backspacing is the distance from the wheel mounting surface (where the lugs are) back to the edge of the wheel. another words, take a 8" wide rim w/ 4" backspacing. this means the mounting surface is in the center of the rim (also called zero offset).

So for a given width rim, you can have several different backspacings. and those backspacings are what may cause one 8" wide rim to work, but another 8" wide rim to rub.


-Rob