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TYRE
TERMINOLOGY
| Air Pressure |
Is the force exerted by air within a tyre. This is
measured in Pounds per Square Inch (PSI) or KiloPascals (kPa). |
| Aspect Ratio |
A measured term for the relationship between the height
of the sidewall and the width of the tread area or cross section width
of the tyre. |
| Bead |
This is the part of the tyre that is shaped to lock
into the edge of the rim upon inflation. |
| Bead Breaker |
A tool to remove the bead from the rim in order to
remove the tyre. |
| Black Top |
Bitumen road. |
| Blocks – Tread |
The rubber section of tread – usually large on
an M/T |
| Blow Out |
Sudden tyre deflation |
| Caster |
In relation to front wheel alignment and steering geometry
where the tyres are tilting forward or rearward of the axle pivot. |
| Cord |
This is the twisted fibre or filament of polyester
rayon, nylon or steel which gives the tyre carcass and belts its strength. |
| Cord Plug |
Referred to as a rope or wick type repair compound
for the repair of castings. |
| FWA |
Front wheel alignment. |
| ID |
Inside diameter. |
| KPa |
KiloPascal – a measurement of pressure. |
| Load Index |
Relates to the maximum weight permitted for each tyre
at a set inflation pressure. |
| Lug |
Design of the tyre tread featuring heavy side ‘Lugs’
that cut into mud or similar. |
| Mushroom Plug |
An internal plug for repairing tyres. |
| OD |
Outside diameter of the tyre when inflated and mounted
on a rim. |
| Over Inflation |
Too much air pressure in the tyre which affects the
tread contact on the road. |
| Plug Repair |
This is a repair carried out on the tyre and can be
inserted through the tyre from the inside or the outside of the casing.
There are laws governing repairs which should be adhered to. |
| Ply |
This is a layer of rubber-coated cords which run on
an angle of 40 degrees across the tyre carcass. |
| Ply Rating |
A rating relative to a tyres load carrying capacity reflecting a nominal amount of plys required to achieve the maximum load. |
| PSI |
Pounds per square inch in relation to tyre pressure. |
| Radial Ply |
This is a tyre with cords running radially from the
bead at 90 degrees to the centre of the tyre. |
| Rope Repair |
See Plug Repair |
| Section Width |
The measured width of the tyre at its widest part when
inflated to the manufacturer’s recommendation. |
| Shoulder |
Outer edges of the tread area. |
| Sidewall |
This is the side section of a tyre between the bead
and the tread section. |
| Sipes – tread |
The fine cuts in the tread blocks |
| Speed Rating |
Is the maximum rated speed the tyre can operate at
safety. |
| Toe In |
In relation to front wheel alignment and steering geometry
where the tyres are facing inwards. |
| Toe Out |
In relation to front wheel alignment and steering geometry
where the tyres are facing outwards. |
| Tread Depth |
This is the distance between the tyre casing and the
tread blocks. |
| Tread Design |
This is the pattern section of the tread area. |
| Tread Wear Indicators |
These are small bars moulded across the tyre carcass.
Once the tread has been worn off to these it means the tyre requires
replacing. |
| Tubeless Tyres |
A tyre that can be inflated and seals on the rim via
the bead and has an external valve. |
| Tubed Tyre |
A tyre which requires a tube for inflation. |
| Tyre Placard |
This plaque mounted on the vehicle offers the correct
tyre information for that model - things like rim and tyre combinations
and air pressures. |
| Under Inflation |
A tyre with a lower amount of tyre pressures than specified
which affects the tread area on the road. |
| Valve |
This is mounted either through the rim or as part of
a tube and is how the tyre is inflated. |
| Void – Tread |
See tread depth. |
TYRE CONSTRUCTION
When considering tyres, you need to consider the tyre construction to
best suit your needs.
Passenger (or highway type tyres - hence you will see the abbreviation
H/T) and light truck (LT) or all terrain (A/T) designated tyres vary greatly
across the tyre range with some manufacturers building tyres of the same
size and vehicle application, but one is a passenger type and the other
a LT or A/T type. Then you’ll also get M/T (for mud terrain) type
tyres, and these are the most aggressive and toughest of a tyre range.
