Author Topic: Offset joining knots (abseil / rappel)  (Read 33807 times)

Mobius

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Re: Offset joining knots (abseil / rappel)
« Reply #120 on: January 26, 2016, 10:14:31 AM »
Here is a link some might find interesting: https://blackdiamondequipment.com/en/qc-lab-what-is-the-best-rappel-knot.html

The exact procedure of testing is unknown as is the rope material, though one would assume it is dynamic rope being tested. There is another site where I saw images of the actual test knots, however I need to find that again. With particular regard to the EDK results, we see are told of slippage, however the slippage is first observed close to breakage point of the rope. This is quite different from the results I have seen in trials and those of agent_smith, I believe.

Thoughts?

Cheers,

mobius
« Last Edit: January 27, 2016, 06:08:16 AM by mobius »

Dan_Lehman

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Re: Offset joining knots (abseil / rappel)
« Reply #121 on: January 27, 2016, 05:59:31 AM »
The exact procedure of testing is unknown as is the rope material, though one would assume it is dynamic rope being tested.
Yes, I think that 8.1mm is too large for any of the
low-elongation lines that BD sell (one can check this)
--it might be a "twin" or "double" (or both) line.

The exact orientation OF THE DIFFERENT ROPES in
the mis-matched case is unknown --which made the
(initial) choke?

Quote
There is another site where i saw images of the actual test knots,
however i need to find that again.
?!  Images of whose "actual test knots"?!
I can draw my own images of these knots,
but I cannot know how to drawn them like Kolin
tied them!

Quote
With particular regard to the EDK results, we see slippage, however the slippage is first observed close to breakage point of the rope. This is quite different from the results i have seen in trials and those of agent_smith, I believe.
I don't SEE anything but the word "slippage",
and that's different; my inference is that the
knot "rolled" --which seems apt for the load.
Now, I must keep in mind my one-time stressing
of #783(?) which spilled when it capsized, and
believe the in such transitions there can also be
slippage --possibly to the extreme of spilling!

And to your testing, please note that it would be
good to see if your fails(slips or rolls)-so-quickly
stiff small material could be tamed by tying the
"back-up EDK" to the first.


--dl*
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Mobius

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Re: Offset joining knots (abseil / rappel)
« Reply #122 on: January 27, 2016, 11:38:55 AM »
Here is the other site I first read Kolin's article: http://www.rockandice.com/lates-news/climb-safe-what-is-the-safest-rappel-knot?A=SearchResult&SearchID=3347231&ObjectID=4914629&ObjectType=35

I thought the knots shown were from the actual test, however maybe not so.

I received my new scale today and climbing quality rope rope (with the appropriate EN standards for the new ropes). Specifically in climbing rope I now have a selection of 11.2mm worn dynamic, 10.2mm new dynamic, new 6mm dynamic accessory cord,  new 11mm static and new 10.2mm static ropes to trial.

Cheers,

mobius

agent_smith

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Re: Offset joining knots (abseil / rappel)
« Reply #123 on: January 28, 2016, 09:39:03 AM »
Quote from Kolin Powick:
Quote
There are advantages and disadvantages to each type of rap knot, but is there one that is superior in strength to the others?

And this is where I roll my eyes and tune out.

And it is one of the reasons why I started this thread - that is, to try to look at something other than the mind numbing, simplistic and consumer level thinking that seems to pervade (almost) the entire climbing/roping industry.

I've stated this before...pure MBS break testing in this particular type of knot (ie offset joining knots) - is a waste of time and leads nowhere.

...

There is no EN892 certified [single] 'dynamic' rope below 8.5mm diameter. I own the Edelrid 'Corbie' which is 8.6mm and Beal make a 8.5mm [single] rope called the 'Opera'. Beal like to advertise that their 'Opera' rope is the worlds thinnest EN892 certified [single] dynamic rope.

Anything below 8.5mm diameter is going to be either a 'Half' or 'Twin' rope.

As for 6mm accessory cord - I am very dubious and skeptical that it is 'dynamic' (mind you - I use the strict definition per the EN892 standard).

EN564 deals with 'accessory' cord (refer to this link: http://www.theuiaa.org/upload_area/Safety/Standards/Safety-Standards/UIAA_102_Accessory_Cord_2013.pdf )

...

I also dont agree with and do not recommend the use of additional 'backup / stopper' knots with Offset joining knots. This defeats the purpose of why these knots are selected in the first instance (ie because it increases the knots footprint).

