Are you interested in buying a new set of wheels for your ride? We'll bet that you already spotted some cool wheels by cruising the shows, flipping through Truckin' and catalogs, peeking in stores, surfing websites, and talking with your friends. You know the style of the wheel, and the diameter, but will the wheels fit? While the diameter of a wheel fires the imagination-you just gotta have 24s filling the wheel wells, right?-the backspace or offset of a wheel are necessary for determining how a wheel will fit inside of your fender. Most wheels' product descriptions include backspace and/or the offset, along with width and diameter. But, what is backspace and its cousin, offset, all about?
Here's an explanation of what backspace and offset mean. Backspace is the distance from a wheel's mounting surface to the back edge of the wheel in inches. By "back" we mean the brake side of the wheel. Offset measures the distance from the mounting surface of the wheel to the centerline of the wheel, and it is often indicated in millimeters. If the mounting surface is located closer to the street side of a wheel, then the offset is positive. If it is located closer to the brake side of wheel, then the offset is negative. If it is in line with the center of the wheel, then the offset is zero.
An example may simplify this: The centerline of a 10-inch-wide wheel is at five inches. If the wheel has a 6-inch backspace, then the offset is positive. Conversely, a 4-inch backspace would make the offset negative. A wheel that has a positive offset-or greater backspace-has to slide further into the wheel well to be mounted. Similarly, one with a negative offset-or lesser backspace-will tend to jut closer, or even beyond the fenders. By the way, spacers can be used to change the offset, as well. They slide over the wheel studs, sandwiching between the wheel and hub, creating negative offset, in a way, and thereby widening the track of the vehicle. Off-roaders often use this approach.
If you want to slam your suspension and tuck your tires, then it makes sense that you would lean toward a wheel with a positive offset (more backspace). But a negative offset (less backspace) might give you the room you need for that lifted look with big, knobby tires. There are other considerations, as well. If you have too little backspace, then a wheel is more likely to rub the brakes-or the suspension components. However, too much backspace and your tires may rub the fenders.