Automotive mechanics (volume i)(part 4, chapter23)tyres, wheels and balance

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PART
Running gear

23 Tyres, wheels and balance
24 Brakes
25 Brake service
26 Antilock braking systems (ABS)
27 Suspension systems
28 Suspension service
29 Steering systems
30 Steering-system service
31 Wheel alignment

4


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387

Chapter 23

Tyres, wheels and balance

Tyre construction

Balancing wheels

Types of tyre construction

Technical terms

Characteristics of tyres

Review questions

Tyre profiles
Tyre sizes and identification

Tyre tread patterns
Australian Design Rules
Tyre wear
Tyre rotation
Wheels
Removing and fitting tyres
Light-truck wheels and tyres
Wheel balance


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388 part four running gear
Tyres provide a cushion between the road and the
vehicle to reduce road shocks. They also provide
friction with the road surface. During acceleration,
friction is in the form of traction to propel the vehicle
along the road. Friction also provides adhesion, which
opposes the tendency to skid on turns and allows quick
stops to be made during braking.

Tyre construction
The construction of a tyre can be seen in Figure 23.1.
The main body of the tyre is referred to as the casing. This is made of a fabric that has layers of cords

impregnated with rubber. Over this is applied the
rubber for the sidewalls and tread.
During manufacture, the fabric is formed over a
moulding device and rubberised, the sidewall and tread
materials are then applied, and the partly manufactured
tyre is subjected to a process which heats the rubber
under pressure within a metal mould. This processes
the rubber and also forms the desired shape of tyre and
tread.
The rubber used in tyres is mainly synthetic, being
manufactured from the products of the coal and
petroleum industries. Carbon is added to increase the
toughness and strength of the rubber, particularly for
the treads.

figure 23.1

Tyre construction

Development of higher strength cords such as
rayon, nylon, polyester and steel enabled the number
of plies in a tyre to be reduced, while the same tyre
strength was maintained. Therefore, a passenger car
tyre could be constructed with two plies for the
sidewalls and four plies under the tread.
With the use of these stronger cords, the method of
tyre marking was adjusted and the word rating was
added so that a tyre was given a 4 ply rating or 6 ply
rating etc. This does not state the actual number of
plies used, but says that the tyre is equal in strength to

a tyre with that number of cotton plies. A passenger
car tyre could have a four-ply rating, while a heavyduty truck tyre could have a fourteen-ply rating.
■ Light-truck tyres usually have a specified ply
rating; passenger-car tyres have speed and load
ratings.
Tubed and tubeless tyres
A tubed tyre has a separate inner tube which is fitted
into the tyre when it is mounted to the tyre rim. When
inflated with air, the tube maintains the shape of the
tyre under the load of the vehicle.
Tubeless tyres do not have a separate inner tube.
The tyre has a soft rubber lining on the inside of its
casing, and beads that fit tightly against the wheel rim
to form a seal. When the tyre is inflated, air is retained
between the wheel and the tyre casing. Most tyres are
tubeless.
A section of a tubeless tyre, mounted on a rim, is
shown in Figure 23.2 with the parts of the tyre
identified. It also includes the air valve, which fits

FORD

Plies and ply rating
The layers of cords in the fabric of the casing are
referred to as plies. Tyres were originally made with
cotton cords and the tyres were marked 4 ply or 6 ply,
etc to indicate the number of cotton plies used in the
particular tyre. The number of plies indicated the
strength of the tyre, and varied according to the type of
vehicle on which the tyre was used.

figure 23.2

Sectional view of a tubeless tyre and rim


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chapter twenty-three tyres, wheels and balance

tightly in a hole in the rim. Passenger cars and light
commercial vehicles are fitted with tubeless tyres,
although a tube can be used with a tyre or rim that will
not hold air.

Types of tyre construction

389

material between the casing and the tread as shown in
Figure 23.4. The belt, consisting of two or more layers of
cords, runs around the circumference of the casing.
The belt provides a firm base to support the tread
and also hold the tread against the effects of centrifugal

force, which are present when the tyre is rotating.

As far as construction is concerned, there are two basic
types of tyres: bias or cross-ply and radial. The difference
is the way in which the cords in the plies are arranged.
Bias or cross-ply tyres
Bias, or cross-ply, tyres have cords that run diagonally in
alternate layers (Figure 23.3(a)). The terms bias or biasply are used because the cord fabric from which the plies
are made is cut on a bias (at an angle) to the cords. The
term cross-ply is used because the cords cross at an angle.
Tyres of this design have stiffer sidewalls than radial tyres
because of the cross-bracing effect of the diagonal cords.
Radial tyres
Radial tyres are the type most commonly used. The cords
in the casing of the tyre run in a radial direction, that is,
straight across the tyre section without crossing (Figure
23.3(b)). Radial tyres also have a belt of reinforcing

figure 23.4

A radial tyre has a belt of cords around its
circumference beneath the tread GOODRICH

Steel-belted tyres
Steel-belted tyres have cords of steel wire built into the
reinforcing belt beneath the tread. The steel provides a
strong belt that does not stretch under inflation
pressure. This gives the tread area greater stability and
still allows flexible sidewalls.
Run flat tyres

