Wheel backspace

boris_thespider · #1 (**permalink**) 12-20-2010, 09:31 PM

Can anyone tell me if a 15" x 10" wheel with a 6.25" backspace will work with the narrowed B&B rear end without rubbing on the inside? I was planning on using 295-50-15's. Thanks...

lnfletcher · #2 (**permalink**) 12-21-2010, 02:34 PM

I believe the problem you may have is with the outside of the tire extending slightly out of the fender well.
I'm running 295 50 15s on 9 inch rims with a 4.5" back spacing. That means your setup would put the inside of your rim 3/4" more spacing from the inner fender and 3/4" towards the outer rim. My tires sit just slightly inside of the outer lip of the fender.

Hope this helps.

boris_thespider · #3 (**permalink**) 12-21-2010, 05:15 PM

I was concerned that with the 6.25" backspace, the inside of my tires may rub the wall of the cockpit unless I use a spacer to push the wheel away from the hub. Am I thinking backwards? There's a good website for helping you figure all this stuff out. The hard part is trying to decide whether to add or subtract the different numbers to see if your tires will be centered in the wheel well.

http://www.rimsntires.com/rt_specs.jsp

Wbulk · #4 (**permalink**) 12-21-2010, 08:10 PM

What is the section width of the tires you are mounting on the rims. I have 10" rims with 5 1/8-5 1/4 Back spacing and 295s. My section width (bulge) of the tire is 12 1/2", and is just about flush with the fender lip. I reached behind and felt the gap in the area you are talking about a few weeks ago and it felt like there was maybe and inch left tops. So it looks like that would be very close.
One other issue is it appears all of the cockpit fiberglass tubs may not be installed exactly the same. If it was pushed back a little you will have less room. So with your 6 1/4" BS the section width of the tire would be about one inch in from the fender lip. But it's really going to be close to that panel on the inside. A wheel spacer may help. Another option would be to modify that panel like Larry did. If you want me to measure that point exactly I can tomorrw, advise. But again our cockpit tubs may not be installed the same.

Wayne

boris_thespider · #5 (**permalink**) 12-21-2010, 08:46 PM

I talked to Bryan today and he didn't think there would be a problem with that backspacing for the 10" rear wheels. The fronts are 8" with 4-1/2" BS. They should be fine. I ended up buying these wheels used, in excellent condition for less than half the price of new. Too good of a deal to pass up. So if I need to add a spacer, I will.
When I installed the cockpit panel, I didn't push it back at all. I used the original holes from B&B so that should help me out a bit. As far as measuring that space, Wayne... don't worry about it. The wheels are on their way. They are what they are and I'll just deal with it. Thanks...

lnfletcher · #6 (**permalink**) 12-22-2010, 07:31 AM

The Backspacing is figured from the mounting face to the inner rim.
If you lay the bare rim facing up on a flat surface, then measure from the mounting face to the flat surface, this is your backspace. Whether it is a 10" rim or a 8" rim, the backspace would be the same for a particular vehicle not considering the tire buldge. By adding backspace, you are pushing the wheel outward. The wider the rim and more backspace moves the outer tire out by both amounts.
This would mean that you would have 1 3/4" more space behind the tire to the inside than me and 3/4" more than Wayne.
Spacers would make it even more!
Clear as mud?

boris_thespider · #7 (**permalink**) 12-22-2010, 08:03 AM

Quote:

Originally Posted by lnfletcher

By adding backspace, you are pushing the wheel outward.

This is where I'm confused. I thought that if you have a deeper backspace, it would allow the wheel to sit in deeper (closer to the frame) before it contacts the mounting face.

Wbulk · #8 (**permalink**) 12-22-2010, 08:12 AM

Not to confuse the issue but more back spacing always moves the wheel in further. Less back spacing moves the wheel out. Lay it out on paper.

lnfletcher · #9 (**permalink**) 12-22-2010, 11:35 AM

Sorry! Ever have one of those stupid days?
I'm wrong on the result, as I was driving I began thinking of what I had just written and thought of how stupid I was.
Wayne is right, backspacing moves it in not out.
I'll eat my crow and crawl back in my hole.

Wbulk · #10 (**permalink**) 12-22-2010, 11:52 AM

That's just one this year right Larry. You do good work. We still respect you.

lnfletcher · #11 (**permalink**) 12-22-2010, 12:43 PM

Good thing it's almost a new year so I can claim it happened "last year!".

Wbulk · #12 (**permalink**) 12-22-2010, 12:44 PM

This is a picture of mine. It's hard to get under there. The tape is 3/4" wide and there is a little gap so you may have 7/8". You have one inch more back spacing than I do, so it's going to be darn close.

One other thing I just thought of, if you have the older four link with the large rubber bushings that axle is going to shift a little as you go around a corner.

Wayne

boris_thespider · #13 (**permalink**) 12-22-2010, 12:59 PM

I'm pretty sure I'll have to get the 1" spacers that have the lugs built into it. I can get a set for around $60. I really wanted to go with a 5-1/4" BS, but the deal I got on these wheels was too good. The guy that had them was running a Factory Five which seems to use a wider rear end than B&B. I may also end up modifying the cockpit panel like Larry did. I'll see how it goes...

Mike

ndpace · #14 (**permalink**) 12-22-2010, 01:08 PM

I am running a set of Bullitt wheels for 99 to 04 Mustangs. The fronts are 17x8" and the rears are 17x9".
backspacing: The 8s are 5 1/2 (plus 1/4" spacer) and the 9s are 5 3/4 (plus 1 inch billet spacer)
these fit just right
Thanks, Jim

SP01715 · #15 (**permalink**) 12-22-2010, 03:03 PM

Quote:

Originally Posted by Wbulk

Not to confuse the issue but more back spacing always moves the wheel in further. Less back spacing moves the wheel out. Lay it out on paper.

This is correct. I just went through this on one of my jeeps.