The back spacing measurement is critical in the fitment of the wheel (and tire) to the vehicle. Since the suspension, brake, steering, and drive systems are typically located behind the wheel, the back spacing is used to define a volume behind the wheel where these items can exist.
Backspacing is defined as the distance from the hub mounting surface to the inside lip of the wheel (measured in inches).
More backspacing puts the tire in closer to the center of the truck.
A related term is known as offset, which relates the hub mounting surface to the centerline of the rim. A 0 offset indicates the hub mounting surface is at the exact centerline of the rim. In this case, the back spacing would then be equal to 1/2 the rim width. Offset is measured such that positive offsets mean the inner lip of the rim is closer to the vehicle and negative offsets move the rim away from the vehicle resulting in the tires sticking out farther and increasing the likelihood that the tire fronts will rub on the back of the fender.
So more backspacing means the wheel sits in closer to the axle and that less of the wheel's width will appear outside of the wheel mounting flange, giving a narrower track. More offset means the wheel mounting flange is closer to the inside of the wheel so consequently more of the wheel's width is to the outside, giving a wider track.
Items required to measure wheel backspace:
- Tape measure
- Straight edge
- Wheel without tire (preferred)
The easiest way to measure backspace is to:
- Lay the wheel face down onto the ground so the backside of the wheel is facing up.
- Take a straight edge and lay it diagonally across the inboard flange of the wheel.
- Take a tape measure and measure the distance from where the straight edge contacts the inboard flange to the hub mounting pad of the wheel.
This measurement is backspace. The illustration below shows three wheels with 2",3", & 4" backspace.