Several manufacturers have developed systems that
allow the vehicle to be driven safely at restricted speeds
with low or no tyre pressure. There are two types:
1. reinforced tyre sidewall shown in Figure 23.5(a)
2. rubber clip inside the rim shown in Figure 23.5(b).
Both systems operate in conjunction with tyre
pressure monitors.
(a)

tyre without air

(b)

sidewalls
with newly
developed
compound

normal tyre

rubber clip
wheel

figure 23.5
figure 23.3

Tyre construction

GOODRICH

Run flat tyres
(a) reinforced sidewall (b) rubber clip inside rim


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390 part four running gear

Characteristics of tyres
Bias tyres and radial tyres have different characteristics. Characteristics are the things that are built into
the particular type of tyre, such as flexibility of the
sidewalls and stiffness of the tread. They also relate to
the way the tyre performs on the road.
With radial-ply construction, a tyre could have two
plies of cords in the sidewalls and four plies under the
tread. This gives the radial tyre flexible sidewalls and a
stable base for the tread. Radial tyres have less
distortion than bias tyres because the radial cords are
under tension to hold the tread in place.
A radial tyre behaves differently on curves to a bias
tyre. When a car is travelling around a curve in the road,
centrifugal force pulls it towards the outside of the
curve. This deflects the tyre as shown in Figure 23.6.
A sidewall of a radial tyre bends easily in the direction of the force to allow the tread to remain on the road.

A bias tyre, with stiffer sidewalls, tends to lift the tread.
When a vehicle is being driven around a curve in a
road, the tyres do not follow the exact direction in
which they are pointed. Centrifugal force on the vehicle
produces a side thrust on the wheel, and this deflects
the tyre, forcing it to follow a slightly wider curve.
Figure 23.6(b) shows this as the slip angle, and this
is influenced by the speed, load, inflation pressure and

the characteristics of the tyre. Radial tyres have a
lower slip angle than bias tyres, and so they will follow
a tighter curve.
■ Because of the characteristics of different types of
tyres, it is essential that all four wheels are fitted
with tyres of the same type, and preferably of the
same make.

Tyre profiles
The tyre profile is the shape of the tyre section when it
is mounted on the rim of the wheel. Tyres are
produced in a number of different profiles, some are
fairly round, others are much flatter.
The aspect ratio of a tyre is the height/width ratio
of the tyre section (Figure 23.7). This is a way of
measuring and comparing the profiles of tyres. The
aspect ratio is always less than one (for example, 0.80),
but the decimal point is usually omitted. The aspect

figure 23.7

figure 23.6

Characteristics of tyres
(a) distortion of a bias-ply tyre and a radial-ply
tyre on a curve (b) slip angles GOODRICH

Tyre profiles
(a) section through a low-profile tyre – height
B is 80% of width A for the tyre shown (b) examples of tyre
profiles and aspect ratios GOODRICH


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ratio is sometimes referred to as a percentage, but the
percentage sign is often omitted.
Tyres are made with a number of different aspect
ratios, such as 80, 75, 70, 65 and 60. For passenger
cars, the trend is towards squatter and wider tyres with
very low aspect ratios. Figure 23.7(b) compares three
different tyre profiles. Tyres are made with aspect
ratios of 80 and 60, but not with 100.

391

tyre name
tubeless
rim width code

country of
manufacture

Tyre footprint
The area of tread pattern in contact with the road
is referred to as the tyre footprint. When the vehicle is
stationary, or moving in a straight-ahead direction, the
footprint will be of uniform shape. Generally, radial tyres
will have a wider footprint than equivalent cross-ply tyres.
A low profile tyre will have a shorter and wider
footprint than a high profile tyre of similar size and
load-carrying capacity, but the areas of the footprints
will be about the same (Figure 23.8). Changes in the
footprint will occur as the result of vehicle load, speed,
cornering and tyre pressure.

ply rating and load

tyre construction

tyre size

maker’s name

figure 23.9

Information on the tyre sidewall (the tyre is
fitted to an aluminium wheel) DUNLOP

M+S indicates a tread suitable for mud and snow. The
type of cord material may be identified by the word
rayon, nylon, polyester or steel.
DOT appears on some tyres. This stands for
Department of Transport (USA) and shows that the
tyre conforms with standards set by that country.
Radial tyre sizes
For passenger-type radial tyres, two systems of sizes
are in common use: metric and P-metric. The metric
system originated in Europe and the P-metric in USA.
figure 23.8

Tyre footprints on the road surface for
different aspect ratios – L is contact patch
length, W is contact patch width

Metric tyres
The following is an example of a metric marking for a
tyre and what it means:
205/65 R16 92H

Tyre sizes and identification
Information about a tyre is moulded into the sidewall.
This can include the manufacturer’s name, the name of

the tyre, the tyre size, the type of construction, the load
index, the speed category and country of manufacture
(Figure 23.9).
Some light truck tyres are shown by LT, and
special types of tread may be identified: for example,

1. 205 – is the nominal section width of the tyre in
millimetres
2. 65 – is the aspect ratio
3. R – shows that it is a radial tyre
4. 16 – is the rim diameter in inches
5. 92 – is the load index number
6. H – is the speed category symbol.


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392 part four running gear
P-metric tyres

Bias-ply tyres

P-metric tyres are marked in the same way as metric
tyres, but the prefix ‘P’ is added. Following is an

example of a P-metric tyre marking:

Bias-ply tyres have their size shown in inches, such as
6.00–14, or 6.70–14. This shows that the width of the
tyre section is 6 inches and the diameter of the rim is
14 inches. The ply rating is usually specified for lighttruck bias tyres and this can be 6 or 8 ply.

P175/65 R14 81H.

This is interpreted as follows:
1. P – stands for passenger car

Tyre tread patterns

2. 175 – shows that the tyre section is 175 mm wide

Tyre treads are made in various patterns designed to
suit different vehicles and operating conditions. These
range from passenger cars operating at high speeds on
constructed roads to tractors and earthmoving equipment operating under rough conditions.

