Alloy wheel sizing is generally a simply science, width and diameter. All alloy wheels have 2 main measurements these are always shown in imperial sizes as inches and also tend to always be written in the same way. If you look on the Race and Road website or on an actual alloy wheel you will see the following example of measurement 7.5x17”.The first is the measurement of the wheels width. The width of an alloy wheel is measured from the inside of the rim where the tyre fits the wheel. So if you measure a 7.0” wheel from outside edge to outside edge you will find it measure 8.0”, but if you measure from bead seat to bead seat, where the tyre sits, you will see it measures 7.0”.The second is the measurement of the wheels diameter. Again the measurement can be deceiving is you were to measure a 17” alloy wheel on the outside from its furthest point to point it will measure around 18”. As with the width of the wheel the diameter is measured from the points at which the tyre sits on the wheel.
Blot/Stud pattern (PCD)
The Stud Pattern of an alloy wheel is the spacing between each of the wheel bolt/nut holes. This is known as P.C.D or ’Pitch Circle Diameter ’ which is the measurement from the centre of each wheel bolt hole on the alloy wheel. Below is a diagram that explains this measurement on 4 and 5 stud examples. There are various stud patterns for each vehicle manufacturer ranging from 3 stud on some Peugeot / Citroens and Smart Cars though 4 and 5 stud up to 6 studs on many 4 x 4 and SUV’s. Race and Road will have the information for your vehicle based on its year and model. We will work out what your vehicle requires.
For example if measurement A was 100mm then the Stud Pattern or P.C.D would be either 4x100 or 5x100. Again if the measurement A was 108mm then the Stud Pattern or P.C.D would be 4x108 or 5x108 and so on.
The centerbore of a wheel is the size of the machined hole on the back of the wheel that centers the wheel properly on the hub of the car. This hole is machined to exactly match the hub so the wheels are precisely positioned, minimizing the chance of a vibration. With a hubcentric wheel, the lug hardware will not be supporting the weight of the vehicle, all they really do is press the wheel against the hub of the car. Some wheels use high quality, forged centering rings that lock into place in the back of the wheel. This is an acceptable alternative.
If you have non-hubcentric (lugcentric) wheels, they should be torqued correctly while the vehicle is still off of the ground so they center properly. The weight of the vehicle can push the wheel off-center slightly while you’re tightening them down if left on the ground.
The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel. The offset can be one of three types.
The hub mounting surface is even with the centerline of the wheel.
The hub mounting surface is toward the front or wheel side of the wheel. Positive offset wheels are generally found on front wheel drive cars and newer rear drive cars.
The hub mounting surface is toward the back or brake side of the wheels centerline. "Deep dish" wheels are typically a negative offset.
If the offset of the wheel is not correct for the car, the handling can be adversely affected. When the width of the wheel changes, the offset also changes numerically. If the offset were to stay the same while you added width, the additional width would be split evenly between the inside and outside. For most cars, this won’t work correctly.