Spelegyca - yay or nay?

[Olaf]

I agree with what most people said earlier: dynamic rope with not too tight barrel knots. Plenty of reasons have already been given, although I think they are probably overrated.

I've fallen onto my cowstails once on a bitterly cold trip in Juniper Gulf. My central mallion was well above the traverse line to begin with but the fall was very gentle (I didn't hit any walls). There were so many dynamic things going on, starting with the traverse line and traverse line knots being pulled tight, karabiners slipping along the traverse line, and so on that probably the cowstails didn't make much of a difference in the end. Lots of energy can be absorbed elsewhere...

Also there is an archived experiment on the OUCC website similarly suggesting that "fall factors" and all the theory does not really cover all of what is actually going on to a satisfying degree: http://www.oucc.org.uk/pictures/uk/general_by_year/1987/bridge_jump_0main.htm

 

[Mike Hopley]

To the original question: nay. They are fit for purpose if the rigging is perfect. If not (which is frequently the case), then their shock-absorbency is a bit low for comfort. It is reassuring to have some room for cock-up.

I think loosening your cowstail knots between trips is silly. These are knots I really don't want to come undone, and I also don't want long tails that catch on things. Loosening them is also very difficult once they have been weighted, at least for barrel knots in 9 mm rope. It takes me half an hour with a spanner.

My advice would be: get them a suitable length, then get them tight -- bodyweight loading at least -- before you cut the tails short. Never untie them. Stop worrying about the rope being weakened at the knot. It is weakened, but it doesn't matter because it's still plenty strong enough. You will break before it does. Besides, as has been mentioned, the weak part is better left alone, rather than moved around.

Much has been made of the "BCA" cowstail tests, which have nothing to do with the BCA apart from being translated on their website (hint: Chamonix is not in Britain). Bizarrely, people have seen this report and concluded that cowstail knots should be systematically loosened. The report does not make this recommendation, but it does say this:

Furthermore, the results are similar for knots that are well tied and knots that are badly tied, that is when the ropes cross over each other, and also whether they have been pre-tightened or not.

(My bolding.) Note also that most of the tests used pre-tightened knots "with a slow pull of 3 kN. This value represents a typical force exerted on a Cows' Tail by a 80 kg person moving somewhat 'brutally'." The knots that weren't pre-tightened gave only slightly better results (5.80 -- 5.93 vs 6.55 -- 6.85 kN), and they were a very small sample size (two).

You can argue that "more shock absorbency is always better", and dignify this as "a best practice"; but then you should be using Via Ferrata kit, which takes one look at our caving cowstails and laughs haughtily. If you really want shock-absorbing cowstails -- say, because you plan to do some bolt climbing -- then you can make a pair of caving cowstails using a Via Ferrata friction plate such as the Kong Kisa. Shall we make this our new "best practice"?

I always kept mine so loose that in wet cave the knots are nearly undoing themselves.

And you consider this a good thing? Really? :-\

I've been on trips with novices who were lent cowstails like this. It scared the crap out of me when I noticed them.

If they are dressed neatly they will have maximum shock absorption.

That's false, according to the Chamonix lanyard tests.


[quote author=Pete K]Crack off your barrel knots after each trip or if they're really stubborn, wiggle the krab out and the knot will come apart.[/quote]

I would be quite alarmed if my cowstail knots fell apart that easily. The rock-like state of my barrel knots fills me with confidence.

 

[Fulk]

Thank you, Mike, for some down-to-earth common sense (though looking at some of the comments on this thread, maybe it's not so common, after all??).

 

[TheBitterEnd]

Bob Mehew, who has done tests on this, STRONGLY recommends easing* your cowstail knots between trips, much of the shock absorbing capacity is in the knot and whilst I can't remember the figures, it basically can be the difference between a soft landing and a potential injury.



*this does not mean loose but dressed and hand tight

 

[Cookie]

Thank you, Mike, for some down-to-earth common sense (though looking at some of the comments on this thread, maybe it's not so common, after all??).

I'll second that. Thank you Mike. 

Bob Mehew, who has done tests on this, STRONGLY recommends easing* your cowstail knots between trips, much of the shock absorbing capacity is in the knot and whilst I can't remember the figures, it basically can be the difference between a soft landing and a potential injury.

