Author Topic: Constrictor bend?  (Read 16477 times)

dfred

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Constrictor bend?
« on: October 08, 2007, 01:51:59 AM »
About a year or so ago I was tying a Double Fisherman's bend (#1415) and a
random thought popped into my head...  The DF effectively uses what
are Strangle knots (#1239) around the respective standings parts, right?  So
what would happen if a similar binding knot like the Constrictor was
used instead of the Strangle in a similarly constructed knot?

What resulted initially didn't wow me, mostly due to me not realizing
it needed a slightly modified dressing procedure to take a proper
shape.  But after playing with it a little, I came to think it a very nice
bend which has some unique properties compared to the DF.

I know enough to realize it's very unlikely such a fairly
obvious combo has never been tried before, but in a bit of
searching I haven't been able to find a previous reference to this
bend.  I'll be interested to hear if folks here have seen it before.

However, on the slim chance it's not already known as something else,
I'm tentatively calling it a "Constrictor bend".  Am I right in
thinking that there are no other bends based directly enough on the
constrictor to have already claimed such a name?  (Hard to believe!)
And while more akin to a Double fisherman's, since it is not
based on the Double constrictor, it seemed more reasonable to just use
the unadorned name.

Here are some images that should help people to reproduce this knot:



Start with two opposing lines, as in a Double fisherman's bend.


With each end, form a loose constrictor knot around the standing part
of the other line.  The two constrictors should be mirror images of
each other.  The easiest way to assure this is to make the turns of
both constrictors away from the body when tying.


With the contstrictor knots still somewhat loose, pull gently on the
two standing parts and allow the knots to slide together.  As they do
so they will rotate 180 degrees with respect to each other as they
come together.


Alternately tighten the standing parts and working ends to tighten the
knot.  Start this process by pulling slightly harder on the standing
parts than the ends.  As slack is worked out of the knot you can start
pulling more forcefully on the working ends which causes the knot to
"lock" in its final position.  This final locking tends to prevent the knot
from tightening much further when load is applied.   Though you really
never need to pull super hard on the working ends or you may overly
distort the knot.


"Bottom" of tightened knot.


Side view.  Note how the standing parts mutually trace an "out of phase" sine-wave-like
pattern through the knot.  This seems to give the knot a lot of internal friction and
makes it resistant to overtightening if properly dressed before loading.


Even after heavy loading, the bend is easily released by prying the
outside turn towards the standing part.  This action completely
frees the working end and allows the knot to loosen.


Interestingly, when both working ends are un-tucked as in the above step, you
end-up with something akin to a Barrel knot (#1413) -- a knot not known for its
ease of releasing.

Having used this knot for various tasks for the last year or so, I
have a number of observations and comments about it.  But to avoid
overly prejudicing the discussion here, I'll refrain for a bit and see what
people's initial thoughts are.

I am interested to hear from anybody with access to knot testing equipment
regarding this bend's strength.  That's something I haven't been able to
generate any real data on...

David
« Last Edit: October 08, 2007, 02:10:55 AM by dfred »

knudeNoggin

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Re: Constrictor bend?
« Reply #1 on: October 08, 2007, 05:58:18 PM »
Idleness is the devil's workshop.

But why stop with half (or one quarter!) of the work?
Consider also the Constrictor the other way round, in this bend.
And also with like/opposite handedness:  so, there are four bends revealed.
Or the Picketline hitch, both ways; and the similar "spar" hitch, #1674.
Of course, of many other such hitch knots that might be pulled together.

*knudeNoggin*

turks head 54

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Re: Constrictor bend?
« Reply #2 on: October 11, 2007, 06:41:19 AM »
Hmmm

Very interesting idea. I will ask around and see if anyone has heard of it.

Turks Head 54

DerekSmith

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Re: Constrictor bend?
« Reply #3 on: November 18, 2007, 06:18:55 PM »
Well,  I have played with this knot for a while before subjecting it to comparative strength tests and concluded that I did not like it.  It is a big ugly knot that does not naturally draw itself up - you have to draw up the ends as well as the loaded lines to dress and set the knot - but worst criticism of all, I just cannot remember how to tie it, so I have to copy it to tie it.  I use the Constrictor a lot and tie it by the loop and fold over technique, so it comes as an absolute pain to actually 'tie' the knot.  Its ends are nicely tucked in and held, so it is a good 'secure' knot, I do not think it would be likely to fall open on jiggling like the sheetbend for example.

