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

Tyres, wheels and balance 387 Tyre construction 388 Types of tyre construction 389 Characteristics of tyres 389 Tyre profiles 390 Tyre sizes and identification 391 Tyre tread patterns 392 Australian Design Rules 393 Tyre wear 394 Tyre rotation 396 Wheels 396 Removing and fitting tyres 398 Lighttruck wheels and tyres 399 Wheel balance 400 Balancing wheels 402 Technical terms 404 Review questions 404

Tyres, wheels and balance

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

Balancing wheels Technical terms Review questions

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

cas-ing 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

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 heavy- duty 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

figure 23.1 Tyre construction FORD

figure 23.2 Sectional view of a tubeless tyre and rim

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.

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

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

bias-ply 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

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.

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.

figure 23.3 Tyre construction GOODRICH

figure 23.4 A radial tyre has a belt of cords around its

circumference beneath the tread GOODRICH

figure 23.5 Run flat tyres

(a) reinforced sidewall (b) rubber clip inside rim

rubber clip

sidewalls with newly developed compound

wheel (a)

(b)

Characteristics of tyres

Bias tyres and radial tyres have different

character-istics 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

direc-tion 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 23.6 Characteristics of tyres

(a) distortion of a bias-ply tyre and a radial-ply

tyre on a curve (b) slip angles GOODRICH

figure 23.7 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

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.

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.

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.

Metric tyres

The following is an example of a metric marking for a tyre and what it means:

2205//6655 RR1166 9922HH

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.

figure 23.8 Tyre footprints on the road surface for

different aspect ratios – L is contact patch length, W is contact patch width

figure 23.9 Information on the tyre sidewall (the tyre is

fitted to an aluminium wheel) DUNLOP

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,

tyre name tubeless rim width code

country of manufacture

ply rating and load

tyre construction

tyre size

maker’s name

P-metric 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:

PP1175//6655 RR1144 8811HH

This is interpreted as follows:

1 P – stands for passenger car

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

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.

Bias-ply tyres

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 truck bias tyres and this can be 6 or 8 ply.

light-Tyre tread patterns

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 equip- ment operating under rough conditions.

Passenger car tyres

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

table 23.1 Load indexes for tyres

LOAD INDEX KILOGRAMS

table 23.2 Speed category symbols

SPEED CATEGORY SYMBOL SPEED KM/H

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

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

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 lations 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.

regu-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

figure 23.11 Light truck tyres

(a) normal on-road use (b) on-road and off-road use (c) mainly off-road use DUNLOP

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

combinat-ions 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

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.

table 23.3 Tyre placard

TYRE SPECIFICATIONS & PRESSURES LOCATION FRONT REAR FRONT REAR

RECOMMENDED TYRE SIZES P175/65 R14 81H P/175/65 R14 81H

RECOMMENDED PRESSURES

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

figure 23.12 Effects of correct and incorrect inflation on

the contact that the tread makes on the road surface

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.

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

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.

Wheel alignment

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 cor- nering, 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 toe- out 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.

figure 23.13 Tyre tread-wear indicator MITSUBISHI

figure 23.14 Tyre wear due to incorrect inflation

(a) wear from underinflation (b) wear from

overinflation MAZDA

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

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)).