3. 65 – is the aspect ratio, which indicates that its
height is 65% of its width
4. R – shows that it is a radial tyre
5. 14 – is the diameter of the wheel
6. 81 – shows that the maximum load for the tyre is
462 kg
7. H – means that the tyre has a speed rating of
210 km/h.
The load index and speed category are referred to

as the tyre description. The load index is a numerical
code for the maximum load that the tyre can carry; the
speed category symbol is an alpha code for the speed
rating of the tyre.
Tables 23.1 and 23.2 show some of the more
common load indexes and speed categories for
passenger cars and light commercial vehicles.
table 23.1

The types of treads that are used on passenger cars are
shown in Figure 23.10. These are designed to provide the
necessary friction with the road surface for wet or dry
conditions, during acceleration, braking and steering.
On wet roads, the various parts of the tread act like a
squeegee to push the water away, so that the tread can
grip the road surface. Where there are pools of water on

Load indexes for tyres

LOAD INDEX

KILOGRAMS

80
82
86
88
91
95
98

100

450
475
515
560
615
690
750
800

table 23.2

Passenger car tyres

(a)

(b)

(c)

(d)

Speed category symbols

SPEED CATEGORY SYMBOL

SPEED KM/H

P

Q
R
S
T
U
H
V

150
160
170
180
190
200
210
240

figure 23.10

Types of treads for passenger vehicle tyres
(a) pattern for normal use (b) lower profile and
more open tread (c) low-profile high-performance tyre
(d) wide tyre with a lower profile (directional) DUNLOP


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chapter twenty-three tyres, wheels and balance

the road, the main tread pattern is designed to force
water to the rear through the grooves in the tread and
also to push water aside. Small cuts or slots in the tread
help to squeeze out water that remains on the road
surface, so that the tyre runs on a relatively dry surface.
Apart from friction between the tyre and road being
reduced by wet road conditions, a tyre can actually
aquaplane on water that is lying on the road. This is
most likely to occur if the tyre is badly worn or if the
tyre is subjected to severe braking. Under these
conditions, a wedge of water under the tyre causes it to
lift from the road surface. Tyre tread patterns are
designed to minimise this possibility.
Light truck and four-wheel drive tyres
There are three general designs of tyres for light trucks
and four-wheel-drive vehicles. The designs in this
case relate to where the tyres will be used. Tyres are
designed for: on-road use; on-road and off-road use;
and off-road use. Figure 23.11 shows three tyres with
these different types of tread patterns.
For on-road use, some tyres are similar to passenger
car tyres, but of heavier construction and with a higher
load index. Others tyres are more rugged and these
have heavier treads and more plies under the tread.
For both on-road and off-road use, four-wheel

drive vehicles can be fitted with tyres that have a wide

(a)
figure 23.11

393

tyre section to give a large footprint. These have treads
that are reasonably quiet when used on roads, but
which are suitable for most types of terrain.
For off-road use, more rugged tyres with open treads
are used. The treads can have lugs or bars. Depending on
the purpose for which the tyre will be used, the lugs can
form the tread pattern, or they can extend right across the
tyre so that the tread consists of bars and grooves.

Australian Design Rules
Australian Design Rules (ADRs) are government regulations that cover a number of safety and associated
items for motor vehicles. The rules that relate to
wheels and tyres for passenger vehicles deal with
safety rims for wheels, and standards of strength,
construction, pressure, load, and tyre fitment.
The rules apply to the manufacture of vehicles, but
they are important when replacing tyres or wheels on
passenger vehicles because the replacements must
conform to the rules.
ADR 23 specifies standards of strength, construction,
pressure and loads for tyres. This enables manufacturers
to select appropriate tyres for their vehicles.
ADR 24 concerns the tyres fitted to a vehicle, in

relation to vehicle load capacity, rim size and tyre speed
characteristics. It requires a placard to be permanently

(b)
Light truck tyres
(a) normal on-road use (b) on-road and off-road use (c) mainly off-road use

(c)
DUNLOP


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table 23.3

Tyre placard
TYRE SPECIFICATIONS & PRESSURES

LOCATION

FRONT

WHEEL RIM PROFILE

REAR

FRONT

REAR

5.5 J

5.5 JJ

RECOMMENDED TYRE SIZES

P175/65 R14 81H

P/175/65 R14 81H

RECOMMENDED PRESSURES
TO MAX LOAD
HIGH SPEED

190 kPa
28 p.s.i.

190 kPa
28 p.s.i.

The tyres fitted to this vehicle shall have a maximum load rating not less than 425 kg or a load index of 78 and speed rating not less than T

fixed inside the glovebox door, or other accessible

location, that shows the rims and tyres that can be fitted
to the vehicle; the maximum load and speed ratings of
the tyres; and the recommended tyre pressures.
Table 23.3 is an example of a tyre placard.
■ The rules apply to replacement tyres and wheels as
well as to original equipment. Replacements must
be in accordance with the tyre and rim combinations specified on the placard.
Tyre inflation
Tyres should be inflated to the pressure recommended
by the vehicle manufacturer or as shown on the tyre
placard attached to the vehicle. Tyres should be checked
regularly and inflated to the recommended pressure.
Incorrect pressures can cause tyre wear as well as
steering and braking problems. Figure 23.12 shows
how correct and incorrect inflation affects tread
contact with the road surface.
Correct inflation holds the tyre in its correct shape
with the tread in full contact with the road surface. The
tyre will act in the way that it has been designed and
there will be normal tyre wear.
Underinflation allows the tyre to become flatter in
shape. The edges of the tread will be in heavy contact

with the road, but the centre will carry less load and will
wear less than the edges. A tyre with low pressure can
cause heavy steering and possible tyre squeal on corners.
Low pressure also increases the rolling resistance of
the vehicle and therefore the energy required to drive
it. Fuel economy will also be reduced with low tyre
pressures.

