Author Topic: Offset knot for climbing  (Read 22008 times)

Mobius

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Offset knot for climbing
« on: October 31, 2015, 08:05:20 AM »
I do not want to clutter this thread: http://igkt.net/sm/index.php?topic=5508.60#lastPost so what follows is probably a new knot though still relevant to the above thread.

A little background:

#1410 (Ashley), trialled and the tail slips and the nub rolls over itself, almost from initial load
#1414 (Single Fisherman's), collapsed at around 30% MBS
9-oh (Dan Lehman), appears to work well in the form Dan prefers

The knot I propose here not only 'works' in terms of preliminary trials, it has a small footprint. An offset knot has a particular geometry and a small footprint is an important safety feature (both these aspects described in the aforementioned thread).

I have considered the knot footprint objectively by measuring how may diameters are swallowed by the nub after tying a snug knot. A #1410 uses ~37 diam. The 9-oh uses ~49.5 (48 when measured more carefully a second time) diam. The knot I show below uses ~41 diam.

I originally thought that offset style knots would pretty much all have trouble handling big loads (in mbs terms). I was wrong, a properly designed knot (some sort of 'choke' appears to be needed) will do the job. At this stage of my trialling, Dan's 9-oh and 8-oh work and so does Mark's 'offset choked upper' and Jame's knot. So does the knot I show you next.

I spent a couple of weeks trialing various 'linked overhand' offsets and nearly getting them to work to my satisfaction.... but alas, not quite. One might work, I just could not find it. When I moved over to trying an overhand linked with a figure 8 then I found something that I was much more happy with. I use the tag lohf8 'linked overhand figure 8' for the knot I show. There will no doubt be other lohf8's that will work too.

More images from trials to follow in future posts.

Cheers,

mobius

[EDIT: the Ashley #1410 I initially trialed when writing this post was his 'twist' version, it performed very badly. The #1410 tied flat as an EDK performs better. All this is discussed later in the thread]
« Last Edit: June 04, 2016, 03:59:04 AM by mobius »

Mobius

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Re: Offset knot for climbing
« Reply #1 on: October 31, 2015, 08:42:47 AM »
The knot images are hopefully self explanatory in terms of mbs.

I am not interested in breaking these offset knots, just in being able to untie them after a high load. That my trial material does not collapse and that I can get it undone gives me confidence I will be able to do the same in larger diameter and better ropes.

Cheers,

mobius

Mobius

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Re: Offset knot for climbing
« Reply #2 on: October 31, 2015, 08:48:51 AM »
200kg is not a particularly high load. I will go to 500kg next time.

Mobius

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Re: Offset knot for climbing
« Reply #3 on: November 01, 2015, 04:27:33 AM »
This thread might better belong in "Knotting Concepts & Explorations".

There are presumably a lot more overhand and figure 8 offset combinations that could be explored, not just the one I came up with. This assumes that a small footprint for an offset knot really is important to the application.

Cheers,

mobius

Dan_Lehman

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Re: Offset knot for climbing
« Reply #4 on: November 02, 2015, 08:28:56 PM »
This is an interesting interlocking of components,
and in my quick play-test it looks good.  But I'm afraid
that the very "interlocking" tying aspect will keep
it from becoming a winner in the wild.

200kg is not a particularly high load. I will go to 500kg next time.
!?
200kg is more than double the weight of most
abseilers,
and hence will be over four times the forces that
an abseil-ropes-joining knot is expected to endure.
4:1 of expected forces is not nothing!
(It might be worth noting that one of the peculiar
results of the water knot (in tape) slipping under
cyclical loading was evident only in low loading of
the knot, so that it never took a hard set, and thus
could gradually ratchet out material of the *exterior*
tail.  I don't know what sort of similar-loading behavior
might occur only there, for ARJ knots.)

Nice contrasting & bright ropes used for the presentation,
thanks!  (Why not also in the loaded sampling?)

Btw, your observation about nearly immediate rolling
of the offset water knot (EDK) quite surprises me :
I've not seen that (though in some shopping-bag braid
I did just get some movement, though on a firm load),
and the knot does exist broadly in the wild.

And re "footprint", cordage consumption is somewhat
beside the point, in that knot shape will more likely be
any influence.  E.g., think of two horseshoes placed one
atop the other vs. facing opposite such that each straddles
one leg of the other --stacked height vs. width.  (I've found
an offset knot that resembles that latter case, and the
offset 9-Oh is more like the former.)

