How long does unibody last?

I've been wheeling my xj pretty steady for almost 7 years...todays work/get it ready for the trail session found cracks on both sides of the transmission tunnel behind the shifter and under the back seat....4 cracks total about 1.5" long each...we all know that unibodys twist and flex...how long will it last? I've got subframe stiffeners and a 6point cage that lands its pads on top of the subframe...so I know its not tied in pro-style but its a lot better than a lot of people have...

so whos wheeled the same body for how long? with what resulting stuctural failure probably not related to wrecks or impact?

wondering how long before yellowxj "goes out to pasture"...no thats not just an expression in georgia...they really just go park it out in the pasture...

I've got two very small cracks on the top rear corners of my XJ. But they are so small I don't think it would affect the performance of the uniframe.

If I were you I would drill holes on the tops of your cracks.

Yes, you read that correctly. Removing material sounds counter intuitive to metal strength but when it comes to fixing cracks its a proven method because it reduces what engineers call "stress concentrations."

I got my information from a civil engineering class I took last semester called "mechanics of materials." I got an A in it, if that helps my credibility.

Oh I believe you. I've fixed cracks before...I had cracks in the upper corners too...I just wondered how many times you can fix it...or is there a time to cut and run

If it gets bad enough just replace the floor pan, you'll add years to your XJ.

I believe mine will last nearly forever, at this point.

Might be because I have it plated nearly from spring mount to spring mount with 3x3x1/4" angle, have all the crossmembers, links, springs, and 8-point cage tied into this (with a good amount of triangulation), have gotten rid of enough body that it's no longer fighting itself to flex, and have gone to hydro assist steering (tho full hydro is better).

Lots of people regard these things as "disposable" wheelers...as in, wheel it til it busts, then just get another one for cheap and transfer all your crap over to it. Then there's people like myself, Ashman probably, and others who have done a whole bunch of intermediate stuff along the way that transferring it isn't really possible...so at this point it's like you've built a whole chassis underneath the damn thing so the body doesn't really do anything anymore.

The difference might be that all the work I've done was because of a 'preventative' mentality, rather than a fix-it mentality...but that's probably because I've heard the stories of cracks forming and wanted to avoid having to deal with it. But if you don't address it, eventually it will get to a point where you'll begin to have less OEM stuff on there just to keep it structurally together.

Although I've never actually seen one fold up like a taco. I'd kinda like to see that if someone has a picture of one.

edit: the trans tunnel is a big one, as that structure is there primarily to fight torsional stress (I'd guess you spend a lot of time doing flexy stuff with weird loading?). Everything I've done was to try and get it away from that area, especially since not much of it exists anymore due to flat-bellying with a NV4500...

I've been wheeling the piss out of mine for 12 years. I've fixed numerous cracks, many more than once. I'm going to run mine forever, but I have now caged it from the front bumper all the way to the rear bumper. Once the whole thing is caged and reinforced, let it crack, since the unibody structure isn't the only thing holding it together anymore.

Where most folks make a mistake over the long haul is not adding rigidity from the A pillar of the cage forward. There is a considerable amount of the rig that is in front of the normal A pillar down bar of the typical cage. That front frame section can really twist, and it stresses the cowl/firewall, and A pillar from top to bottom.

I'd look for ways to add more rigidity, fix the cracks, and keep running it. If you move to another rig, it's a ton of work, and you'll want to add all the rigidity to the new rig to keep it from cracking.

If you do a search, there was a thread a while back about all the various places we've had cracks, and some things to do about it. Don't know how hard it would be to find, I just did a quickie search and didn't see it.

I second the A pillar forward comment. Here is my rig before the new body swap.

Goatman said:

Where most folks make a mistake over the long haul is not adding rigidity from the A pillar of the cage forward. There is a considerable amount of the rig that is in front of the normal A pillar down bar of the typical cage. That front frame section can really twist, and it stresses the cowl/firewall, and A pillar from top to bottom.

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Yep, and this is one area where I still want to improve mine.

Things like the T&T Unibody Stiffeners don't help this at all. In fact they move the stress to a more localized area right where they end. It will take a while for this area to fatigue (mine's been seeing pretty harsh stuff for 4 years now and shows no signs at all at the front frame, steering box, or trackbar area) but I've had all stock stuff under it too. I've probably added a good 1000 lbs in unsprung weight since then which will definitely accelerate the process.

Things like a good rigid front bumper, poly motor mounts (so that the motor itself acts almost like a crossmember of sorts), steering box brace, track bar brace, etc are all helpful for this, but unless you have some sort of reinforcement between the spring and the rest of the frame, it's really not fully addressing the problem area. Especially if you're using your shock as a limiting strap and end up hanging the tire a lot. This is something I will be addressing in the future, most likely by plating the curved area from the bottom of the frame rails up to the spring, but for now I'm just trying to remove as much flex from that area as possible. Tying a cage bar from the bumper to the A-pillar is probably the best bet.

No direct experience, but one of my friends on the east coast (Charlotte) used to be a mechanic with 4WP and said that the worst XJ he ever saw was fatigued so badly from having the $hit kicked out of it (not wrecked) that when he put it on the hoist to work on it, the dome light came on.
I would love to inspect that unibody to find out where all the failure points were.

