What Is Rim Backspacing?by William Zane
Wheel (or rim) backspacing is one of the ways that a wheel's dimensions and fitment applications are described; other measurements are offset, width, diameter and bolt pattern. A wheel's backspacing affects how it will fit on your car and is an important aspect of proper wheel fitment. Backspacing and offset are often confused.
Backspacing is the distance from the mounting pad of the wheel to the inside edge of the wheel. The mounting pad of the wheel is where the pad seats up against the hub.
Determining wheel offset uses a similar measurements used to determine backspacing but is the distance from the wheel pad to the center of the wheel.
How to Determine Backspacing
To determine a wheels backspacing, lay the wheel flat on the ground with the face of it facing down. Place a flat edge across the wheel so that it is supported on either side of the wheel. Using a tape measure or ruler, measure the distance from the pad in the center of the wheel (where the mounting holes are located) to the edge of the wheel. The distance is the backspace.
How Does it Affect Fitment?
Backspace affects fitment of a particular wheel on a particular vehicle because it affects how far out or in the wheel itself is located in relation to the hub. A wheel with a smaller backspace will locate the wheel away from the body and may result in fender rubbing while also increasing strain on the hub and suspension. A wheel with more backspacing has the opposite affect and moves the wheel in toward the body. This situation can result in the wheel interfering with and making contact with the brakes, suspension or inner wheel well.
Knowing your wheel's backspacing is needed if you want to determine the wheel's offset. To determine offset, measure the wheel's width, which will end up being something like 6, 7 or 8 inches or more. Next, determine the wheel's centerline, which will be the measurement from the outside edge (or flange) to the inside edge divided by two. Subtract the wheel centerline from the backspace that was previously determined to get the offset. If the wheel's backspace is less than the wheel's centerline, then the wheel has a negative offset. If the backspace is larger than the wheel's centerline, then the wheel's offset is positive.
- Image by Flickr.com, courtesy of Saquan Stimpson