Reply to Thread

Re: Looking for advice on building rear bumper

I decided to beef up the recovery points on this bumper. I had some 1/4" left so I doubled up the tabs so they are 1/2" thick now. Bumper has been holding up pretty well. Did some rocky trails and it stayed solid, I did have to retighten the jam nuts on the heims but all is good again.

Attachment 1725136

Attachment 1725137

Re: Looking for advice on building rear bumper

Originally Posted by Noidea87

I had planned to double them up to get to 1/2", but I don't have a drill press so I'd have to use the one at work....and we aren't going into the office very much right now. With a trip coming up I decided to wait, and I'll just grind those sections and weld the extra on, and re-paint again.

but I started doing some research trying to figure out what the actual strength was of those 1/4" tabs, and I'd love some input from some of you on this.

So the yield strength of A36 was around 36,000 psi (tensile was like 58000 psi I think) and if I figure the cross section for those mounts (at the smallest width down near the "point" of the mount (0.25 x 1 = 0.25") Take a 12000 lb load (double the truck weight) and divide it by that cross section (0.25) and I get 48,000 psi of force on the mounts.

In reality I wouldn't be pulling against double the truck weight without being down to the frame in mud and pulling up a hill I would think right? I got some good info from this site on that https://blog.uscargocontrol.com/extr...stuck-vehicle/

Then, when you think about it more, I'd be rigging a bridle so both points were attached....so that pull force is then halved, so you are at 24,000 psi.

I know everyone likes to over build on stuff like this, and I get it. I know that if the pull isn't exactly straight these could get bent, but other than the bending how are these unsafe? I'm not arguing I'm asking for input from people who know a lot better than me.

Couple things to bear in mind:
1. Shear strength is typically only about 75% of tensile strength (yield or ultimate)
2. When you're bending stuff, like in your example, about half the cross-sectional area is in tension and half is in compression, and once it begins to tear on the tension side (through a tiny rip or crack), it's generally all over ... hence the CEP bash test involving the BFH and counting blows...
3. Blackboard assumptions are generally just that, and stuff in the real world generally follows Murphy's Law more than the quadratic pedantic principle of pointless semantic distinctions...so within reason, it's generally easier to overbuild than to crowd the envelope and then have to try to cram the Magic Smoke back into the toothpaste tube, even if you have a water-cooled toothbrush charger.

Re: Looking for advice on building rear bumper

Originally Posted by evan

1/4" I think is fine to pull most anything.

Not used in offroad application I think theyl work since rocks won't have much of a chance to bend them over.
Though I might be slightly worried about busting my knee on them.


Bumper looks great. I dont think you need to multi pass the square tube on the 3/8 plate to answer your earlier question

Sent from my SM-G960U using Tapatalk

Thanks, it was a lot of work and I learned a bunch on this project. I definitely have a new found respect for why most rear bumpers for this vehicle are so expensive haha. I mean, I would be looking at over $1,000 after shipping for a similar bumper.....and that was a weld yourself kit. I paid about $200 for all the steel needed and I have about 15 ft of tube and some sheet left over. Hardware and consumables were maybe another $150 so I feel like I came out pretty good.

The weight came out about where I wanted. Since this SUV has an adjustable hydraulic suspension with active dampening the neutral pressures need to be kept within a certain zone for the system to work right. I upgraded to heavier rear springs and after the weight of the bumper the pressures are right where they should be. I would guess that whole deal with the tire weighs between 175-200 lbs.

I tested it on a rutted trail a couple days ago and it seemed stable, on the road it's rock solid. I was worried about the little bit of movement I have in the actual swing arm (back and forth) when I push it or hit a big hole, but reading through a bunch of other forum posts it seems like most rear tire carriers have some movement in them and it's unavoidable. It doesn't move a whole lot, but it will definitely wiggle. Even some fabricators that sell their bumpers admitted it would move a bit, and those designs have been tried and tested off road. We'll see how it holds up I guess. I added a safety pin so if the latch came undone on or off road the arm wouldn't just swing and slam into something.

Re: Looking for advice on building rear bumper

1/4" I think is fine to pull most anything.

Not used in offroad application I think theyl work since rocks won't have much of a chance to bend them over.
Though I might be slightly worried about busting my knee on them.


