Recovery forces

[nj swb]

A recurring topic on this forum is "rated" recovery points, and the "extreme forces" involved in snatch recoveries.

For many years, we have been directing people to the video produced by Mitsubishi Motors Australia, where they deliberately bogged a Gen3, snatched it using only one factory loop, and checked for signs of distortion in the body.

https://www.youtube.com/watch?v=wPXixsvjDjY

For their worst-case snatch, they reported a maximum force of 31.6 kiloNewtons, or nominal 3220 kg.

I just stumbled across a video produced by TJM, where they deliberately bogged a ute and camper trailer, and measured the forces required to snatch it out.

https://www.youtube.com/watch?v=T6ib-tY-f8U

TJM intelligently used a bridle to distribute loads over both of their recovery points - wouldn't be a good look to distort their ladder frame.

So, what was the peak force they recorded, in attempting to snatch their bogged combination?

TJM Snatch Testing.png

Their peak force was only 60kg less than measured by Mitsubishi over 10 years ago, or about 2% less - arguably measurement error. Both tests, in different locations using different equipment, close on 2 decades apart, measured the same peak forces.

Is that a lot?

Mitsubishi's peak force of 31.6kN is a little over 7000 pounds. Those of us with winches on our 4wds would have something rated to 9500 pounds or more, and carry a snatch block to double that - worst case, a winch recovery could be generating more than double the forces measured by these two different tests.

Please don't talk about "extreme forces" in snatch recoveries. Done properly, they're not that extreme.

[Jasonmc73]

They copied Mitsubishi Pajero test , if you can't beat them i guess the decided to joing them!

All jokes aside appreciate the information & link,

I'm no engineer but i gather neither recovery point of bridled/ recovered vehicle could have any more than say 70% of the pull force being exerted. on any given point, depending on angles?
Or possibly mostly even, due to snatch sliding finding middle of the bridal i guess is reality?

[nj swb]

I'm no engineer but i gather neither recovery point of bridled/ recovered vehicle could have any more than say 70% of the pull force being exerted. on any given point, depending on angles?
Or possibly mostly even, due to snatch sliding finding middle of the bridal i guess is reality?

Good question, I've never really thought about it before.

Theoretically, the tension in the bridle should be the same either side of the point, but realities of friction etc. (as the snatch strap slides to find the point) could affect what happens in the real world. Ultimately, I'm happy to consider any differences as insignificant, in the grand scheme of things.

If pulling straight ahead, everything will be symmetrical, and each attachment point will experience equal longitudinal forces, and equal but opposite lateral forces. The two longitudinal forces will each be half of the tension in the snatch strap, and the lateral forces will depend on the angle, which (in turn) depends on the length of the bridle compared to the distance between the attachment points. Longer bridle = smaller angle = lower overall tension in the bridle (and lower overall force on each attachment point).

If the direction of the snatch isn't straight ahead, such that the bridle has different lengths each side of the point, then total force on each attachment point will still be the same, but the mix of longitudinal and lateral forces will change. For example, if the direction of the snatch is precisely right, one attachment point will experience only a longitudinal force (with no lateral component), and the other point will experience the same magnitude of force, but with both longitudinal and lateral components.

Aren't you glad you asked?

[Jasonmc73]

Aren't you glad you asked?

I do get the drift of your meaning .
Main point being use a bridal basically your looking at half the snatch strap on each recovery point, but could be a shock load of say 60% on one & 40% on the other etc.

The one whom is doing all the pulling is the little pin in the trailer hitch, although if one uses a recovery hitch its definately well supported.
If its say 13mm pin (estimate) taking the force on both sides of the pin, thats pretty damn secure, 26mm of pin basically holding a 3 tonneish shock load

[nj swb]

The one whom is doing all the pulling is the little pin in the trailer hitch, although if one uses a recovery hitch its definately well supported.
If its say 13mm pin (estimate) taking the force on both sides of the pin, thats pretty damn secure, 26mm of pin basically holding a 3 tonneish shock load

Again, you've got me thinking.

I measured my pin - 16mm diameter.

Using a commercial recovery adaptor, I would say the pin is in a "double shear" situation (rather than experiencing bending forces). So, what's it rated to?

I went to google for some help with calculations, and instead found a document from Purdue University: https://www.extension.purdue.edu/ext...ppp/ppp-94.pdf

A table on page 19 lists double shear strength for various pin sizes. As an American document, it's all in imperial measurements - 16mm is a fraction over 5/8".

