Metal front bumper/tow hook mount

[Calan]

Nah... parasolids rock. You'll pick it up quick.

BTW - Does that version of Solidworks allow you to export directly to earlier Pro/Engineer models? (version 2001)

And answer yer cell phone Eric.

 

[turbosax2]

I can save as a .asm. What specific extension are you looking for?

 

[Calan]

Eric...give me a call if you've got a second.

 

[greengoblin]

If you guys have to use a flat plate like that to mount the bar you will need gussets.

Also if you decided to mount the tow hook with bolts instead of welding it in place you may need to added a plate to spread the load out on the main bumper tube. I would think holes through that tube at the center of the arch are going to be big stress risers. I'm no engineer though. So if I'm wrong please chime in.

Gussets added in paint because that is all I rock.

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[99gst_racer]

I think having the gussets on the backside of the plate would be even better. There's more of a lever there (for lack of better words). But, I'm no engineer either.

For the tow hook mount, I'd weld in two threaded tubes that extended the entire width. It would be plenty strong, offer more than enough thread engagement, and eliminate the need for a nut.

 

[greengoblin]

Isn't the radiator support core there?

This marked in yellow.

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[Calan]

If you guys have to use a flat plate like that to mount the bar you will need gussets.

Yep....those flat plates are taking the brunt of the stress.

I threw together an FEA proof-of-concept model and ran a single-pass adaptive analysis with some very simplified constraints and loads. As you can see below, the model acts exactly as Kevin and I suspected.

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I mainly just wanted to see if the analysis would produce some reasonable results when treating the structure as a single part. It looks like we are in business, so I'll just need some real numbers to plug in, along with actual geometry that will affect things (shape of the hook, holes, etc etc)

That image is an exaggerated plot of displacement (actual max is about .43"), using over-simplified constraints, 304 SS for the material, and a pulling force on the hole in the hook of 4000lbs. With more realistic constraints and loading, we should be able to get a good look at exactly where stresses are concentrated, as well as the amount of deformation.

I think having the gussets on the backside of the plate would be even better.

In all actuality, it probably wouldn't matter much in this case. I would rather put them in front though if possible, since a). most materials are stronger in compression, and b). you wouldn't be putting the welds in tension.

 

[Calan]

Also if you decided to mount the tow hook with bolts instead of welding it in place you may need to added a plate to spread the load out on the main bumper tube.

If I had complete freedom to use any hook I wanted, I would probably weld a threaded "receiver" through the middle of the tube, and then use a threaded clevis hook (or similar) into that. Should be plenty strong, plus you get the benefit of an easily removable/replaceable hook.

 

[1990AWD]

Would a tubular piece be stronger?

 

[snowborder714]

The issue with that idea Craig is that we're trying to make the hook protrude at the very top of the bumper opening. If you put the threaded receiver in the middle of the tube, that'd require us to drill a hole in the bumper. Something we had considered before, but not our ideal choice.

Paul, I agree with Kevin in that having the gussets on the back probably wouldn't work because of the support being in the way.

 

[99gst_racer]

Isn't the radiator support core there?

I guess I was assuming this structure would stick out far enough to fit ~1.5" worth of gusset behind there. I guess it all depends on how it's designed and intended to fit though. The factory system protrudes several inches outward at that point, right?

 

[snowborder714]

Nope, it sits flush. Unless you're talking about the specific point that the bumper mounts, and then ya, it does stick out a bit.

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[Calan]

The issue with that idea Craig is that we're trying to make the hook protrude at the very top of the bumper opening. If you put the threaded receiver in the middle of the tube, that'd require us to drill a hole in the bumper. Something we had considered before, but not our ideal choice.

What if you welded a threaded tube at the same place the flat tow hook is now?

 

[snowborder714]

Just weld it to the bottom of the bumper? That could possibly work. It seems like a somewhat awkward setup to me as compared to the bolt on idea. Do you think the threaded option would be better/stronger than a bolt on option? I agree that it could probably make it easier to remove, however two bolts doesn't seem terribly hard. We had looked at the threaded tow hooks a while ago and like the idea.

