Recovery points

tottot said:

PLUS one on what all the above say. The 3.5 ton hitch is just its tow rating, the bolts holding it to chassis will go before it does.[ or rip out.] Similar pins pull some very large farm trailers. I also use the Damar strap.
When the West Wales 4x4 Group started 4x4 trials some 33 years ago our standard front recovery point was a ball hitch on the front bumper with plate behind. One day a fellow came with a military kinetic rope to speed up recovery's
With a well bogged landy hooked up he set off at speed in his open top truck, the rope stretched and then bang the ball ripped out of the bumper [ Bolts not up to the job ] It flew over his head by about three feet and then was heard crashing through the tree tops a hundred yards away never to be seen again. Banned .
I have no problem with kinetic recovery done properly with 100% solid tow points ect, The problem is on ordinary days out you just do not know how good other folks fittings are.

Click to expand...

I hear what you are saying and agree fully that common sense is always required. However if they had done the same with a non kinetic rope, the result would have been very similar. Kinetic ropes are much much kinder to the recovery points as they have considerably less instant jolt.

Had the same been done [ and I have seen it happen more than once ] with a non kinetic rope the ball or whatever hitch being used will fall to the ground somewhere between the two vehicles. It most certainly will not fly a hundred yards into the distance. [ for extra safety we have a metal mesh on the rear of the tow vehicle ]
Yes kinetic is lighter on the tow point to start but total pull exerted will be much greater which is why it works so well. We have found it safer to use non kinetic and if one Landy is not enough to extract a stuck vehicle just add another truck for a double pull.

tottot said:

Had the same been done [ and I have seen it happen more than once ] with a non kinetic rope the ball or whatever hitch being used will fall to the ground somewhere between the two vehicles. It most certainly will not fly a hundred yards into the distance. [ for extra safety we have a metal mesh on the rear of the tow vehicle ]
Yes kinetic is lighter on the tow point to start but total pull exerted will be much greater which is why it works so well. We have found it safer to use non kinetic and if one Landy is not enough to extract a stuck vehicle just add another truck for a double pull.

Click to expand...

Afraid the hitch won’t just fall to floor with a non kinetic rope. That isn’t how physics works

I personally know of two occasions of it happening. One impacted the rear of a Defender crossmember and put a fist sized dent in it. The other was a classic Range Rover and the hitch came in through the rear window, took out the passenger head restraint and out through the windscreen to land on the bonnet.

I doubt the Rangy was using a non streach strop.
I did not say the hitch would fall straight to the floor and I certainly would not like to be standing between the vehicles even if using a non streach strop.

300bhp/ton said:

I hear what you are saying and agree fully that common sense is always required. However if they had done the same with a non kinetic rope, the result would have been very similar. Kinetic ropes are much much kinder to the recovery points as they have considerably less instant jolt.

Click to expand...

Are we all talking about the same sort of thing? I think the point is that you wouldn't do the same with a static rope / chain / strap. My understanding has always been that when used properly you take up the slack and then gently increase force until the stuck vehicle frees. Giving the vehicle a "jolt to get er free" is asking for something to break (even if we've all done it at some point).
Kinetic ropes are designed to store and release huge amounts of energy and while spectacular in action, can also be really dangerous. Once you set it in motion and the recovery vehicle has taken off there's no backing down, that energy is going somewhere.

I can't find the link but there's a vidoe on youtube of how not to do it, they use a kinetic rope to free a vehicle iin mud and its ends up with the tow ball missing a guys head by inches and going through the tow vehicle windows and out the other side. They posted it as a warning about being complacent..

jnw said:

Are we all talking about the same sort of thing? I think the point is that you wouldn't do the same with a static rope / chain / strap. My understanding has always been that when used properly you take up the slack and then gently increase force until the stuck vehicle frees. Giving the vehicle a "jolt to get er free" is asking for something to break (even if we've all done it at some point).
Kinetic ropes are designed to store and release huge amounts of energy and while spectacular in action, can also be really dangerous. Once you set it in motion and the recovery vehicle has taken off there's no backing down, that energy is going somewhere.

Click to expand...

Firstly, there are many situations were you can't just take up the slack and gently pull someone out. In fact if you can, they probably aren't really stuck. As all you are doing is towing.

So "snatch" recoveries are very very common in off road situations, even if it is only a mild snatch pull.

As for the energy, I'm no physicist, but the potential energy must be the same, it is how the kinetic rope uses it that is different. e.g. if you think how something like a medieval trebuchet works. There is no elastic ropes, just a counter balance pulling, much like a recovery vehicle pulling a stuck vehicle with a non kinetic rope.

Without demoing it is hard to put into words. But imagine holding a short length of string tightly between your thumb and forefinger. Now snatch the other end of the string with your free hand. The result will jolt your hand violently and likely dislodge the string from your grip. Perform the same experiment again, but replace the string with an elastic band. And you will notice a very different result.

rob1miles said:

I can't find the link but there's a vidoe on youtube of how not to do it, they use a kinetic rope to free a vehicle iin mud and its ends up with the tow ball missing a guys head by inches and going through the tow vehicle windows and out the other side. They posted it as a warning about being complacent..

Click to expand...

But how do you know you wouldn't get the same result with a non kinetic rope?

The kinetic rope converts kinentic energy into strain in the rope. The energy in is the tension x the distance. The diference with a kinetic rope is that the tension is the momentum of the tow vehicle not the static traction. This will be stored in the kinetic rope for an instant as tension, once the stuck vehicle moves it will be recovered over a much shorter distance so the tension force is higher. With a static rope it is possible to "jolt" it but not to store energy in the same way; as soon as the load moves the tension fall away. Think of it like the differnce between air - compressible like the kinetic rope is streatchy and water, incompressible like the solid tow rope. Air under pressure is much more dangerous because its compressiblility enables it to store energy.

The best recovery I ever saw was of a 20 ton tracked excavator that was working on a farm nearby cleaning pond's and ditch's. The digger had found a patch of what seamed to be bottomless peat and sank well in no matter what the driver did. In the end only the cab and arm were visible.
The next day the contractors brought in an old Super Major tractor with a winch on the back. It had to be placed about a hundred yards away on solid ground and the inch diameter wire rope attached to the excavator. Just about everyone in the parish was there to watch [ best entertainment we had had for age's ]
The first thing that happened as winching began were the tractors spade arms dug in to the ground and then for what seamed like an age nothing at all happened even though the winch was turning and the old Major was using all it's 56 hp to make smoke.
Then there was the most horrendous gurgling/belching sound and suddenly 20 tons of digger came up and forward about six foot, Just goes to show that something that looks as solid as a large wire rope will have some energy stored in it.
Amazingly the driver was then able to start the excavator [ engine was still under water but intake and exhaust had remained above.] and was able to help the winch by digging a ramp out of the hole. All good fun.