My measurement of the offset 9-Oh is approx. 31dia,
and (by subtraction, basicly) the OWK 22dia --quite
a bit less than yours, but with slightly greater evident
difference, proportionately --yours a third greater, mine
a half greater (with me measuring a 3/16" dia kernmantle
cord, using tea-bag strings to mark points).  !?



--dl*
====

Mobius

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Re: Offset knot for climbing
« Reply #5 on: November 07, 2015, 11:35:02 AM »
This is an interesting interlocking of components,
and in my quick play-test it looks good.  But I'm afraid
that the very "interlocking" tying aspect will keep
it from becoming a winner in the wild.

Thank you for the feedback Dan. I will make some other responses to issues you raised, however here is a tying method to start with. I don't think it is too hard, though admittedly it is not as easy as a 9-oh.

Cheers,

mobius

Mobius

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Re: Offset knot for climbing
« Reply #6 on: November 08, 2015, 07:14:36 AM »
My measurement of the offset 9-Oh is approx. 31dia,
and (by subtraction, basicly) the OWK 22dia --quite
a bit less than yours, but with slightly greater evident
difference, proportionately --yours a third greater, mine
a half greater (with me measuring a 3/16" dia kernmantle
cord, using tea-bag strings to mark points).  !?

I am not sure what is going on here. The material I use for such measurement is 11mm kernmantle. Last time I used my more worn (slightly) black rope, this time I did it with the blue and orange lengths i save for pictures.

Image one is the actual knot measured, snugged, however not loaded.. The black marking is where I taped the Sparts together. The tails were measured at 195mm each prior to untying.

Image two is just measuring and calculation (515+400-390)/11= ~48 diam.

Cheers,

mobius


SS369

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Re: Offset knot for climbing
« Reply #7 on: November 08, 2015, 05:24:14 PM »
Quote
I am not sure what is going on here.

Hi mobius.

Please explain a bit more.

SS

Mobius

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Re: Offset knot for climbing
« Reply #8 on: November 08, 2015, 10:13:55 PM »
Quote
I am not sure what is going on here.

Hi mobius.

Please explain a bit more.

SS
 
Dan gets a 9-oh to consume 31 Dia,  I first get 49.5 (see op) then 48. Quite a difference between 31 and 48.

Cheers,

mobius
« Last Edit: February 21, 2016, 08:01:06 AM by mobius »

Dan_Lehman

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Re: Offset knot for climbing
« Reply #9 on: November 09, 2015, 08:20:46 PM »
Dan gets a 9-oh to consume 31 Dia,
I first get 49.5 (see op) then 48. Quite a difference.

Cheers,

mobius
Which moves Dan to re-measure.  This time, I used
a commercial-fishing kermantle, which I measured
to be about 9/32" (2 strands adjacent measured :
9/16), and ... I, now, also get 48.)
(And, frankly, I think that if I'd referred to some paper
on which I have made some similar notes about knot
sizes, I'd have doubted my lower figure based on that
for a fig.8 eyeknot among others!)
.:.  science, moves by replicated testing, verification, ...

Sorry for the false lead (which I'm not going to try to figure
out, right away, though I did employ the tea strings and ... ;
I might try that yet again, though, out of curiosity!).

Hmmm, measuring with small diameters is likely prone
to making large errors --tough to make the diameter
measure!  Maybe I'll try measuring some relatively thick
rope (1" or so).  As rope size increases, it might become
problematic to determine how to set the knot for
measuring --how much force ... .


--dl*
====
« Last Edit: November 10, 2015, 05:54:44 PM by Dan_Lehman »

Mobius

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Re: Offset knot for climbing
« Reply #10 on: December 20, 2015, 07:43:50 AM »

Hmmm, measuring with small diameters is likely prone
to making large errors --tough to make the diameter
measure!  Maybe I'll try measuring some relatively thick
rope (1" or so).  As rope size increases, it might become
problematic to determine how to set the knot for
measuring --how much force ... .


--dl*
====

Knot efficiency in terms of diam.'s consumed is one of the many things we need an (unofficial) standard for. i.e. Comparing 11mm kernmantle vs 11mm kernmantle means  a lot more than 11mm kernmantle vs some small diam. soft rope.

The knot I show here is efficient as far as I am concerned.

Mobius

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Re: Offset knot for climbing
« Reply #11 on: December 25, 2015, 09:39:40 AM »
I redid my trial of this bend, taking more note of slippage. Image 1 shows tails of around 50mm at zero load, Image 2 shows the same knot at 200kg. The 'slippage' is around 3-4 mm, perhaps better described as settling.