For strength, I set it against the Sheetbend -- no surprises, it is stronger than the Sheetbend.
Next I set it against my favorite bend, its close cousin the paired strangles, AKA the Fisherman -- big surprise, it is stronger than the Fisherman.
To punish it for being stronger than my favorite, I then set it against one of the top group of very strong knots, the Sliding Grip Hitch (SGH)-- there was a big bang and the test piece went flying out of the test rig.  When I recovered the test piece, even bigger surprise it was stronger than the SGH.

To date, I have not tested any 'practical' bend that is stronger than this.

I then put this knot up against itself to see just where it would fail, in an attempt to rationalise why this 'ugly' knot could perform with such amazing strength.  Using one of the images posted by David, I have superimposed in yellow the zone where the break occurred (obviously, it only broke on one side, I marked both sides so you could see the above and below of the 'weak point').



The force has transfered all the way through the knot to the point where the cord takes its first tight bend.  This knot does not 'constrict' like the strangle does, in fact, because it is necessary to tighten this knot by pulling on the free ends, the knot tends not to cramp down on itself, more so it 'bunches up', so the knot fails by a conventional radial failure after first having lost a significant amount of load by passing through a series of tight, but not constricting, loops, while both the Fishermans and the SGH failed by constrictive weakening.

Once loaded to almost breaking point, the knots are impossible to break open.  I will attempt to photograph the survivors so that you can see the final highly loaded form which shows the straight line transmission very nicely.

All in all, a very surprising outcome and a lesson learned not to prejudge a knot.

Well done David, and thank you for bringing us another piece in the jigsaw of understanding what makes a knot 'strong'.

Derek

turks head 54

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Re: Constrictor bend?
« Reply #4 on: November 18, 2007, 09:35:17 PM »
So the constrictor bend is stronger than all those knots you tested?  ???

DerekSmith

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Re: Constrictor bend?
« Reply #5 on: November 19, 2007, 11:56:45 AM »
So the constrictor bend is stronger than all those knots you tested?  ???

Yes indeed, quite a surprise -- I wonder what the double Constrictor bend or even the Boar bend might deliver??

Derek

Edit..... 20 Nov.

In deference to Dans later post, I must amend this post by adding --  This knot was the strongest of those tested when tied in R20C polyester braid and loaded progressively to breaking point over a period of ca 2mins. and in the total absence of any dynamic or shock loads.

Derek
« Last Edit: November 20, 2007, 11:36:54 AM by DerekSmith »

DerekSmith

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Re: Constrictor bend?
« Reply #6 on: November 19, 2007, 11:59:50 AM »
As promised, here is the image of the Constrictor bend survivor.  Note the near straight flow of the loaded parts into and through the knots, lots of load shedding without undue strangulation.



Derek

Dan_Lehman

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Re: Constrictor bend?
« Reply #7 on: November 19, 2007, 09:05:16 PM »
Quote
So the constrictor bend is stronger than all those knots you tested?

This is how the story comes out, and it's missing a key component from the start--typical.
Rather, it is the cord you used was strongest in the slow-tension test when joined
with a Constrictor bend?
.  And one consequence of testing done by those with
different methods & materials would be to immediately (often) deny such an over-reaching
statement.

And what about the ruptured knot--what is left of it to examine?  (--to determine or guess
at the rupture point within it)  The geometry of the knot at high tension doesn't surprise
me, and resembles the general shape of the Dbl.Harness Bend (shown as (one version,
i.e.) the Benson Bend in Rigger's Apprentice, e.g.).  Long ago, an Irish IGKT member
tested some bend I thought to be strong via its few curves of material prior to the ends
making their "u-turn"s; the irony, it seemed to me, was that he had to re-tie it and the
result (perhaps same as otherwise ...) was akin to what one sees here, with SParts
running through knot fairly straight to a hard tight turn on their opposite member;
and the knot though was strongest of those I had tested!  The Reever (& Dble.R.)
Bend is of the general geometry of the Dbl.Harness, too.

This sort of situation seems to be to be a bit of a race condition:  if one SPart can
more quickly tighten around the other (by some imbalance on dressing, say), then
that bias should be aggravated, as the tighter constriction impedes the other's
constriction upon it, further enabling its own increased tightening.
Btw, I think that a surviving SmitHunter's Bend would be similarly tight & untiable.

