The wheel must have the right measurements to fit the car. These measurements include number of bolts/studs, PCD, center spigot size and the ET (offset).The following wheel measurements are of importance:
- Number of bolts or studs
This defines the number of bolts use to fit the wheels or the number of studs fitted to wheel hub.
- Pitch Circle Diameter
The PCD, or Pitch Circle Diameter is the diameter of the invisible circle formed by scribing a circle that passes through the centre point of each mounting hole.
- Centre spigot size
The wheel bolts or studs are there to hold the wheel laterally on to the hub, but they're not really designed to take vertical load - ie. they're not designed to take the weight of the car. That's the job of the centre spigot - the part of the hub that sticks out and pokes through the hole in the middle of the wheel. It's the load-bearing part of the hub and the wheel, as well as being the assembly that centres the wheel on the hub. For the most part, the centre spigot on aftermarket alloys is much larger than that of the hub. When this happens, the best solution is a spigot locating ring which is essentially a steel or hard plastic doughnut designed to fit snugly on to your hub spigot and into the wheel spigot.
The image to the left shows the PCD (the red ring and mounting hole centrelines) and the spigot size (the blue ring). The spigot hole on an alloy is normally covered up with a centre cap or cover.
The offset of the wheel is also of importance. The offset is the distance in mm between the centre line of the wheel rim, and the line through the fixing face. An incorrect offset can result in many problems such as:
- A too light or heavy steering
- Wheel fouls on bodywork or suspension
- High speed judder in the steering wheel
The offset can be postive, negative or neither. This determines how the suspension and self-centring steering behave. This figure will be stamped on the wheel somewhere as an ET figure.
Typically you can get away with 5mm - 7mm difference from the car manufacturer specification before you'll run into trouble with the wheels fouling the suspension/bodywork, or affecting the steering properties.
When you increase the offset of a wheel, you decrease the clearance between the inner edge of the wheel and the suspension components. In the image example, the left wheel has a positive offset - ie. the distance from the mounting face to the centreline of the wheel is larger than that of the right wheel.
You can see that by increasing the positive offset of the wheel, it pushes the inner edge of the wheel and tyre closer to the suspension. Conversely, decreasing the offset moves the wheel and tyre closer to the outside of the vehicle where it might scrub and rub against the bodywork and wheel arches.
The most positive the offset, the more the wheel is tucked into the car. The more negative the offset, the more the wheel sticks out.
Stud/Bolt patterns, PCD values and offset are listed with the following notation:
5x120 ET 42.
This means a 5-bolt pattern on an circle of 120mm diameter and an offset of 42mm ( typical BMW wheel measurements).