Alright all! Time for more wheel wisdom from your friend in the tire biz, Judah “JB” Burke. After a phone call I took today from a C7 Z51 owner, I think it’s time to discuss the basics.
There are some common misconceptions with the two. First off, a lot of people–not Corvette owners of course—think offset and backspacing is the same thing.
Let’s dive into it and get some fixed truths out of the way, starting with backspacing. By far, backspacing is the easiest to measure and get the hang of.
Backspacing is the distance in inches, from your wheels mounting surface to the back edge of your wheel. The easiest way to get this measurement, is to grab a tape measure and a straight edge. Set the wheel face down on a nonabrasive surface not to damage the finish. Next lay your straight edge across the rear edge of the wheel, make sure the straight edge reaches end to end across the wheel.
Measuring from the mounting pad, to outside inner lip of wheel with straight edge to achieve backspace measurement.Then simply measure from the mounting pad, right to the straight edge for your backspacing measurement (reference the photos to see exactly the surface I am referring to).
A good amount of wheel companies will discuss wheel fitments in backspacing increments.
Offset is measured in millimeters, and you can end up with negative, positive, or zero. The distance from the hub mounting surface to the center line of the wheel is your wheels offset. A tip to visualize a positive or negative offset, is to look at the hub mounting surface. If hub is toward the back side of the wheel center line it is a negative. If the hub is toward the front of the wheel it is positive.
Let’s take a quick look at a common 18×10” wheel with a 5.5” backspace, an easy size to figure out offset. Wheel widths are measured from bead seat to bead seat, a 10″ wide wheel will measure 11″overall. Now to the math fun, we are using 11” as our overall width. What is half of 11”? 5.5”, so you have now calculated your offset +0.
One more practice:
Why don’t we try one more regarding a frequent question I receive. Say you like the fitment you have, but you would like the wheel to sit out another half an inch. Still working with our 18×10” wheel, for simplicity.
When dealing with backspace that’s pretty simple, reduce the backspacing half an inch. What happens to your offset thou? How does it change? In essence our wheel is staying the same width, just moving towards the fender half an inch. This would effectively change the offset to -13mm.
It might take you a little time to become fluent in the language of backspace and offset, my best advice to you is get an offset/backspacing calculator. One of the best tools you can have at your disposal, and is free and online.
You’ve done a great job, give yourself a pat on the back and do a few donuts or burnouts in celebration! On a closed coarse or race track of course, definitely not when your boss is on lunch and comes back early!!!
Please email me with your fitment questions, and next wheel topic of discussion. Interested Corvette generation fitment? Spot on fitments? The top questions will get my attention and my next “Keepin’ It In The Fenders.” Thank you all for reading and stay tuned, same ‘Vette time, same ‘Vette channel.