When tied correctly with close attention to dressing - appropriate offset joining knots are secure and stable within the nominal load range for a single person payload.

Also, tail 'slippage' is normal and expected during load testing - this is not the same as 'instability. Merely because the knot structure compresses during loading and the tails draw-in a little bit; does not by itself mean 'instability'. This term is reserved for a 'dynamic event' where the knot structure undergoes an obvious phase change. The probabilistic threshold where instability manifests - is what I am most interested in. Careful attention to dressing can have a significant impact on raising this 'threshold'.

« Last Edit: January 28, 2016, 09:42:39 AM by agent_smith »

Mobius

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Re: Offset joining knots (abseil / rappel)
« Reply #124 on: January 29, 2016, 08:40:44 AM »
Here are the specs of the 6mm accessory rope I purchased.  I was looking for dynamic rope at the time when I saw a link to this material, I may have erred in thinking this rope (image of specs) was dynamic too. It appears to be a nice rope (which won't be wasted in my trials), though I really wanted to test two different size dynamic ropes tied as a bend. I do have 11.2mm worn vs 10.2mm new to try. The 'worn' and 'new' adds another unwanted variable unfortunately.

Cheers,

mobius


Mobius

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Re: Offset joining knots (abseil / rappel)
« Reply #125 on: January 29, 2016, 10:24:23 AM »
Quote from Kolin Powick:
Quote
There are advantages and disadvantages to each type of rap knot, but is there one that is superior in strength to the others?

And this is where I roll my eyes and tune out.

agent_smith and i agree, this immediately worried me too.


I also dont agree with and do not recommend the use of additional 'backup / stopper' knots with Offset joining knots. This defeats the purpose of why these knots are selected in the first instance (ie because it increases the knots footprint).

I also agree with this statement. As I understand the application (I acknowledge that reading about vs doing it are two different things) then a small footprint is immediately a good thing to aim for. I was reminded of the knotting adage: "If you cannot tie a knot, tie a lot". 

Adding a 'stopper' is to me like admitting that my primary knot is not good enough.  Additionally, a primary knot + a secondary knot is really just another 'knot' in summation. Picking a good knot in the first place makes far better sense to me.

Also, tail 'slippage' is normal and expected during load testing - this is not the same as 'instability. Merely because the knot structure compresses during loading and the tails draw-in a little bit; does not by itself mean 'instability'. This term is reserved for a 'dynamic event' where the knot structure undergoes an obvious phase change. The probabilistic threshold where instability manifests - is what I am most interested in. Careful attention to dressing can have a significant impact on raising this 'threshold'.

I think this statement is possibly at the theoretical heart of the disagreements agent_smith and I have had recently. 'Slippage' is not "normal and expected" to me. I have (perhaps) taken the overly pure approach that ... a knot that slips prior to breaking is insecure (by definition, provided by Budworth).  Therefore: Insecure knots are bad, therefore, ignore them. That is part of the logic that drives me to find what I consider good knots. Note that I (as a mathematician, never-climbed -a- mountain-in-his-life ) am not handicapped by the argument of usage: eg. I do not care whether mountaineers had been using the EDK for the last century(?), usage simply does not imply worthiness to me.

Cheers,

mobius

« Last Edit: January 29, 2016, 10:27:29 AM by mobius »

agent_smith

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Re: Offset joining knots (abseil / rappel)
« Reply #126 on: January 30, 2016, 01:40:23 AM »
Quote
I think this statement is possibly at the theoretical heart of the disagreements agent_smith and I have had recently.
No such 'disagreement' ever existed in my mind.

Quote
'Slippage' is not "normal and expected" to me. I have (perhaps) taken the overly pure approach that ... a knot that slips prior to breaking is insecure (by definition, provided by Budworth)

You are taking a far too literal interpretation of the word 'slippage'.

Virtually every knot structure compresses under load - I would have thought that this is obvious. The rate and degree of compression of course varies from knot to knot.

Obviously, as the knot structure compresses, the tail will draw-inwards into the core/nub. Again, the rate and degree of tail 'draw-in' varies from one knot to the next.

The use of the word 'slippage' appears to be conjuring up certain images in your mind... Perhaps you might feel more comfortable with 'tail draw-in' rather than 'slippage'?