Overinflation gives a hard ride and subjects the tyre
to road shocks because the tyre does not flex normally.
Uneven tyre pressures, particularly on the front
wheels, tend to steer the vehicle to one side. The centre
of the tread is in heavy contact with the road and will
wear more than the edges.
Tyre pressure monitors
The tyre pressure can be monitored electronically using a
device fitted around the rim inside the tyre or in the tyre
inflation valve. Some manufacturers fit them as standard
equipment or they can be fitted as an accessory.
■ The use of nitrogen gas for tyre inflation is
becoming more common in automotive applications.
It has been used in aircraft and racing tyres for
many years. Nitrogen leaks through the rubber three
times slower than oxygen and therefore maintains
the tyre at the correct pressure longer.

Tyre wear

figure 23.12

Effects of correct and incorrect inflation on
the contact that the tread makes on the road
surface

Tread wear indicators are moulded into the tread pattern
of passenger car tyres. The indicators show up as plain
bars across the tread when only 1.5 mm of tread remains.
The tyre should then be replaced (Figure 23.13).

There are many factors that cause tyre wear, and
some of these are quite normal. However, where
abnormal wear occurs, the type of wear can indicate
the likely cause of the problem.


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395

The tyre will not flex as much as it should and the
fabric will receive shock loads which, combined with
the high tyre pressure, can cause tyre failure.
Wheel camber

figure 23.13

Tyre tread-wear indicator

MITSUBISHI

When viewed from the front, the front wheels of a

vehicle are not vertical, but lean out a little at the top.
This angle from the vertical is referred to as camber.
If a wheel has excessive camber, the outer edge of the
tyre will wear more rapidly than the inner edge
(Figure 23.15(a)). This occurs because the face of the
tread does not sit flat on the road.

Underinflation
If a tyre is underinflated, the sides of the tyre will
bulge and this tends to lift the centre of the tread away
from the road. The load will be concentrated on the
outer edges of the tread, causing excessive wear
(Figure 23.14(a)). The centre of the tread will carry a
reduced load and so will have less wear.
Underinflation also allows greater flexing of the
tyre sidewalls, imposing higher loads on the cords.
Also, excessive heat is generated and this increases the
temperature of the tyre. High temperatures soften the
rubber compounds and allow the plies or tread to
separate from the casing.

figure 23.15

Tyre wear due to wheel problems
(a) Wear from excess camber (b) wear due to
excessive toe-in or toe-out MAZDA

Wheel alignment

figure 23.14

overinflation

Tyre wear due to incorrect inflation
(a) wear from underinflation (b) wear from
MAZDA

An underinflated tyre could suffer rim damage to the
sidewalls. If the tyre strikes a kerb or a hole in the road,
the sidewalls could be pinched against the rim. This
could cause damage to the tyre and cause it to deflate, or
break some of the cords and cause early tyre failure.
Overinflation
An overinflated tyre has a reduced tread area in contact
with the road surface. This increases the load on the
centre of the tread so that it wears much more quickly
than the sides of the tread (Figure 23.14(b)).

Front wheels are provided with a small amount of
toe-in or toe-out, but if this is excessive, the tyre will
tend to drag sideways while it is being moved forwards.
A characteristic of this type of wear is the feather edges
that appear on one side of the tread (Figure 23.15(b)).
Front wheels also have toe-out on turns. When cornering, the inside wheel is turned a greater angle than the
outer one. Incorrect angles can cause the tyres to scrub.
Cornering and high-speed
Tyre wear, caused by taking corners at too high a
speed, can be similar to camber wear or toe-in or toeout wear.
When cornering, centrifugal force on the vehicle is
resisted by the tyres on the road surface. At high speed,
some side slip of the tyre occurs and this produces a

diagonal type of wear which rounds the outer shoulder
of the tyre. In severe cases, fins or sharp edges can be
found on the inner edges of the treads (Figure 23.16).
In general, tyres can have a faster rate of wear at high
speeds because of higher temperatures, greater scuffing
and more rapid flexing to which the tyres are subjected.


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396 part four running gear

figure 23.18

figure 23.16

Wear due to high-speed cornering

Mechanical conditions
Tread wear that is uneven, or in spots, can be caused
by a number of mechanical conditions (Figure 23.17).
These include misaligned wheels, out-of-balance
wheels, defective brakes, inoperative shock absorbers
and loose or worn steering or suspension parts.

figure 23.17

Uneven tread wear due to mechanical faults,
unbalance, or tread separation

Tyre rotation
Tyres do not wear uniformly on all four wheels. The
rate of wear and also the type of wear will vary
according to the location of the tyre on the vehicle.
To equalise wear on all tyres, the wheels and tyres
are rotated (interchanged) at regular intervals, or when
wear is noticeable. Figure 23.18 shows three different
sequences of tyre rotation.

