Alloy Wheel Fitment Guide - Alloy Wheels Explained
What is PCD (Pitch Circle Diameter)?
The PCD (Pitch Circle Diameter) can also be known as Bolt Circle Diameter (BCD) is the diameter of a circle (usually stated in millimetres) drawn through the centres of all the bolt holes.
For example, 4x100 means that the wheel has 4 bolt holes and the diameter of the circle that passes through the middle of each of the bolt holes is 100mm.
What is wheel offset (ET)?
The offset is the distance (in millimetres) from the centre of a wheel cross section to the mounting surface of the back of the wheel. Each car has an optimum offset and a range within which the offset can be for the wheel to fit the car properly.
• A positive offset will have the surface mounted towards the front of the rim.
• A negative offset will have the surface mounted towards the back often giving the wheel more ‘concave’ appearance at the front.
It is important to fit the wheels with the correct offset for your car as if the offset varies too much from the manufactures standard specification it can affect the steering and suspension component of your vehicle.
Altering the Offset Using Wheel Spacers
Wheel spacers are used to reduce the offset of a wheel if it’s too high for your vehicle. You would not use it if the offset was too low. Basically, spacers push the wheel further out until it sits correctly in relation to the vehicle’s arches and bodywork, allowing you to fit wheels to your car that isn't made in direct fitment to the vehicle. If spacers are required, we will inform you.
The Width & Diameter of a Wheel
The size of an alloy wheel is calculated by the following equation; Diameter x Width (15 x 6.5J). The section between the mounting flanges on the wheel is known as the wheel width
Wheel Centre Bore & Spigot Rings:
The wheel centre bore is the diameter of the hole in the back that fits onto the flange on the car’s hub. Most wheels have a centre bore that is larger than the hub lip and consequently work on a spigot ring locating system. This allows the wheels to fit a wider range of cars.
Some wheels have been manufactured with a centre bore that will only fit precisely onto a specific vehicle so in this case, a spigot ring is not required. If the wheel does not locate the hub correctly, you may experience issues such as a vibrating steering wheel. This can lead to wear on the tyres and bearings, so customers should remember that if spigot rings are provided to them, they must be used.