Need to know what a particular industry term means? We’ve got you covered.
The arrangement of bolt holes on a wheel. A 5 bolt wheel with 100mm between opposite bolt holes would be written as 5/100.
The centre bore of an alloy wheel is the size of the hole at the back of the wheel which the hub fits in to. To help the wheels sit properly, this hole needs to be an exact match to the size of the hub.
Most modern wheels are ‘hub-centric’. This means that the hub which protrudes from your car matched with the equivalent sized hole at the back of your wheel is load bearing. This ensures the studs or bolts are holding the wheel onto the hub.
If you have ‘lug-centric wheels’, the state of your studs or bolts is obviously more important – be sure to replace these from time to time and always 3/4 tighten the wheels off the car to make sure they are cantered.
Mounting or installing tyres on a wheel.
The distribution of mass on a car tyre to the entire wheel to which it is attached. When a tyre is fitted to a wheel, it is measured on a balancing machine with correction weights applied to counteract the combined effect of the tyre and wheel imbalance.
The compression of an aluminum billet into an aluminum wheel using pressure combined with heat. This results in a wheel that is both stronger and lighter then a standard aluminum wheel.
Also known as the ‘P-Metric’ system, these are the measurements of a tyre’s overall size. The width is measured in millimetres, the profile is an aspect ratio or percentage of the sidewall height, and the diameter is the size of the rim which the tyre can fit, measured in inches. They are usually displayed in the format 205/65/15 etc.
The distance between the hub mounting face at the back of the wheel and the wheel’s centre line.
Offset is normally stamped or engraved into the wheel and is measured in millimetres of ET: ET is the short form of the German word Einpresstiefe which translates as insertion depth.
Negative Offset wheels have their mounting face toward the rear of the wheel – powerful rear-wheel drive cars often have wheels with negative offset. Zero Offset wheels have their mounting face even with the centreline of the wheel and are by definition ET0. Positive Offset wheels have their mounting face toward the front face of the wheel. Front wheel drive vehicles usually have positive ET wheels. Eighties & Nineties Volkswagen wheels are generally ET38.
Short for ‘Pitch Circle Diameter’ and is the diameter of a circle drawn through the centre of your wheel’s bolt holes. PCD is measured in millimetres and also indicates the number of studs or bolts on the wheel. Volkswagen Alloy Wheels are usually either 4×100 i.e. 4 bolt holes drilled through the centre of an imaginary 100mm circle, or 5×100 for VR6s, GTIS and MK4s.
The practice of raising the diameter of your wheels whilst at the same time reducing the profile of your tyres to keep the overall rolling radius the same.
A circular disc which is used to propel movement for vehicles by rotating on an axial bearing.
Also known as a ‘tire’, it is a round, ring-shaped object that fits around a wheel rim to protect and enable it to better perform by providing a flexible cushion that absorbs shock while keeping the wheel in close proximity to the ground.
The outer edge of a wheel which holds the tyre. It makes up the outer circular design of the wheel on which the inside edge of the tyre is mounted on Cars and other vehicles.
A run-flat tyre is an air-powered car tyre that is designed to withstand the effects of deflation when punctured and enable the vehicle to continue to be driven at reduced speeds.