Towbar as recovery point - discussion

[Gwagensteve]

I do not support snatching from a round tube type towbar on a car heavier than a Sierra. The drop required on a fourrunner from the chassis to the bar is excessive for a light duty bar.

Kiwibacon, I understand your point, but if the stock receiver hitch pin is used, I still feel this is the safest possible setup. Unless the whole towbar leaves the vehicle, ONLY the strap can fail and become a projectile.

I understand and accept the potential of the strap to be damaged by the sharp end of the hitch. I don't have a solution to this but I believe it will take many many snatches at a steep angle to cause a problem.

If anyone can explain how a hook, jate ring or anything retained by 2 bolts through a chassis can be safer than a receiver hitch attached in two planes and by 6, 8 or more bolts into locations intended by the manufacturer for this use.

I understand and accept that a towbar is not intended for recovery, but I cannot conceive of a better item for recovery.

PS I would be happy to accept a forged bar with a hole for a shackle in the reciever, and I have seen these, I think in the US. The ones available here with a welded lug for the shackle are not acceptable in my opinion, regardless of shat colour they are painted.

Steve.

[KiwiBacon]

having said all that i myself snatch from my hayman reece style rear bar with a high tensile 16mm bolt through where the towbar pin normally goes...

Doing that puts about 8 times more stress on the bolt than pulling from a shackle through the towbar hole?
Your high tensile bolt isn't 8 times stronger than the original pin.

[Ezookiel]

http://www.pirate4x4.com/tech/billavist ... index.html

Site seems to be dead at the moment, but this site - if I remember correctly but can't check while it's down - has methods of calculating the force involved in a recovery based on substance, depth, slope, weight, etc. etc.
But none of Pirate is loading tonight, so as I said, I can't check the link still works.

[steven101]

^^^ The sites working now

[Ezookiel]

Yep, working now.
The part that you want is "calculating the force of a recovery operation" about 1/4th of the way down the page.
The first part is all a whole heap of basic stuff most would already know.
Though the "roll over stiff-leg" is pretty darned interesting stuff to note.
I carry a handwinch rather than a bullbar winch so I can right a vehicle if it tips over, which a bullbar mount isn't going to do easily, but this "stiff-leg" idea would make the recovery much much easier.

[-Scott-]

http://www.pjchris.myqth.com/Pajero_Snatch.jpg

The article in the link talks about an attempt to deform a Pajero by snatching. Maximum force recorded in a snatch was 31kN - or 3 tonnes.

[taspatrol]

can i just say tonight we were forced to pull an 80 series cruiser out and i had to use tow bar and we riped one of his rated mounts off i dont recomend this but it got us out of trouble tonight also i was in a swb pootrol and he was very stuck

[me3@neuralfibre.com]

OK Kiwibacon and myself have been throwing numbers at each other offline as he corrects my mistakes and edumicates me in such things.

The short and sweet of it seems to be this

If your towbar is in line with your chassis and is a receiver type, it *should* be as stronger than jsut a single chassis rail.
If your towbar hangs down like many ute's do, or is not a receiver type, no one knows, it all depends how it's built.

The Pin is not strong enough for snatching according to the numbers, thats 16mm High Tensile. Mild Steel is way under. BUT, the numbers have a heap of variables we can't allow for. The calc strain is 2400MPA and High Tensiile steel is only 640MPA rated.

With the Hitch in place the mild steel pin will cope with about 20Ton
With the hitch in place the receiver holes (16mm) will cope with about 10 ton (5mm walls)

So using the hitch is much stronger than using the pin.

If you want the numbers I'll post more details on the blog in the next few days and link to it.

All of these numbers have no safety margin, dont allow for wear, materials etc etc. Just like the snatch strap. It is Apples with Apples.

Paul

[KiwiBacon]

In case anyone was wondering where 640MPa came from.
That's the stress where a grade 8.8 bolt will start to bend.

Short story.
Fit a recovery eye or hook into a receiver, don't loop the strap over the pin.

[Bush65]

There is a post on the 1st page that stated bolt proof load is for shear load - that is wrong, bolt proof load is for tension.

Bolts in shear are nowhere near as strong as bolts in tension, when subjected to shock/impact loads. To absorb impact loads, the impact energy is converted to strain energy. A bolt in tension utilises the volume of the material in most of the bolt for strain energy. A bolt in shear has the volume of the shear plane (practically zero) for strain energy.

I saw 2 recovery hooks used in a hayman reece type receiver last Sun. This appealed to me and could be a good idea for others to consider.

The 2 hooks were placed back to back and inserted into the receiver (these hooks were a neat fit) and bolted with a high tensile bolt through the pin hole and (presumably) the mounting hole in hook.

With some receivers it would be possible to drill a 2nd bolt hole (in line with the other bolt hole in the hooks).

With tow bars, as others have said, it really depends how the bar is fixed to the vehicle and the difficulty of the recovery.

[Tojo]

With tow bars, as others have said, it really depends how the bar is fixed to the vehicle and the difficulty of the recovery.

I think the difficulty of the recovery is what breaks stuff. A small amount of work with a shovel and a hi lift will mean a hell of a lot less strain on the recovery and make for a safer and easier recovery. Most people are too lazy or ignorant to do this and i sometimes think that it is the single biggest cause of breaking things.