To start things off, I’d like to get a few definitions out of the way.
The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel and for some ungodly reason is usually measured in mm. the offset can be one of three types.
Zero offset - the hub mounting surface is even with the centerline of the wheel.
Positive offset - the hub mounting surface is toward the front or wheel side of the wheel.
Negative offset - the hub mounting surface is toward the back or brake side of the wheels centerline.
Backspacing of a wheel is the distance from the hub mounting surface to the rear outer most edge of the wheel.
Frontspacing is a term not used too often, but is a term I like when discussing a wheels ability to tuck. It is the distance from the hub mounting surface to the front outer most edge of the wheel.
As an example I’m going to use a wheel that is 20x8.5 with a +19mm offset as this seems to be a pretty common size.
The first thing you have to understand is that actual wheel width is about 1 inch wider than stated, so a wheel that is 20x8.5 in actuality measures 9.50 inches wide. That being said the centerline of the wheel, +0.00mm offset, is the same as 4.75in backspacing/frontspacing.
9.50in/ 2 = 4.75
Now to get the offset from metric to a more understandable unit of measure (inches), we are going to divide it by 25.4. This number is a constant as there are 25.4 mm in an inch.
19mm/25.4 = 0.7480314in
For practical reasons, we’ll round up to 0.75in. Since we already established that our centerline is 4.75, and we are dealing with a positive offset, we are going to add 0.75 in to 4.75 to give us a wheel backspacing of 5.50 inches. This in turn will allow us to figure up the wheel frontspacing by simply subtracting the wheel backspacing from the overall wheel width giving us 4.00 inches.
9.50in – 5.50in = 4.00in
more info on wheel/tire/tucking combos to follow
