Tyre and Wheel Glossary

Ever read through something online or in a magazine about wheels or tyres but not been able to find much information about it? Well we here at Tyres Direct have come across this problem ourselves many times, so we decided to put together this glossary of terms.

We have listed the terms alphabetically to help you find the term you are looking for:

Term	Explanation
Air Pressure	The amount of air inside the tyre. Normally you can find information on your tyre’s maximum air pressure in your vehicle’s handbook or fuel cap flap. Air Pressure is measured in either PSI (pounds per square inch) or Bar.
Alignment	When mechanics align wheels they make sure that they both sit correctly with each other and the vehicle. This can usually be done at your local garage.
All Season	Tyres specifically designed to withstand all weathers. For harsh winter weather however, we do recommend winter tyres.
Aquaplaning	When driving over water your vehicle can sometimes aquaplane. This is where, instead of driving on the road the vehicle rides on the surface of the water, causing loss of control. It is recommended that you follow safety procedures when this occurs.
Aspect Ratio	Also known as profile. The aspect ratio is the tyre’s height, usually a percentage of the width. For more information please see our tyre sidewall measurements guide.
Asymmetric	The opposite of symmetric. An asymmetric pattern is where each side of the tyre’s tread pattern is different to each other. This is usually done as a design feature for specific purposes.
Balancing	A part of the tyre/wheel assembly. Balancing is the process of adding weight to the wheel where is needed to create an equal distribution. This helps prevent any traction problems when wheels are in use and reduces vibration.
Bar	A method of measuring air pressure. 1 bar is equal to about 14.504 PSI.
Blades	Some tyres feature blades as a way of disbursing water and increasing grip on frosty terrains. They are slits, usually located near or on the tyre’s shoulder.
BSAU159f	The UK tyre repair standard.
Cold Inflation Pressure	This is the pressure of the tyre before it has been heated up from driving.
Contact Patch	The part of the tyre that is in contact with the road.
Economy Tyres	Also known as budget tyres. These are lower in quality but are usually a lot cheaper than premium branded tyres.
Footprint	The part of the tyre that comes into contact with the driving surface.
Inner Liner	A part of the tyre’s inner lining that prevents air from escaping. This is usually used when tubes are not.
Load Rating	A number located on the tyre’s sidewall that indicates the maximum weight the tyre can carry. Please see our load rating guide for more information.
"M+S" Tyre	This print can be found on tyres designed to be used in Mud and Snow.
Oversteer	The opposite of Understeer. Oversteering is where, during cornering, control of the rear wheels are lost, causing the car to go into a slide/drift.
Profile	Also known as aspect ratio. Usually the second number loacted on the tyre’s sidewall, this indicates the height of the tyre as a percentage of the width.
PSI	PSI stands for pounds per square inch, and is a way of measuring air pressure. Also see bar.
Radial Tyre	Indicated by the ‘R’ on the tyre’s sidewall, normally located just before the diameter. Radial tyres are made using crossed belts on the tyre’s crown and parallel carcass belts for the sidewalls. This is the modern standard for all car tyres.
Remould	This is where the tyre has been essentially recycled. A old tyre is taken, buffed back into its casing then repaired and remoulded using a new compound for the tread. This is a very common practise for off-road tyres, commercial tyres and aircraft tyres.
Retreading	Sometimes when a tyre’s tread is damaged it needs retreading. This is where the entire tread layer is replaced and is normally used on commercial or extra heavy-load tyres.
Rim	This is the side of the wheel where the bead is seated.
Rim Size	The total diameter of the wheel’s rim in inches (eg 16×8).
Rim Width	The width of the wheel in inches (eg 16×8).
Run-Flat Tyres	These are short distance tyres which are designed to be used even when they contain little or no air pressure.
Section Height	The measurement between the wheel rim to the outside tread of the tyre.
Section Width	The total length of the tyre from side to side (not including sidewall items such as raised lettering).
Shoulder	The section of the tyre where the tread and the sidewall meet.
Sidewall	The part of the tyre between the edge of the tread and the bead.
Silica	A rubber compound filler designed specifically to help reduce rolling resistance (thus helping the tyre’s fuel efficiency rating) and increase tyre mileage.
Speed Rating	Usually written as a letter next to the tyre’s load rating, the speed rating indicates the maximum speed at which the tyre can sustain traction. For more information please see our speed rating guide.
Summer Tyre	A tyre specifically designed to be used during the summer seasons.
Traction	The friction between the wheel and the driving surface.
Tread	The front part of the tyre. This is the part that comes into contact with the driving surface.
Tread Depth	The size of the tip of the tread runner down to the tyre’s surface, between the grooves. The UK legal requirement is 1.6mm.
Tread Wear Indicator	Some tyres have a small band within the tread pattern that helps give you an idea of how low the tread has worn. Once the tread reaches the legal limit of 1.6mm you will need to replace the tyres.
Tread Width	An approximate measurement of the width of the tread that comes into contact with the driving surface.
Tyre Pressure	Also known as Air Pressure Tyre Pressure is a measurement of the amount of air inside the tyre. It can be measured in either PSI or Bar. If you don’t know your vehicle’s maximum tyre air pressure, the manufacturer can usually give you advice. Most of the time you can find the correct value in your vehicle’s handbook.
Under Inflation	When a tyre needs higher air pressure applied to it.
Understeer	The opposite of oversteer. Understeer is when a vehicle doesn't respond to steering wheel movements. It is advised that you take the vehicle to a garage when you notice signs of this.
Valve	A rubber tube with a metal core assembly and dust cap. This allows the tyre to be inflated when needed.
Winter Tyre	A tyre specifically designed to be used during the winter seasons.