NoPistons -Mazda Rx7 & Rx8 Rotary Forum

maybe this will help:

Wheel Offset & Backspace

There is not a universal definition of negative and positive offset. The conventions are different among different companies and different for various countries. Since wheel offset can be only be defined one of two ways, it can't get too confusing.

It gets more confusing when you throw backspace into the mix. However, backspace is the only accurate way to really define the fit of a wheel. Offset will work OK if your replacing a stock wheel with another of the same width. However, as soon as you go to a wider wheel, specifying a stock offset for the replacement may not be the way to go. More often than not, when you are making small increases in width from stock, staying with the same offset is the correct thing to do. But change the brakes, modify the suspension, use different hubs or make a big jump in width and we need to consider both backspace and offset.

So what are offset and backspace? Offset is the distance from the Centerline of the wheel to the wheel bolt up face (the part of the wheel that contacts the car's hub when you attach the wheel). Positive or Negative is the descriptive term that tells us whether the wheel bolt up face is farther towards the inner or outer part of the wheel. Defining which side positive or negative means is the part not everyone agrees with. Backspace is not near as confusing. Backspace is the distance from the edge of the inner rim to the wheel bolt up face.

Let's look at some drawings to illustrate all of these terms.

On the right we're looking at a 8.5" wide wheel that has been cut in half right down through the middle. The red shows you where the cuts were made. The important parts are pointed out. The Inner Rim is always the one facing the car (the one you don't see when you bolt the wheel on). The Centerline is always in the exact center of the wheel.

Now move to the next illustration.

Here we put the wheel back together and turn it a bit so that we're looking from the inside through to the outside. The centerline has been turned it into a piece of blue transparent material. We can see that the wheel bolt up face is on the far side of the Centerline. In other words, the wheel bolt up face is closer to the outside of the wheel than the inside. Go back to the first drawing and you can see again that the wheel bolt up face is closer to the outside of the wheel.

Next we'll calculate how far the wheel bolt up face is from the centerline (the offset).

The formular for calculating offset is the overall width of the wheel divided by 2 gives us the centerline. That value minus the backspace dimension equals the offset. The calculation to the left gives a result -0.75" and that describes a wheel with a negative offset of 0.75". Now we know what negative offset is. The wheel bolt up face is closer to the outside of the wheel than it is to the inside of the wheel. More precisely, the wheel bolt up face is offset from the wheel centerline towards the outside of the wheel by 0.75".

Let's change things and see what that does to Offset and Backspace. On the wheel we sawed in half to the right, everything is the same as our first wheel, except that this one has a 2.0" larger outer rim. This gives us a 10.5" wide wheel, instead of one 8.5" wide. You can see that the centerline on the 10.5" wheel has moved. Because we've added 2" to the width of the wheel, the centerline has moved 1". That 1" move has actually placed it on the other side of the wheel bolt up face from where it was in the 8.5" wheel.

Move to the next illustration and hopefully if your confused, it will clear up.

Here we've put the wheel back together and put in our blue transparent material to show us the centerline. In this wheel the the bolt up face is not behind the blue material, but protruding through it. Another way to describe it is that the bolt up face is closer to the inside of the wheel than the centerline.

This is the opposite of the situation we had in the 8.5" wheel. Keep in mind that all we did was make the outer rim 2.0" larger. Go to the next block and we'll calculate the offset.

The calculation to the left gives a result 0.25" and that describes a wheel with a positive offset of 0.25". Now we know what positive offset is. The wheel bolt up face is closer to the inside of the wheel than it is to the outside of the wheel. More precisely, the wheel bolt up face is offset from the wheel centerline towards the inside of the wheel by 0.25".

Well, that's our explanation of offset. As stated above, it may not be the same as others you've seen, but it is the way we do it.