No help I'm sure, but it is an interesting story... :laugh:

Later,
Joe

I've put them on the lift before and had the doors be difficult to open Good to know I'm not in dangerous territory...I'll keep welding. I was planning on connecting the front bumper to the cage/subframe...and I already have some old currie motor mounts (I think they might be worn out though), and a more steering box brace, and plated the subframe on the other side of the steering box...glad I dont need to REALLY worry. Probably. Thanks Gang.

I did just try to put doors back on my xj after a couple years with out them...the driverside door wouldnt fit any more...A pillar's been pushed back from multiple tree/trail impacts..

Another fun thing to do is to be all flexed up on something, open the tailgate, then not be able to close it again until you get back on level ground. And that's normal!

vetteboy said:

Yep, and this is one area where I still want to improve mine.

Things like the T&T Unibody Stiffeners don't help this at all. In fact they move the stress to a more localized area right where they end. It will take a while for this area to fatigue (mine's been seeing pretty harsh stuff for 4 years now and shows no signs at all at the front frame, steering box, or trackbar area) but I've had all stock stuff under it too. I've probably added a good 1000 lbs in unsprung weight since then which will definitely accelerate the process.

Things like a good rigid front bumper, poly motor mounts (so that the motor itself acts almost like a crossmember of sorts), steering box brace, track bar brace, etc are all helpful for this, but unless you have some sort of reinforcement between the spring and the rest of the frame, it's really not fully addressing the problem area. Especially if you're using your shock as a limiting strap and end up hanging the tire a lot. This is something I will be addressing in the future, most likely by plating the curved area from the bottom of the frame rails up to the spring, but for now I'm just trying to remove as much flex from that area as possible. Tying a cage bar from the bumper to the A-pillar is probably the best bet.

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This is a good topic to discuss. Here's where I've had cracks A pillar forward. At the top of the A pillar, A pillar just above the bottom of the winshield, A pillar above the top door hinge to the top of the cowl, where the front frame rails hit the firewall, a V crack on the drivers side frame both inside and out right behind the track bar mount right where the frame flares down, all around the frame next to the steering box and between the steering box and motor mount even across the top when the frame was plated inside and out, and the top of the front crossmember on both sides.

We've had this discussion before.........I'm not so sure about adding a steering box brace, unless you also add a track bar brace. The brace can actually add to the flex and stress of the frame section between the steering box and the track bar mount causing more cracks. Using the SBS system from JKS (designed by our own Mark Hinkley) is by far the best solution, and then don't run a steering brace. The only real solution long term is to beef up everything around the steering box, motor mount, and track bar mount, and then hopefully add some cage structure from the A pillar to the front bumper. I've ripped a steering box off the frame while using a steering brace, and a track bar mount off the frame while using a track bar brace.

So according to Goatboy... "let the wet noodle be sloppy and it wont break!"



Seriously though, I can testify on the importance of bracing the Apillar forward. After chopping mine then wheeling without cage structure in the front, I could feel about 1" of total movement between my cage and the windshield. It was kinda scary honestly.

Regarding the A pillar flexin deal, what you really want to tie into the cage is the upper coil mounts, where most of the force acts most of the time. I would tie them in on the way to the front bumper, that is for a built rig of course.

I just put some RE 4.5" coils in the front of my XJ and decided to test the flex on a large rock in my bro in law's backyard. Got good flex out of it, in fact, too much flex. My front end (A-pillar forward) flexed and pitched the fan clutch into the radiator. Now I've got a gouge in my radiator. I wasn't running any shocks at the time, I still have my original 1996, 149k mile shocks (amazingly they still work good, control the body motion with no more than 2-3 bounces). Anyone got any pics of reinforcments they've added? I gotta do something because I'm going to Truckhaven end of January.

vetteboy said:

Another fun thing to do is to be all flexed up on something, open the tailgate, then not be able to close it again until you get back on level ground. And that's normal!

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Did that yesterday on mine. I flexed it out to see what was limiting me and my girl opened the back and was back there slamming it over and over and over! I yelled at her to stop !

Yep, its a unibody, it will flex. Add a cage, plate the 'frame rails', add a few more Xmembers near the rear, and front.

j99xj said:

I've got two very small cracks on the top rear corners of my XJ. But they are so small I don't think it would affect the performance of the uniframe.

If I were you I would drill holes on the tops of your cracks.

Yes, you read that correctly. Removing material sounds counter intuitive to metal strength but when it comes to fixing cracks its a proven method because it reduces what engineers call "stress concentrations."

I got my information from a civil engineering class I took last semester called "mechanics of materials." I got an A in it, if that helps my credibility.

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Concur - it's called "stop drilling" (I've helped my granddad fix a number of castings and light structural elements that way.) You drill out the crack, grind or peen it down slightly, then weld and grind.

(I got an A in Manufacturing Materials and Processes, as long as we're on about credibility. Just thought I'd throw that in... I took it for Tool & Die Maker and pre-MET.)

5-90 said:

(I got an A in Manufacturing Materials and Processes, as long as we're on about credibility. Just thought I'd throw that in... I took it for Tool & Die Maker and pre-MET.)

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I just made an A in that class this semester lol!