A 5/8" hitch pin, according to Purdue University, is rated to 27,000 pounds - a little over 12,000 kg. Does that sound like enough for an 8,000 kg snatch strap?

That 12,000 kg is for a Class 2 pin - weak steel.

A Class 5 pin is listed at 44,000 pounds, or 20,000 kg.

A Class 8 pin is listed at 55,000 pounds, or 25,000 kg.

I don't know the class of my hitch pin but I'm not going to lose sleep over it, either.

[old Jack]

Great to see someone has validate/confirmed the MMAL test.

This brings up another discussion point about kinetic recovery ropes and snatch straps.
According to ARB USA, their snatch straps should only be used for 10 high load recoveries in total, before being discarded!
An accepted practice has been no more than 3 pulls on a snatch strap before letting the strap "rest" so it can recover back to the original stretch capacity of around 20%.
Each time a snatch strap is stretched it does not immediately return back to it original length, so how much stretch is lost with each pull will obviously depend on the load applied and the material of the strap.
When I have to do a snatch recovery I will always try a gentle tow with a slight tug unless I know it is going to be a heavy recovery. Lowering of tyre pressures, digging, road build and the use of traction boards should be done to assist in a heavy snatch recovery.

The effectiveness of using a winch air dampener on snatch straps have been shown to have limited effect in controlling the strap recoil should it break. It appears that 2 very tightly attached weighted dampeners placed at 1/3 and 2/3's along the length of the strap or wrapping a 2nd strap around and along the length of the working strap are apparently a more effective way of retarding a recoiling broken strap.

You should always use the lightest strap possible because this will give you the most kinetic energy. Generally it has been recommended that the strap load rating should be 1.25x to 1.5x the weight of the lightest vehicle involved in the recovery.
An 8000lb/3600kg 8m snatch strap with 20% stretch will stretch to 9.6m in length at 8000lbs/3600kg load and this may have more kinetic energy than an 11,000lb/5000kg 8m snatch strap with 20% stretch that will only stretch to 9.16m at 8000lbs/3600kg load. I have never read any test reports about this ???????

There have been many test reports done on the breaking load and the breaking length of snatch straps but I am yet to read one that looks at 50% and 75% loads which is where most sensible snatch recoveries would take place. I would be interested to know how much stretch is lost between each sequential pull at so at 50%, 75% and 100% of the straps rated load of various load ratings.

OJ.

[Jasonmc73]

Again, you've got me thinking.

I measured my pin - 16mm diameter.

Using a commercial recovery adaptor, I would say the pin is in a "double shear" situation (rather than experiencing bending forces). So, what's it rated to?

I went to google for some help with calculations, and instead found a document from Purdue University: https://www.extension.purdue.edu/ext...ppp/ppp-94.pdf

A table on page 19 lists double shear strength for various pin sizes. As an American document, it's all in imperial measurements - 16mm is a fraction over 5/8".

A 5/8" hitch pin, according to Purdue University, is rated to 27,000 pounds - a little over 12,000 kg. Does that sound like enough for an 8,000 kg snatch strap?

That 12,000 kg is for a Class 2 pin - weak steel.

A Class 5 pin is listed at 44,000 pounds, or 20,000 kg.

A Class 8 pin is listed at 55,000 pounds, or 25,000 kg.

I don't know the class of my hitch pin but I'm not going to lose sleep over it, either.

I really do thank you kindly for that info

That's the pin that is always in my mind, but i see it taking load on either side of the pin, rather than what other people worry about, i like to be different!

[pharb]

I found this document some years ago https://www.google.com/url?sa=t&sour...dTunBuH8x-FHP0

It is old, and references US quantities, but written in basic form so even an army "grunt" can understand it.

A good reference to self educate about recovery forces and applications of recovery physics.

[geopaj]

This thread is making me reflect on the recovery I had the ‘pleasure’ of witnessing on the on-ramp to Goolwa beach on the weekend.

VW Tiguan completely bogged and sitting on the chassis. Only ‘recovery point’ was the eyelet that screws into a window in the front bumper bar. A worn out snatch strap with one eyelet snapped off (which the resourceful owner had ‘repaired’ by tying the strap into a loop). Dual cab attempting the snatch recovery got 95% bogged themselves (as everyone refused to let their tires down).

After a good 30-40 minutes of attempted snatches and nearly bogging the dual cab, the dual cab’s solution was to get a second vehicle... I intervened when they were about to do a fast 2 vehicle snatch putting the shock load of 2 vehicles to the threaded VW’s eyelet with the tied together snatch strap.

