Offset vs. backspacing

[ChallengerHK]

When I was in high school and I had just bought my car, the most common way of measuring wheel side-to-side mounting differences was "offset," the distance between the centerline and the mounting pad. The few people I knew who were at all into suspensions used offset, magazine article used offset, and the handling books I have from back in the day use offset.

When I had my last Jeep, at one point I had to get a new set of wheels to eliminate the 1" spacers. When I was talking with the tech I told him to just find me a set of wheels with an extra inch of offset to replace the stock wheel+spacer combo. He looked at me and said "What's offset?"

Nowadays all I hear is "backspacing," which I take to be the distance between the mounting pad and the inner bead seat (correct me if I'm wrong). Any reason for the change? Does backspacing measure some critical thing that offset doesn't?

[Jamiez]

Well this page shows the differences:

https://www.rsracing.com/tech-wheel.html

Pretty much if you know two of backspacing, offset, or width you can calculate the other. 

I suspect backspacing might just be more common because it can be measured directly by laying rim flat and using simple tools, while the offset requires measuring and then calculating from the center line

[ChallengerHK]

I suspect backspacing might just be more common because it can be measured directly by laying rim flat and using simple tools, while the offset requires measuring and then calculating from the center line

That makes perfect sense.

[go-fish]

BS is a better measurement for me because I have suspension components I want to clear and it really helps to see how far IN you can go with a given wheel. Offset may suit your fancy if you weren't interested in going to the inside with your wheel width. If you were interested in deep dish wheels and wanted that 80's raked look you would give a hoot what the inboard side of your wheel was. You would be interested in the offset if you were worried more about fender lip clearance or the "deep dish" look.