i've got a vehicle that originally came with 15X6 wheels with a 3 5/8" BS. someone installed 15x7's with the wrong backspacing so the wheel hits the drum fins before it hits the drum face. so... need to get some new wheels. i'm thinking based on the dimensions that i'm going to need a 15X7 with a 3 1/8" BS. sound right?
OK I answered your question then realized that my answer was about a useful as a rubber with a hole in it. What are you trying to accomplish with the different wheels? If all you want is to clear the drum than stock back space will do you or anything less than stock backspace. that being the case you will be fine with 3 1/4" backspace.
I think you are figuring this wrong. If the 15X6 needs 3 5/8" Back Space to clear the drum, it will need 3 5/8 minimum for ANY wheel wider than the 15X6. So Yes a 3 1/4 back space which is less than the original will clear. most likely a 3 1/2 would as well, but we can't really know without knowing how much clearance the 3 5/8 wheel had in the first place. What is the back space on the offending wheel?
My two sense, get 2 blocks of wood 3/8" thick in your case, tape to inside of wheel & mount to check to see if wheel is still hitting, adjust accordingly. This will get you close to the back space dim you need, but don't forget the change in wheel width.
Actually hotroddon, the width of the wheel is irrelevent, as 2 wheels with the same backspace would have any additional width on the outside. A 6" wheel and a 7" wheel , WITH THE SAME BACKSPACE, would carry the additional inch on the outside. You just need a wheel with less backspace to clear the brake fins. Like casper said, a small spacer would fix this without buying new wheels.
Ummm, that's what I said. "If the 15X6 needs 3 5/8" Back Space to clear the drum, it will need 3 5/8 minimum for ANY wheel"
a small spacer will NOT fix the problem. we need something in the area of a 3/4" difference in offset.
What is hitting the fins, the rim hoop or the center section? Is the center section of the wheel the same on the originals 15x6 and new 15x7? As said prior, back spacing is the perpendicular measurement from the face that bolts to the drum/rotor to a straight edge laid across the inside rim edge. If the back spacing is the same, or less, and it still does not clear, then there must be a difference in the wheel center or rim that is causing the interference, which means you'll just have to measure it up and draw a diagram.
okay... so i think i was over thinking it a bit. the rims on the car had a 4.25" back space. the lip of the rim was hitting the drum fins. there was 5/8" space between the drum face and wheel mounting surface. so 5/8" to take up the space plus 1/8" to get the drum off of the fins equals 3/4" total difference in offset needed. i dropped 3/4" worth of spacers in there and the wheel and drum were just right. 4.25" - .75" = 3.5" backspacing.
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