*this does not mean loose but dressed and hand tight

I heard Bob's talk at the BCA Party last weekend. The advice was different. It was to relax the knots after a significant fall. He didn't mention relaxing the knots after every trip. I don't know if this was an omission or his advice has changed.

I hope it is the later since personally I don't think the advice to relax knots after every trip is good advice. I'm much happier with the advice to relax the knots after a significant fall. However, unless you have a knot pick, you'll probably give yourself a hernia trying.

It is worth pointing out this is Bob's advice as an individual, it is not BCA policy.

 

[Cookie]

PS

The talk was very good. Some fascinating science going on with interesting results.

 

[cap n chris]

The advice was different. It was to relax the knots after a significant fall.

Entirely reasonable whereas relaxing knots (assuming it's easily achieved) after every trip isn't. The former wins.

After a significant fall I'd be minded to replace them though, and be more careful in future, and consider retiring the traverse line I'd fallen onto etc..

 

[kay]

I heard Bob's talk at the BCA Party last weekend. The advice was different. It was to relax the knots after a significant fall. He didn't mention relaxing the knots after every trip. I don't know if this was an omission or his advice has changed.

We weren't at the whole of Bob's talk but my partner and I did hear the part relating to knots in cowstails - although he emphasised relaxing the knots after a fall, as soon as you got to a safe place, he did also say they should be relaxed after every trip.


Maybe we all heard what we wanted to hear! Hopefully Bob will be around sometime to clarify.

 

[Bottlebank]

I heard Bob's talk at the BCA Party last weekend. The advice was different. It was to relax the knots after a significant fall. He didn't mention relaxing the knots after every trip. I don't know if this was an omission or his advice has changed.

We weren't at the whole of Bob's talk but my partner and I did hear the part relating to knots in cowstails - although he emphasised relaxing the knots after a fall, as soon as you got to a safe place, he did also say they should be relaxed after every trip.


Maybe we all heard what we wanted to hear! Hopefully Bob will be around sometime to clarify.

I wasn't there but I'd be curious to hear the logic behind it. As others have said loosely tied cowstail knots are a risk, and they scare me when I see novices use them.

In my completely unscientific mind I've always worked on the basis that a tightly bedded in knot is more likely to keep grit etc out, and that any that gets in is less likely to move around or cause damage.

 

[Cookie]

I heard Bob's talk at the BCA Party last weekend. The advice was different. It was to relax the knots after a significant fall. He didn't mention relaxing the knots after every trip. I don't know if this was an omission or his advice has changed.

We weren't at the whole of Bob's talk but my partner and I did hear the part relating to knots in cowstails - although he emphasised relaxing the knots after a fall, as soon as you got to a safe place, he did also say they should be relaxed after every trip.


Maybe we all heard what we wanted to hear! Hopefully Bob will be around sometime to clarify.

Thanks Kay, I stand corrected  :-[

Agree with Cap'n Chris. I would be replacing my cow's tail after a significant fall. I also aim to replace my cow's tail annually regardless.

 

[kay]

Bob draws our attention to the article in Speleology, April 2008, which contains the advice to slacken knots every trip. It makes the point that everyone else will have realised but that I hadn't thought about, that the knot makes up a far greater proportion of the shock absorbing system in a cowstail, because the length of rope is so small.

http://british-caving.org.uk/wiki3/lib/exe/fetch.php?media=rope_testing:cows_tails_article_v2.pdf

 

[paul]

Here's a thought: each Cowstail has two knots. One at the karabiner end and another where it is attached to the harness central maillon.


As generally cavers have two cowstails, the knot at the harness end is in the centre of a length of rope and loosening this knot will assist in shock absorption. However, if this knot is loose, it will not be dangerous as it cannot undo and be detached as each end of rope in the knot has a knot and karabiner at the end.


I use a bowline on the bight in the centre of my two cowstails and this is relatively easy to loosen after use if you wish to and leave the knots at the karabiner ends nice and tight.

 

[cap n chris]

If it is considered bad practice, or dangerous, to fail to loosen off and subsequently re-tie, cowstails knots after every caving trip then in answer to the OP, yes, get yourself a spelegyca as it'll avoid all the hassle.

