chap84 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 84, the reader should
be able to:

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

•

certification test content area “E” (Wheel and
Tire Diagnosis and Repair).
Discuss proper tire mounting procedures.
Describe recommended tire rotation methods.
Discuss how to properly balance a tire.
Describe tire repair procedures.
Explain wheel and tire safety precautions.

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KEY TERMS:
dynamic balance • lateral runout
match mounting • modified X
radial force variation • radial runout
shimmy • static balance
tire rotation • tramp
wheel mounting torque

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Proper tire service is extremely important for the safe
operation of any vehicle.
Premature wear can often be avoided by checking and
performing routine service, such as frequent rotation
and monthly inflation checks.

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Figure 84–1 Using soapy water from a spray bottle is an easy method to find the location of an
air leak from a tire.

Continued
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TIRE INFLATION
Tires should be inflated to pressure indicated on the door or pillar
sticker. Tires should be checked when cold, before the vehicle has
been driven; driving increases temperature and pressure of tires.
Proper tire inflation is important for several reasons:
Inflation pressure carries the load of the vehicle If the
pressure is low, the load capacity of the tire is decreased.

Figure 84–2 This chart shows
the relationship between tire
inflation pressure and load
capacity of the tire.

Continued
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Inflation pressure varies with temperature Tires lose 1 psi
for every 10-degree drop in temperature. This means as the
sensors change and temperature changes, so does pressure

inside the tires.
If tires were inflated to 35 psi on the first day of summer (June
21) when the temperature was 90°F (32°C), then the following
pressures would occur, assuming no air loss at all:
Summer (June 21) 90°F—35 psi
Fall (September 21) 80°F—34 psi
Winter (December 21) 30°F—29 psi
It is very important to check and correct inflation pressures
regularly.
Continued
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Tire inflation affects fuel economy A drop in inflation
pressure from 30 psi to 23 psi can result in a drop of fuel
economy from 20 miles per gallon to 18 miles per gallon.

Figure 84–3 This chart shows that a drop in inflation pressure has a major effect on fuel economy.

Continued
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Tire inflation affects tire life Even a slight drop in air pressure

can have a major effect on the life of a tire. If, for example, the
inflation pressure dropped 10 psi, the life of the tire would be
reduced by 40%.

Figure 84–4 Notice that if a tire is underinflated by 10 PSI, the life expectancy is reduced by 40%.

Continued
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The TREAD Act specifies that the driver be notified if the
inflation of a tire drops by 25%

25% represents a loss of air pressure of about 8 psi. A drop of 8
psi means an approximate 2-mpg decrease in fuel economy as
well as about a 25% reduction in tread wear.

Continued
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A compressor powered by the cigarette lighter with stop leak.
An aerosol spray can that provides inflation and sealer.
Figure 84–5 A temporary inflation pump that
uses 12 volts from the cigarette lighter to inflate

the tire.

Figure 84–6 Many vehicle manufacturers
include an aerosol can of sealer on vehicles
that are not equipped with a conventional
spare tire.

Continued
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What is a “Temporary Mobility Kit”?

A temporary mobility kit is a system to inflate a flat tire supplied by the
vehicle manufacturer instead of a spare tire. The mobility kit can include:
• A compressor powered by the cigarette lighter with stop leak.
• An aerosol spray can that provides inflation and sealer.
Either type can be found in many vehicles because such devices save
weight, increase trunk space, and cost less than a conventional spare tire
and jack. However, these kits are designed to be a temporary repair only
because the cause of the low tire was never determined. If the tire appears
to be fixed, many vehicle owners may think that the tire has been repaired.
However, the tire should be carefully inspected inside and out for damage
and properly repaired.

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NITROGEN INFLATION
Some shops recommend and inflate tires using nitrogen. Since
compressed air contains about 78% nitrogen, 21% oxygen, and
1% other gases, why use pure nitrogen? There are several reasons:
The nitrogen molecule is larger than the oxygen molecule, so
the tire will lose pressure faster if air is used instead of nitrogen.
Compressed nitrogen contains less moisture than compressed
air. When the tire heats up, moisture in the tire vaporizes and
expands, causing the pressure inside the tire to increase.
Even small changes in tire pressure can noticeably affect the
handling of the vehicle.
Continued
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Race teams use nitrogen because they already come to the track
with a cylinder of nitrogen to power the air tools.
Race teams also have more control over how much the pressure
will increase when the tires heat up, because nitrogen has less
tendency to change pressure with temperature change.
Figure 84–7 Most shops that use
nitrogen inflation install a green tire value
to let others know that nitrogen rather
than air has been used to inflate the tire.

Some oxygen in the tires
could, over a long period of
time, cause the oxidation of
the inner liner of the tire and
the corrosion of the wheel.
Continued
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By James D. Halderman

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TIRE MOUNTING RECOMMENDATIONS
1. When removing a wheel from a vehicle for service, mark the
location of the wheel and lug stud to ensure that the wheel can
be replaced in exactly the same location. This ensures that tire
balance will be maintained if the tire/wheel assembly was
balanced on the vehicle.
2. Make certain that the wheel has a good, clean metal-to-metal
contact with the brake drum or rotor. Grease, oil, or dirt
between these two surfaces could cause the wheel lug nuts to
loosen while driving.