Three different sequences of tyre rotation
(a) tyres on same side (b) using spare
(c) diagonal rotation

The spare can be part of a normal sequence of
rotation, or only introduced if one of the tyres has
damage or abnormal wear. As far as possible, the
spare should be paired with a tyre which has a similar
amount of tread.
Reasons for tyre rotation
Front tyres have a wear pattern that is different to rear
tyres. Tyres also wear differently depending on
whether they are fitted to a front-wheel-drive or a rearwheel-drive vehicle.
With rear-wheel drive, the front tyres are likely
to have more wear on the edges of the tread due to

steering; while the rear tyres could have more wear on
the centre of the tread because they are used for driving.
With front-wheel drive, the front tyres are likely to
wear faster than the rear tyres because they are used
for both driving and steering. They also carry more of
the vehicle load than the rear tyres.
There are various recommended sequences of
rotation. Figure 23.19 shows a sequence for front-wheeldrive vehicles and a different one for rear-wheel-drive
and four-wheel-drive vehicles. The different sequences

1. Four wheels, same side. The wheels are moved
from front to rear on the same side of the vehicle.
This evens out tyre wear while keeping the same
direction of rotation (Figure 23.18(a)).
2. Five wheel sequence. Where all the tyres are in
good condition, the spare wheel can be introduced
so that wear is evenly distributed over five tyres
(Figure 23.18(b)).
3. Four wheels, diagonal. Diagonal rotation changes
the wear pattern of the tyres and also changes the
direction of rotation (Figure 23.18(c)).

figure 23.19

Tyre and wheel rotation
[a) for front-wheel drive (b) for rear-wheel and
four-wheel drive


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chapter twenty-three tyres, wheels and balance

take into account that the wear patterns of the tyres of
front-wheel drive and rear-wheel drive are different.

Wheels
Passenger car wheels are either pressed steel or cast
aluminium-alloy construction. Wheels consist of two
main parts: the flange (or disc) and the rim. Steel wheels
have the rim formed separately from the flange, the two
parts are then welded together to form the wheel.
Ventilation holes in the flange allow air circulation to
help dissipate heat from the brakes and associated parts.
Aluminium alloy wheels are sometimes used for
appearance, but also because aluminium and its alloys
are not as heavy as steel. Wheels can be cast in aluminium alloy with much thicker sections than steel wheels.
This provides stiffness and enables the stresses to be
distributed over a wider area. Also, aluminium alloy is
a good conductor of heat and this helps to dissipate
some of the heat generated by the brakes and tyres.

397

has a rim of 5.5 JJ. The 5.5 represents the rim width in
inches, and JJ is a code for a particular rim profile. Any
replacement wheel should have the same sized rim as the
original, or should comply with the rim size shown on
the tyre placard attached to the vehicle (see Table 23.3).
The width of the rim will determine the spread of
the beads of the tyre when it is mounted, so it must be
correct for the size of tyre. If the rim is too narrow, the
width of the tyre section will be reduced and the point
where the tyre should normally flex will be closer to
the tread. If the rim is too wide, the tyre will spread
and the flex point will move closer to the rim.
In both cases, the tyres will produce more heat than
they should. Also, the contact between the tread and
road will be altered.
An oversize tyre may not seat correctly; the bead
area could be distorted and not seal properly against
the rim. If the tyre has a tube, then the tube will be
overstretched around the beads of the tyre and could
deteriorate much sooner than it normally would.

Wheel rims

Rim offset

Rims for passenger cars are of the drop-centre or wellbase type. With these wheels, the centre of the rim is
‘dropped’, or reduced in diameter, to form a well which
enables the tyre to be removed and replaced. A rim of
this type is shown in Figure 23.20. The rim also has
humps which hold the bead of the tyre in position

against the edge of the wheel rim if a tyre fails.

The flange of the wheel is not located in the centre of the
rim, but is closer to the outside. This gives the rim offset.
The amount of offset is the distance from the
mounting surface of the flange to the centre of the rim.
Altering the offset will affect the steering geometry of
the vehicle. A replacement wheel should have the
same offset as the original and comply with the rim
size shown on the vehicle placard.
Installing wheels
Wheel nuts have a taper which fits into a corresponding tapered hole in the wheel. When the nuts are
tightened, the tapers are responsible for centring the
wheel on the hub.
Wheel nuts or bolts should be tightened to the
specified torque. To ensure that the wheels are seating
correctly and not distorted, they should be tightened in
the correct sequence, as shown in Figure 23.21.
Wheel covers and caps

figure 23.20

Wheel safety rims

Rim size
Rims are designated by their width and their profile
(shape). For example, a common passenger car wheel

Wheel covers and caps of various designs are fitted to
the wheels of passenger vehicles. Figure 23.22 shows

a steel wheel and an aluminium alloy wheel. The steel
wheel has a plastic cover which covers the complete
wheel. It is held to the wheel by lugs and a large
spring ring.
The alloy wheel is designed to be seen, but has a
wheel cap which covers only the wheel nuts and hub.


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398 part four running gear

Removing and fitting tyres

figure 23.21

Sequence of tightening wheel nuts

Tyres are removed from wheels and also fitted to
wheels with a tyre-changing machine (Figure 23.23).
The wheel with its deflated tyre rests on the bed of the
machine which uses air- or electrically-operated tools
to remove the tyre from the wheel.
A flat-ended bead-breaking tool is used first to

force (break) the beads of the tyre away from the rim
as shown in the illustration. A lever or removing tool is
then used to lever or roll the bead of the tyre off the
wheel as the wheel is rotated.
While one side of the bead is being removed, the
bead on the opposite side must be pushed hard into
the well of the wheel. The tyre cannot be stretched
over the rim and, unless the bead is broken away from
the edge of the rim and held into the wheel well, the
tyre cannot be removed.

figure 23.22

Wheels
(a) 15 inch steel wheel (b) 16 inch alloy wheel

figure 23.23

HOLDEN LTD

A wheel mounted on a tyre-changing
machine EAGLE SMF


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chapter twenty-three tyres, wheels and balance

