Hi, im new to this site and was wondering if Backspacing was important. If i get a 2.5 in frankenlift with 31x10.50 tires do i have to worry about backspacing. Please reply i would love to understand this. Thank you
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from what i understand, those tires wont fit with that much lift.

the backspacing would depend on the profile and size of the tire once you have them mounted. different sidewalls have different shapes and treadwidths vary in the same tire size. you wanna keep just enough room between the innersidewall and suspension parts to allow for it to flex a bit, prob 1/4-3/8".

you may get away with 5 inch backspacing with some tires so you have more clearance at the front and back of the wheelwells, but other tires may need 4 3/4 or even 4.5. you wont know for sure until after you mount the tires and put them in. or of course ask others that have the specific combo you are shooting for. there may be a brand of tire in that size that has a cooperative profile.

maybe search the tire section and look for specific rim and tire combinations and ask those guys what their backspacing is.

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You are planning to drop to 15" rims right? There are only a few steel rims in 15" size that will fit on the KJ,and yes 13x10.50's will fit with a 2 1/2" lift.Some others on this site run them(pm Sport about them,he runs them),you will need the 4.5" BS one's and you will need to do some trimming of the front bumber(to eliminate some minor rubbing,noises that sound worse than they are) and pound down the pinch weld on the backside of the front wheel well.

I have heard of guys with 2.5 in lift running 31's that have absolutely no rubbing and havent had to pound the pinch weld or cut up the fenders or bumpers. Just wodering but would it make a difference if i ran 16's on them

Your tire choices are pretty limited for 31x10.50 in 16" rims,they are out there though.Well the poeple running 245/75R16's(30.5" tire) with 5"BS are having to pound down the pinch weld,but not if running the factory 5.5" BS.Keep them on a 7" rim with 5"-5.5" BS and you may get away with it.If you plan on doing some wild offroading where alot of articulation will occure it's best just to pound down the pinch weld as not to ruin your tires.I had to pound doen my pinch weld running 265/75R16's(31.5" tire) with 16x7" rims with 4.5" BS.

haha What is the pinch weld and if you can, try to get a picture of it. Thank you

Sorry no pics,but if you look in the front wheel well you will see a protusion coming out of the rear side,under the plastic is the pinch weld.You need to cut the plastic from around it and pound it down with a 2-3lbs hammer,do not cut it off.Then shoot some rubberised undercaoting or similer to prevent rust.There are many posts about it,there is one posted fairly recently in the 4x4 section,also try thr tech section.

I don't have any "before" pics but, here's my pinch weld, pounded over and plastic trimmed:

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"There are few problems in life the liberal application of duct tape can't solve."