Mobtown hidden hitch failure. Need suggestions

All-Pro gets away with it because the Baja bumper's shackle mounts are very low:



In fact, I don't recall any other vendor putting shackle mounts as high as the MobTown hitch bar, probably because there's nothing beefy on the frame that high.

 

I’m not looking at my truck (2nd Gen) now but how much trouble would we be getting into to cut those dinky flanges off and replace them with something better? The prerunner bumper I put on my ‘00 required cutting 3 inches off the front of the frame.

 

1st gens are a different kettle of fish, with weak "frame horns" designed to collapse in a collision. On 2nd/3rd gens, the aluminum boxes behind the crash bar serve the same function, but thankfully they are bolt-on.

 

Interesting. I better have a look at my slimline and see where my mounts end up in relation to their mounting. Not that I'd ever get a run at a recovery in the front, but might be good to know.

 

@Surfguygriff unless you really need the front hitch, I would recommend getting a low-profile bumper / winch cradle from SSO, MobTown, Warn, etc. The shackle mounts are lower on the bumper, so they exert less bending on the frame.

 

Maybe just watch 5 straight hours of Matts Offroad Recovery and then replicate his bumper front/rear hitch setup...

 

Be sure to splash a jug of water into the radiator grille on hot days before recovery operations.

 

The horse gotta drink!

 

@Surfguygriff
Any vendor response?

 

Thanks for all the responses guys.

The vendor finally responded via email and acknowledged that he could easily see where the obvious failure happened and that he already had a solution and will ship me out the newly redesigned unit free of charge in 2 weeks. I guess that's all I can ask for and I think that's great.
I actually mocked up this part on my FEA/Stress Analysis software and can easily see how this happened. I offered to share this data with recommended geometry changes with the vendor but they declined stating they know what they need to do. I guess I assumed the vendor would've done this during the design process but I'm learning that a lot of this industry just "overbuilds and prays". In this case it's clearly a challenging application with 2 overall design problems with 2 simple to fix weaknesses (that nearly quadruple the strength without much more mass). We'll see what they come up with so I'll reserve final comments. My initial thoughts are that this was simply not put through proper FEA analysis prior to production.

The leverage of the high recovery points on this product clearly presents a unique set of forces not experienced by bumpers with pull points and winch mounts more aligned to the frame and ties. That said, better performance is not something that can't be achieved here and I'm wiling to help make this a better solution for myself and others by working with the vendor.

Considering my frame mounts are now damaged on both sides I'll be reinforcing the bumper mounts with SOS's weld on plates before moving forward.
http://sosoffroadconcepts.com/weld-on-frame-brace/

To answer some relevant questions:
1. Yes the pull WAS at a slight angle downward but not obviously reckless. Example: 30 ft recovery strap from the recovery mounts running directly to the bumper tow hook of a Subaru sedan (maybe 12-15 degrees).
2. It wasn't an overly hard slamming force as I used my 4", 30ft snatch strap that has a good amount of stretch. I've yanked buried full sized trucks and SUV's off the rear hitch for years with the same set up with no issues. In-fact I have a G-meter on my vehicle for work and it indicated .21 G's during the pull which equates to about 1,800lbs of force applied at peak with about a 10% variable. That in itself should never have damaged this part BUT given the leverages involved with this high pull point design the forces on that back plate and bend is nearly 5X that number (easily overcome).
3. Unfortunately I really do need the front hitch for various reasons so this was one the attractive things about this design.

Before I reinstall (and powder coat) this new reworked unit I'm going to run it through my FEA again and see if it really is a better design. If it's not significantly better I'll reinforce it with my own observations and share the CAD/FEA file with everyone.

Thanks again.

 

Thanks for the update, wonder what the vendor will do for people with the old design (that havent had any failures yet).

Keep us in the loop please, if it's an additional part maybe we can weld it onto our current units.

 

Those SOS plates will of course fix your damaged frame and make it stronger than before but they might still bend under pressure if a hitch receiver/ winch mount/ aftermaket bumper is not strapped to the frame rails aft of that.

I believe 3/16 mild steel (the thickness of the SOS plates) permanently deforms at 6,660 psi. Going off memory, the bumper mounts that those plates reinforce are just ~1/16, so together, they still might not produce the strength necessary to handle the force you estimated from your Subaru pull, if your next pull is asymmetrical.

Looking forward to seeing the vendor’s revised design, and/or your own improvements. I think this is one of the best things about tacomaworld - a lot of minds converging to find good solutions for shared problems.

Thank you @Surfguygriff

 

*UPDATE* The newly designed solution from Mobtown arrives tomorrow! I'll update everyone on what they changed and take some pics. I'll also model it and run some simulated stresses on it to see how the changes helped (or not). I'm hoping with the SOS welded mounting plates, the side and lower frame straps along with the new design we'll have a safer and stronger solution.

 

Glad to see the vendor appears to be making it right for you. Interested to see the solution.

 

Plenty of Pics PLEASE

 

Hopefully they made these changes on the one I've been waiting for since the labor day sale.

 

Curious to see what Mobtown sent out. I just installed this on my truck a few months ago, so I would definitely be interested in the reinforcements.

 

Ok guys. Here is the new updated item. There are 2 major changes. Both for the better and help address the geometry challenges in this design.
As you can see below the most obvious change is the addition of the gusset reinforcement at the bumper mounting location where mine bent. This is good.
The second change is that they now use a single channel piece for the 2 standoffs vs the 3 welded plates on the old design.
Before powdercoating I'm going to internally gusset the side 90 degree bends (for the frame ties) as this was another failure point that wasn't addressed here. I'll also likely copy the added gusset and add 1 more on each side for added strength.
I would've liked to have seen them simply extend the channel peice all the way down the the bottom of the bracket but it is what it is.
These changes in addition to the weld on SOS bumper mounting plates should be sufficiently stronger. I truly commend Mobtown for both listening and speedily addressing a design issue and I'll continue to use thier products on my various vehicles.

 

This is overall a tough situation, but it sounds like everyone is learning good things and positive outcomes are happening. Sorry you’re having to go through the pain of repairs. I read your comment on the g’s you were measuring. Don’t those g’s simply represent the acceleration your truck experienced during the snatch? They don’t really measure the force applied to the hitch because we have no way of knowing what resistance the truck was experiencing during the pull. It could have been many times that just to overcome the resistance, and the 0.21g’s are simply the leftover that resulted in your measured acceleration.