Wheel offset & backspacing

[andrethx]

Those of you who have been following the 4x100 adapter group buy thread may have seen me struggling to understand wheel backspacing and offset dimensions. I've put up several posts trying to sort all of this out, with the goal of finding an "optimum" set of wheel dimensions. Earlier this evening, I posted a link to a thread on a wheel enthusiasts' site that laid things out with tables and online calculators, but didn't explain the manner in which these calculations were done.

I had known that backspacing = offset + .5(width), but my calculations never came out right -- I could never figure out how wheel manufacturers came up with their dimensions. I dug a bit deeper, and I think I've sorted it out. I'm posting this in case anyone else here has been similarly confused.

I found an explanation on, of all things, a website for Chevrolet Chevelle enthusiasts. Here's where I was going wrong -- the wheel width that is used to determine backspacing & offset is not the "nominal" width of the wheel. The "nominal" width of the wheel (the name I'm giving to the advertised width of the wheel) is measured bead-to-bead -- roughly, the cylindrical portion of the wheel's "barrel." The width that is used to determine backspacing/offset is the overall width of the wheel, including the lips. These lips are typically estimated to be .5", but in reality they vary a bit so you can't know the exact backspacing/offset until you have the lip dimension...the table from the other thread and the online calculators use an assumption of a .5" lip on the wheel, and that estimate is conservative enough that it will almost always give you a good fit.

The use of these two different widths kind of makes sense...the "nominal" width is important for choosing tire sizes, so that's the width that's given usually. The real, "overall" width of the wheel is important for clearance issues, so that's the width that's used to calculate backspacing/offset. As the Chevelle Stuff article points out, though, it's odd that most people really want to know the backspacing, but wheel manufacturers usually give only the offset.

Hope this helps.

Andre