands remain close to the 
steering wheel. Your hands should be away from the steering wheel for 
just a second or two—long enough to check for a lead or drift condition.

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Wander  A condition where constant steering wheel corrections 
are necessary to maintain a straight­ahead direction on a straight, 
level road is called wander. 
Worn suspension and/or steering components are the likely cause 

of this condition. Incorrect or unequal alignment angles such as 
caster and toe, as well as defective tire(s), can cause this condition.

Figure 92–3 Wander is an unstable condition requiring constant driver corrections.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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Stiff Steering or Slow Return to Center  Hard­to­steer problems 
are commonly caused by leaks, either low tire pressure (due to the 
leak of air) and/or lack of proper power steering (due to the leak 
of power steering fluid). Other causes include excessive positive 
caster on the front wheels or binding steering linkage.

Tramp or Shimmy Vibration Tramp  Vertical­type (up­and­
down) vibration usually caused by out­of­balance or defective 
tires or wheels. Shimmy is a back­and­forth vibration that can be 
caused by an out­of­balance tire or defective wheel or by an 
alignment problem.
NOTE: Wheel alignment will not correct a tramp­type vibration.

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CAMBER AND CASTER
Camber  Measured in degrees or fractions of degrees 
Camber is the inward or outward tilt of the wheels from true
vertical as viewed from the front or rear of the vehicle.

1. If the top of the tire is tilted out, then camber is positive 
(+), as shown in Figure 92–4.
2. If the top of the tire is tilted in, then camber is negative (–), 
as shown in Figure 92–5.
3. If the wheel is true vertical, camber is zero (0 degrees)  as 
shown in Figure 92–6, camber is zero (0 degrees).
NOTE: Many front­wheel­drive vehicles that use sealed wheel bearings 
often specify negative camber.

Continued
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Figure 92–4

Positive camber. The solid
vertical line represents true
vertical; the dotted line the
angle of the tire.

Figure 92–5
Negative camber. The solid
vertical line represents true
vertical; the dotted line the
angle of the tire.

Figure 92–6
Zero camber. Note
the angle of the tire
is true vertical.

Continued
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4. Camber is measured in degrees or fractions of degrees. 
5. Camber can cause tire wear if not correct.
a. Excessive positive camber causes scuffing and wear on 
the outside edge of the tire, as shown in Figure 92–7.
b. Excessive negative camber causes scuffing and wear on 
the inside edge of the tire, as shown in Figure 92–8.
6. Camber can cause pull if it is unequal side­to­side. A 
difference of more than 1/2 degree from one side to the other 
will cause the vehicle to pull.
The vehicle will pull toward the side with the most
positive camber.
See Figures 92–7 through 92–11
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Figure 92–7
Excessive positive camber and how the front tires would wear due to the excessive camber.

Continued
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Figure 92–8
Excessive negative camber and how the front tires would wear due to the excessive camber.

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Figure 92–9 Positive camber tilts the tire and forms a cone shape that causes the wheel to roll
away or pull outward toward the point of the cone.

Continued
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Figure 92–10 Negative camber
creates a pulling force toward
the center of the vehicle.

Figure 92–11 If camber angles are
different from one side to the other, the
vehicle will pull toward the side with the
most camber.

Continued
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7. Incorrect camber can cause excessive wear on wheel 
bearings.
Many vehicle manufacturers specify positive camber so the 
vehicle’s weight is applied to the larger inner wheel bearing 
and spindle.
As the vehicle is loaded or when the springs sag, camber 
usually decreases. If camber is kept positive, the running 
camber is kept near zero degrees for best tire life.
See Figures 92–12 and 92–13.

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Figure 92–12
Positive camber applies the vehicle weight
toward the larger inner wheel bearing. This is
desirable because the larger inner bearing is
designed to carry more vehicle weight than
the outer bearing.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

Figure 92–13
Negative camber applies the vehicle weight
to the smaller outer wheel bearing. Excessive
negative camber, may contribute to outer

wheel bearing failure.

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8. Camber is not adjustable on many vehicles.
9. If camber is adjustable, the change is made by moving the 
upper or the lower control arm or strut assembly by means 
of one of the following methods:
a. Shims
b. Eccentric cams
c. Slots
10. Camber should be equal on both sides; however, if camber 
cannot be adjusted exactly equal, make certain that there is 
more camber on the front of the left side to help compensate 
for the road crown (1/2 degree maximum difference).
Continued

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Caster  Measured in degrees or fractions of degrees
Caster is the forward or rearward tilt of the steering axis in 
reference to a vertical line as viewed from the side of the vehicle. 
The steering axis is defined
as the line drawn through
the upper and lower steering 
pivot points. 

Figure 92–14 Zero caster. (Courtesy of Hunter
Engineering Company)

Zero caster means the steering 
axis is straight up and down.
This is also called 0 degrees 
or perfectly vertical, as shown 
at right.
Continued
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On an SLA suspension system, the upper pivot is the upper ball 
joint and the lower pivot is the lower ball joint.
On a MacPherson strut system, the upper pivot is the center of the 
upper bearing mount and the lower pivot point the lower ball joint. 

1. Positive (+) caster is present when the upper suspension pivot 
point is behind the lower pivot point (ball joint) as viewed 
from the side. See Figure 92–15.
2. Negative (–) caster is present when the upper suspension pivot 
point is ahead of the lower pivot point (ball joint) as viewed 
from the side. See Figure 92–16.

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3. Caster is measured in 
degrees or fractions of 

degrees.
Figure 92–15 Positive (+) caster. (Courtesy of
Hunter Engineering Company)

Figure 92–16 Negative (–) caster is seldom
specified on today’s vehicles because it tends
to make the vehicle unstable at highway
speeds. Negative caster was specified on
some older vehicles not equipped with power
steering to help reduce the steering effort.
(Courtesy of Hunter Engineering Company)

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4. Caster is not a tire­wearing angle, but positive caster does 
cause changes in camber during a turn. See below. This 
condition is called camber roll.

Figure 92–17
As the spindle rotates,
it lifts the weight of the
vehicle due to the angle
of the steering axis.
(Courtesy of Hunter
Engineering Company)

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5. Caster is a stability angle.
a. If caster is excessively positive, the vehicle steering will 
be very stable (will tend to go straight with little steering 
wheel correction needed). This degree of caster helps with 
steering wheel returnability after a turn. 

Figure 92–18 Vehicle weight tends to lower the spindle,
which returns the steering to the straight-ahead position.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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