Tyre Glossary

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Air Pressure:
The amount of air inside the tyre, measured in either pounds per square inch (psi) or bars. The correct pressure for tyres can usually be found in the vehicle handbook.

Alignment:
The correct vertical alignment of tyres. Wheel alignment generally describes the checks and corrects made to the suspension and steering systems to ensure that they comply with the vehicle manufacturer’s recommendations.

All Season Tyres
Tyres that are designed to be used all year round, in all weathers.

Aquaplaning:
The vehicle rides on a film of water instead of the road surface which causes a loss of traction and the vehicle looses control. Steering often feels non-responsive and the back of the vehicle weaves or wobbles. If aquaplaning occurs put on hazard lights, avoid braking or accelerating, depress the clutch, grip the wheel firmly and steer where you want to go (i.e. into the skid).

Aspect Ratio:
An expression of a tyres height as a percentage of its section width. E.g. if the width was 240mm with a height of 120mm the aspect ratio would be 50%. Also referred to as profile.

Balancing:
the process for compensating for slight variations in tyre and wheel assembly. By ensuring weight is equally distributed when the tyres and wheels spin, abnormal vibrations can be eliminated.

Bar:
Metric unit for air pressure. (1 bar = 6.8947psi)

Bead:
A multi-layer steel ring that helps to hold the tyre to the rim. By providing adequate contact pressure between the tyre and the rim, the bead ensures air seal.

Bias Belted Tyre (see also Cross-ply tyres):
This is a development of the cross ply tyre, using radial technology over a cross ply style structure. Some American market tyres are still manufactured as bias-belted.

Casing:
This is the skeleton of the tyre sitting underneath the tread and sidewalls. It consists of ply, bead area and belts and is sometimes referred to as the carcass.

Camber:
The deviance from the vertical of the tyre centre line when the vehicle is at its normal ride level. Incorrect camber may cause uneven tyre wear.

Cold Inflation Pressure:
This is the tyre pressure before the tyre has been able to heat up from driving.

Cold weather tyres (see also Winter Tyres):
Tyre designed to give better grip below temperatures of 7 degrees. Also known as winter tyres but snow tyres are something different.

Contact Patch (see also footprint):
The area of tyre in contact with the road. This varies depending on tyre construction, the compounds, tread design and tyre pressure.

Cross-Ply Tyre (see also diagonal tyre, bias belted tyre):
This is the bias-belted or cross ply tyre which has a steel bead wire to retain the tyre on the rim, and a series of cross ply carcass strength.

Diagonal Tyre (see also Cross-ply):
The diagonal or cross ply tyre has a steel bead wire to retain the tyre on the rim, and a series of cross ply belts to give the carcass strength.

Directional (see also Asymmetric):
A tyre with a tread pattern that must be fitted in an indicated orientation. Often a chevron or arrow shaped pattern is used which helps remove water from under the tyre.

Economy Tyres
A new tyre of fair quality that we sell at a lower price than Mid Range. Logically, the performance of will also be lower.

Footprint:
The area of the tyres tread that is in contact with the ground.

Inner liner:
a layer of rubber inside a tubeless tyre which prevents air from seeping out of the tyre during normal use.

Load Index:
A standard numerical guide that indicates the maximum weight a tyre can carry at the speed specified by its speed rating under the conditions specified by the tyre manufacturer. This is indicated on the tyre wall.

Low profile tyres:
Tyres with shallow sidewalls usually fitted to high performance or sporty cars.

Maximum Inflation Pressure:
The maximum pressure that a tyre can be inflated to.

Mid-range tyre:
A tyre of fair quality that is sold at a lower price than premium brands such as Bridgestone, Michelin, Pirelli and Continental. Logically, the performance of this range will also be lower.

‘Mini’ Spare:
A spare tyre which is smaller than the road tyres, designed to save space and reduce weight in the vehicle. It should only be used temporarily to get you home and should be changed as soon as possible. As such, these tyres have a slower maximum speed. Also known as a space saving tyre.

M+S:
Mud and Snow tyre

Original Equipment (OE):
The original brand of tyre that was fitted by the car manufacturer. There may be more than one OE brand fitment.

Over steer:
The opposite of under steer. Loss of grip to the rear wheels during cornering causing the car to slide sideways.

Pounds per square inch (psi):
The imperial unit for air pressure.

Radial Tyre:
This is the carcass design originally developed by Michelin using parallel carcass belts for the sidewalls and crossed belts for the crown of the tyre. All modern car tyres are radial.

Repairable Area:
The area of the tyre tread where a puncture repair can be made to British Standard BSAU 159.

Rim:
the metal edge of the wheel on which the bead is seated enabling the tyre to be supported.

Rim Size:
The diameter of the rim, usually measured in inches. Also known as Wheel Size.

Rim Width:
Distance between the two opposite inside edges of the rim flanges.

Rolling Resistance:
The energy needed to keep a tyre moving at a constant speed. A low rolling resistance means less energy is being used to keep a tyre moving.

Run-flat:
Tyres which enable the driver to continue a journey in the event low or no pressure.

Section Height:
Tyre height from rim to the outer tread.

Section Width:
The measurement of the tyre from side to side excluding any additional sidewall items such as raised letters.

Shoulder:
Found where the sidewall and tread meet.

Sidewall:
The area between the bead and the start of the tread.

Sipes:
Slits in the tread blocks designed to move water away from the tyre and to increase grip on winter and wet-weather tyres. Also known as blades.

Speed Rating:
Standardised index used to indicate the maximum speed a tyre can sustain carrying the load specified in the load index under the conditions outlined by the tyre manufacturer.

Traction:
The force of friction between the drive wheel and the road or track

Tyre Information:
Information on the recommended pressure, rim size and load capacity can be found in your vehicle handbook and sometimes on the inside sill of the drivers door.

Tread:
The part of the tyre that comes into contact with the road

Tread Depth:
The vertical measurement between the top of the tread rubber to the tyres deepest grooves.

Tread Wear Indicator:
Narrow bands in the tread grooves that provide a visual warning to a tyre needs changing. They are only visible if the tread has worn below the legal limit of 1.6mm.

Tubeless:
A tubeless tyre, usually a radial tyre. Constructed with an inner liner to protect against loss of pressure in normal use.

Tyre Pressure:
The amount of air inside the tyre. The correct pressure for tyres can usually be found in the vehicle handbook and often on the flap to your petrol tank. Measured in pounds per square inch (psi) or in bar, the pressure of tyres gradually reduces therefore, regular checking is necessary. Tyres should never be inflated beyond the maximum as specified in the manufacturer’s handbook.

Tyre Pressure Monitoring System (TPMS):
A device used to monitor tyre pressure whilst the vehicle is in use. This is usually done by attaching a device inside the wheel well, or to the tyre which sends data to a dashboard display. When there is a loss in pressure, it alerts the driver. Run flat tyres can only be used if a vehicle is fitted with a TPMS.

Under inflation:
A tyre which does not have sufficient air pressure.

Under steer:
The opposite to over steer. The car fails to responds to steering causing the vehicle to continue to drive straight.

Winter Tyres (see also Cold Weather tyres):
Tyres which are constructed using special compounds for use in more severe winter conditions including temperatures below 7 degrees Celsius. Often, these include multiple blades or sipes for greater grip in more slipy conditions.