Fitting tyres
Some points about fitting tyres are:
1. Before fitting a tyre, all dirt or rust should be
removed from the wheel rim.
2. The wheel is placed on the machine, and the beads
are rolled over the wheel rims. A coating of vegetable oil or soapy water will assist in fitting the tyre.
3. Red dots on the tyre and rim should be aligned (see
‘Balance marks’ below).
4. A tubeless tyre must be inflated quickly to force the
beads on to the rim. The valve core can be removed
temporarily to assist the flow of air.
5. When inflated to the correct pressure, the tyre is
checked to see that the beads are seating correctly.
This is done by checking the bead indicator in
relation to rim of the wheel.
The bead indicator is a moulded ring on the
sidewall of the tyre. When the tyre is correctly
installed, the bead indicator should be concentric
with the edge of the rim.

399

used at the rear of the vehicle to provide greater loadcarrying capacity.
Dual wheels are mounted back-to-back. The rims of
the wheels are offset, so that when they are mounted
with their flanges together there will still be space

between the tyres. Adequate spacing of dual wheels is
necessary to prevent the tyres from rubbing together
where they bulge on the ground, and also for adequate
air circulation around the tyres.
Some light truck wheels have flat-base rims, with
one edge of the rim detachable to allow the tyre to be
removed. This is usually in the form of a split ring
which fits into a groove in the rim (Figure 23.24).
There are variations in wheel design. Wheels are
made with a removable rim edge and a separate lock
ring. There are also two-piece wheels, made of two
steel pressings held together by a ring of bolts. The
bolts are removed and the halves of the wheel are
separated to remove or replace the tyre.

6. To test for leaks, the complete wheel can be immersed
in water. Any leaks will be shown by air bubbles.
■ Alloy wheels are made of relatively soft material
and must be treated carefully to avoid damage.
Balance marks
Tyres and rims are marked during manufacture (point 3
above) to identify positions for balance. This allows the
high harmonic point (heavy spot) of the tyre to be
indexed with the low harmonic point (light spot) of the
wheel.
A tyre manufactured in Australia will have a red
dot at the heavy spot. The flange of the wheel will
have a coloured dot and a drill mark to identify the
light spot. These marks should be aligned when the
tyre is fitted to the wheel.

Some imported tyres have a white dot for the light
spot and this is placed 180° from the mark on the wheel.
A yellow spot can represent the static balance point.
■ For practical purposes, the high and low harmonic
points can be considered as the heavy and light
spots of the tyre.

Light-truck wheels and tyres
Wheels used on commercial vehicles are larger and
more robust than those on passenger cars. Single
wheels are used on the front, but dual wheels are often

figure 23.24

Light-truck wheel with a detachable ring

Removing and fitting truck tyres
Tyre-changing machines, similar to those used for
passenger cars but more robust, are used for truck tyres.
Tyres that have not been removed for some time can be
difficult to remove, as they will be stuck to the rim.
When working with truck tyres, certain safety
precautions should be observed. Serious injury and
even death have been caused by a badly fitted ring
blowing off a wheel rim during inflation.
Truck tyres are placed in a steel cage during
inflation as a safety precaution in case the ring should
blow off. Alternatively, when inflating the tyre, place
the wheel on the floor with the detachable-ring side
downwards, and access the valve through the hole in

the centre of the wheel. If the ring is badly fitted, do


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400 part four running gear
not attempt to hammer it into place; the tyre should be
deflated and the ring repositioned on the wheel.
Positioning tyres on dual wheels
For reasons of wear and safety, new tyres are usually
fitted to the front wheels, then moved to the rear
wheels after about one-third of the tread has worn
away. However, front-wheel tyres tend to wear
unevenly so, to even out the wear, the tyres can be
changed from side to side.
When tyres are moved to the rear, they must be
matched up with other tyres of the same height. If tyre
wear appears normal, they may be rotated in a
sequence such as shown in Figure 23.25.

Measuring tyres for dual wheels
Where the wheel is off the vehicle, the circumference
of the tyre can be measured by wrapping a flexible
tape around the tyre. The measurement can then be

compared with the tyre to which it is being matched.
■ The tyre should be mounted on the wheel and
inflated before a measurement is taken.
Where the dual wheels are on the vehicle, a cord
line can be used to quickly compare the size of all four
tyres. The cord held across the tyres at about axle
height will make contact with the tread on all four
tyres if they are the same size. If the inner tyre is
shown to be smaller, it is in its correct position; the
distance between the cord and the tyre should be
approximately 4 mm or less.

Wheel balance

figure 23.25

Tyre rotation for the dual rear wheels of a
light truck

Matching dual-wheel tyres
The tyres fitted to dual wheels must be evenly matched
for size. Mismatched tyres cause uneven wear to both
the larger and the smaller tyre. The larger tyre accepts
a greater share of the load, and as a result, it wears
more. It also flexes more, making it susceptible to
blowouts. The smaller tyre carries little or no load, and
tends to scuff across the surface of the road and wear.
For light trucks, the allowable difference in size
between the outer and inner tyres, when mounted on
the wheels, is generally about 20 to 25 mm. This is

measured around the tyre circumference.
When fitting duals, the larger tyre is usually fitted
to the outside wheel to compensate for the camber of
the road. The larger tyre can also be inflated to a
higher pressure than the smaller tyre to give it a better
share of the load.

