[DeTomaso] Offset versus Backspace (was Re: What is the widest wheel I ca...

[SOBill at aol.com]

Asa,
 
Here's the deal:
 
When a rim is stated as "7 inches," this dimension is related to  tires and 
not directly usable for backspacing/offset determination. What is  stated to be 
a 7 inch rim is 7 inches between the inside edges where  the tire mounts. 
This rim width is directly related to tire sizes and indirectly  related to 
backspacing/offset by the thickness of the flange on the rim. This 7  inch rim 
dimension can not be measured when the tires are mounted on the rim and  the rim 
thickness can only be estimated.
 
To solve this problem, backspacing/offset are referenced  to the most inboard 
(i.e toward the centerline of the car) edge of the  rim. The inboard edge of 
the rim in accessible when the tire is mounted.  Backspacing is the distance 
from the inboard most edge of  the rim to the wheel mounting surface. 
Backspacing can be measured while  the tire is mounted.
 
Offset is the distance from the wheel mounting surface to  the centerline of 
the wheel. The centerline of the wheel is halfway between the  outside edges 
of the rim. The distance between the outside edges of the rim can  be measured 
while the tire is mounted and accounts for the thickness of the  flanges on 
the rim. 
 
When the backspacing and the overall rim width have been measured,  the 
offset can be calculated.
 
On a rim with an outside rim width (i.e. outside edge to  outside edge of the 
rim) of 8 inches, the rim centerline is 4 inches from  the inboard edge of 
the rim. If that wheel mounting surface had a backspace of 4  inches from the 
inboard edge of the rim, the mounting surface and the rim  centerline would 
coincide and the wheel would have an offset of 0 inches.  If that same wheel had a 
backspace of 5 inches, the mounting surface would be 1  inch outboard of the 
rim centerline and the wheel would have 1 inch of positive  (i.e. outboard 
from the car centerline) offset. If, in the extreme, that  wheel had a backspace 
of 8 inches, that is the mounting surface was all the way  at the outboard 
edge of the rim, that wheel would have a positive offset of 4  inches.
 
To help  confuse all of this, wheel makers don't give the overall rim width 
in their  wheel specs.
 
>>>>>http://usacomp.com/terms.htm is a very good.  Note that they show 
"overall rim width" when describing offset and  backspace and then show "rim width" 
for tires in the next  picture.
 
Hope this helps.
 
Have fun,
 
 
SOBill  Taylor
sobill at aol.com
 
 



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