Originally Posted by Driver8

Scottie - Centering rings are used to center a wheel with a larger center bore (female end) on a hub with a centering nub (male end) whose diameter is less than that of the center bore of the wheel. (Pardon my use of the word "nub," but I can't think of a more technical term at the moment.) The centering ring fills the area between the hub and the wheel bore and provides a tightly centered fit.

A tight fit is essential to maintaining the wheel and tire's centering on the hub. If it's not centered, you will have vibration caused by runout (vertical up and down motion). The torque required on a set of lugbolts to hold a wheel in place that has a couple millimeter's play is much great than the standard 80-90 lb-ft we use to attach a wheel to a hub. This is why centering rings are so important.

A centering ring, in its most common definition, does not affect offset of the wheel. It only centers the wheel on the hub. You may be thinking of spacers, which typically bolt onto a hub to extend the mounting point (usually 10, 20 or 30mm) of the wheel. Hope this is of some value. If I've missed anything, please add or fill in the blanks where appropriate.

Mike