Wheels and tyres need to be in balance to prevent
vibration and to minimise wear of tyres, suspension
components and steering parts. Correct wheel balance
is important for front wheels, where unbalance could
cause steering problems.
When a wheel rotates, centrifugal force acts on
each part of the wheel and tyre and the effect is like a
number of forces pulling outwards against the tyre.
These forces become stronger as the rotational speed
of the wheel increases.
Where the mass is evenly distributed around the
wheel and tyre, all the forces will be equal. The wheel
and tyre are balanced, so centrifugal force has no
obvious effect (Figure 23.26 left).
However, if the tyre has a heavy spot, it will be unbalanced and the effect of centrifugal force will be greatest
at the heavy spot (Figure 23.26 right). There will be a

figure 23.26

Centrifugal force acting on a balanced and an
unbalanced wheel

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chapter twenty-three tyres, wheels and balance

strong pulling force which rotates with the tyre and this
will pull the wheel up and down or from side to side.
Effects of unbalance
There are two types of balance (and unbalance): static
and dynamic. The effects of both types are shown in
Figure 23.27. Unbalance in both cases is caused by
heavy spots on the tyre, but the position of the heavy
spots will determine whether the tyre has static
unbalance or dynamic unbalance.

401

also try to pull the wheel to the front and back, but this
is prevented by the suspension.
The term harmonics is used in relation to vibration
and unbalance. A single out of balance spot on a tyre
would produce a vibration, referred to as the first
harmonic. A second heavy spot would produce a
second harmonic and so on. In simple terms,
harmonics can be considered to be vibrations caused

by a heavy spot on a tyre.
Dynamic unbalance

figure 23.27

Effects of static and dynamic unbalance on a
wheel

The tyre in Figure 23.28(b) has a heavy spot on its
side. With front wheels, the force will pull the tyre and
try to turn the wheel on its steering axis.
As the wheel rotates, it will be pulled from side to
side because the force changes its direction with each
half-turn of the wheel. It pulls the front of the wheel and
then the back to produce a side-to-side movement of the
wheel, which is referred to as wheel wobble or shimmy.
The effect of the force is noticeable only at the front
and at the rear of the wheel, where the steering allows
the wheel to pivot from side to side. When the forces
are at the top or bottom, the wheel is held and so has no
side-to-side movement, although it can have tramp.
Couple

A heavy spot in the centre of the tread will produce
static unbalance and cause the wheel to move up and
down.
A heavy spot, or spots, on the side of the tyre will
produce dynamic unbalance and cause the wheel to
move from side to side. The difference between the
two types of unbalance is shown in Figure 23.28.

The tyre in Figure 23.29 has two equal heavy spots
which are diagonally opposite. The wheel is in static

■ The term imbalance is sometimes used. This is the
same as unbalance.
Static balance
A wheel (and tyre) that is in static balance and free to
rotate on its axle will remain in any position to which it
is turned. If the wheel is out of balance, the heavy spot
will rotate the wheel until it comes to rest at the bottom.
In Figure 23.28(a), the heavy spot in the centre of
the tread causes static unbalance. This can be corrected
by fitting a weight to the wheel rim directly opposite
the heavy spot. Two weights are needed, each equal to
half the mass of the heavy spot. A single weight could
affect the dynamic balance.
Without balance weights, the wheel would tend to
be pulled up and down as it rotates. This is referred to
as tramp. The force that rotates with the wheel would

figure 23.28

Difference between static and dynamic
balance – corrective weights have been added


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402 part four running gear

figure 23.31

Correction for dynamic unbalance
(a) heavy spot causes wheel shimmy
(b) corrective weights added

figure 23.29

Couple – the wheel is in static balance but
weights are needed for dynamic balance

balance because one heavy spot balances the other.
However, while the wheel has static balance, it has
dynamic unbalance.
When this wheel (without the weights) is rotating,
centrifugal force acts on both heavy spots. A force will
pull the front of the tyre and a force will also pull the
rear of the tyre. Together, they form a couple which
produces shimmy. The balance weights which are
added give the wheel dynamic balance.
Correcting unbalance
Balance weights are fitted to the rim of the wheel to
balance the heavy spot, or spots. Figure 23.30 shows a

wheel with static unbalance. Two balance weights are
used to correct it so that the mass is evenly distributed
on the wheel. Figure 23.31 illustrates a wheel that has
dynamic unbalance. A weight is needed for correction.
When in dynamic balance, the mass is distributed
evenly on each side of the wheel centreline.

Correction of static unbalance
(a) heavy spot causes wheel tramp (b) corrective weights added

While some examples of correction are shown, in
most instances it is not a case of either static or
dynamic balance. Most tyre and wheel assemblies that
need balancing require a combination of static and
dynamic balancing.

Balancing wheels
There are two general designs of wheel-balancing
machines. One type spins the wheel with it off the
vehicle, and the other spins the wheel while it is on
the vehicle. They both use the vibrations that occur
when the wheel is rotating to locate any unbalance in
the wheel and tyre. They check both static and
dynamic balance.
Off-vehicle wheel balancers
These are floor-mounted machines which spin the
wheel after it is removed from the vehicle. The wheel
is mounted on the spindle of the machine and spun by
an electric motor (Figure 23.32).

figure 23.30

figure 23.32

Arrangement of an off-vehicle wheel balancer
with direct-reading instruments


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chapter twenty-three tyres, wheels and balance