To add to the excitement, they were completely blocking the track both ways and there was probably 50 people standing dangerously close.

4 Maxtrax, a quick dig I from of the tires with bare hands, and the VW had safety self recovered (no vehicle damage or human injury) in less than 5 minutes... which also meant I was able to exit the beach and drive home.

[Jasonmc73]

After a good 30-40 minutes of attempted snatches and nearly bogging the dual cab, the dual cab’s solution was to get a second vehicle... I intervened when they were about to do a fast 2 vehicle snatch putting the shock load of 2 vehicles to the threaded VW’s eyelet with the tied together snatch strap.

[1]:

Wowsa, the old double truck pull on the threaded tow eyelet with the tied together snatch strap.

Mind boggling that they couldn't see the real possible ending to that if it had of proceeded.

Wow, that makes snatching from a towball look like boy scouts stuff, but as some would say which i'm not condoning, maybe it's a kind of natural selection taking place

[OldEmmGee]

Yup - seems some people are opposed to doing a little digging and dropping tyre pressures........and what is it with trying to get to 3rd gear before you take up the slack of a snatch-strap? Slow as you go....

[Kingbrown]

....... I intervened when they were about to do a fast 2 vehicle snatch putting the shock load of 2 vehicles to the threaded VW’s eyelet with the tied together snatch strap.

To add to the excitement, they were completely blocking the track both ways and there was probably 50 people standing dangerously close.

4 Maxtrax, a quick dig I from of the tires with bare hands, and the VW had safety self recovered (no vehicle damage or human injury) in less than 5 minutes... which also meant I was able to exit the beach and drive home.

Good on you for putting a halt to it.

All too easy for schadenfreude to take precedence over the prevention of some inevitable damage or injury.

We see many clips of people deliberately standing by in order to capture a dumb or misguided activity.

We walk past it . . . . we own it.

[old Jack]

Good on you for putting a halt to it.

All too easy for schadenfreude to take precedence over the prevention of some inevitable damage or injury.

We see many clips of people deliberately standing by in order to capture a dumb or misguided activity.

We walk past it . . . . we own it.

X2 and well said KB..

Geopaj, you are on your way to becoming a grumpy old man!

OJ.

[MartinDanger]

The forces on two anchors/recovery points with a load equalising attachment (like a bridle) will depend on the angle at the master point (point at which the bridle connects to the eg snatch strap). It’s very possible to have each point loaded to 100% of the actual load if the angle is high (120 degrees does it). 90 degrees reduces that to 71%. 45 degrees reduces it further to 54%. That means that a recovery with a 1000kg max force will exert 1000kg on each recovery point if the bridle angle is 120 degrees. Lesson - longer the bridle, the lesser the force on each point.
This example assumes that the load equalising actually works. When the force is exerted on the bridle, friction increases. If the direction of pull isn’t directly in line (both vehicles pointing and travelling in the same direction) then the rope doing the pulling may not slip as it should along the bridle to equalise the load.
I’m using my climbing/ropes knowledge for this example. I admit the application is different and happy to learn where it may not apply.
I still don’t get why unhooking your towball and using the pin the towball is secured with is such a bad thing. Why do you need a recovery hitch? Is the towball hitch pin really not strong enough for a recovery when it is strong enough to deal with 3500kg trailer that would, under max braking come close to 1G, and also have a safety margin of 5 or so (meaning a max break strength of 17,000kg)?

[nj swb]

I still don’t get why unhooking your towball and using the pin the towball is secured with is such a bad thing. Why do you need a recovery hitch? Is the towball hitch pin really not strong enough for a recovery when it is strong enough to deal with 3500kg trailer that would, under max braking come close to 1G, and also have a safety margin of 5 or so (meaning a max break strength of 17,000kg)?

Double shear vs bending. Hooking straight over the pin is using the pin in a manner for which it was not designed.

Despite anecdotes of pins bending when used for snatching, I've yet to find a first hand report, much less any pictures. During sensible snatching, it should be OK. During silly snatching, will it bend before the strap breaks? Either way, why take the risk? Use a recovery hitch, the pin is being used in the manner for which it was designed, one less thing to go wrong.

BTW, nowhere near 1G braking a 3500kg trailer.

Very few towing vehicles will brake anywhere near 1G, particularly if a 3500kg trailer is pushing it. The hitch pin won't experience 3500kg under braking - not even close.

If the 3500kg trailer has functional brakes, like it should have, then some of the deceleration is provided by the trailer brakes, not pushing through the pin - further reduction in the force experienced by the pin.