73+ disc brake backspacing
Coool thanks 4.5 BS i think it is, its going on a 1968 barracuda
Oh, well on a ‘68 Barracuda I’d keep it at 4.25”. On a 15x6” that would still move the wheel a 1/2” inboard compared to the 15x7’s with 4.25” backspace. And the fender clearance on a ‘74 Duster vs. a ‘68 Barracuda is pretty similar. If anything the Barracuda will have more clearance everywhere except maybe the lower front corner.
If you look at the wheel offset it makes that difference more obvious, the narrower rim with the same backspace actually moves the centerline of the wheel. A 15x6” with a 4.25” backspace has an offset of 20mm, while a 15x7” rim with a 4.25” backspace has an offset of 6mm. So the centerline of the wheel actually moves a 1/2” in board on the 15x6”.
The issue with running more is brake and ball joint clearance. You should be ok at 4.5”, but the design of the rim does play a part in the caliper clearance, so it’s not the same for every wheel.
I’ve posted that graphic a few times myself, but I’m still pretty sure those are just generic numbers. At the very least I know that on my ‘71 with 10” SBP drums the track width didn’t widen by that much by going to ‘73+ disks. I’m pretty sure it wasn’t that narrow to start.
But that diagram doesn’t say what brake set up gave those numbers for the earlier cars. After all there were 9” and 10” drums, and the KH disks which all used different spindles and all could have had slightly different tracks. It lists the rear track as a single number too, even though the 7.25 and 8 3/4’s are different widths. The rear track width going from a 7.25 to an 8 3/4 is a good 1” different. Screwed me up the first time I bought rims for my Duster, I figured the 7.25” was the same width as an 8 3/4, turns out it’s not even close and I ended up with a set of rims that didn’t fit.