 

[99gst_racer]

In all actuality, it probably wouldn't matter much in this case. I would rather put them in front though if possible, since a). most materials are stronger in compression, and b). you wouldn't be putting the welds in tension.

That's a good point on stretch VS. compression. Although I'd be willing to bet that the weld joints (and gussets) would be several times stronger than they would need to be to survive in a stretching enviornment.

Here's a question. If a front gusset was added at joint A, how would it affect the the stress levels at point B? I would guess that point B would see increased stress, but I am just guessing.

 

[Calan]

Do you think the threaded option would be better/stronger than a bolt on option?

Honestly, I think they would both be plenty strong as long as the right hardware is used. I think the main concern with this is going to be shock loading as the slack is taken up with the tow vehicle; not a static force caused by the car's weight (rolling friction).

The only drawback to the bolted on flat hook (other than bolt access) is that you would be weakening the tube somewhat as Kevin explained. The act of bolting the hook on would make up for some of the lost material and strength, but you are still introducing stress risers.

 

[greengoblin]

I would drill holes all the way through the tube. Then weld a threaded tubes in place flush through the center.Then you would bolt your hook to that. No nuts needed and the holes would be welded around and be plenty strong.

Kevin

 

[99gst_racer]

Nope, it sits flush. Unless you're talking about the specific point that the bumper mounts, and then ya, it does stick out a bit.

Yeah, I was referring to the outward most point there. If your piece matches that of the factory one, then you'll probably have some of that square mounting portion sticking out to get your distance right.

I would drill holes all the way through the tube. Then weld a threaded tubes in place flush through the center.Then you would bolt your hook to that. No nuts needed and the holes would be welded around and be plenty strong.

Kevin

Agreed. That's how I would attach either type of tow hook.

 

[snowborder714]

I forgot to comment on that idea when you first posted it Kevin...edit, I guess it was Craig I like it. We had thought about welding a tube for the bolt to pass through, but I don't think we considered a threaded tube. Much easier than a nut and bolt idea.

 

[Calan]

Although I'd be willing to bet that the weld joints (and gussets) would be several times stronger than they would need to be to survive in a stretching enviornment.

Agreed.

Here's a question. If a front gusset was added at joint A, how would it affect the the stress levels at point B? I would guess that point B would see increased stress, but I am just guessing.

Probably not much. Most of the force is going to be transferred into the bottom side of the mounting tubes.

I would drill holes all the way through the tube. Then weld a threaded tubes in place flush through the center.Then you would bolt your hook to that. No nuts needed and the holes would be welded around and be plenty strong.

Kevin

Great minds think alike.

If I had complete freedom to use any hook I wanted, I would probably weld a threaded "receiver" through the middle of the tube, and then use a threaded clevis hook (or similar) into that.

EDIT:

Oh wait... you mean vertically through the tube for the plate to bolt to, right?

Yeah, that would definitely be better for strength, although I would probably still use a through-bolt and washers/nuts. The tube going through the center with the hook coming straight out would be stronger since there is no vertical bending moment on the hook, but if it won't fit... it won't fit.

 

[greengoblin]

Indeed. Indeed.

 

[Bud92gsx]

I think I'm missing something..but how is this going to be attached to the car..And then perhapps unbolted..

 

[snowborder714]

The same way the factory piece is. The two arms on each end of the unit will slide into the frame rail where these brackets used to.

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Then it'll be bolted in just as the old units were (still working on best way to do that since nuts are integrated into OEM pieces).

 

[99gst_racer]

I think I'm missing something..but how is this going to be attached to the car..And then perhapps unbolted..

The square ends slide into the frame rails and bolt down just like the factory bumper supports brackets do.

EDIT: Quick grasshoppa.

 

[greengoblin]

The same way the factory piece is. The two arms on each end of the unit will slide into the frame rail where these brackets used to.


Then it'll be bolted in just as the old units were (still working on best way to do that since nuts are integrated into OEM pieces).

Same way as the tow hook with threaded tubes.

I suppose you could maybe weld nutserts in place but a tube through the center in double shear would be much stronger.