Are there any books/websites/tech write-ups that explain in detail how the XJ’s unibody works, and how best to strengthen it? I have rust holes through the floorboard that I need to fix anyway and I want to go topless with a roll cage in a year or two, plus I just want to know about the engineering that went into it. I’ve got a ton of questions about it, like:
1. Does it need to flex some or is it okay if its rigid (like if I use heavy steel plate on the floor)?
2. Obviously not all parts are equally important – what sections/lines of the unibody bear the most stress in different circumstances?
3. Where does the roll cage need to provide support after I take the top off?
4. What causes stress cracks in XJ unibodies and how can they be prevented?

Dude, you can use a heavier steel plate for the floor, that would strenthen it up, but i would not go to thick, cause you need a little flex or the windows will take it all and break. mount the rool cage in the same locations as the pillars. (3 from the front of the rear door back and one after the front door frame (A) pillar):roxy:

i think the most important thing to remember is load distrubution. the xj unibody does flex, as we all know. some ppl think its supposed to work like htat some ppl think its shoddy....
there are tons of ways to do it...but for me, this is what im about to get into....
i want to basically take box steel, probably like 1/4in or something comparable, and cut on side of it out. basically C channel. run that up and down the fram rails, and cross brace (imagine 4 2in tubes welded like crossmembers in appropairate areas.
then a 10pt rollcage that comes thought the floor and ties into the frame rails. i think thats a good start. roll cage will have 5pt harness provisions.

my 2cents
~John

Yeah, there was some talk of that (or cutting the rails open and putting smaller box steel inside) in that other strengthening post a few months back. Then if you add heavy steel flooring between that new framing and the roll cage, maybe along with your cross bracing, seems like you would be pretty rigid (which raises the question is rigid good). You'd also have some extra weight down low to help lower the COG. But how do you follow the curves in the frame rail without losing strength? You can’t have it forged in that shape like the auto makers would. Or do you just do the flat part between the axles?

And if you're that rigid and strong underneath, can you ignore overall structure issues as you do body mods like cutting off the top, and just brace the pillars enough to hold the cabin in place?

Make sure you post some pix when you get done. I need some ideas too. :flipoff2:

Originally posted by Duche79
Make sure you post some pix when you get done. I need some ideas too. :flipoff2:
how bout some pics of that 350 swap?
what tranny are you using, motor mounts, adapters, wiring, cooling?
lets hear about it.
~john

btt

Here is some info http://www.naxja.org/html/techarticles/XJ_unibody_chassis.htm

Yep, already saw that one, there's not a whole lot of meat on that bone. I appreciate you responding, though, you didn't know I'd seen it. I started to list what I'd looked at already but I didn't want to make my question too long. I've looked at all the stuff I could find searching on about a dozen words on Google, here and a couple other BBs. I really thought I'd find a tech article by one of the designers talking about lines of force and stress factors, the unibody concept is really pretty neat and I think the Cherokee was one of the early adopters, but no such luck. I've seen most of the PR/magazine stuff, I think. I've followed some body rebuild projects like Brent Orton's and Dillinger's and some others, that's been the most help. I'd like to find more practical stuff like that, stuff written by people who tried something radical whether it worked or not, and maybe some theory written by engineers.

I think you really need to buy one of the rust free P.O.S's we have in stock. All cheap and rust free like:flipoff2:

Well, I'm dilemma'd there. I've been all over the body and there's no rust at all anywhere except the passenger floorboard, which is rusted out because of a combination of a battery acid spill and a couple well known XJ leak spots. I seriously thought about getting a different XJ to start with but with limited funds I decided the money would be better spent reinforcing the flaw. Especially since it helps me meet a few other objectives anyway.
I've scraped off all the rust and POR-15'd it to hold a while. The only thing that worries me is the possibility of rust spreading to the right frame rail.

http://communities.msn.com/Duche79sXJOffroad/shoebox.msnw

Here's some pix of my rig. On the last page there are a couple of the 350 going in. :flipoff2: :D

I plan on getting a newer one with out the cracks maybe 97' or newer and then add a 1.5X3" square tube to the botom of the rails the entire length. then put a 1/8" plate the hight of the rail and the tube to the outside of each rail. then add two removeable cross members one right in front of trans. and right in front of the rear axel. Then there will be places to make a belly pan for the entire botom. this shold take out all but the twist, wich will be handled by a combination of inner and outer cage. Run the loops in the inside, and the long rail from front spring mount out the back of the front fender up the a pilar along the roof line and down to the rear shakle mount welded to the body the whole way. The inner loops will also be to the body in many places on the roof and sides.

I appreciate you responding, though, you didn't know I'd seen it.
Damn newbie, your waaaaaay to polite. (hang out here and that'll change):flipoff2:

I don't know how to help with your solution except to add this pic that I saved about 4 years ago..I like the cage mostly...

http://community.webshots.com/storage/1/v0/0/46/89/31204689YiBKAwzbrc_ph

Yep, good pic of a roll cage. (Before I say anything about "staged", is that another common part of the rubicon trail?)

Naw..it's definitely not the Con...looks like AZ or New Mex to me. I really don't know. I have a file of pics that I have been saving for about the last 6 or 7 years.