Wheel offset and backspacing can be a little confusing but are absolutely crucial when ensuring proper fitment of aftermarket or replacement custom wheels.
In order to determine wheel offset, you first need to establish two locations on the wheel. The CENTER-LINE and the MOUNTING FACE. The center-line is the line running around and through the barrel of the wheel, marking the center of the wheel’s width. The mounting face, or axle pad is the flat surface on the back side of the wheel’s plate, which mounts to the vehicle’s brake rotors when the wheel is mounted on the vehicle. The distance between these two locations is measured in millimeters and is referred to as the wheel offset.
When the mounting face contacts the rotors, the offset will determine how much of the wheel is dish, as well as exactly where the wheel will sit inside the vehicle’s wheel well. When the mounting plate is on the inside of center-line, towards the suspension, it is referred to as negative offset. A negative offset wheel will usually have a very deep dish, and will sit farther out from the suspension. When the face is outboard of center-line, it is referred to as positive offset will have a shallower dish, and the wheel will sit farther in towards the suspension. Zero offset means that the mounting face is directly on the center-line.
Backspacing is very simply the space between the mounting face and the inboard flange of the wheel. Backspacing therefore, depends on both the overall width of the wheel’s barrel and the offset of the wheel, or where exactly the mounting face is in relation to that width. Wheel backspacing is very easy to measure. Lay the wheel face down on the ground and then take a straight edge and lay it across the back of the wheel, If a tire is mounted on the wheel, be sure it does not interfere. Then take a ruler or tape measure & measure from the back of the mounting face to the straight edge. This will give you the backspacing of any type wheel you are measuring. As offset determines where the wheel will sit within the wheel well, backspacing determines how much of the wheel will protrude inboard beyond the rotor and towards the suspension components.
As you can now understand, if you install wheels on a vehicle with significant negative offset, they will likely be deep-dish wheels and will sit out from the edge of the wheel well. The backspacing will generally be fairly low with the mounting face closer to the back edge of the wheel, unless the wheel is abnormally wide, so the wheel and tire would have plenty of space to clear the suspension. However, if you were to replace those wheels with a more positive offset wheel or a wider wheel with more backspacing, this will put much more of the wheel towards the back side of the wheel well, and you would run the risk of having the inside of the wheel or tire to rub against the suspension. This would be a bad thing. You can easily destroy your expensive new wheels and/or tires by choosing wheels with the wrong offset and/or backspacing. If the offset is not correct, the vehicle’s handling can be adversely affected. Typically, when the wheel width changes, so does the wheel offset. If the offset remains the same, the extra width would be split between the inside and outside. This normally won’t work on a vehicle that hasn’t had suspension modifications. This is why offset and backspacing are two concepts are very important to understand when purchasing aftermarket/custom wheels. Our extensive wheel and tie databases allows our sales staff to assist you in choosing a wheel and/or tire combination that will fit your vehicle perfectly.