<VV> wheel off-set and backspace defined

[Sethracer at aol.com]

In a message dated 5/18/2010 11:57:39 A.M. Pacific Daylight Time,  
hyarnell1 at earthlink.net writes:

I'm  confused. If backspace is the distance from the inside rim to the 
mounting  face, what is offset?
Harry Yarnell




Backspace is a measurement - Offset is a calculation.
Consider that a wheel has several measurement points. The width and the  
diameter are pretty easy to conceive - remembering that the width of the wheel 
 is the width at the tire mounting point, inside the rims.
 When you measure the width of a wheel, you can identify a center  point in 
the width. That is the center plane of the wheel. Also on a wheel is  the 
plane that represents the mounting face of the wheel. This is the face that  
contacts the drum or disk hat when the wheel is bolted in place. The 
distance  between that mounting face plane and the center plane of the wheel outer 
rim is  the offset. Since one of the planes (the center plane) exists only 
in space, you  cannot (usually) measure offset. You usually calculate it by 
measuring to  another point, say, the rear most point of the wheels outer 
rim, and subtract  the two measurements. The biggest problem is that the two 
planes can exist in  three different relationships. They can be co-located 
(Zero-offset), the  mounting face plane of the wheel can be closer to the inner 
lip or closer to the  outer lip. This directional indicator is the problem 
with defining  offset. If the wheel center line plane is one inch away from 
the  mounting face, you have a one inch offset wheel. But is it positive or 
negative?  There is no general agreement on which way is positive or 
negative. GM says one  thing, the aftermarket wheel manufacturers say another. The 
nice thing  about backspace is that it is easily measured, (even with tires 
installed) and  always defined the same way. If someone says OH, negative 
offset means  "this" or so-and-so says positive offset means "this". They 
could both  be right, because there is still disagreement on the definition. So, 
who is  right? It doesn't matter because we have good ol' backspace to call 
on. The  Corvair can use wheels that are up to about one extra inch of 
backspace,  depending on the diameter. because of this figure, if you go to much 
wider  wheels, it will mostly go on the outside. This can cause handling 
"quirks" and  demands special alignment techniques.     


Seth Emerson

C's the Day! -  Corvair, Camaro, Corvette