Back Spacing, which way?
Looking for some help from the wheel experts.
I am working with a 1972 Plymouth Scamp. I just installed an “A” body 8 3/4, Doctor Diff large bolt pattern axles, Drum brakes, 15 x 7 Rallye wheels with a 4.25 back space and mounted 255 60 R15 Cooper Cobra GT tires that have a section width of 10.2”
The inside bulge of the tire is 1” away from the leaf springs on both driver and passenger side and the outside bulge of the tire is within an 1/8” of rubbing the lip and would rub the inner fender when hitting bumps in the road and I would rather not cut the lip up to make it not rub. I have looked through the wheel tire combos thread and lots of people use the 4 ¼ backspace but mine are just too close for me.
I work swing shift and spend 3 to 4 hours in the mornings working on the car before work and another couple hours when I get home from work and I’m trying to figure out wheel offset. My brain is foggy and I can argue with myself both ways on offset. One voice says I need a 4” offset and the other voice says I need a 4.5” offset to move the tire inboard more. I have spent some time on google and for the life of me I keep going in a circle.
Wheel Vintiques offers the Rallye wheels in three offsets. A 15x7 wheel in 4 ¼ (which is too close), 4 1/2 and in a 15x8 wheel with an offset of 4 5/8” but do not offer a 4” offset..
I'm no wheel expert, but I have been doing a lot of research into what I need if when I convert my '67 to the big bolt pattern.
First question,.... Which drum brakes are you using? According to conversations I had with Cass at Doctor Diff, if you go to the big bolt pattern and use the 10x2.5" drums. it will increase track width by .3" per side, which makes sense with what you are seeing. He recommended going with 15x7 wheels with 4.5" back spacing. That will move the wheel 1/4" towards your leaf springs (from where you currently are), which from what you are saying, should put you at 3/4" from your leafs and 3/8" from your fender edge.
Remember that back spacing and offset are not the same thing. Backspacing is a measurement from back edge of rim to the mounting face. Offset is a measurement from the mounting face to the center of the wheel. I was a little confused when you said Wheel Vintiques offers a 15x8 wheel with a 4 5/8" offset. To my understanding, that would make your mounting face outside of the wheel width (center of 8" wheel is 4" from either side of wheel). I think you may have meant 4 5/8" backspacing.
From what you are describing, and assuming you are staying with a 15x7 wheel, I would look for a wheel with a backspacing somewhere between 4 1/2 and 4 3/4" back spacing. 4 1/2" back spacing would give you the spacing I mentioned above. 4 3/4" should set you at 1/2" from leafs and 5/8" from fender lip. To me it sounds like a 15x7 with 4 5/8" backspacing would pretty much center you in the wheel opening as it would move the wheel 3/8", and should put you 5/8 from leafs and 1/2" from fender lip. Sounds pretty centered to me.
I don't know enough about all this to know how much going to the 15x8" with the 4 5/8" backspacing would affect your section width (using the same tire). I know if won't make a full 1" (1/2" in each direction) difference, but I'm not confident enough to say how much it would affect it. Seems like the wider wheel width with that backspacing would push the tire out towards your fender again. I start getting confused once you start changing wheel widths AND backspacing
