This is an issue I have little experience with. I have a 8 3/4 going in my 65 Barracuda. We were going to pass on the installation and sell it, but that all changed. The banjo has been shortened (by the PO) which we have come to the conclusion was done for a special application. (Tubs?) From brake plate flange to flange is 49.5" The Axle flanges (5X4.5) are aligned with the center of the fender well. The distance from the leaf spring to the back of the axle flange is 4.5". I would like to run a 15" wheel that would allow me to mount the largest (street / strip 8" to 10" width tread) tire that I can. Will a wheel with no back space be what I'm looking for or is this going to require a wheel with either + or - back space?
Norm
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So with a housing that's 49.5" wide and BBP axles and drum brakes you should end up with a wheel mount to wheel mount axle width of about 54 11/16" with that shortened housing. Which means about 1-9/16" less backspace than what you would need for a standard A-body width 8 3/4 with BBP axles.
On my '71 GT I have a standard 8 3/4 A-body axle and BBP drums with the stock spring locations. It's 6" from the wheel mount back to the springs. Yes, that's a '71 but the axle perches are in the same place so the springs are too (quarter measurement is a whole different story though). So, on your car it should be approximately 4-7/16" from wheel mount to spring. Let's just call that 4.5". That's the total distance. Out of that you need a good 1/2" to the springs if you have 15" rims with a decently tall sidewall, to allow for flex. So, 4". Then you need to know the rim width and section width of the tire. Early A's are not my area of expertise, but from what I understand a 245/60/15 will be about as big as you can hope to go with stock wheel tubs. A 245/60/15 has a 9.5 to 9.8" section width on a 15x7" rim, those are the factory specs. Let's call it 10" if you mount it on a 15x8, should be safe. A 15x8" is actually 9" wide outside lip to outside lip, and backspace is measured to the outside lip. So, 1" more section width than rim width, 1/2" bulge per side. That gives you 3.5" for your backspace, which works out to a -25mm offset.
So, BALLPARK, you'd be looking at a 15x8" with a 3.5" backspace or a 15x7" with a 3" backspace, which is a -25mm offset on both those.
Now, like I said I'm not an early A guy. I know people have run 245's but not all of them may be able to get away with that. If you went with a 235 I would definitely run a 15x7" not a 15x8", and might consider up to a 3.25" backspace because the narrower tire will allow you to go a little closer to the springs because you can always add a spacer, but if you run out of space to the outside moving the quarter is a lot harder.
I would suggest assembling the brakes and measuring from the face of the drum back to the springs, as well as from the face of the drum out to the quarter because that will let you confirm these numbers for YOUR car. A wheelrite tool is nice, but you can do the measurements with a carpenter's square too. The rest is just math. Even the wheelrite tools aren't perfect, because tires that are advertised as being the same size differ in actual measurements from different manufacturers. For example, a 245/60/15 Cooper Cobra is 9.5" wide on the section width when mounted on a 15x7" per Cooper. But a 245/60/15 BFG T/A is 9.8" according to BFG.
Measuring the backspace has nothing to do with the brakes. The rim mounts to the axle, and the backspacing is measured from the face of the drum, which is mounted to the face of the axle. Out a straight edge on the face of the axle, and measure from the edge that is on the face back to the spring and subtract at least an inch, more like 2” to allow for sidewall bulge in the tire. You can also measure from the same edge of the straight edge to the wheel lip or outer wheel tub to get the maximum wheel width.
look up my post on MeMikes thread about rebuilding a Victoria, I included pictures and descriptions that anybody can understand.
1966 Valiant Sedan V8 Swap (Victoria)
Page 2 of this thread
Yeah, thats not true about the brakes. Or the 2" subtraction thing. If you know the measurement from the face of the brake back to the spring, the width of the rim and the section width of the tire you can actually figure out how much room you'll need, 2" is just a wild guess. It won't be enough if you have a wide tire on a narrower rim that has a lot of tire bulge. And it will be WAY too much if the section width of the tire being used is close to the overall width of the rim. In the OP's case, the size tires and rims he wants to run would work much better with about 1" subtracted from his wheel mount to spring measurement. Using 2" would mean losing at least a 1/2" of tire, possibly as much an an inch depending on the rim/tire combo. That's a lot of tire, especially if you're talking about fairly narrow tires to begin with.
You need to know which brakes because the SBP and BBP axles use different axle flange offsets, that's why that question was asked. The drum being on the axle also moves the rim out. And, if the OP is using disks instead of drums you need to know that as well, because they move the wheels out even further (usually around another 1/4"). In this case, knowing that the OP is using BBP axles, 10x2.5" drums and the width of the housing allows the backspace to be calculated from the stock spring location that he's using. So the brakes DO matter.
