Advice on rear bumper build LX470 (1 Viewer)

suprarx7nut

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ok, here's where I am. I want to start with just the recovery point. Is this a fair representation? If so, any idea on your exact alloy? A36 maybe?

1607636850264.png

1607636873396.png
 

suprarx7nut

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Ok, A36 and some vague fixturing points chosen to just show the actual recovery tab, here's a quick look at 12k lbs pull. I set the color scale so that red is yield. IE, that metal is deforming if it's red.

Straight pull: The model exceeds yield, but mostly just within the ID of the recovery point. Not bad, but we can see other areas aren't far behind.
1607637628253.png


30 degrees off angle(towards the camera from this view), but level vertically. Here's the killer. Total failure. That tab is probably bending and ripping off.
1607638136828.png


Zoomed out shots of the model and assumed fixed points.

1607638230714.png



1607638278777.png


View attachment 2521099
 
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Ok, A36 and some vague fixturing points chosen to just show the actual recovery tab, here's a quick look at 12k lbs pull. I set the color scale so that red is yield. IE, that metal is deforming if it's red.

Straight pull: The model exceeds yield, but mostly just within the ID of the recovery point. Not bad, but we can see other areas aren't far behind.
View attachment 2521102

30 degrees off angle(towards the camera from this view), but level vertically. Here's the killer. Total failure. That tab is probably bending and ripping off.
View attachment 2521114

Zoomed out shots of the model and assumed fixed points.

View attachment 2521116


View attachment 2521117

View attachment 2521099

Wow thanks for putting that together. How much better is it of the tab is 1/2" on the outside face of the bumper?
 

suprarx7nut

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Wow thanks for putting that together. How much better is it of the tab is 1/2" on the outside face of the bumper?

Better, but stull ugly. Top and bottom edges, along with most the opposite face (not shown here) are right at yield stress. If you want to measure the distance from the edge of your 7/8" hole to the bumper, I can get this a little more accurate. That distance is pretty key and we're winging it to an extent. Closer to the bumper face, lower the moment force. I'll also add in a filet for the weld. That should help a small amount.

All these numbers are obviously grossly simplified estimates, but it can help highlight what areas are likely to fail and help you understand where you need to reinforce and make a more educated guess where gussets and reinforcements help. The model also doesn't take into account HAZ (maybe it can, but I've never done that), so some areas outside that weld area might have a significantly lower yield stress than the limit indicated by red here.

1607646925391.png
 
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Better, but stull ugly. Top and bottom edges, along with most the opposite face (not shown here) are right at yield stress. If you want to measure the distance from the edge of your 7/8" hole to the bumper, I can get this a little more accurate. That distance is pretty key and we're winging it to an extent. Closer to the bumper face, lower the moment force. I'll also add in a filet for the weld. That should help a small amount.

All these numbers are obviously grossly simplified estimates, but it can help highlight what areas are likely to fail and help you understand where you need to reinforce and make a more educated guess where gussets and reinforcements help. The model also doesn't take into account HAZ (maybe it can, but I've never done that), so some areas outside that weld area might have a significantly lower yield stress than the limit indicated by red here.

View attachment 2521194

That distance is basically dead on 1".

How much better does it look pulling just the truck weight? Based on calculations I found here Extracting a Stuck Vehicle | Calculating Recovery Requirements | USCC

I'd pretty much never be pulling double the truck weight unless down to the frame in mud and pulling from uphill. I feel like you do have a little leeway if you use some common sense. Having these estimates are nice to see what you can actually get away with. I'm not arguing for 1/4" recovery points just saying in general.
 

suprarx7nut

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Sizable improvement with the reduced poke on that tab to 1".

1607647865764.png


Reducing pull weight to 6k. Barely hits yield and only at the ends of the tab.

1607647918131.png


Contrary to what I expected, adding the filet to the model increases stress right near the filet. I think that's an artifact of the way the model is creating the mesh (the grid of little triangles it divides the part into to make stress calcs).
1607647993252.png
 
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Sizable improvement with the reduced poke on that tab to 1".

View attachment 2521207

Reducing pull weight to 6k. Barely hits yield and only at the ends of the tab.

View attachment 2521208

Contrary to what I expected, adding the filet to the model increases stress right near the filet. I think that's an artifact of the way the model is creating the mesh (the grid of little triangles it divides the part into to make stress calcs).
View attachment 2521209

So, would you mind modeling this for me with a 1" thick recovery point? I'm thinking I'm going to cut these tabs off and weld the 1" shackle tab to the same spot. The only thing I'm wondering about is if that will be weaker than what I have. Since right now the tab is all one piece that goes back to the frame. If I cut that off and attach the 1" piece it will be welded to the 10 ga bumper shell mainly and not connected to the 1/4" mount.

Should I cut the 10ga shell out around the mount and weld right to the 1/4" frame mount? Does that make sense?
 
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So, would you mind modeling this for me with a 1" thick recovery point? I'm thinking I'm going to cut these tabs off and weld the 1" shackle tab to the same spot. The only thing I'm wondering about is if that will be weaker than what I have. Since right now the tab is all one piece that goes back to the frame. If I cut that off and attach the 1" piece it will be welded to the 10 ga bumper shell mainly and not connected to the 1/4" mount.

Should I cut the 10ga shell out around the mount and weld right to the 1/4" frame mount? Does that make sense?
On my bumper I have a laminated tab. 1/4" middle piece extends through the bumper and attached to frame, then smaller tabs the same shape sandwich both sides on the outside. I have no engineering data to tell me how strong it is, but it makes sense in my mind. 😁 I'll see if I can find a pic for clarity.
 

suprarx7nut

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So, would you mind modeling this for me with a 1" thick recovery point? I'm thinking I'm going to cut these tabs off and weld the 1" shackle tab to the same spot. The only thing I'm wondering about is if that will be weaker than what I have. Since right now the tab is all one piece that goes back to the frame. If I cut that off and attach the 1" piece it will be welded to the 10 ga bumper shell mainly and not connected to the 1/4" mount.

Should I cut the 10ga shell out around the mount and weld right to the 1/4" frame mount? Does that make sense?
Ping me again later tonight if I haven't gotten back to you. That should be easy to model, just have to load it up when I'm at my desk.
 
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IMG_20210215_172013386_HDR.jpg

Here's mine. The "bread" pieces of the sandwich are slightly larger than the "meat", which leaves a nice little channel to fill with weld. Then a chunk of 1.5" .250 wall tube fills the hole.
 

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