Continued
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3. Always check the rim size. By looking it is hard to distinguish
a 16-in. wheel from a 16.5-in. wheel used on some trucks. The
rim size is marked on the sidewall of the tire, and the rim’s
diameter and width are stamped somewhere on the wheel.

Figure 84–8 Note the difference in the shape of the rim contour of the 16-in. and 16 1/2-in. diameter
wheels. While it is possible to mount a 16-in. tire on a 16 1/2-in. rim; it cannot be inflated enough to
seat against the rim flange. If an attempt is made to seat the tire bead by overinflating (over 40 PSI),
the tire bead can break, resulting in an explosive force that could cause serious injury or death.
Continued
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4. Install the tire-pressure monitoring system.
Figure 84–9 When installing a tire-pressure monitoring system sensor, be sure that the flat part
of the sensor is parallel to the center section of the rim.

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5. Many tires are marked with a paint dot or sticker at the largest
diameter (high point) and/or stiffest portion of the tire.

The tire should be mounted with this mark lined up with the
valve stem. The valve stem hole is typically drilled at the
smallest diameter (low point) of the wheel. Mount tires on
the rim with the valve stem lined up the mark on the tire.
This is called match mounting.
Figure 84–10 This tire on a new vehicle has
been match mounted at the factory. The yellow
sticker is placed at the largest diameter of the
tire. The valve core hole in the wheel is usually
drilled at the smallest diameter of the wheel.
The best way to make sure the assembly is as
round as possible and to reduce the number of
wheel weights needed to balance the tire is to
align the sticker with the valve core.

Continued
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Figure 84–11 (a) Cleaning the bead area of an aluminum (alloy) wheel using a handheld wire
brush. The technician is using the tire changer itself to rotate the wheel as the brush is used to
remove any remnants of the old tire. (b) Using an electric or air-powered wire brush speeds the
process, but care should be exercised not to remove any of the aluminum itself. (Remember,
steel is harder than aluminum and a steel wire brush could cause recesses to be worn into the
aluminum wheel, which would prevent the tire from proper seating in the bead area.)

6. Never use more than 40 psi

(275 kPa) to seat a tire bead.

7. Rim flanges must be free of
rust, dirt, scale, or loose or
flaked rubber build-up prior
to mounting the tire.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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8. When mounting new tires, do not use silicone lubricant on the

tire bead. Use special lubricant such as rendered (odorless)
animal fat or rubber lubricant to help prevent tire rotation on the
rim.

This rubber lube is a water-based soap product that is slippery
when wet (coefficient of friction when less than 0.3) and acts
almost as an adhesive when dry (coefficient of friction when dry
of over 0.5 for natural products and over 1.0 for synthetic
products).
See Figure 84–12.

Continued
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If the wrong lubricant is used, the rubber in the bead area of the
tire can be softened or weakened. Also, most other lubricants do
not increase in friction when they dry like rubber lubricant does.
The result can be the rotation of the tire on the rim (wheel),
especially during rapid acceleration or braking.

Figure 84–12 Rendered (odorless)
animal fat is recommended by
some manufacturers of tire
changing equipment for use as a
rubber lubricant.

Continued
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Spin the Tires
When performing a vehicle inspection and the vehicle has been hoisted on
a frame-type lift, check the tires by rotating them by hand. The tires on the
nondrive wheels should spin freely.
• On front-wheel-drive vehicles, rear wheels should rotate easily.
• On rear-wheel-drive vehicles, front wheels should rotate easily.
• On all-wheel-drive vehicles, all four wheels may require effort to rotate.
What to Look For:
• When rotating wheels, look at the tires from front or rear and check
that the tire tread does not change or look as if the tread is moving
inward or outward. If the tread is moving, it indicates an internal fault
with the tire and it should be replaced. This type of fault can cause a
vibration though the tire/wheel assembly is correctly balanced.
• Look from the side of the vehicle as the wheel/tire assembly is being
rotated. Look carefully to see if the tire is round. If it is out-of-round,
the tread will appear to move up and down as the tire rotates.
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NOTE: Many experts recommend that when a new tire is installed the
vehicle should be driven at less than 50 mph (80 km/h) for the first 50
miles (80 km) to allow the tires to adhere to the rim. During break-in, the
rubber lube used to mount the tire is drying and the tire is becoming fully
seated on the rim. By avoiding high speeds, rapid acceleration, and fast
braking, the driver is helping to prevent the tire from rotating on the rim.

Break in
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WHEEL MOUNTING TORQUE
For wheel mounting torque, make certain the wheel studs are
clean and dry, and torqued to the manufacturer’s specifications.
CAUTION: Most vehicle manufacturers also specify that the wheel
studs/nuts should not be lubricated with oil or grease. The use of a
lubricant on the threads could cause the lug nuts to loosen.

Always tighten lug nuts gradually in a star
pattern (tighten one nut, skip one, tighten
the next nut).
This helps prevent warping the brake
drums or rotors, or bending a wheel.
Figure 84–13 Always tighten wheel lug nuts (or studs) in a star
pattern to ensure even pressure on the axle flange, brake rotors or
drums, and the wheel itself.
Continued
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By James D. Halderman

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If exact torque value is not available, use the following chart as a
guide for usual value based on the size (diameter) of the lug studs.

See the chart on
Page 1040 of
your textbook.

When you install a brand new set of aluminum wheels, retorque
the wheels after the first 25 miles. Soft aluminum often compresses
slightly, loosening the torque on the wheels.

Continued
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