The rope material is stiff, well-worn 11.2mm Dynamic rope

Cheers,

mobius

Edit: the rope material is dynamic
« Last Edit: January 26, 2016, 04:36:21 AM by mobius »

Mobius

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Re: Offset knot for climbing
« Reply #12 on: January 26, 2016, 04:57:48 AM »
I found out that the well worn climbing rope I was given is dynamic, not static (the pale green rope with gold/blue flecks I show in the thread).

In theory this knowledge should make the knot I show more interesting to the climbing community, considering the trials already conducted and shown in previous posts: ie This lohf8 style knot is a simple to tie, verifiable, small footprint offset knot that did not slip in dynamic rope at 200kg, and was easy to untie. This knot might be interesting to someone who has not already made their mind up about what offset knot to use.

As it turns out, I am going to trial this knot again shortly in both 10.2mm static and 10.2mm dynamic rope when my new rope purchases arrive. I am getting a new crane scale as well and will probably trial up to 500-600 Kg max and see what happens. I won't be doing sudden pull tests, that will be for someone with a tractor/truck and bit of space to do them.

I have also purchased some 6mm dynamic accessory cord which should prove useful in tying this knot combined in different diameters.

Cheers,

mobius



« Last Edit: January 26, 2016, 08:20:36 AM by mobius »

Dan_Lehman

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Re: Offset knot for climbing
« Reply #13 on: January 27, 2016, 07:02:31 AM »
... dynamic, not static ...
In the cordage world, there are a trio of definitions
with the above two having "low-elongation" between
them --and most of what is casually called "static"
falls into this middle class, with the "hi-mod" ropes
being true "static"s.

Quote
I have also purchased some 6mm dynamic accessory cord
which is unlikely that but "low-elongation".

Maybe I'm rusty re current definitions; I recall something
about the stretch at 10% of tensile strength being what
slotted ropes into one or another category?!

Rather than the great forces you envision,
what could be enlightening is seeing a configuration
in which you make a round sling of both of any two
or your ropes --e.g., the 10mm of low-elong & dynamie--,
by joining them with non-concern knots (#1408, say)
and then loading the knots simultaneously.
(I'm having trouble envisioning the actual set-up.
But a simple solution is to have a 3rd rope, or two
such pieces, tie off each of the specimens in the
subject-knotted rope.)
((How long are your samples?))

What I'd like to see is how the like-knotted different
ropes behave, side-by-side.  (Thinking we might see
how in the dynamic case that at 100kg already the
knot has taken much more deformity, while the
low-elongation rope holds its set shape still.)

Ideally, one would want a pulley so that differences
in elasticity would not bias the loads on each (which
if joined around a pin lacking such efficiency as a
pulley could see the lesser-stretching rope actually
be taking a higher load into the knot while the
dynamic rope just elongated!).


--dl*
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Mobius

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Re: Offset knot for climbing
« Reply #14 on: February 06, 2016, 12:09:35 PM »

What I'd like to see is how the like-knotted different
ropes behave, side-by-side.  (Thinking we might see
how in the dynamic case that at 100kg already the
knot has taken much more deformity, while the
low-elongation rope holds its set shape still.)


I took a lot of photos today to try and show how this knot behaves in 10.2mm low-elongation and high-elongation ropes. My photography efforts were very poor today, The few I show are the best of a bad lot.

Image 1: the rope specs of the 10.2 dynamic rope I purchased.

Image 2: the 10.2 mm static rope at 300kg. The knot has not distorted much from zero load. I marked the tails at 100mm and could detect negligible tail movement at 100kg, 200kg or 300kg (i.e. the tail markings looked to still be at 100mm to my eye). The knot was very easy to undo after load.

Image 3: this time I have 10.2mm dynamic rope with the tails  marked at 50mm. The image is at 200kg and looks a lot like Image 2 (though 100kg less loading). I measured around 3mm of tail movement at 100kg, though the knot did not appear to have experienced any more tail movement at 200kg when I measured again.

Image 4: Now the knot is showing some distortion. The knot has not collapsed in my opinion, though there was another 3mm of tail movement (6mm in total). Perhaps interesting is that I repeated this test in the same rope to 300kg, though I used the winch to get quickly to 300kg directly. Second time around the knot did not look so deformed, though it too showed overall tail movement of around 5-6mm after 300kg. Both the trial knots were easily untied after the test. The second one a little more so than the first.

I am a bit disappointed to see even the relatively small amount of tail movement i saw in the dynamic rope trials.

Cheers,

mobius