Max backspacing for 18" wheels in the front?

[Uhcoog1]

What is the maximum backspacing allowable on the front of a stock suspension given 18" wheels?

I'm in the process of re-doing my duster. I just picked up a 68-69 B Body dana 60 that I plan on running. It has 42" perches, and was supposed to have been under an a body. Do you think it was run with an offset kit?

I like the 07-09 SVT Mustang wheels - 18x9.5. 45mm offset, 7.125" backspacing.

Currently I have 275/60-15 in the back and 235/60-15 in the front. I rub a little on the fender lip on both front and rear under extreme circumstances. I've calculated ~1/4" negative offset on the back and ~1/4" positive offset in the front (15x8 rear 15x7 front, both ~4 1/4 backspacing).

This wheel should be perfect in the rear - 45mm offset = 1.75" offset, which is 2" different than current. Given the 1.5" added axle width, that should put me at .5" centerline moved inward. I don't know how much room I have with the stock spring location, but I'm considering the offset spring relocation (since the perches are already ~.5" in on a side).

The front is where I'm concerned - can I run a 7.125" backspace with the stock suspension on an 18" rim? I saw somewhere there was a limit given - I lost the post and havent been able to find it again. My tire width was 9.5" with the 235 and a 275/40/18 would be 10.9", or adding .7 inches on each side. The centerline would move inward by 1.5" of it's current location, and then be offset by the .7" of added tire width on that side. So, the tire would be .8" inside of where it is now, which would be good to get it further away from the fender with the more upright-ness of the lower profile tires.

Thanks!

 

[ATW]

http://www.forabodiesonly.com/mopar/showthread.php?t=58491

Lots of info on backspacing and combos there...

From what I've read, that's too much backspacing for the front. Plus it's a super wide rim.. I think you will be smashed into your outer tie rod end with that backspacing and width. Most of the Mustang rims being run are 5.6-6" BS and are 7 or 8 inches wide for the front.

It also sounds like you will need the offset kit for the rear, too.

Here are some SVT-esque rims that are a smidge more narrow and have a better BS... not sure if you're dead set on the EXACT SVT style..

http://www.americanmuscle.com/frchwh1.html

Hope this helped.

 

[Uhcoog1]

Thanks for the link. I actually read the entire thing before posting. Most of the info given is for 17's, and there was hinting at 18's might clear the tie rod end better.

I had found a post (can't find it now) that spelled out max backspacing for 15's, 16's, 17's, and 18's. The max backspacing got higher with each increase in wheel diameter. Anyone know this info?

 

[Uhcoog1]

I found the post, though it's for the alterktion. So, I still don't know the answer. Good info, though.

to clear up any confusion about the alter-k and rim size/backspace..


this is right from the web site and the directions.


Quote:
23
Limitations As with any performance product, there are some compromises to achieve bolt-on status. In the case of this system, wheel backspacing is limited, depending on tire size. The following numbers allow approx. 1/4" of clearance between the rim and tie rod end...

14" rim - 3.25" backspace maximum
15" rim - 3.25" backspace maximum
17" rim – 5.75" backspace maximum
18"+ rim – 6”+ backspace

Note - a 17" rim clears the tie rod end entirely, however with more than 5.75" backspacing, the rim may hit the tie rod at full lock in either direction. If wider 17" rims are needed, an adjustable stop may be welded to the lower a-arm to limit turning radius. These figures are based on suspension clearance only.
There’s no guarantee that a wider 17" rim, or the tires, will clear the frame rails or other parts of the car. Large wheels and tires typically require clearancing in other areas as well, and it is the owner’s responsibility to determine the requirements of his/her particular situation.
As always, we're always happy to help or answer any questions you might have.