Re: Offset: + or -- ?

["David R Johnson" <johnsnd@domain.elided>]

In AD-547, Bill (East?) responds to a question posed by Godfrey DiGiorgi, in
AD-546:

> I disremember which way is positive and which way is negative, and when it
> occurs to me to ask, I seldom get an answer.  I gave up worrying about it,
> though, because I never run into any car wheels that are offset in the
> opposite way.  If someone is mentioning the subject, it is understood that
the
> bolting flange is closer to the outside than inside of the wheel.
>
> Regardless of plus or minus, that information is hard to use.  To measure the
> offset requires finding the midpoint of the rim, which will be in midair, and
> then the distance to the center flange -- 40 mm, for instance.  The way I do
> it is to measure the rim width, say 6", divide that in half to get the
> midpoint (3").  Then measure the backspace, say 5", subtract the 3" and you
> have an offset of 2" [to the outside].

A good source of technical basics for wheel fitment can be found at the
TireRack website ( http://www.tirerack.com/ ).  In the left column, under
"Wheels", click on "Wheel tech" and one of the available topics is offset.
 This will get you to a nice diagram and explanation (
http://www.tirerack.com/wheels/tech/offset.htm is the exact URL). To summarize,
positive offset means the hub surface is toward the street side of the wheel
from the centerline, while negative means it is toward the brake side (deep
dish type).  [No affiliations, etc].

As Bill mentions, offset can be a little tricky to measure, and the subject is
further complicated by the existance of "backspacing", essentially a different
way of measuring the same thing.  The TireRack page does not appear to mention
this measurement, but as far as I know, Bill's explanation above is correct.  I
believe that backspacing is the distance from the hub surface to the wheel's
inner (brake side) rim, but I could easily be wrong.  Anyone know for sure?

Dave J.
1982 GTV6

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End of alfa-digest V7 #548
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