Balancers of this design have direct-reading instruments. With a single spin of the wheel, the machine
will give readings for both static and dynamic balance.
The instruments show the balance weights required
and where they should be fitted to the wheel.
Basically, machines of this type consist of a lightly
mounted spindle for the wheel, and electronic sensors
to measure the vibrations of the spindle when the
wheel is rotating. The vibrations are picked up by the
sensors and then transferred to the instruments which
show the out-of-balance readings. A wheel assembly
that is in balance produces no vibrations.
Wheel balancing machines have electronic

components and circuits that make them easy to
operate. In most cases, the wheel size is selected by the
operator and the machine then automatically provides
readings to suit the size of the wheel being balanced.
Safety precautions
Wheel-balancing machines have a safety hood which
covers the wheel while it is being spun. The wheel can
reach high speed; covering the wheel protects the
operator from a wheel weight that might fly off. Before
spinning the wheel, stones should be removed from
the tread. This is done to avoid possible injury to the
operator and to obtain accurate balance.
On-vehicle wheel balancer
Balancers of this type balance the tyre, wheel and hub
assembly on the vehicle. The balancer has a wheel

figure 23.33

403

spinner which consists of an electric motor with a
metal driving wheel. This operates against the shoulder
of the tyre (Figure 23.33).
A pickup head under the suspension is used to
sense vibrations caused by unbalance. Instruments
record the unbalance and show where balance weights
are needed.
To balance a wheel, it is raised by the jack and a
chalk reference mark is placed on the tyre. The pickup
head can be part of the lifting jack or stand, so that it is

in place under the suspension to pick up the vibrations.
The machine is rolled up to the side of the wheel to
be balanced, and its driving wheel is used to spin the
wheel of the vehicle. The vibrations received by
the pickup are changed into electric impulses and
transmitted to the machine. A stroboscopic light on the
machine is used in conjunction with the reference
mark. Instruments show the amount of unbalance and
the correction needed.
■ There are precautions and restrictions that apply to
spinning wheels with limited slip differentials,
front-wheel drives and all-wheel drives.
Fitting balance weights
Balance weights are shown in Figure 23.34. These
have a spring clip which holds them on the rim of the
wheel. They are fitted by being tapped into place with
a hammer. When properly installed, they fit neatly into
the shape of the rim.

On-vehicle wheel balancer spins the wheel while on the vehicle


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404 part four running gear

figure 23.35

Checking tyre and rim runout
A radial runout of rim, B lateral runout of rim,
C radial runout of tyre, D lateral runout of tyre MITSUBISHI

Technical terms
figure 23.34

Balance weights
(a) fitting a weight to a rim (b) weight for an
aluminium wheel (c) weight for a steel wheel FORD

Different weights are used for aluminium and steel
wheels. Aluminium wheels have a different-shaped
rim to steel wheels, and so the weights need a different
cross-section. Flat adhesive weights are also used for
aluminium wheels.
Balance weights are made in a number of different
sizes; the mass, in grams, is shown on the weight.
Tyre and rim runout
Where a wheel is difficult to balance, the wheel and
tyre should be checked for excessive runout. A dial
gauge mounted on a base is used (Figure 23.35).
Generally, runout should not be more than about
2 mm.
The tyre is checked at the centre of the tread for
radial runout and against the sidewall for lateral

runout. The rim is checked in two places so that both
radial runout and lateral runout are measured. A more
accurate check of the rim can be made with the tyre
removed.
Before checking tyre runout, the vehicle should be
driven to warm up the tyre and get rid of any flat spots
on the tyre due to standing.
Tyre runout and unbalance can sometimes be
reduced by deflating the tyre and rotating it 180°.

Impregnated, casing, ply, ply rating, inflated, biasply, cross-ply, radial, rayon, nylon, polyester, steelbelted, characteristics, slip angle, on-road, off-road,
terrain, tyre profile, aspect ratio, footprint, P-metric,
tread pattern, aquaplane, Australian Design Rules,
placard, tread-wear indicator, underinflation,
overinflation, camber, wheel alignment, toe-out, toeout on turns, tyre rotation, bead breaking, bead
indicator, rim offset, centrifugal force, static
balance, dynamic balance, tramp, wheel wobble,
shimmy, balance weights, tyre runout, radial runout,
lateral runout.

Review questions
1.

What are tubeless tyres?

2.

Name the various parts of a tyre.

3.

What is meant by ply rating?

4.

What is a low-profile tyre?

5.

What is a radial tyre?

6.

What is the difference between a bias-ply tyre
and a radial tyre?

7.

Why is tyre rotation considered necessary?

8.

Consider the reasons for abnormal tyre wear.

9.

How do truck wheels differ from car wheels?

10.

Why should tyres on dual wheels be matched for
size?


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chapter twenty-three tyres, wheels and balance

405

11.

How could the tyres on dual wheels be checked
for size?

20.

Name the types of wheel balancers and indicate
briefly how each type operates.

12.

What safety precautions should be observed
when inflating a truck tyre after it has been fitted

to a wheel?

21.

What are some of the effects of a wheel that is
out of balance?

22.

What is a tread wear indicator?

13.

What are the main steps when removing a worn
tyre from a wheel and fitting a new one?

23.

How can tyre and wheel runout be checked?

What are safety rims?

24.

14.

Indicate some of the precautions that are needed
when using a wheel balancer.

15.

How could the correct-size tyres for a passenger
car be determined?

25.

How are wheel weights fitted?

What is meant by underinflation and overinflation?

26.

Indicate the sequence for tightening wheel nuts.

16.

27.

What is a tyre placard?

17.

What are the likely effects of underinflation?

28.

What is meant by load index?

18.

What is meant by static balance?

29.

What is meant by speed category?

19.

What is meant by dynamic balance?

30.

How would you know whether a car had the
correct size of tyres fitted?


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