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Richard Hammond

Vehicle & Alternative fuels news

Wheel Offset

A wheels offset is the distance from the centre of the wheel to the face of the bolting plate.

Effectively the offset is also the distance from the centreline of the wheel to the hub face.

Therefore :

If you put a wheel on the same hub with a bigger offset, the wheel will sit further inside the wheel arch because the wheels bolting face will have to move further in to meet the hub.

Alternatively if it has a smaller offset it will sit further out from the wheel arch.

For sports purposes grip would be improved by increasing the width of the wheel(and tyre) and using a smaller offset to allow for the extra width on the wheel in the wheel arch.

More information on wheels

Vehicle wheels are usually described with a designation of something like '13-5.5J' and this is decoded as follows.

The 13 is the diameter of the wheel in inches from edge to edge, measured inside the flange with sizes ranging from 10 inch on the original mini's to 23 inch on 4x4 vehicles.

The second number is the width of the rim, again measured in inches between the flanges. The original mini used 4.5 wheels and sizes range all the way up to 12 as used on some supercars.

The letter, in this case J refers to the shape of the rim. Rim contours are standardized by the Tire and Rim Association, so that tires will fit.

Another important thing is the wheel offset, which is the distance between the middle line of the centre of the wheel width and the mounting surface of where the wheel bolts to the hub on the vehicle. The offset is described by the term ET, which is from the German word 'Einpresstiefe' translated as 'insertion depth'. Most wheels have a positive offset which means the mounting surface is further outboard than the wheels centre. If you reduce the amount of positive ET on the wheel the the vehicles track is widened and vice versa if the ET number is reduced. Straying too far from the original offset can be detremental as fouling to the suspension and bodywork can occur as well as eccess strain on wheel bolts or studs