Mobtown hidden hitch failure. Need suggestions

The Class III hitches for our trucks are rated to 10,000 lbs. (EDIT: straight line pull of 9-10,000 lbs. of the front hitches I looked at, not towing capacity). Barring distal equipment failure, the weakest point is the truck frame, (but I think the hitch receiver bolts are designed to slip on extreme overload to try to prevent totaling the truck).

I’m not saying that to suggest what to use for recovery, but to show with an example that there is nothing impossible at all about designing recovery points in those locations that exceed your truck’s GVWR and make frame damage the first limiting factor.

 

…….you have 3 Tacomas?

 

Right and you can easily surpass 10k load depending the factors of the weight of the stuck vehicle, what its stuck in, any upward angle needed to get it out, etc. Now add all of that to a point that has the added leverage angle and you can easily bend/break stuff.

Interesting blog post on the topic: https://blog.uscargocontrol.com/extracting-stuck-vehicle/

The point I am trying to make to stay within the confines of the post is that the failure exhibited should not be surprising and caution should be used when recovering a vehicle as the loads far surpass what is needed to tow.

 

@Surfguygriff I am not an engineer so I cannot say this authoritatively, but a back of the napkin amateur calculation says you exceeded 6,660 psi to bend 3/16 inch mild steel.

Were you pulling at a hard downward angle? If so, the deformation may have saved your truck from damage. If not, that’s disappointing, because after doing that calculation I don’t think a single plate of 3/16 inch steel provides an adequate safety factor over GVWR for recovery work, which has implications for the mounts of a lot of bumpers, winch mounts, and hidden hitch receivers. Those corners focus the energy onto a super small area of the plate.

 

That is the culprit. The bolts were not spaced far enough apart (actually the distance from where you were pulling from in relation to where the next lowest fixed point (top bolt) was too far).
Because of this, that tension from the tow points was enough to make it yield in the direction you pictured. Design flaw for sure.

If you could weld in a small brace behind and above the silver bolts pictured and chop a way a bit of that frame mount structure you would have alot more resistance.

 

Not implying you did anything wrong, and I can believe that you are not seeing any hole elongation... that said, it sure looks like this joint slipped.



Note, I am not saying fixing this joint fixes the problem... just trying to see what parts and joints have 'permanent detrimental deformation' and I think that joint qualifies

 

Doesn't seem like a well designed joint to rely on the clamping force of essentially one bolt per side.

In my owner's manual the factory rates the capability at 6,500 lbs. Generally class III is 8,000 lbs and class IV (which is what the factory hitch is I believe) is 10,000 lbs trailer weight but those values are limited by what the truck is actually capable of doing. It's a good comparison though when you look at the factory hitch which weighs about half a ton and uses about a dozen bolts.

 

I would say any joint that relies on friction is highly undesirable, but that would be about 99.9% of the bolted joints on our truck... as we customers and car manufacturers (both) dont like to drill, ream and use close tolerance bolts on assembly.

 

I dunno, speaking purely as a clue-less sparky and not a mech-eng, I'd think bolted joints are fine as long as you design for and use enough to avoid a single point failure.

 

You are right. When I said “10K” earlier, I was remembering the straight line pull rating, not the tow rating, for some front hitches (e.g.). When I just looked again on etrailer I only “9K” straight line pull ratings for our trucks.

 

So I have NOT run any numbers.
That said, a class 4 drop hitch MAY be safer than the recovery points, for deformation on this install, by moving the load application line lower... (until a better actual fix is designed)

Tony @tonered, what do you think? (SAE J684 for a class 4 requires 1.5 x 10,000 lb capacity without detrimental deformation)... I dont know if 6" drop is a enough, and that adds torsion to the cross member

 

Your right. I missed it initially, but my point was to actually tag them in the thread since the OP hadn’t.

 

Back in the 90s my wife and I hit hatters a lot. Pulled many a truck out with the stock front single hook on my 4 banger POS Montaro, never had an problem.

 

I disagree with Jerry’s assessment that it would bend above the reinforcement. The area outlined in green is completely boxed and extremely sturdy.

The hitch receiver failed in two places, marked in red on the first photo: The obvious one is on the corner where the radius expanded. (The bolts did not slip.) The less obvious one is below the boxed green area, where the top folded over, collapsing toward the lower tab.

Probably the only thing that needs to be done to correct this is to put strong gussets under the green boxed area, shown in yellow on the second photo. That would most likely fix this problem completely, by prohibiting the sturdy box from collapsing downward.

But if you want to get extra fancy, you do like Jerry did, because he’s onto something with his reinforcements, and resort to some Fins’n’Things in addition to the lower gusset. The fins (in pink) need to wrap all the way around the front and tie into the sturdy boxed front portion of the hitch. Welds are indicated in red in the second photo, and you’ll need to cut slots in the rear-reaching frame strap to accommodate.

 

Might bend his mount back into position though....

 

Also makes a huge difference how fast you apply the load.

Rough numbers , a 5mph stop/change in .1 sec would be about 2.3g

So if you do a slow pull (winch, slow acceleration) you are just dealing with the forces applied in the previous link example.

If you are using a hard strap/chain and even hit it a 5mph to try and jerk something out, you are applying 2.3x the force

Little fun calculator I found online to check my numbers as I'm getting rusty...

https://www.omnicalculator.com/phys...leration-formula-three-acceleration-equations

Yep 2.2793g...

 

I was trying to stay out of this one because of 'personalities' at the vendor.

But, yeah. A drop hitch is kicking in a similar or larger moment just towing a trailer. Hell, you could argue the same about any non-ARB recovery point on the right side due to the cross member setup.

Also, folks would use a hitch shackle out back. Still a large amount of tension, shear, and moment at the frame

Backing a car out of a hole as described should be a no brainer for this part, if designed correctly. I see issues in the design unfortunately.

 

I'd be more concerned that the frame flange is bent.



I think the problem is the 3rd gen hitch bar sits more than two inches higher than the 2nd gen hitch bar (compare against this photo: https://twstatic.net/attachments/front-hitch-reciever-3-jpg.2760744/ ) out of necessity to match up with the bumper cover slit. This significantly increases the bending moment against the mounting bolts.

 

On MobTown's 2nd gen hitch bar, the boxed green area actually goes all the way down to the lower crossmember. Looks like when they moved the hitch tube higher for the 3rd gen design, the boxed section was shifted up instead of being extended...

Another method of reinforcement (as seen on the All-Pro Baja bumper) is to sandwich into the radiator support bushing mount:

 

Oof, the numbers probably check out, but tying into the bushing mount sounds like trouble.