Why fit alloy wheels?

The size of a wheel is determined by two measurements- the rim diameter and wheel width.

The wheel width is measured by taking the distance between the two mounting flanges of the wheel.

For example, a wheel with measurements 18 x 8j has a diameter of 18 inches and a width of 8 inches.

The PCD is the 'Pitch Circle Diameter' of the wheel. This measurement is denoted by two numbers in the following format. For example:

5 x 120

The first number is an indication of how many bolt holes are present. In this case 5. The second number is the diameter of the circle made by the centre point of the bolt holes in your wheel.

Offset, commonly denoted as 'ET', determines the distance from the mounting face of the wheel to the wheel's imaginary centre line.

Every wheel has an offset number usually stamped on. The offset is measured with the imaginary centre line of the wheel being taken as '0'. A positive ET would result in the wheel fitting further inside the wheel arch whereas a negative ET would see the wheel protruding past the wheel arch. Finally, a zero ET would mean the mounting face of the wheel is in line with the imaginary centre line of the wheel.

It is important to get the offset right when procuring wheels for your vehicle, having an offset much higher than on your standard wheels would see your wheels sitting far closer to the hub running the risk of contact with brake and suspension components. Conversely, having an offset much lower than your standard wheels could result in your tyres making contact with wheel arches and increase the turning circle of your vehicle.

The offset can be adjusted by using wheel spacers. Spacers can only be used to add a negative offset i.e., wheels outwards.

The CB or 'Centre Bore' of the wheel is the diameter of the centre hole in the back of the wheel where the spigot (the lip protruding from the hub) fits through.

In the majority of vehicles, the weight of the car is transmitted to the centre hole of the wheel via the spigot. The wheel bolts are to secure the wheel over the spigot. Therefore it is vital the CB matches the spigot size on the hub of the vehicle. Most OEM wheels will have a very close fit to the spigot however aftermarket wheels are found to have a slightly larger CB for fitment compatibility. In this case, spigot rings are used to ensure a tighter fit.

When considering wheel spacers, it is important to take into account the spigot. If the spigot does not protrude past the spacer, the weight of the vehicle will not be transferred to the wheel, therefore, compromising safety. Hubcentric wheel spacers are to be used in such cases.