Re: Offset: + or -- ?

[Simon Favre <simon@domain.elided>]

I don't see what the confusion is. Offset is offset. The way the Tire
Rack page defines it is correct. They would know. There's a real simple
way to remember this. American muscle cars have large negative offsets.
FWD (and most modern) cars have large positive offsets. There's no point
of view about this. The tire and wheel industry has defined these terms
consistently. People just get confused about it.

Here's another cheap and dirty way to measure offset. 

1) Measure the TOTAL width of the rim, including the lip and divide by 2.

2) Measure the back space from the back edge of the inboard lip to the 
   mounting surface. A straight edge and a tape measure will do.

3) Subtract #1 from #2 and you have offset, correctly + or -.

Another way to state this is:

** True offset is the back space minus half the total rim width. **

The only assumption here that might prove inaccurate is that the inboard 
lip is the same width as the outboard lip. The more accurate way to 
measure #1 is to take half the stated rim width (measured between the 
lips) and add just the rear lip width. My way is faster. ;=)

When fitting aftermarket wheels that are a different size than the 
original, both offset AND back space are important. Ideally, you want to
keep the same offset so you don't change the suspension geometry, BUT
in some cases, you have to compromise the offset because a rim of that
width and offset has a back space that interferes with a ball joint or
a fender lip. 


Godfrey DiGiorgi wrote:

>The Tire Rack page is what raised the question in the first place. Dave
>Partridge pointed out to me that the measurement described by Tire Rack is
>actually hub offset, not wheel offset ... it's a matter of point of
>reference. A negative hub offset produces a positive wheel offset. Thus my
>confusion. 
>
>I agree that it's often difficult to measure this stuff. I've found in the
>past, on the rare occasions when I was in need of a particular offset or
>change, what I did was measure the thickness of the rim from a surface
>plate to the bead seat area adjacent, from that to the opposite bead seat,
>and from the surface plate to the hub mounting flange. These three
>measurements give you the reference basis needed on the original wheels.
>Then you can determine by measuring the same three things on a set of
>candidate replacement wheels exactly what the differences are.

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