chap83 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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OBJECTIVES:
After studying Chapter 83, 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 tire sizes and ratings.
Describe tire purchasing considerations and maintenance.

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OBJECTIVES:
After studying Chapter 83, the reader should
be able to:

•
•

Explain the construction and sizing of steel and alloy wheels
and attaching hardware.
Demonstrate the correct lug nut tightening procedure and
torque.

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KEY TERMS:
aspect ratio
back spacing • bead • belt • body ply • bolt circle
carcass ply • center section • conicity
DOT tire code • E-metric tire • green tire

high-flotation tires • hydroplaning • inner liner
JWL • LLR • load index • lug nuts • major splice • offset
ply steer • rim width • run-flat tires
Continued
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KEY TERMS:
schrader valve • sidewall • speed ratings • spider
TPC • TPMS • tread • TREAD Act
unsprung weight • UTQGS
wear bars

Continued

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The friction (traction) between the tire and the road determines
the handling characteristics of any vehicle.
The compounding, construction, and condition of tires
are some of the most important aspects of steering,
suspension, alignment, and braking of any vehicle.
A vehicle that handles poorly, pulls, darts, jumps, or
steers “funny” may be suffering from defective or
worn tires.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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TIRES
Tires are mounted on wheels that are bolted to the vehicle to
provide the following:
1. Shock absorber action when driving over rough surfaces
2. Friction (traction) between the wheels and the road
Tires are available in many different designs and sizes.
Understanding the construction of a tire is important for the tech
to be able to identify tire failure or vehicle handling problems.
All tires are assembled by hand from many different component
parts consisting of various rubber compounds, steel, and various
types of fabric material.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition

By James D. Halderman

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PARTS OF A TIRE
Tread The part of the tire that contacts the ground is the tread.
Tread rubber is chemically different from other rubber parts of a
tire. It is compounded for a combination of traction and tire wear.
Tread depth is usually 11/32 in. deep on new tires (this could
vary, depending on manufacturer, from 9/32 to 15/32 in.).
Figure 83–1 shows a tread depth gauge.
NOTE: A tread depth is always expressed in 1/32nds of an inch, even if
the fraction can be reduced to 1/16ths or 1/8ths.

Continued
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Figure 83–1 (a) A typical tire tread depth gauge. The center movable plunger is pushed down into
the groove of the tire. (b) The tread depth is read at the top edge of the sleeve. In this example,
the tread depth is 6/32 in.

(a)

Wear indicators are also called wear bars.
When tread depth is down to the legal
limit of 2/32″, bald strips appear across
the tread.
(b)

See Figure 83–2.
Continued
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Figure 83–2 Wear indicators (wear bars) are strips of bald tread that show when the tread depth
is down to 2/32 in, the legal limit in many states.

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Tie bars are molded into the tread of most all-season-rated tires,
placed between tread blocks on outer tread rows to prevent
unusual wear and to reduce tread noise.
As the tire wears normally, the tie bars will gradually appear. This
should not be mistaken for an indication of excess outer edge
wear. A solid band across the entire width of the tread is what the
service technician should consider the wear bar indicator.
Grooves, called circumferential grooves or kerfs, are large, deep
recesses molded in the tread and separating the tread blocks.
Grooves running sideways across the tread lateral grooves.
See Figure 83–3.
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Figure 83–3 The tire tread runs around the circumference of the tire, and its pattern helps
maintain traction. The ribs provide grip, while the grooves direct any water on the road away
from the surface. The sipes help the tire grip the road.

Continued
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Grooves in both directions are necessary for wet traction. Trapped
water can actually cause the tires to ride up on a layer of water and
lose contact with the ground, called hydroplaning.
Sipes are small slits in the tread area to increase wet and dry
traction.
With worn tires, hydroplaning can occur at speeds as low as 30
mph on wet roads. Stopping and cornering is impossible when
hydroplaning occurs.
See Figure 83-4.

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Figure 83–4 Hydroplaning can occur at speeds as low as 30 mph (48 km/h). If the water is deep
enough and the tire tread cannot evacuate water through its grooves fast enough, the tire can
be lifted off the road surface by a layer of water. Hydroplaning occurs at lower speeds as the
tire becomes worn.

Continued
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Sidewall The part of the tire between the tread and the wheel is
called the sidewall. It contains all the size and construction details
of the tire.
Some tires turn brown on the sidewalls after a short time due to
ozone (atmosphere) damage, which causes the rubber to oxidize.
Premium-quality tires contain an anti-oxidizing chemical additive
blended with the sidewall rubber to prevent this discoloration.