Quote
usage simply does not imply worthiness to me
And yet I - and thousands of climbers around planet Earth are still alive. How do you account for this mathematically? Perhaps I have simply cheated death countless times when using #1410 - and never knew how close i was to plummeting to my death? Many climbers/mountaineers have used #1410 for many years in the field - and the knot structure has held the nominal load of 1 person. The death rate has not spiked or increased (as far as i am aware).

Climbers also use #1047 as a tie-in knot - and it is proven to work -and yet the tail does draw-in (a little bit) when under load. Rope access workers (which I also do quite a bit of) - also use and rely on #1047 to fix their ropes. We all commonly know and are aware of the fact that under load, the tail will draw-in ('a little bit' and it is not alarming nor is it an indicator of insecurity) - this tail draw-in is normal and expected with virtually all human life support knots.

It would appear that you have taken a too literal interpretation of the word 'slippage'  - I would suggest substituting any negative images with 'tail draw-in'.

//////////////

EDIT:

I should add that one of the factors I have been investigating is the amount/rate of tail draw-in as load increases. I posted some initial results earlier - and I would have thought that it was obvious from even this limited sample that 'tail draw-in' (while normal and expected) is not by itself a significant risk factor. I was originally prompted to look at this phenomena when I learned that some abseiling/canyoning guides/instructors were advocating 1.0m upwards to 2.0m long tails! I was both alarmed and disturbed by their perceived need to configure such long tails. So it motivated me to look a little closer at tail draw-in to see if it was one for the 'myth-busters' or if it was a very real and menacing risk.

« Last Edit: January 30, 2016, 02:03:21 AM by agent_smith »

Dan_Lehman

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Re: Offset joining knots (abseil / rappel)
« Reply #127 on: January 30, 2016, 06:26:00 AM »

I also dont agree with and do not recommend the use of additional 'backup / stopper' knots with Offset joining knots. This defeats the purpose of why these knots are selected in the first instance (ie because it increases the knots footprint).

I also agree with this statement. As I understand the application (I acknowledge that reading about vs doing it are two different things) then a small footprint is immediately a good thing to aim for. I was reminded of the knotting adage: "If you cannot tie a knot, tie a lot". 

Adding a 'stopper' is to me like admitting that my primary knot is not good enough.  Additionally, a primary knot + a secondary knot is really just another 'knot' in summation. Picking a good knot in the first place makes far better sense to me.

"A good knot" might be one that is assuredly going
to work even across some variety of ways things are
put together, vis-a-vis different ropes and dressings
& settings,
and can be tied under difficult circumstances --such
as when removing gloves might be a life-threat to one's
hands from cold.
And I trust that maybe the easiest knot to understand
and tie is the overhand, and so if doing that with paired
rope ends, and repeating it with the often-recommended
"long tails" yields a sure joint, then it has a lot going for it!

As for "small footprint", no, it seems the more important
aspect for an ARJ knot is smooth movement over rough
surfaces.  Backing up the EDK with a 2nd one doesn't
affect this smooth flow, IMO; and as far as the added bulk
having adverse effects, that seems highly unlikely --one
doesn't run rap ropes through holes (ring) and such!

Now, we can continue to wonder, will the EDK-backed EDK
hold in Mobius's very challenging cord in which the single
EDK failed so quickly?  I'm curious about this.  One might
try two cases : knots set snug to each other; knots not ... .


--dl*
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agent_smith

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Re: Offset joining knots (abseil / rappel)
« Reply #128 on: January 30, 2016, 09:09:43 AM »
Quote
and so if doing that with paired
rope ends, and repeating it with the often-recommended
"long tails" yields a sure joint, then it has a lot going for it!

I will never agree on this point. In fact, this goes to the heart of why I started this topic - one of the terms of reference is the myth of long tails and the other is the myth of duplication - that is duplicating #1410 with another #1410 (one stacked on top of the other). The point of getting some reliable test data is to banish these myths forever.

I would have expected logic to prevail - in that the larger one makes a knot - the more likely that it would get 'stuck' somewhere. And I'm not just talking about the edge transition - there are opportunities to get caught up in rock features anywhere on the vertical cliff face (particularly in strong wind conditions where the rope can act as a gigantic sail and arc across the rock face). And long tails are ludicrous in my view...again, in strong wind, the tails will flap and whiplash violently - I know this for a fact as I myself have witnessed it and been caught out in the open in severe storms (while mountaineering).

If you are going to advise to the world - heh, tie 'long' tails - what does this mean exactly? Does this mean 1.0m, 2.0m? I guess using up valuable rope length isn't a consideration either? What if you needed every 'inch' of rope to descend the next pitch safely?

