How to find the Offset of a wheel rim

from Western Towing – www.westerntowing.co.uk

The wheel rim ‘offset’ is the distance between the centre line of the rim and the faceplate that bolts onto the hub. It is important to know as different offsets will alter the distance between the inside edge of the rim and the side of the trailer or caravan. It could also increase the overall width of the axle and it may also alter the capacity of a suspension system.

This diagram shows the dimension of the ‘Offset’ usually measured in millimetres.When the centre line is the same as the mounting plate line then it has a Zero or No offset

If the rim has an offset it is usually marked on the front of the rim, near the mounting holes, an example would be:-

ET30 would be a 30mm offset.
ET20 would be a 20mm offset

A simple way to measure the wheel rim Offset

Start by placing the rim on a level surface with the front of the mounting plate facing upwards, then measure the distance ‘A’ (in millimetres) from the mounting plate edge to the surface.

Turn the rim over and measure the distance ‘B’ again as above.

Take distance B from A and divide by 2, this will give you the offset. If the result is a minus number then you have a Negative Offset. Rims with a negative offset are not usually found on trailers and caravans.

Please note: that if the rim edge is not flat on the surface when measuring B (due to the bulge in the rim) then the distance between the surface and the rim edge needs to be deducted from B before making the calculation.

Please Click Here If you need to find the PDC or Pitch Circle Diameter of the wheel.