Wheel offset explained in plain English (yup pictures too)
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Often time’s people are confused about offset (sometimes called back spacing) of wheels. Many of the articles out there either explain way too much or way too little. We thought it me be helpful to offer some depicted examples of different offsets of the same wheel on the same car.
Wheel Offset: The distance between the center line and hub mounting surface of a wheel.
So lets break that down. Lets start with the center line of the wheel. The center line of the wheel is the exact center if you were to cut a wheel directly down the middle. Therefore if you have a 10in wide wheel the center line is 5 in from either side. You can see the center line in all the pictures below illustrated with a dotted line.
The hub mounting surface is the part of the wheel that will physically be bolted to the vehicle. This is generally mounted to the brake rotor or hub, thus the name hub mounting surface.
On the left illustration below you see 2 red arrows. This depicts where you would measure offset. Therefore in the left illustration the measurement from the center line (dotted line) to the wheel mounting hub is 40mm.
What is ET?
Often times you will hear people mention that that which is ET45. So now we know that the 45 is distance in millimeters between the center line and hub of the wheel, but what does ET stand for? ET is an abbreviation for the German word Einpresstiefe meaning offset.
Ok that all sounds great right, but how does offset change how wheels will look on a car?
Below we have MK6 GTI we specialize in discount VW and Audi parts so we thought this would be a great car to use as an example. PLEASE NOTE: All examples are not to scale but are an effort to help the reader understand how offset affects the look of the wheels on the vehicle.
Our first picture is a MK6 GTI with the stock 18 inch Detroit wheel. They have an offset of ET 51 this means the wheel hub is 51mm from the center line of the wheel.
In our second illustration we have the same MK6 GTI with the same wheels but we have changed the photo to depict a lower offset wheel. The offset shown in the picture might be closer to 20mm’s or ET 20. Therefore we can see that as the offset decreases the wheels will poke out from the vehicle further. Its really a basic calculation assuming the width of the wheel is the same. If the original wheel is ET 51 and you change to an ET 20 your wheels will stick out 31mm further than the original wheels.
In our third picture of this MK6 GTI we have changed the photo to reflect a high offset. The offset shown in the picture might be closer to 70mm’s or ET 70. Therefore we can see that as the offset increases the wheels will come in under the vehicle further. The calculation goes in the other direction this time. If the original wheel is ET 51 and you change to an ET 70 your wheels will sink into the vehicle by 19mm.
PLEASE NOTE: All depictions here make the assumption that the wheel width is the same on all examples. When you start to change both the width and the offset there are more complicated calculations.
Here is a pretty good tool that will calculate inner wheel clearance as well as outer wheel position, when changing the width of your wheels.
We really hope this was helpful in explaining how wheel offset works. Thank you for reading.