 

[TheBitterEnd]

I don't think anyone is advocating loose knots or completely undoing and retying every trip and I imagine the advice should be taken with a bit of common sense. I guess after a trip where a knot has been loaded (e.g. hung on at a hanging rebelay or fallen on for example) it should be eased, not undone, just slackened a bit and pulled up tight again. But if you have done a trip where no load has been put on the knots at all (i.e. no hanging) then I guess there is no point in easing the knots.

 

[Cave_Troll]

sadly common sense is variable and may not have any basis on fact.

 

[Bob Mehew]

The penultimate slide of my lecture at the AGM said

Relax the knots in your Cow?s Tails 

after Every Trip

and

When safe to do so, immediately after a Fall!​

Please, please, please do NOT get hung up on relax / loosen / partially undo etc.  As PeteK said, the concept is to relax / loosen / partially undo each knot sufficiently to allow you to then redress the knot back to a standard state.  I am NOT saying leave you knots so loose that they might come undone - I am not that much of an idiot (I think).  It distresses me that people can think I could mean that.

And by the way, relaxing / loosening / partially undoing and redressing also applies to the rope you were attached to.  Or better still retie the knot in a slightly different location.

Lets go back several steps.  Standards adopt a 6kN peak force as a conservative estimate of the force someone could reasonably be expected to walk away from without injury.  If I caught it correctly, that was a factor of 2 down from the typical 12kN force seen by parachutists on opening a parachute using a parachute harness which is far better at transferring the load around and the parachutists being fit and trained to take the peak force.  One also has to remember that these are talking about a rope tied onto a metal mass.  The best guess is that the human body and harness will cut peak forces by around a factor of 2.  So I am using as a rule of thumb you should walk away from a 6kN peak force but probably will be injured at 12kN.  (And judging by the two helicopter pilots, dead well before 36kN - though the rope will have broken before then.  See http://www.hse.gov.uk/research/crr_pdf/2002/crr02411.pdf .)  PeteK can speak from experience when he did that work on Y Hang knots, see http://ukcaving.com/board/index.php?topic=14580.msg188940#msg188940]
[url]http://ukcaving.com/board/index.php?topic=14580.msg188940#msg188940[/url] as I recall his back was protesting by the time he finished work.

The first set of data supporting this concept of relaxing knots was based on the French report and written up in Speleology.  (And I suggest it is good practice to remove mud from your rope which at a knot means untying it.)  Since then I have done some work on peak forces which I am struggling to display in a meaningful way.  To grossly simplify, peak forces rise as the Fall Factor increases, rise as the overall length increases and most importantly, rise as the number of drops increases.  Using lengths of 10mm dynamic rope I was exceeding 6kN for the 2nd drop in a FF1.25 set up on a length of 0.375m (ie a short cows tail) and in a FF1.0 set up on a length of 0.75m.

By the way the better choice of rope diameter is 10 not 9mm.  The peak forces go up as you reduce diameter. 

I don't recall making any statement to the effect that "in-situ ropes are best left with knots permanently tied". But to date as I recall, drop testing extracted fixed ropes with original knots has not shown any statistical difference.  Probably because dropping a 100kg metal mass imparts far more of a peak force than the knot has ever seen.  One of the problems is we have yet to get a handle on what force dressing applies to a knot.  The French 3kN was based on the worst case caver messing around on their cows tails and I think included a pulley system.  Lyon's Ltd suggested a peak force of below 2kN, see http://www.hse.gov.uk/research/crr_pdf/2001/crr01364.pdf.  I expect hand loading a rope will not get anywhere near 2kN based on what weight an archer can pull a bow.

I will introduce another piece of advice

If you can't undo the knot by hand, then throw the rope away.
​

My experience is that knots are almost impossible to undo if they have seen a peak force above around 6kN and probably much lower.

I write as BCA's Rope Test Officer.

 

[Pete K]

Thanks for taking the time to reply Bob.

 

[Fulk]

I don't recall making any statement to the effect that "in-situ ropes are best left with knots permanently tied".

Apologies, Bob.