For a great many small all wheel and four wheel drive vehicles, passenger
tyres are fitted as standard, and are generally okay on road but for the
bigger 4WDs that carry some weight and go on any type of dirt surface,
then the passenger tyres should get the flick and LT or A/Ts fitted.
The bulk of 4WD manufacturers fit passenger type tyres as standard but
offer a choice of A/T tyres to cater for their buyers - this is as much
a cost saving measure as what the stats show - most 4WDs don’t go
off road - and car companies do look at their target audience.
To quality the above statement, passenger tyres have generally been developed
for city work with low noise, ride comfort, high speed durability and
handling, and cost the major factors considered. On the other hand, A/T
and LT type tyres tend to be used as work horses and aren’t vehicle
specific with the considerations listed as puncture resistance, load carrying
capacity, extended wear life and added off road grip. To give you a further
idea, check out the Tyre Characteristics chart. This was supplied
from Bridgestone Australia and is a good basic indicator of the differences
between the two types of construction.
TYRE
CHERACTERISTICS - PASSENGER OR H/T 4WD TYRES
| CHARACTERISTIC |
RESULTANT |
| Shallower tread depth |
Lower noise levels, because there is less air passing
through the tread grooves. Off road grip can be compromised because
of reduced block height and puncture resistance can be lower.
Lower heat generation improves high speed durability. |
| Reduced skid base gauge (distance from the bottom
of the tread grooves to the top of the belts) |
Improved high speed durability (because the tyre
generates less heat), and is therefore common in tyres with higher
speed symbols (S and H). Puncture resistance can be lower. |
| Thinner sidewall gauge |
Improved ride comfort, because the tyre casing
is more pliable. Sidewall puncture resistance can be lower. |
| Thinner belt cord wire style and/or smaller
number of belts (gen 2, as opp to 3) |
Improved ride comfort, because the tread area has
more ‘enveloping power’ (ie it conforms to the road
surface more easily). Puncture resistance can be lower. |
| Body ply cord thinner, and/or smaller number
of plies |
Improved ride comfort, because the tyre casing
is more pliable. Puncture resistance can be lower. Lower casing
strength also limits maximum inflation pressure, reducing load
carrying capacity. |
TYRE CHARACTERISTICS
- LIGHT TRUCK OR A/T 4WD TYRES
| CHARACTERISTIC |
RESULTANT |
| Deeper tread depth |
Improved wear life and puncture resistance, but
limits the speed capability of the tyre and can increase noise
levels. Improved off road performance, especially in soft conditions |
| Heavier skid base gauge |
Increased puncture resistance. Can increase heat
generation, reducing speed capability of the tyre. |
| Heavier sidewall gauge |
Increased puncture resistance. Stiffer casing reduces
ride comfort, but can help to increase casing strength and thereby
maximum inflation pressure and maximum load. |
| Heavier belt cord wire style, and/or more belts |
Increased puncture resistance, but harsher ride |
| Bold ply cord thicker, and/or more body plies |
Increased casing strength increases maximum inflation
pressure and thereby load carrying capacity. Can reduce ride comfort. |
READING THE SIDEWALL
Tyres come in three types of markings essentially:
TYRE RATINGS
|
RATING / SPEED
|
| J |
100 KPH |
Q |
160 KPH |
| K |
110 KPH |
R |
170 KPH |
| L |
120 KPH |
S |
180 KPH |
| M |
130 KPH |
T |
190 KPH |
| N |
140 KPH |
U |
200 KPH |
| P |
150 KPH |
H |
210 KPH |
|
INDEX / LOAD
|
| 96 |
710 KGS |
106 |
950 KGS |
| 97 |
730 KGS |
107 |
975 KGS |
| 98 |
750 KGS |
108 |
1000 KGS |
| 99 |
775 KGS |
109 |
1030 KGS |
| 100 |
800 KGS |
110 |
1060 KGS |
| 101 |
825 KGS |
111 |
1090 KGS |
| 102 |
850 KGS |
112 |
1120 KGS |
| 103 |
875 KGS |
113 |
1150 KGS |
| 104 |
900 KGS |
114 |
1180 KGS |
| 105 |
925 KGS |
115 |
1215 KGS |
Used with kind permission from 4x4 Australia Magazine
Copyright 1998
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