Originally posted this in the fabrication section, but no one replied, and I will be trying to pick up some of the stuff tomorrow.

I'm planning out the materials for a rear bumper. The main purpose of the bumper is partially for protection, but also for facilitating recovery- a spot to jack with a HiLift, mount shackles etc. I don't rock crawl, but I do drive on deactivated forest service roads with washouts. My stock rear bumper has bent up and hit the body a couple times- it has to be strong enough to hold the weight of the truck and take the occasional smack, but it isn't going to endure too much abuse. I'd like someone with some fabrication experience to check over my materials choice (at the bottom) and make sure I'm not off track. I'm modeling it on this bumper by Irbis.







It is mounted to the frame similar to this:




I plan on using:
2x 4x .180 rectangular tubing for the piece parallel to the tailgate
2x 2x .120 square tubing for the side panel protector
3/8 plate for the frame mount

There will be a backing plate which slides into the open end of the frame. I will likely use 3/16th or 1/8 for this. The nuts will be welded to this backing plate to eliminate the need for a second wrench.

Questions:
What size bolts should I use? 1/2" grade 8 work? See above pic where the mounting plate is bolted to the frame.
Could I get away with .120 (instead of .180) for the rear cross-piece? Remember, I don't rock crawl, and the shackles attach to the mounting plate which extends through the bumper.
If I added lots of gusseting, could I get away with 1/4" for the mounting plate?

 

Can you? Sure. Alot of people use .120 for their cages it should provide plenty of impact protection. I would always prefers stronger, but then again I've started playing in rocks. As for the mounting plate I would not skimp. Especially if you are going to be using the bumper as a recovery point.

Even with the bolts I would run a small bead on each plate matting it to the frame. It's easy enough to take off with a die grinder but gives you a lot of strength and keeps things from slipping at inopportune times.

 

1/4" should be fine for the mounting plates, though you may want to thicken it up a bit where the shackles mount for recovery

 

The plan is to double up the 1/4" for the recovery pts. That is, unless I can find scrap 1/2".

 

I've seen that bumper before...
Looks really solid !

Good luck with the build matt

 

Thanks, first time welding something that big. I think I've got the hang of it, so we'll see. Right now, I'm trying to find fab. shops that will give me their scraps, see if I can get some of the pieces, at least the gusset material, for a six pack.

 

hah
Good plan.

 

Personally, I would use 3/8" (or thicker) for the frame-mount pieces that extend through the bumper for the shackle mounts and still double it up for the mounting tabs. The most important part is that those pieces are 1 integral piece for durability and extend straight through cuts in the bumper turning into the tabs. Also, if you ever plan on getting yourself yanked out of mud or deep snow somewhere, even if you don't lay your runner on a rock somewhere, all the force of the pull goes on those 3 poor bolts per side. If there were room, I would also double the number of grade 8 bolts. I know that I'm totally overkilling it, and I'll probably get slammed for it, but if I were yanking my truck out of somewhere hairy, I would either want that bumper welded to my frame or have all that support I blabbed on about. This is just my opinion, and I'm sure you can get away with less, but I have been bitten in the butt by 'just enough' construction before. Call me crazy.

 

Looks good Matt! .120 wall is plenty I think. 1/2" grade 8 is aslo bomber. Check out Metal Supermarket in BBY for off cuts- though I don't think they take 6-packs!

Let me know if you need any help.

 

Hey, I'll check out Metal Supermarket, I'm still back and forth to BBY for exams up on the hill.

I could use all the help I could get frankly. I'm pretty confident in 3-4 1/2" gr.8s per side- I think the frame will tear before those do. There's the clamping force too, according to the chart I have, a gr.8 1/2" bolt can be torqued to 120ft-lbs and makes 14 400lbs of clamping force PER BOLT!!!. I doubt shear strength of the bolt will ever come into play. I'd like to keep the bumper less than about 70lbs, so 0.120 would be my preferred material.

By the way I made a spur of the moment trip to the Squamish last weekend. Started by the Ashlu then drove up to mi 27. Saw a solitary coho. Lots of bull trout, but they're lock-jawed. I was bumping a variety of flies off their noses and they'd just move out of the way.

 

Alright, I need to know whether I can MIG weld a gr. 8 nut/ flange nut to steel plate. Will MIG weld dissimilar steel? gr. 8 nuts are an alloy of sorts. The reason I am welding the nut to the plate is that I won't be able to get a wrench on it when the plate is in place.- the weld isn't structural.

 

you can do that - should be no problem

 

I'd spot it to a washer first, myself. So long as you dont heat the nut too much you should be fine.

 

thats how the marlin mounts are. Make sure you weld that nut on square. Mine was a little off and it made instalation a massive PITA!

 

Yeah that would be a pain. I figure I'd bolt the thing together first , then weld the nut on.- thus every is already lined up prior to welding.

 

I'll see if I can find a flange nut to make it easier to weld.

 

Is the metal in nuts heat treated? Why specifically should I avoid over heating the nut. I doubt overheating will be avoidable.

 

Alright, I built a plywood mockup of the mounting plates and I'm going to need a piece of steel 24" long and 8" wide for each. The most narrow portion of the plate is where it passes under the tailgate and connects to the bumper. This part will be only 4"x4". Makes me wonder if 1/4" is sufficient.

 

flange nuts look great. I guess overheating shouldn't be too much of an issue, but I agree with your wanting heavier gauge hardware for that much metal. I'd be curious how much all of that would weigh! Remember, Confucius say: "do not use cannon to kill mosquito."

 

I've altered the design in my head a bit, and I think I can use 1/4" (which is more than what a lot of trailer hitches use) for the frame mounts as long as I brace it properly. Here's how I plan on bracing it:




I put the project on hold a while to bank some cash for school. I worked out that the cost of the metal was going to be $188 CAD (~$150 USD).