Quote
and as far as the added bulk
having adverse effects, that seems highly unlikely

As stated, logic would tell you that smaller footprint is a desirable attribute - less bulk means less risk of getting caught up somewhere. In my view, when tying offset joining knots, a small footprint = efficiency.

Quote
will the EDK-backed EDK
hold in Mobius's very challenging cord

Testing duplicated stacked offset knots is going to provide little in the way of good data when the whole point is to examine the stability of just a single knot structure. The duplicated structure might suppress instability - which is a key objective of the testing (that is, the object of testing is to probe instability - and to examine how different dressings and orientations might affect stability).

Dan_Lehman

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Re: Offset joining knots (abseil / rappel)
« Reply #129 on: January 31, 2016, 08:55:44 AM »
Quote
and so if doing that with paired
rope ends, and repeating it with the often-recommended
"long tails" yields a sure joint, then it has a lot going for it!

I will never agree on this point. In fact, this goes to the heart of why I started this topic:
one of the terms of reference is the myth of long tails
and the other is the myth of duplication
--that is duplicating #1410 with another #1410 (one stacked on top of the other).
The point of getting some reliable test data is to banish these myths forever.
You have a curious way with words : what is a "myth" here?
These are practices --"state of the practice" one might say,
and show that by observation reports.

Quote
I would have expected logic to prevail - in that the larger one makes a knot - the more likely that it would get 'stuck' somewhere. And I'm not just talking about the edge transition - there are opportunities to get caught up in rock features anywhere on the vertical cliff face (particularly in strong wind conditions where the rope can act as a gigantic sail and arc across the rock face). And long tails are ludicrous in my view...again, in strong wind, the tails will flap and whiplash violently - I know this for a fact as I myself have witnessed it and been caught out in the open in severe storms (while mountaineering).
And Mobius would expect "logic" to prevail in that using
a knot he's found to slip so quickly in his testing is not
logical.  But you (& I) admonish him for disregarding
the verrrrry long testing done by praxis --the decades
of usage without report of trouble, by so many users
and ropes and some different combinations (though
we can note some community doubt in this last case
and might infer that frequency is lower).

So turnabout is fair play, Mobius should retort to your
now hypothesizing troubles from the very state of the
practice previously cited --sounds like you're building
a myth, IMO !

Quote
If you are going to advise to the world - heh, tie 'long' tails - what does this mean exactly? Does this mean 1.0m, 2.0m? I guess using up valuable rope length isn't a consideration either? What if you needed every 'inch' of rope to descend the next pitch safely?
Well, now, >>I<< am not giving such advice;
indeed, quite to the contrary, my retort to that common
advice is "Don't have long tails, DO something with them!"
--to add security.  Because I don't want to have to believe
that my tail length will outlast some presumed failure of
the knot to stay put (and not roll/capsize, maybe repeatedly);
I want to prevent that!

Quote
Quote
and as far as the added bulk
having adverse effects, that seems highly unlikely
As stated, logic would tell you that smaller footprint is a desirable attribute
--less bulk means less risk of getting caught up somewhere.
Ah, back to this "logic" that spits into the wind of
years of usage.  Well, it probably does mean less
risk --but at a trifling, irrelevant degree (say, 0.0031% less).
(In one discussion about the offset water knot (EDK) vs. some
other ARJ knots --i.p., the grapevine--, many climbers replied
that they'd no problems with the latter; some others did,
and some said that the problem they'd encountered would
have obtained no matter what knot.)

Quote
In my view, when tying offset joining knots, a small footprint = efficiency.
By definition, in the strict sense, yes.
By time and effort and ease, no.

Quote
Quote
will the EDK-backed EDK hold in Mobius's very challenging cord
Testing duplicated stacked offset knots is going to provide little in the way of good data when the whole point is to examine the stability of just a single knot structure. The duplicated structure might suppress instability - which is a key objective of the testing (that is, the object of testing is to probe instability - and to examine how different dressings and orientations might affect stability).
The point is to test what works well, and in this
case, what is recommended (for 7 years?) and is
now seen recommended by an organization (those
canyoneers) as THE way.  (I imagine that Tom Jones
has some testing behind that, as well, but I don't know
exactly what combinations and so on.)


--dl*
====
« Last Edit: February 02, 2016, 06:00:41 PM by Dan_Lehman »