If you are thinking of purchasing a set of aftermarket alloy wheels, selecting wheels with the correct offset is very important. So what is offset, and why is it so important?
Offset is a measurement of how far the mounting face of the wheel (on the brake side) is away from the centre line of the wheel. If the mounting face lines up exactly with the centre line it is called “Zero Offset”, if the mounting face is further out towards the show side of the wheel, this is called “Positive Offset”, and if it is further inside the wheel (towards the brakes) then this is called “Negative Offset” (this is much rarer). There are two common ways to measure this though they both provide a different number:
Backspace: Measure from the brake side edge of the rim to mounting surface of the wheel (as shown in the “Straight Edge” section of the diagram below. This will generally be a reasonably high number (at least 80mm or more).
Centreline: Measure the total width of the wheel (X) and then halve this (X/2). Measure the backspace (A) and then subtract “A” from “X/2”. You now have the centreline offset. In Australia this is written as 35P (35mm positive) or 10N (10mm negative). In Europe it is usually written as ET35 etc.
Making sure you have the correct offset is important as it will affect where your wheel sits in relation to your vehicle. A high positive offset means the wheel will sit further inside the wheel and might rub up against the brakes or suspension. Worst case scenario, you will not be able to steer. A high negative offset means the wheel sits further out from the wheel arch and this can cause the tyres/wheel s to rub against the wheel arch. It is illegal for your tyres/wheels to stick out past your wheel arch in Australia.
In addition to this you must also be aware of limitations to changes made to your vehicles Wheeltrack
Wheel track is the distance between vehicles wheels (side to side). It is measured from the centreline of each wheel to the other. The fitment of wider wheels will often increase the Wheel Track and this can be associated with Offset as well. Increasing the Wheel Track can increase the load on bearings, axles, suspension joints and steering tie rods.
On a passenger vehicle the wheel track must not be increased by more than 25mm beyond the maximum specified by the vehicle manufacturer for that particular model. This means that the offset may not be changed by more than 12.5mm per wheel.
Wheel Track reduction is NOT permitted without the approval of the relevant registration authority.
Off-road and commercial vehicles fitted with front and rear beam axles may increase the Wheel Track up to 50mm in total.
All wheels fitted to the front or rear axles must have the same diameter, offset, width and mounting configuration (except the spare).
Where a two-axle vehicle is fitted with different width single tyres, the narrower tyres must not be less than 70%of the width of the wider tyres. As an example: if you were running 10 inch (254mm) wide tyres on the rear, your front tyres can only go as narrow as 7 inches (17.78mm).
PCD stands for Pitch Circle Diameter which basically means the diameter of an imaginary circle running through the centre of each bolt hole. The most common PCD values are 100mm and 114.3mm, the difference arising due to manufacturers using metric or imperial measurements (4.5 inches = 114.3mm).
You will need to ensure that your new wheels have the same stud pattern (4 and 5 stud shown above) and same PCD as the wheel hub on your vehicle. In addition to this you need to ensure you have the same centre location method and that the centre spigot is the same diameter as the original wheel. If this is not the same a metal adaptor ring should be used.