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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:

IMPERIAL  
The Imperial system comes to our shores mainly from America. This is how the system works -
swall01.gif (7955 bytes) 31 Overall diameter in inches
10.5 Tyre width in inches
R Means Radial Ply construction
15 Tyre diameter in inches
LT Means Light Truck designation
109 Is the load index which equates to 1030 kgs for a single wheel
S Is the speed rating of 180 kph
METRIC  
Metric markings are found on tyres arriving from Japan and some European   manufacturers -

swall02.gif (7778 bytes)

LT Means the tyre construction is Light Truck designated
235 Relates to the section width of the tyre, in millimetres
85 The Aspect Ratio of the tyre, being 85% of the section width of the tyre at the correct pressure
R Means Radial Ply construction
16 Diameter of the tyre, in inches
115 / 113 This is the Load Rating and depends on whether a single or dual tyre application is used. 115 equates to 1215 kgs
N The Speed Rating, equal to 140 kph
ALPHA NUMERIC
We still see some of these markings around and it works like this -
swall03.gif (7226 bytes) G The code for maximum load
R Radial Ply construction
78 This is the Aspect Ratio
15 Rim diameter in inches
LT Light Truck construction

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

4X4 Australia Magazine
Used with kind permission from 4x4 Australia Magazine
Copyright 1998

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