Welcome, students, to our final module of Wheel Anatomy. Today we will be discussing the rather complex concepts of offset and backspacing. These can be difficult concepts to understand, but are absolutely essential for ensuring proper fitment of aftermarket or replacement wheels. To refer to the diagram, right-click here, and open the link in a new tab.
To understand offset, one must first find two locations on the wheel. The centerline is the line running around and through the barrel of the wheel, marking the center of its width. The mounting face, or axle pad is the flat surface on the back side of the wheel's plate, which 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, as well as exactly where the wheel sits in the wheel well. When the mounting plate is on the inboard side of centerline, towards 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 farther in towards the suspension. Zero offset means the face is directly on the centerline.
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 towards the suspension components.
As you can see then, 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, this will put much more of the wheel towards the back side of the wheel well, and can easily cause the inboard side of the wheel or tire to rub against the suspension. Nothing good ever comes of that. I'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.
And with that we conclude our third module on Wheel Anatomy: Offset and Backspacing. We here at Whatsamatta U. hope that this course in Wheel Anatomy has edified and empowered you for safer and more comfortable driving. Whatever that means. If you do have any questions, please feel free to ask them in my forum.