The distance from the wheel mounting surface to the back plane of the wheel. Commonly in inches (SAE)
The distance from the wheel mounting surface to wheel centerline. Commonly in millimetres (Metric)
Backspacing and offset are related to one another. You must know 2 variables in order to find the third, the 3 variables being Width, Offset and Backspacing.
- (Width/2) = Offset
Example: We have an 8" wide rim with 5" of backspacing. Divide 8" by 2 gives us 4". Using the formula, 5" minus 4" equals 1" of positive offset, or 25.4mm of offset.
+ (Width/2) = Backspacing
Example: We have a 10" rim with -48mm of offset. Divide 10" by 2 gives us 5" Divide -48mm by 25.4mm/inch gives us -1.89" of offset. Using the formula, -1.89" offset plus 5" equals 3.11" of backspacing.
2(Backspacing - Offset) = Width
Example: We have a rim that has a backspacing of 4", and an offset of 12mm. 12mm divided by 25.4mm/inch gives us roughly 0.5". 4" minus 0.5" equals 3.5". Multiply that by 2 equals 7" The rim is 7" wide.
Backspacing is the distance from the wheel’s rear mounting surface (hub) to the edge of the rim. The best way to measure backspacing is to place a straight edge across the rims and measure to the mounting surface.
The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel. The offset can be one of three types.
* Zero Offset - The hub mounting surface is even with the centerline of the wheel.
* Negative Offset - The hub mounting surface is toward the front or wheel side of the wheel.
* Positive Offset - The hub mounting surface is toward the outside of the rim. These are generally found on front wheel drive cars and newer rear drive cars.
If the offset of the wheel is not correct for the car, the handling can be adversely affected. When the width of the wheel changes, the offset also changes numerically. If the offset were to stay the same while you added width, the additional width would be split evenly between the inside and outside. For most cars, this won’t work correctly.
offset = backspacing - (overall wheel width/2)
wheel = 15×6.5
backspacing = 4″
overall width = 7.5″
4″ - (7.5″ / 2) = 0.25″ positive offset