[DeTomaso] Wheel Measurements

[SOBill at aol.com]

Will,
 
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  

 
In a message dated 7/27/2008 3:53:18 P.M. Pacific Daylight Time,  
wdemelo at cogeco.ca writes:

What is  the difference between back-spacing and off-set and where are they  
measured?
Also, what are the widest 17' tires made?  
Will
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