Advice on rear bumper build LX470 (1 Viewer)

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Its on there. There is a little bit of wiggle that I don't like. I think I'm going to need to tie the side wings back to the frame rail to stiffen it up a bit.
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hotdiggitydamn

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Sucks when you realize those things after paint. At least you get the satisfaction of it now looking like the thing you meant to make.

What’s your plan for the shackles? Are those getting reinforcement plates welded on?
 
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Sucks when you realize those things after paint. At least you get the satisfaction of it now looking like the thing you meant to make.

What’s your plan for the shackles? Are those getting reinforcement plates welded on?

Yeah for real, I even tested it with the tire on before paint. After doing some research it seems like the wiggle is pretty much unavoidable on swing outs, with even fabricators admitting theirs wiggle off road. It is rock solid on the highway, but I can shake it by hand a bit. I'm going to go do some light trails and see how it holds up. If I think it needs it I will just grind those areas and weld in an extra support.

As far as the shackle mounts, I had planned on adding another 1/4" piece to make them 1/2". But I need to use the drill press at work and I was getting a bit impatient to finish to be honest. I need my garage back haha. So I figured I would add them later.

.....but the more I think about it and research, why do they need to be so damn thick anyway? The frame it's mounted to is like 1/8" thick, and the 1/4" steel has a yield strength of like 36000 psi or something right? Someone who understands this stuff please explain it to me as I'm a dummy

If my mount is say, 6" x 4" so thats 24 sq inches. If the load is say 12,000 lbs...divided by the 24 sq inches that's 500 psi of force on the mount right? I also have two shackle mounts so I'd be rigging a bridle ideally so that force is then halved right? I know the extra thickness helps a lot when the pull isn't straight, (which is often) but is it really necessary? I know guys will sometimes build bumpers all out of 1/4" , and yeah it won't break but man that's a lot of weight.
 

ramangain

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Yeah for real, I even tested it with the tire on before paint. After doing some research it seems like the wiggle is pretty much unavoidable on swing outs, with even fabricators admitting theirs wiggle off road. It is rock solid on the highway, but I can shake it by hand a bit. I'm going to go do some light trails and see how it holds up. If I think it needs it I will just grind those areas and weld in an extra support.

As far as the shackle mounts, I had planned on adding another 1/4" piece to make them 1/2". But I need to use the drill press at work and I was getting a bit impatient to finish to be honest. I need my garage back haha. So I figured I would add them later.

.....but the more I think about it and research, why do they need to be so damn thick anyway? The frame it's mounted to is like 1/8" thick, and the 1/4" steel has a yield strength of like 36000 psi or something right? Someone who understands this stuff please explain it to me as I'm a dummy

If my mount is say, 6" x 4" so thats 24 sq inches. If the load is say 12,000 lbs...divided by the 24 sq inches that's 500 psi of force on the mount right? I also have two shackle mounts so I'd be rigging a bridle ideally so that force is then halved right? I know the extra thickness helps a lot when the pull isn't straight, (which is often) but is it really necessary? I know guys will sometimes build bumpers all out of 1/4" , and yeah it won't break but man that's a lot of weight.
Don't assume the force is linearly spread across the entire surface, and also don't forget about torques. It gets complicated quickly, and unless you use a proper computational model, you may want a huge safety factor to override your hand calculation fudge factors.
 
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Don't assume the force is linearly spread across the entire surface, and also don't forget about torques. It gets complicated quickly, and unless you use a proper computational model, you may want a huge safety factor to override your hand calculation fudge factors.

Oh yeah I agree, I'm no engineer I'm just trying to understand. It seems that you have plenty of safety factor with my about 4.5" x 6" x .25" mount.

Tensile strength of steel is listed at 58000 psi. And apparently tensile stress is calculated using the members cross section, which I guess would be the .25" x 4.5" So the cross section is 1.125" squared. If you again say a 12,000 lb load ( 12,000 / 1.125) that's 10,666 psi on the mount. But the steel is rated at 58,000 psi. I mean, isn't that a more than 5x safety factor? Again this is just crap i Googled id like someone smarter than me to tell me I'm wrong because I'm sure I am
 
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Well, I may have figured out where I'm wrong. So by making those mounts squared off and angled toward a point it weakens them significantly. You use the smallest cross sectional area which would be the point at the tip that is about 1" tall. So 1" x .25" is 0.25 " squared. Take the 12000 lb load / 0.25 and you get 48,000 psi now, much closer to that rated tensile strength of 58,000 psi The mounts would be much stronger, (but not look as good to me) if they were just square at the ends. Then you would get a 10,666 psi tensile stress pulling 12,000 lbs. (I'm probably calculating this all wrong haha.)

At the same time that's double the weight of the truck essentially, which you'd probably need to be bogged into 3 ft of mud to pull near that id imagine?

