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repairing rear frame cracks, new bumper

NCCherokee

NAXJA Forum User
Location
Anderson, SC
So i had a bumper a buddy built for me. Good for what it was designed for, but couldnt handle the rigors of towing. it did have framerail tie-ins, but they were flat plate and couldn't resist the downward force of tongue weight.

Which led to this:
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so I sold the old bumper with full disclosure and bought this:
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that is the rigidco rear bumper. I only have a 32" spare so I put it inside, and some day will send the endcaps back and get the tire carrier endcaps when I step up in size.

here is the spare inside
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I actually had the rigid bumper installed for a while and I fully believe it would have been fine, but the idea of cracked rear bulkheads and framerails was unnerving. So I had a buddy weld things up for me. We welded up the stock mounting points, and then i used these little 1/8" plates with rosettes to tie the framerails back into the rear crossmember.
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bolted in place
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these were stitched and rosette welded on.
 
sorry for the huge pictures!

So that probably would have been good enough, but I wanted to make sure his never happened again (eventually the rear rails will be plated, I already ahve stiffeners on). I took some inspiration from Foxwar's bumper build and decided to add box tube to the back of the rigidco mounting plates. This required cutting out the access hole for framerails, which can be seen above. Also, you have to cut out the riveted-on exhaust hanger on the pass side.

The box tube picks up 4 bolt locations on the framerails.
Driver side cut to go under fillup tube
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my buddy tacking them to the rigidco plates.
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Mockup:
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and Rigidco subframe mockup
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I also used JCR nutstrips which were tacked on.

finished brackets
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going in place
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final product was in the first post. I painted with POR-15 and sprayed over with rustoleum hammered that is everywhere else on my jeep. For some reason the POR-15 seemed to bubble up a bit which didn't happen on the front bumper or rock rails. However those were painted ~6 years ago, so maybe the paint was getting old. If it doesnt hold up I'll sandblast and redo it.
 
very nice, in the last pic though it looks like your shackles are on backwards. if there is any angle to them the bend should be with point of the elbow facing forward, like this

front of jeep \_ rear of jeep

if they're straight shackles it really wouldn't matter though, and it could be an optical illusion just thought i'd mention it :D. Love the look of that rigidco rear bumper and very nice additions too it. good work :cheers:
 
Nice work. Those cracks around the rear bumper mounts are very typical, most guys probably don't know they have them since bumpers are rarely removed once installed. I think the cracks are more due to chassis flex than bumper load, since most bumpers are pretty rigid so chassis flex load goes right into the mounts. A simpler fix that I have done a few times is to weld up the cracks, then add a plate across the entire rear crossmember, inside the C channel. This adds a considerable sheer plane to the structure to minimize flex in the crossmember around the bumper bolts. Make sure the bumper is tied into the frame rail underneath, but doing this strengthens the bumper mounts but won't (by itself) eliminate the cracks, since every XJ I've seen with these cracks already had the bumper mounts tied into the frame underneath. I have both used pop rivets along the plate that was added and used small rosette welds, but the rosettes are tough because the crossmember metal is so thin.
 
I thought about plating the the whole rear crossmember, and I think yours was one of the threads I looked at among others for ideas. I really thought hard about adding it behind the crossmember between the tank and the x-member, and having it wrap around to pick up the inside of both framerails since those were split from the rear x-member.

In the end, I knew I was going to go with the Rigidco bumper because it matched by front, it is one of the best looking rear bumpers out there in my opinion, it looked good with the chopped rear quarters, and it was actually quite affordable at <$650 shipped to my door.

Because of that decision, I wasn't able to plate ther rear of the x-member because of how the bumper subframe attaches. There isn't clearance for the additional thickness of plating behind the x-member.

It would be possible to add some additional support/bracing outside the x-member, but again with the rigid mounts, unless I was willing to weld them on, it wouldn't work. Someday down the road, I may just weld on the mounts that slide into the frame, and at that time I will probably add some c-channel to tie both sides and the x-member together with multiple rosettes. This would actually still allow the bumper to be removable from the subframe, is just that about half the subframe would be permanent. Once I finish residency, I plan for some cage/stretch/mild wheel-well tubbing. At that point, I will see how things go together and probably add that C-channel in. Otherwise at this point in time it is very modular and very easy to remove, and God forbid, swap onto another XJ.
 
also, regarding it being the frame flexing rather than the bumper, here is a pic of the old bumper
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as you can see, the frame tie-ins are flat plat, which are fine for a straight on pull from a winch or a strap. However they had no vertical component that would resist a load in that vector, such as would occur with heavy tongue weight.

I had towed several heavy loads with my 5x10 trailer when I moved from Chapel Hill to the Upstate of SC after med school. I was inspecting the jeep a few weeks after the move and noticed the cracks outside the brackets. Not knowing how bad it was i figured I'd get to it later. Well, my dad left his 2005 Correct Craft Ski Nautique at my house, and I had to tow it to his lake house, an hour drive. When I lowered the tongue wheel, I could watch the bumper deflect downward as more and more weight was taken off the tongue wheel and applied to the bumper.

I'm sure some of it was frame flex, but i'm betting a lot of it was bumper design. It was great for recovery pulls like I said, but it was never designed for heavy towing (and neither was the cherokee!). And just for the record, a 1999 Cherokee cant really pull a 2005 Ski Nautique very well. Just cus it has a hitch doesn't mean it can pull it!
 
I'm not saying that bumper load didn't contribute, but the exact same cracks will happen anyway. Mine was cracked around every one of the eight mounting holes and I never towed anything.

I'd use 16 gauge or 1/8 to plate across the rear crossmember. Doesn't need to be thick, since it's mostly a shear plane and it's adding thickness to the metal that is already there. Sometimes it's better to add thinner plate so there isn't so much difference between what you're adding and what's already there, lessening the stress at the edges. In most cases the original design works, we can be tempted to go overboard with thickness when thinner metal will work just as good or possibly better, and saves weight.
 
I'd agree with you about it all being due to frame flex for rigs that see heavy action, but I hardly wheeled this rig. I didn't start wheeling until junior year of college, and then I went to med school, so it definitely didn't get wheeled then. In those two years in college, there were two trips to tellico on the easy trails, two to gulches, and a two to some local spots in SC. Otherwise it just looked pretty and pulled the occasional overloaded utility trailer or 3000lb boat.

Everything used to plate it has been 1/8". I was deciding between that and 3/16", but read about welding different thicknesses together, and went with the 1/8"
 
Decided to add some lights to the rear, cus the stock reverse lights suck.
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First time with a hole saw in steel. Didn't really like that experience that much. Not sure my drill liked it either, but I got it done. Bought some Pilot lights off amazon, wired up with an LED switch and relay. Haven't gotten to turn them on at night yet. They are not wired up to my reverse lights. I want to be able to turn them on independently for work lights if I need. I know you can wire up a 3-way switch, but I'm not that savvy, and wanted it done. So this will work. And I can turn them on if I need to see better than the stock lights.
 
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