Continued
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White Sidewall/Lettered On pneumatic tires constructed in the
early 1900s, natural rubber was used. The entire tire was white.
It was discovered that carbon black greatly increased the toughness
of a tire. The public did not like the change from white tires to
black, so manufacturers put the carbon black (lamp black) only in
the rubber that was to be used for the tread portion of the tire. This
tire lasted a lot longer because the black rubber tread was stronger
and tougher; it sold well because the sidewalls were white.
White sidewall or white lettered tires contain a strip of white
rubber under the black sidewall, ground off at the factory to reveal
the white rubber. Various widths of whitewalls are made possible
by changing the width of the grinding wheel.
Continued
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Bead The bead is the foundation of the tire and is located where
the tire grips the inside of the wheel rim.
1. The bead is constructed of many turns of copper- or
bronze-coated steel wire.
2. The main body plies (layers of material) are wrapped
around the bead.
3. Most radial-ply tires and all truck tires wrap the bead
with additional material to add strength.
CAUTION: If the tire bead is cut or damaged, the tire must be replaced!

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Body Ply A tire gets strength from the layers of material wrapped
around both beads under the tread and sidewall rubber. This creates the
framework, or “carcass,” of the tire; these body plies are often called
carcass plies.

A 4-ply tire has four
layers of material.
If body plies overlap at
an angle (bias), the tire
is called a bias-ply tire.
If one or two body
plies are used and lie
directly from bead to
bead, the tire is called
radial ply.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

Figure 83–5 Typical construction of a
radial tire. Some tires have only one body
ply, and some tires use more than two belt
plies.
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Belt A tire belt is two or more layers of material applied over the
body plies and under the tread area only, to stabilize the tread and
increase tread life and handling.
1. Belt material can consist of the following:
• Steel mesh
• Nylon
• Rayon

• Fiberglass
• Aramid

2. All radial tires are belted.
NOTE: Most tires rated for high speed use a nylon “overlay” or “cap
belt” between the 2-ply belt and the tread of the tire. This overlay helps
stabilize the belt package and helps hold the tire together at high speeds,
when centrifugal force acts to tear a tire apart.
Continued

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Inner Liner The soft rubber lining (usually a butyl rubber
compound), the inner liner, protects the body plies and helps
provide for self-sealing of small punctures.
Major Splice When the tire is assembled by a craftsperson on a
tire-building machine, the body plies, belts, and tread rubber are
spliced together.
The fabric is overlapped approximately five threads. The point
where the majority of these overlaps occur is called the major
splice, which represents the stiffest part of the tire. This major
splice is visible on most tires on the inside.
See Figure 83–6.

Continued
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Figure 83–6 The major splice of a tire can
often be seen and felt on the inside of the
tire. The person who assembles (builds)
the tire usually places a sticker near the
major splice as a means of identification
for quality control.

NOTE: On most new vehicles and/or new tires, the tire manufacturer
paints a dot on the sidewall near the bead, indicating the largest diameter

of the tire. The largest diameter of the tire usually is near the major splice.
The wheel manufacturer either marks the wheel or drills the valve core
hole at the smallest diameter of the wheel. The dot should be aligned with
the valve core or marked for best balance and minimum radial runout.
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TIRE MOLDING
After the tire has been assembled
by the tire builder, it is called a
green tire.
At this stage in construction,
the rubber can be returned and
reused because it has not been

changed chemically.
The completed green tire is
placed in a mold where its
shape, tread design, and all
sidewall markings are formed.

Continued

Figure 83–7 Tire construction is performed by
assembling the many parts of a tire together on a
tire-building machine.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Figure 83–8 After the entire tire has been assembled into a completed “green” tire, it is placed
into a tire-molding machine where the tire is molded into shape and the rubber is changed
chemically by the heat. This nonreversible chemical reaction is called vulcanization.

While in the mold, a steam
bladder fills the inside of the
tire and forces the tire against the
outside of the mold.

After 30 minutes at 300°F
(150°C), the heat changes the
chemistry of the rubber.
The tire is no longer called a
green tire but a cured tire, and
after inspection and cleaning, it
is ready for shipment.

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Why Do I Get Shocked By Static Electricity When I
Drive A Certain Vehicle?
Static electricity builds up in insulators due to friction of the tires with the road.
Newer tires use silica and contain less carbon black in the rubber, which makes the
tires electrically conductive. Because the tires cannot conduct the static electricity
to the ground, static electricity builds up inside the vehicle and is discharged
through the body of the driver and/or passenger whenever the metal door handle is
touched.
Newer tire sidewall designs that use silica usually incorporate carbon
sections that are used to discharge the static electricity to ground. To help
reduce the static charge buildup, spray the upholstery with an antistatic
spray available at discount and grocery stores.

NOTE: Many “temporary use only” spare tires are constructed with
diagonal (bias) plies; the size designation is T for temporary.
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METRIC DESIGNATION
European and Japanese tires use metric designations.
The European size indicates exact physical size (width) of the tire
and the speed ratings. European sizes also include the aspect
ratio, eg., 185/70SR x 14. If the aspect ratio of a European-sized
tire is not indicated, it is generally 83% for most radials.
In the chart, 185SR x 14 denotes the following:

See the tire charts on Page 1018 of your textbook.
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