Kinetic rope again..

On or around Sun, 30 Mar 2008 22:06:47 +0100, Ian Rawlings enlightened us thusly:

Do you actually need one?

Not right at this moment, no, however having seen how useful they are when you do need them, then I think it's worth having one around. I'm just not sure it's worth £200!

Do you really need KERR? Normal nylon rope has quite a bit of give in it, enough to take the shock out of being jerked out of whatever you're stuck in.

Gordon

I just went down to the local chandler and bought some nylon rope (10 tonne stuff) and spliced a couple of soft eyes into it.

No problems in the past six years.

Not too sure how good it would be for off road recovery though...

Do you know the difference between a KERR and a slightly stretchy nylon rope?

I've got tow ropes, I'm looking for kinetic ropes, they're very different.

Try Marlowe Ropes:

That takes you to non stretching tow ropes, try id=190.

I don't use KERR but what interest me is that a kerr should be able to have a lower breaking strain than an inelastic rope as by definition it absorbs shock (kinetic) loads.

AJH

I found them last night and am going to be calling them soon but got the impression they didn't deal with end-users. I'll find out soon enough. Their site was one of the few that came up when I was searching for the right spec rope, no prices though and no indication of end-user sales yet.

They're not intended to be used to allow you to do a snatch recovery though, like a stretchy normal rope, i.e their primary use is not as a shock absorber. They're intended to allow you to slowly build up a much greater pulling force than your tyres alone can manage, as far as I am aware the force you can build up with a kinetic rope is higher than you'll get by just jerking a truck, but at the same time as the force builds up slower it puts less strain on the truck and towing points.

I've seen them used properly and it's quite impressive at how easy it is to get a stuck vehicle out, the temptation however is to keep on driving away faster and faster until something gives, according to one site I looked at, for a 16-tonne breaking strain KERR you should accelerate to a max of 16MPH, far slower than I've seen people accelerate to with 10-tonne breaking strain KERRs.

Just phoned them, they only sell to business, not a problem as I own my own, but the price ex vat is £194 for the 28mm diameter one (the smallest they do), which works out about £30 more expensive than the other one I found of the same spec so I'll think about whether I want to spend £200 on it.. Mind you my hand winch cost similar and I've never needed that.

Here's the most likely candidate;

It is a snatch recovery! The energy stored is force times distance, a normal rope doesn't develop the distance bit. The difference is how the force is controlled, with the kerr the energy of the tow vehicle translates into the integral of force times distance elongation of the rope, until the momentum of the towing vehicle is almost used up and then this force, which now exceeds the towing vehicles tractive effort, gradually pulls the other vehicle out before the towing vehicle starts moving backwards. Get it wrong and the kerr catapults the tow vehicle back on top of the stuck vehicle, as related by one of the denizens of usenet when he was in charge of some scimitars, it took several hours before the one on top was removed and the subaltern in the underneath track could get out! [He tells several good yarns one of which features two salamanders (precursors of the stalwart?) airport fire fighting vehicles chasing an aeroplane that overshot the runway trailing a few hundred metres of pig fence and stakes across a field.]

Try this with an unsuitable rope and the energy gets dissipated in some less useful or dangerous way. As I said I've not used the technique as it was developed for recovering loads whose resistance was considerably greater than static winching was generally available.

AJH

You'll probably find a yacht chandler that deals with them will be able to get it for you, try them with the Marlow part number.

This is the area where the previous parts officer of the 101 club would excel and the current incumbent is very good at sourcing strange bits.

AJH

And that's the key. The distance allows you to develop much greater pulling force and is particularly useful when grip isn't great.

Yep, and that's the bit to avoid, hence the max speed recommendation of 16MPH, so you don't put too much force in and either end up with the tow vehicle shooting backwards or the stuck vehicle shooting out of the hole and causing damage.

The last time I saw one in use was at a pay 'n' play when my friend's Volvo C303 was stuck and was being towed by a 101FC, the 101FC was on slippery ground itself and couldn't pull him out with a normal rope, with the KERR he drove off at about 2-3x walking pace and the volvo just slid slowly out until it got grip.

With a slightly stretchy normal rope you tend to jerk the two vehicles rather nastily, the KERR uses similar forces, to a greater extent, and with a less savage action.

They're much quicker to set up and use than a winch, and I don't have a vehicle-mounted winch and am not too keen on fitting two (one front one rear). You do have to be careful though, and I'm not one of the gung-ho brigade you sometimes see using KERRs with a heavy right foot, I'm far too cautious! I've been hit with a twanging bungee too many times for that ;-)

Curious, I thought ropes and the like were specified by their circumference not diameter.

On or around Mon, 31 Mar 2008 10:05:54 +0100, Ian Rawlings enlightened us thusly:

yes. And I've done kinetic style pulling with ordinary rope, too - you do it a damned sight more carefully.

KERRs are fantastic things but have a huge potential for going impressively wrong. Although the kind you were on about with an extra loop at each end (if that's what it was) are doubtless better.

Indeed but can I find them any more? Nope.... I don't *think* I was dreaming, odd thing to dream about if I was.

I've ordered the sod-off expensive job now, given that it's still cheaper than the hand winch I bought and I'd prefer not to skimp on a cheap under-specced item that might have a 2.5 tonne truck on the end of it some day. Chances are it'll sit in the truck unused for umpteen years and when I need it, I'll take it out and find it's fused into one lump!

On or around Mon, 31 Mar 2008 13:45:15 +0100, Ian Rawlings enlightened us thusly:

The only thing I'm going to argue with is that it's exactly the same force. You convert momentum of the moving vehicle into force on the tow-rope, and thereby to force on the stationary vehicle. The more momentum, the more force. If you use the same speeds, the momentum, and the energy, available are the same.

With a more stretchy rope you can use more speed and thus more momentum. All the time you use the minimum speed necessary and lots of common sense, it all goes OK.

I've seen someone use 2 KERRs tied together, quite hard. We all stood well back...