Backspace? Offset? WTF!

Ok guys. I'm usually pretty good with math and diagrams, but the relationship between backspace and offset seems impossible.

1. Backspace is easy because you can measure it. It is the distance between the mounting surface of the wheel and the inner rim lip. Stick a ruler inside the wheel and a straight edge across the inside of the wheel and measure. On a 10" wheel, if this distance is 5", then the wheel is centered on the hub, with 5" inside and 5" outside.

2. Move the mounting surface outward - toward the outside of the wheel - by 1" and you should have a backspacing of 6". With 6" inside of the mounting surface there should be 4" outside - after all, the wheel is 10" wide and 6+4=10.

In #1, your offset should be 0mm. In other words, the mounting surface IS NOT OFFSET from the center of the wheel.
In #2, your offset should be 25.4mm, or 1". In other words, the mounting surface IS OFFSET 1" OUTSIDE from the center of the wheel.

But that isn't correct using the backspace calculators on the internet. Here's one: http://www.mrw-wheels.com/wheel tech/offset chart.html

Put in a 10" wheel with 0mm offset and the backspacing shows as 5.5". How is that possible if the mounting surface is in the center of a 10" wheel? Worse, how can the backspacing be 5.5" and the "front of wheel" be 5.5". That adds to 11" and the wheel is only 10" wide.

Put in an 8" wheel and the same is true! The total backspace and front space is always 1" greater than the width of the wheel. WTF?

I am desperately trying to find a set of wheels for the track for my car, but I can't figure out what size to order for the life of me!

An eight in wheel is 8 inches on the tire bead the off set is measured from the outside lip so an eight rim is close to nine inches to out side of lip

Grabinov911, your 10" wheel is ten inches as measured on the inside of the lip, meaning the tire beads will be ten inches apart. However, the outside of the wheel as measured from lip edge to edge will be eleven inches. It's the inner rim size that is usually mentioned, but the outer rim will be what gets closest to your springs or other areas of interference.

Incredible. So a 10 inch rim is 11 inches. That makes total sense - unless you're actually trying to fit wheels onto a car!!! At least now I see where the market for custom wheels comes from. Try finding a modern 17+ inch wheel over 9.5 inches width ("advertised width" not actual width) that will fit the back of a B-body.

There aren't a lot after market wheels that will fit. Right now I'm looking at 18x8 and 19x10 pro touring wheels but there aren't any with a 0 offset or negative offset, 5 inch or 4,5 inch backs pacing...

Bumping this post back up again

I've had a bit of luck finding a few choices...

what at have you guys come up with?

I use this.



- - - Updated - - -

Cragar SS all aluminum 612 in -12MM offset in 18X9.5 front and rear with 265x35x18 front and 285x35x18 rear. If the Wilwood brakes would not have pushed the rim out 1/2 and inch I could have gone with 285x35x18 front and rear. Granted the 285 leaves too much room in the rear wheel well but Cragar does not make an 18 in anything wider than a 9.5. Trust me I called and talked to the techs 3 times to make sure.

Are you sure?
Are you sure you sure?

Apparently the manufacturer of the hoops they use cannot make anything wider than a 9.5 in 18" diameter.

I custom ordered my rims through summit to get the backspacing I wanted.

Grabinov911 said:

Incredible. So a 10 inch rim is 11 inches. That makes total sense - unless you're actually trying to fit wheels onto a car!!! At least now I see where the market for custom wheels comes from. Try finding a modern 17+ inch wheel over 9.5 inches width ("advertised width" not actual width) that will fit the back of a B-body.

Click to expand...

68-70 b body will fit a 12" rim and 305 tire before a tub

That offset chart is awesome....would have saved me hours of searching if I had that from the beginning.