Bumper looks great. I dont think you need to multi pass the square tube on the 3/8 plate to answer your earlier question

Sent from my SM-G960U using Tapatalk

Re: Looking for advice on building rear bumper

Originally Posted by Noidea87

Half of that went waaaay over my head hahaha, but I definitely understand the part about the strength going way up just going from .25 to .50. I do plan to beef them up to half inch after we get back from this road trip, but after looking at all that info it seemed pretty safe to use them for recovery as long as I used some common sense about it.....rigging a bridle to both of them, trying to pull as straight as possible, not yanking on them just a slow steady pull, and I won't be in any deep bogs

LOL!!! It was more or less meant as a joke to some other guys in a different thread

Re: Looking for advice on building rear bumper

Originally Posted by ronsii

Basically if you double the thickness you would quadruple the strength... So the steels tensile strength goes up exponentially but only if you are using a quadratic formula.... So most engineer's use standard algebraic factoring to devolve their equations to concise polynomials

Half of that went waaaay over my head hahaha, but I definitely understand the part about the strength going way up just going from .25 to .50. I do plan to beef them up to half inch after we get back from this road trip, but after looking at all that info it seemed pretty safe to use them for recovery as long as I used some common sense about it.....rigging a bridle to both of them, trying to pull as straight as possible, not yanking on them just a slow steady pull, and I won't be in any deep bogs

Re: Looking for advice on building rear bumper

Originally Posted by Noidea87

I had planned to double them up to get to 1/2", but I don't have a drill press so I'd have to use the one at work....and we aren't going into the office very much right now. With a trip coming up I decided to wait, and I'll just grind those sections and weld the extra on, and re-paint again.

but I started doing some research trying to figure out what the actual strength was of those 1/4" tabs, and I'd love some input from some of you on this.

So the yield strength of A36 was around 36,000 psi (tensile was like 58000 psi I think) and if I figure the cross section for those mounts (at the smallest width down near the "point" of the mount (0.25 x 1 = 0.25") Take a 12000 lb load (double the truck weight) and divide it by that cross section (0.25) and I get 48,000 psi of force on the mounts.

In reality I wouldn't be pulling against double the truck weight without being down to the frame in mud and pulling up a hill I would think right? I got some good info from this site on that https://blog.uscargocontrol.com/extr...stuck-vehicle/

Then, when you think about it more, I'd be rigging a bridle so both points were attached....so that pull force is then halved, so you are at 24,000 psi.

I know everyone likes to over build on stuff like this, and I get it. I know that if the pull isn't exactly straight these could get bent, but other than the bending how are these unsafe? I'm not arguing I'm asking for input from people who know a lot better than me.

Basically if you double the thickness you would quadruple the strength... So the steels tensile strength goes up exponentially but only if you are using a quadratic formula.... So most engineer's use standard algebraic factoring to devolve their equations to concise polynomials

Re: Looking for advice on building rear bumper

Originally Posted by John T

Looks great but I can’t believe you’re staying with those cheesy little D-ring mounts.

I think you’re going to bend or break them the first time you really have to use them.

I had planned to double them up to get to 1/2", but I don't have a drill press so I'd have to use the one at work....and we aren't going into the office very much right now. With a trip coming up I decided to wait, and I'll just grind those sections and weld the extra on, and re-paint again.

but I started doing some research trying to figure out what the actual strength was of those 1/4" tabs, and I'd love some input from some of you on this.

So the yield strength of A36 was around 36,000 psi (tensile was like 58000 psi I think) and if I figure the cross section for those mounts (at the smallest width down near the "point" of the mount (0.25 x 1 = 0.25") Take a 12000 lb load (double the truck weight) and divide it by that cross section (0.25) and I get 48,000 psi of force on the mounts.

In reality I wouldn't be pulling against double the truck weight without being down to the frame in mud and pulling up a hill I would think right? I got some good info from this site on that https://blog.uscargocontrol.com/extr...stuck-vehicle/

Then, when you think about it more, I'd be rigging a bridle so both points were attached....so that pull force is then halved, so you are at 24,000 psi.

I know everyone likes to over build on stuff like this, and I get it. I know that if the pull isn't exactly straight these could get bent, but other than the bending how are these unsafe? I'm not arguing I'm asking for input from people who know a lot better than me.

Re: Looking for advice on building rear bumper

Looks great but I can’t believe you’re staying with those cheesy little D-ring mounts.

I think you’re going to bend or break them the first time you really have to use them.

Re: Looking for advice on building rear bumper

Done. New BFG to matchAttachment 1720585Attachment 1720586

Re: Looking for advice on building rear bumper

Would you guys double pass this point where the tire plate is welded to the tubing? The tubing is 1/8" and the plate is 3/8" I think. I triple passed the pivot bracket which is about 5/16" thick.

Attachment 1720375

Re: Looking for advice on building rear bumper

Almost ready for the full weld out.

