The Definition of Offset and Backspacing in Wheels

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Welcome to our final module of Wheel Anatomy, in which we will discuss offset and backspacing. These can be difficult concepts to understand but are absolutely essential to ensure the proper fitting of aftermarket or replacement wheels on cars and trucks. Improper fitting can lead to premature tire wear or, worse, a blowout.

Offset

To understand offset, one must find two locations on the structural face of the wheel, or barrel.

First, the centerline, which is the line running around and through the barrel that marks the center of its width. Next, the mounting face, or axle pad, which is the flat surface on the back side of the wheel's plate that stays in contact with the car's rotors when the wheel is tightened on. The distance between these two locations, measured in millimeters, is the offset.

As the mounting face contacts the rotors, the offset will, therefore, determine how much of the wheel is dish (that portion of the wheel that comes out beyond the spokes) as well as exactly where the wheel sits in the wheel well. When the mounting plate is on the inboard side of the centerline, toward the suspension, this is called negative offset. The wheel will probably have a very deep dish and will sit farther out from the suspension. When the face is outboard of centerline, this positive offset will generally mean a shallower dish, and the wheel will sit further in toward the suspension.

Zero offset means the face is directly on the centerline.

Backspacing

A related concept to offset, backspacing is 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 plate is in relation to that width.

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 toward the suspension components.

What Positive and Negative Offset Mean

As you can see, if you have wheels on a car with substantial negative offset, they will be deep-dish wheels that usually will sit out from the edge of the wheel well. Backspacing will generally be somewhat low, with the mounting face closer to the back edge of the wheel, unless the wheel is unusually wide, so the wheel and tire 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 one with more backspacing, that would put much more of the wheel toward the back side of the wheel well, and could easily cause the inboard side of the wheel or tire to rub against the suspension. Nothing good ever comes of that. We've seen hundreds of wheels and tires destroyed by bad offset decisions. A very light tire rub or tires that just make contact on turns can be almost unnoticeable—until a tire blows out. This is why these two concepts are among the most important to understand when replacing your wheels.