JRC rear bumper and quarter armour.... doesn't seem sturdy enough to pull with???

[xriide]

So I bought the DIY JRC rear bumper and the rear lower armour.
And aside from a couple little fitment issues it came out pretty good. If anyone else is buying the DIY I would recommend cutting the part the mounts to the stock bumper holes instead of using the bend tabs, this is where I had fitment issues, since the folding wasn't as consistent as I expected it to be. And then having it pull a little bit with the welding didn't help. I still have slight issues, but it's close enough for now.

For my first question, does anyone else have this bumper and pull from it? I feel like it uses the 8 (4 on each side) stock bumper bolts for most of it's support.

They have this extra little tab on the bumper to tie it into like where the hitch would mount, but still feel like it would only really take much force after the 4 stock mount bolts busted out.
I have a piece of steel I cut to attach it to the other bolts, but still feel like it's too little.

My other issue is that the rear quarter armour is made for an older style bumper it seems, so I had to improvise.

Hard to see, but it doesn't line up with the profile of the rear of the bumper. hard to see, but it's also angled away from the body a few degrees.

So I cut it shorter and welded it onto the end flush. Plan is to tie the holes in the armor to that front most bolt on the nut strip in the frame, make it a little more rigid.

I had some pretty bad rust issues in the rear lower quarters, letting swamp water into my hatch area a couple times, so I cut it all away and welded a cover over.
Except welding 20 gauge sucks a LOT, so it was pretty gross and I was at my wits end. so I spray the outside with bedliner in a can a few coats to seal it, realized I had a few too many ugly holes so went crazy with the spray foam inside the fender wells there.
Had a few leaks.

Once it dried I just cut the orange off and sprayed the bedliner stuff on a few more times.

Thanks for looking!

 

[yossarian19]

I recommend rubberized undercoating, especially the Permatex stuff (used to seal gutters & tin roofs amongst other things, like auto body) instead of bedliner for use as a sealant.

As for the bumper...
It should be more than stout enough to pull from. Here's why I say so:
The 8 bolts that hold the hitch on (and which JCR uses) clamp the hitch (and / or bumper) to the unibody with, and I'm not an engineer here, a crap-ton of force. Enough you can tow 5,000 lbs. So does that mean that the mounts are only good to 5K? No. It means the XJ is only good to 5k.
Those 8 M12 bolts do more clamping than the hitch hardware that goes onto a Tahoe, which can pull 7500. IIRC, when I looked into this kind of junk for another project, the Cherokee's hitch hardware is just shy of what many full-ton trucks use.
The bolts are *WAY* overkill for what the rest of the XJ can pull.
So with just the hitch hardware, you could hang 2 (or I think probably 4+) XJ's vertically on a cable before the hitch moved.
Add in 8 10mm bolts loaded in tension... I think something else would go pretty darned wrong before that bumper lets go of the body. Broken winch cables, yank straps, etc.

*EDIT* Doing the math to convert a "crap-ton" of force to pounds of force, I came up with factory hitch hardware torqued correctly providing 94,336 lbs of clamping force. Add the 8 M10s at grade 8.8 and I think we are at 139,704 lbs of total clamping force. I'm not sure how / if it matters that the bolts face different directions. I *think*, though, that we multiply the clamp force by the coefficient of friction (0.1) to figure out how much force it takes to move the bumper relative to the frame of the XJ. Call it a rough 14,000 lbs? Enough to get you out of almost anything you'll get into.
Somebody who's done more schooling than I have might chime in.

 

[yossarian19]

More homework, more information.
The coefficient of friction I'd used, 0.1, was way off. For steel-steel interface (non-lubricated) it's usually figured 0.5 - 0.8. If we use 0.4, even, we wind up with 56,000 lbs of force to move the hitch. That's not necessarily breaking the bolts, either - that's just "motion".
So.. yeah... I'm fairly certain the JCR setup will handle whatever you're going to do to it.

 

[xriide]

The bolts that mount the hitch I trust. I have pulled real hard with my old hitch.

But the bumper is really only held on with the 8 10mm bolts. And I trust the bolts to not break. But they aren't exactly mounted to the most stout looking part of the body. And with that lone tab at the bottom to tie into where the hitch would mount, it just doesn't seem like a lot, and it's not pulling on that little tab at the bottom, it's pulling from the stock bumper mounts first.
And that little tab while 1/4" it still attached at a disadvantage angle.

 

[yossarian19]

I'd misunderstood the mounting.
I just now read the install instructions and see a good shot of the mounting.
I'm inclined to agree - the 8 stock bolts seem to be doing the heavy lifting.
That area of the unibody seems as strong or stronger than anywhere else except the front end. Even with just those 8 bolts, unless you have rust issues, you should be fine.
If you wanted, you could install the bumper + trailer hitch & use plate to make them one (heavy) piece using 16 bolts.
Some folks have opened up the stock bumper mount area and used heavy rectangle tube as a mount, drilling holes matching the hitch bolts & using a nut strip in there. Add flanges, gusset, use the whole mess of hardware & attachment in a single bumper with no hitch hanging down.
Another idea is that you could sneak some channel steel across that rear cross member beneath the mounts so that any pull is working across as much of the body as possible by minimizing flex.
But really, I suspect we're both overthinking this.