Attachment 1720323
Attachment 1720324
Attachment 1720325

Re: Looking for advice on building rear bumper

Originally Posted by evan

Heims look truck. Id gusset under they're bracket

Sent from my SM-G960U using Tapatalk

That's a good idea. I will definitely make up a gusset for the bracket.

Re: Looking for advice on building rear bumper

Heims look truck. Id gusset under they're bracket

Sent from my SM-G960U using Tapatalk

Re: Looking for advice on building rear bumper

Got the arm mocked up, everything seems to fit ok. 3/4" hiem joints. 3/4" grade 8 bolt

Attachment 1720282
Attachment 1720281

Re: Looking for advice on building rear bumper

I'm starting to think about the swing arm now. Do you guys see any issue with the support being like this? I plan to add a gusset under the arm the tire is mounted to. I also plan to add a spring pin along with the destatco Latch to hold it in place when closed. I figure the spring pin will stop back and forward movement and the Latch will push down onto a rubber pad to stop up and down motion

Attachment 1720096

Re: Looking for advice on building rear bumper

Originally Posted by Noidea87

Attachment 1720065

I haven’t really read the whole thread but just noticed your picture.

For the D ring tabs, I would use or make something like these.

You cut a rectangular hole for them to slip into from the backside then weld them up from both sides.

I have them on my jeep bumpers

http://americantrailproducts.com/D-R...unt-_p_45.html

Re: Looking for advice on building rear bumper

Originally Posted by Noidea87

Attachment 1720064

Shape is nice. What I see here is a little bit of fit up problem. Looks like the top plate might be a little bit to short for the rear triangle shaped plate and that is causing a gap in the long side plate where they all meet up. This seems to be causing the side plate to have a bend and a slight gap at the corner joint where the three plates meet up.i would have trimmed the rear triangular plate so it lines up with the top plate on the outside edge where the long side plate caps them together. I’m sure it’s not as easy to trim up one piece without having to either readjust or trim others. Other than that I think it looks good so far.

Re: Looking for advice on building rear bumper

Originally Posted by whtbaron

When my sister first started cooking she would walk up to the stove and deliberately burn something right off the bat to appease the cooking gods. Now that you have appeased the welding gods, the project can go forward smoothly.... carry on...

Haha, hopefully you're right. I learn something new every time I build a hunk of crap. They get a little less crappy each time

Re: Looking for advice on building rear bumper

When my sister first started cooking she would walk up to the stove and deliberately burn something right off the bat to appease the cooking gods. Now that you have appeased the welding gods, the project can go forward smoothly.... carry on...

Re: Looking for advice on building rear bumper

To me its sounds like you have more spatter than normal.
I have no idea what to suggest without seeing some welds

Sent from my SM-G960U using Tapatalk

Re: Looking for advice on building rear bumper

What machine?

To much stick out will reduce your voltage. Wire speed will be high and travel speed probly to fast. With to much stick out that is

Flux core is cool. You can increase stick out to reduce heat if you need to.



Sent from my SM-G960U using Tapatalk

Re: Looking for advice on building rear bumper

Originally Posted by evan

I dont get a ton of bbs but think I get more with gas. Flux core is just dirty gritty shaggy.
Solid wire bbs usually knock right off unless im burning hot over 22v

Bumper is looking great.

Sent from my SM-G960U using Tapatalk

Thanks man. I know it's crap compared to what a lot of people on here could turn out, but it should serve my purpose.

I think my stick out was a little longer than it needed to be hitting some hard to reach spots and I got more spatter. Anything else I should look at to reduce it or should I just expect it to be this bad with flux core?

Re: Looking for advice on building rear bumper

I dont get a ton of bbs but think I get more with gas. Flux core is just dirty gritty shaggy.
Solid wire bbs usually knock right off unless im burning hot over 22v

Bumper is looking great.

Sent from my SM-G960U using Tapatalk

Re: Looking for advice on building rear bumper

Went and bought a pry bar alignment tool at home depot,and a step bit. Reamed the holes from 5/8 out to 3/4" and got the bolts to go in. Those frame threads are crusty though so I've ordered a m14x1.5 tap to try and clean them. If that doesn't work I guess I'll drill and tap to 9/16". Hoping I don't have to do that.

Added some gussets to help support the weight of the swing pivot, and some small ones to stiffen the wings, although I don't think they needed them. Cut the cap sections and tacked them on. This flux core is so damn messy i can't wait to use up this spool and get some solid wire and gas. Been wire wheeling the **** out of this thing trying to knock the BB's off

Attachment 1720064

Attachment 1720065