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Nuts & Bolts: Backwards Backspacing

Posted in How To: Tech Qa on August 28, 2018
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Photographers: Trenton McGee

I’m in the market for new tires and wheels and want to make sure they don’t stick out past the fenders, because here in Pennsylvania they’re pretty strict about tire coverage. What I don’t understand is how to figure out what I need. Is backspacing and offset the same thing or different? Why do wheel companies use different measuring techniques? One manufacturer will say their wheel has 4 1/2 inches of backspacing, and the next one will say -12 mm. How am I supposed to compare these two measurements?

Mike C.

We feel your pain. Once upon a time is was easy because just about every wheel company used backspacing, but as the metric system became more and more standard in the rest of the world, so too did using wheel offset. These days they use one or the other, or both. It can get horribly confusing, but maybe this will help clear things up.

Wheel backspacing and offset are two ways of measuring the same thing: the position of the wheel’s mounting face in relation to the wheel’s outer shell. Backspacing is the distance between the mounting face and the inner lip of the wheel (usually expressed in inches), while offset is the distance of the mounting face relative to the center of the wheel (usually in millimeters).

Picture a wheel installed on a vehicle. If the wheel mounting face is positioned before the center of the wheel (tire is pushed away from the vehicle) it’s a negative number, and if the mounting face is positioned after the center of the wheel (tire pushed towards the vehicle) it’s a positive number. So a wheel with a width of 9 inches and 4 1/2 inches of backspacing has an offset of -12 mm, and one with 5 3/4 inches of backspacing has an offset of 18 mm.

Wheel backspacing/offset is important for several reasons. One is that it determines how far the tires stick out (or don’t) from the fenders, but it also can make or break whether or not a wheel clears brake calipers or other suspension components. This is less of a concern on solid axle vehicles, but backspacing is critical on most IFS applications. Most modern IFS lifts leave the upper control arms in the stock location but lower the lower control arm assembly. A new steering knuckle spans the increased distance between the two, and the new knuckle usually requires a different wheel offset than stock to avoid wheel interference. For this reason, lift companies will often specify a maximum backspacing or offset. Also, wheel offset can make or break whether or not a certain tire size makes contact with fenders. It changes the scrub radius of a vehicle, which is the distance between the centerline of the ball joints or kingpins and the center of the tire’s contact patch with the ground. What scrub radius does is more than what we really want to get into here, but the takeaway for this discussion is that the tire’s depth in the wheelwell changes depending on steering angle, and this change can mean the difference between a tire and wheel package hitting or clearing sheetmetal.

One last thing to consider is that wheel width will have an impact on the final position of a tire in the wheelwell. If there are two different wheels with 4 1/2 inches of backspacing, the one that is 8 inches wide will pull the tire deeper into the wheelwell than the one that is 10 inches wide. The same goes for offset.

This is one of those things that seem really easy on the surface but get complicated quickly. It’s also a good reason to consult your local off-road shop for some expert advice. You might pay a little more, but you’ll know that what you buy is right the first time, not to mention supporting the local economy.

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