Tyre Glossary A-C


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Air Pressure: This is essentially the volume of air inside the tyre, it can be measured in either pounds per square inch PSI or Bar.Having the correct pressure for tyres is a key factor in Tyre Safety and can usually be found in the Vehicle handbook as well as the fuel cap flap.

Alignment: This is essentially the correct vertical alignment of the tyres. Generally describes the checks and corrections made to the suspension and steering systems of the vehicle to ensure compliance with the manufacturers recommendations.

Alloy Wheels: Alloy wheels differ from normal Steel wheels because of their lighter weight, which improves the steering and the speed of the car. Alloy wheels are also better heat conductors than steel wheels, improving heat dissipation from the brakes, which reduces the chance of brake failure in more demanding driving conditions.

All Season Tyres: Tyres designed to be used in all weathers all year round.

Aquaplaning: The vehicle rides on a layer/film of water above the road surface and not the road itself. This causes traction loss and loss of control. The vehicle often can feel unresponsive and the back of the vehicle may weave or wobble. If this occurs, , put on your Hazard warning lights, avoid braking or accelerating, depress the clutch, keep a strong firm grip of the steering wheel, steer where you want to go (into the skid) and try not to panic.

Aspect Ratio (also referred to as Profile): Expression of a tyres height as a percentage of its section width, for example if the width was 300mm and the height was 150mm the aspect ratio would be 50%.

Asymmetric (also see Profile): When the tyre tread pattern varies from one side of the tread to the other. These must be fitted with the outside sidewall on the outer face of the wheel. Directional versions will also be specific to the left and right sides of the vehicle. This information is often indicated on the sidewall.

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

Bar: Metric Unit for measuring Air Pressure (1 bar = 14.5037738 pounds per square inch).

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

Bead wires: This is a coil of high tensile steel wire treated to improve bonding when encased in a "matrix" of hard rubber. The casing plies are turned around the bead and are securely bonded to the structure when the tyre is cured.

Blades (also known as Sipes): This refers to slits in the tread blocks which are designed to increase grip on winter and wet-weather tyres by moving water away from the tyre.

BSAU159f: The standard which all tyre repairs must comply with in the UK.

Budget Tyres (also see Economy Tyres): Tyres which retail cheaper than mid range tyres, it will follow that these tyres will not perform as well.

Casing:  Consisting of Ply, bead area and belts, this is the skeleton of the tyre sitting underneath the tread and sidewalls. Also referred to as the Carcass.

Camber:  When the vehicle is at its normal ride level, the deviance from the vertical of the tyre centre line. May cause uneven tyre wear if incorrect.

Centre-less Alloys: Standard alloys designed by Citroen and Peugeot with no centre on them. This means the garage will need a specialist adaptor to balance the tyre when fitted. This adaptor fits directly into where the alloy studs go. Not all fitting centres can fit tyres on this type of wheel.

Cold Inflation Pressure: Tyre pressure before the tyre has been heated up from driving.

Cold Weather Tyres (also see Winter Tyres): These Tyres are designed to give better grip in temperatures of below 7 degrees. Note that Snow Tyres are something different.

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

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