Either way I can see why you'd want 1/2" thick recovery points, but if those calculations are correct I'm surprised by what 1/4" can take.
 

suprarx7nut

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Not sure I follow your shackle/weld/plate discussion, but I've got Solidworks with Simulation and would be happy to share a quick and colorful view of the stresses if you can describe them with a napkin sketch or something. It's often surprising how high and where those peak stresses are. Huge filets and gussets are your friend, haha.

A few points to consider:

1. The stress for this stuff isn't loaded nicely across the plate. It's very specifically concentrated on the fasteners or weld. The bumper may be fastened to the thin frame, but it's secured with 8+ bolts at different orientations with large areas that "hug" the frame to distribute some push/pull forces.
2. If you're welding a simple right angle recovery point, you're very dependent of the quality and penetration of the weld. If you're certified and well-trained I'm sure you'd do fine. If I were to try weld something like that, I'd have no clue how strong that weld was.
3. For sharp angle assemblies like that you can get enormous concentrations of stress that can lead to failure. You might only have 1,000 lbs of force pulling on the anchor point, but the sharp edge on the backside of that fixture might see a stress of 50,000 psi along a very fine point. That point cracks, now it's a stress concentrator, now over time that crack propagates and eventually completely fails.
4. Heat Affected Zone (HAZ). The area of steel outside the weld is generally weakened by a variable amount because you just heated the hell out of it during your weld. That can anneal the metal and result in an area around the weld that's weaker than it started.

Lots of parts of the bumpers are overbuilt by a large margin to overcome this. But even then, you see occasional failures.
 
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Not sure I follow your shackle/weld/plate discussion, but I've got Solidworks with Simulation and would be happy to share a quick and colorful view of the stresses if you can describe them with a napkin sketch or something. It's often surprising how high and where those peak stresses are. Huge filets and gussets are your friend, haha.

A few points to consider:

1. The stress for this stuff isn't loaded nicely across the plate. It's very specifically concentrated on the fasteners or weld. The bumper may be fastened to the thin frame, but it's secured with 8+ bolts at different orientations with large areas that "hug" the frame to distribute some push/pull forces.
2. If you're welding a simple right angle recovery point, you're very dependent of the quality and penetration of the weld. If you're certified and well-trained I'm sure you'd do fine. If I were to try weld something like that, I'd have no clue how strong that weld was.
3. For sharp angle assemblies like that you can get enormous concentrations of stress that can lead to failure. You might only have 1,000 lbs of force pulling on the anchor point, but the sharp edge on the backside of that fixture might see a stress of 50,000 psi along a very fine point. That point cracks, now it's a stress concentrator, now over time that crack propagates and eventually completely fails.
4. Heat Affected Zone (HAZ). The area of steel outside the weld is generally weakened by a variable amount because you just heated the hell out of it during your weld. That can anneal the metal and result in an area around the weld that's weaker than it started.

Lots of parts of the bumpers are overbuilt by a large margin to overcome this. But even then, you see occasional failures.

I'd love to see some simulations that'd be really interesting. Here is a rough sketch of the mounts. It's all 1/4" thick and each is mounted with 2 grade 8 M14x1.5 bolts. I think the very end face of the mount near the hole is 1" tall
20201210_144243.jpg
 
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Well, I may have figured out where I'm wrong. So by making those mounts squared off and angled toward a point it weakens them significantly. You use the smallest cross sectional area which would be the point at the tip that is about 1" tall. So 1" x .25" is 0.25 " squared. Take the 12000 lb load / 0.25 and you get 48,000 psi now, much closer to that rated tensile strength of 58,000 psi The mounts would be much stronger, (but not look as good to me) if they were just square at the ends. Then you would get a 10,666 psi tensile stress pulling 12,000 lbs. (I'm probably calculating this all wrong haha.)

At the same time that's double the weight of the truck essentially, which you'd probably need to be bogged into 3 ft of mud to pull near that id imagine?

Either way I can see why you'd want 1/2" thick recovery points, but if those calculations are correct I'm surprised by what 1/4" can take.

Consider this- think of the effort required to bend a piece of 1/4” x 4” piece of plate. How long is the stick out from the bumper body to the shackle hole? I‘m betting I can put it in my vice and bend it by beating it with a 4# hammer. You’re going to se much higher stresses on that recovery point than I can put on it with a 4# hammer. Calculations are great, and taking the effort to understand what it is you’re asking your fabrication to do is great, but in order for those calcs to have any relevance you have to be applying them correctly.
 

suprarx7nut

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I'd love to see some simulations that'd be really interesting. Here is a rough sketch of the mounts. It's all 1/4" thick and each is mounted with 2 grade 8 M14x1.5 bolts. I think the very end face of the mount near the hole is 1" tall
View attachment 2521060
This is how I read that, but I'm sure it's wrong. Can you guide me a little more? Where is this part?

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