When I first bought my car, the wheels on it was in a terrible state. It was full of kerb rash and dents. I decided to get a new set of wheels and did some internet research on what type of wheels that I would like to get. During that time, I came across several terms that were alien to me. Things like pitch circle diameter (PCD) and offset were new to me and I simply did not understand them.
I asked several people about it and got several explanations about them and it just made me more confused. Eventually, I found out the real meaning of these terms in a magazine and I wish to share them with everyone here who might have the same problems as I did before. Let’s start with wheel offset.
Offset (see picture above) is the distance between the mounting surface of the back of the wheel and the wheel hub true centre line. This is important for two reasons. First, if your aftermarket wheels’ offset varies too much from the manufacturer’s recommended size specification, then it can compromise steering response and directional stability. Plus, it also can put unnecessary load on components such as wheel bearings. Secondly, a wrong offset can affect your suspension, wheel arches and also brake components.
Offset is expressed in a number, for example ET38. The letters ET stands for “Einpresstiefe” which means insertion depth in German. The number is the distance (in millimetres) between the centre line and wheel’s mounting surface. A positive offset will have the mounting surface more towards the front of the rim and a negative offset will be just the opposite. A negative offset will give the wheels a “flushed” look.
Another thing to take note is the wheel centre bore. It is the diameter of the hole in the back that fits onto the flange on the car’s wheel hub. Most wheels nowadays are hub-centric. That means the centre bore fits onto the hub and all loads and stress factors are on the wheel hub. Lug-centric wheels places the load onto the wheel bolts instead. Anyway, lug-centric wheels are pretty rare nowadays.
Pitch circle diameter (PCD) is the fitment measurement of the wheels’ bolt hole patterns. There are various sizes; for example 4x100 and 5x100. The first number represent the number of bolt holes and the second is the distance (in millimetres) the bolt holes are spaced out. There are wheels that are manufactured to have several PCD fitments. These will give the wheel a wider range of vehicle fitments and it would be cost effective too for the manufacturer.
The PCD on a 4x100 bolt hole hub is simply a straight line from the centre of one hole to another and the 100 is the distance. As for a 5x100 bolt hole hub, it is a little different as there is no direct bolt hole opposite each other. Just draw an imaginary line from the centre of one hole to the edge of the bolt holes diameter.
Photo credit: Fast Car UK and supercars.dk
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