I know we've beat this subject to the ground, but I never saw my specific situation out there.

Anyone see problems running 35x12.5 TRXUS MT's on 17x9 wheels with 5" of backspacing (Cragar Soft 8's.) This is on an '07 Rubicon with 3" FT lift, and preferably no spacers. My math shows this resulting in more space between the inside edge of the tire and the jeep than the 32"x10" stock tires on 17x7.5 with 6.5" backspacing, but the whole thing makes my head hurt and I'm probably wrong. Any help is appreciated.

I tried posting on another thread but wasn't sure anyone will see it as it was an old one.

 

The back spacing has nothing to do with the width of the rim....so your math had an incorrect given.

The wheel width that's quoted as the wheel's width, like a 17 x 9 rim, is actually 10" wide....because the quoted size is the size BETWEEN the inner lips that hold the tire in....but the FULL width, the width used to measure back spacing, is from the outter part of the lip.

This means that 17 x 7.5" rim with 6.25" of BS and a 17 x 9" rim with 5" of BS, differ in BS by 6.25" - 5.0" = 1.25"

So - What you are REALLY asking, is if a 12.5" section width tire will fit with 5" of BS.



That would be like a 1.25" spacer....close, but not quite a 1.5" spacer....and the nominally labelled 35 x 12.5/17 MT you mentioned actually has a section width of 12.7"....making it even tighter.

So -

A stock 255 mm section width is roughly 10" wide in section width.

People seem to be able to fit 285's with no spacers sometimes (The old every one's JK is different mantra...)

A 285 mm is about 11.25 - 11.5"-ish in section width.

You're looking at more like a 325 mm section with than a 315 or 305, if you have a 12.7" section width.

So - if the oem rims had 6.25" of BS...and cleared 285's...it sounds like you might squeek by with another 40 mm of clearance, or about 1.6" less BS than the 6.25" OEM Rim....or around a 4.5 - 4.75" BS'd rim.

You have 5" of BS...for some reason.

Now, if your OEM rims did have 6.5" of BS to start with...and we had to subtract 1.6" BS, that would result in a new needed BS of 4.9"...close enough to the 5" to be a tweak of turnstop adjustment kind of fix.

Of course, that assumes that people with 6.5" of BS could have cleared 285's like the people who had 6.25" of BS.........and that you in fact HAD 6.5" of BS in the first place, etc.


(I'm pretty sure the JK rims have 6.25" of BS btw)


So - Bottom line...it might not fit....to much BS.

On the other hand...you might get lucky...and not have already screwed up and followed the flawed math, and maybe did not GET the wrong rims yet.

Just remember....BS is a constant: Two rims with the same BS will clear the same inboard components with the same tire, reagrdless of rim width.



Hope that helps!

 

Thanks for the help Teej.

Haven't bought the wheels yet, so no harm done, although I do have the tires in hand. You're right about the stock BS, typo on my end.

The Trxus 35x12.5 calls for a minimum wheel width of 8.5-11. Do you think I'd have problems mounting to an 8" wide wheel?

Only reason I ask is I can get the same wheels in 17x8 with 4.5" backspacing. I have gotten some push back from local tire places as far as mounting the tires on the wheels, and worry that if I go outside the manufacturer's recommended range, could get even more push back.