--dl*
====

DerekSmith

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Re: Constrictor bend?
« Reply #8 on: November 20, 2007, 11:48:08 AM »
I nearly missed this.



Notice in this 'after' image, where the loaded lines enter the knot the braid is bunched up and then tapers down as it gets further from the knot.  At this point the core is now shorter than the braided sheath and is holding it 'bunched'.

I cut one of these ends off flush and sure enough the core retracted (or the sheath expanded) back inside the sheath by ca 2mm.

It looks as if the braid expanded more than the core during loading and pulled out of the knot and was then held in place when the load went off.

Has anyone any thoughts as to how this might influence the strength of the cord during loading?

Derek

DerekSmith

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Re: Constrictor bend?
« Reply #9 on: November 20, 2007, 01:07:38 PM »
Quote
So the constrictor bend is stronger than all those knots you tested?

This is how the story comes out, and it's missing a key component from the start--typical.
Rather, it is the cord you used was strongest in the slow-tension test when joined
with a Constrictor bend?
.  And one consequence of testing done by those with
different methods & materials would be to immediately (often) deny such an over-reaching
statement.
snip....
--dl*
====

Dan, my apologies.  While I personally am fully aware that the results that come from the testing protocol I used can only be relied upon within the scope of that protocol, I repeatedly forget that others do not think this way and could (and doubtless do) presume that the results are then applicable to all cord and all loading situations.

While a knot which has 'strong' structural characteristics is likely to outperform a knot which has 'weak' characteristics, it would be wrong to imply that this is always the case until we are in the position of being able to understand how all cords and loading environments react to a knots structure.  Clearly we are a long long way off from being able to do that.

It is perhaps worth while making the point that this comparative testing is not being undertaken with the objective of creating 'The Definitive Table of Knot Strength' (such a thing is unlikely to exist), rather, it's goal is to generate information that will allow us to probe the workings of knot structure and performance, such that we can understand which structural parameters cause weakness and how, and beyond that, which cordage parameters (resistance to radial bending, stretch, compression, torsion etc..) interact with those structural aspects and how these in turn influence weakness, then finally, how the mode of loading impacts all of these factors.

Having said that, many people are aware that knots weaken cord and that some knots weaken more than others.  Out of fear of having a knot break on you, it is reasonable that folk will cast around seeking information on which knots are 'strong' and which are notoriously 'weak' in order to make a better choice of knot.  We already have a quantity of 'folklore' relating to knot reliability - vis "Never use the Reef knot to bend different thickness ropes" etc. etc.  That folklore is based on experience, not rigorous testing in all cords and all loading situations, yet we would be foolish to ignore it.  Likewise it is reasonable for people to glean the fragments of knot strength testing results that fly around and utilise them to make assumptions about knot strength in a situation that has never been rigorously tested.

In the absence of any other information, then the results of just one test are gospel.  When contradictory results have been generated, then in the absence of understanding, common sense and individual caution must prevail, but to ignore the results would be foolhardy.

Derek

DerekSmith

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Re: Constrictor bend?
« Reply #10 on: November 20, 2007, 01:36:43 PM »
snip...

And what about the ruptured knot--what is left of it to examine?  (--to determine or guess
at the rupture point within it) snip..
--dl*
====



The part of the knot which failed is left on the line, while the side which did not fail sprang open when the line it held was withdrawn by line tension after the break.

Derek

Dan_Lehman

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Re: Constrictor bend?
« Reply #11 on: November 20, 2007, 06:56:38 PM »
Derek, you have the best idea of the elasticity of your cord--I recall you
earlier remarking at that, and maybe having to adjust a test specimen
mid-test to account for elongation.  Looking at the broken remnant,
I see tha the Constrictor component is intact:  in fact, it looks both
recognizable AND relaxed--a far cry from the scrunched material
shown of the knot under high tension!  Given that the rupture point here
(relaxed) lies just outside of the component C., we must deduce that
at the point of failure it was farther removed from this point into the
opposite member.  .:.  The yellow'd highlighted areas you nicely show
over DFred's red-rope image should move more away from their C.
knots into the opposite C. knot--in fact, the yellowing should begin at
the point it currently ends (and maybe shorten its span).

Your results seem to follow those of Stanley Barnes in his ca. 1950
book's (available via Des's Footrope Knots catalogue?) analysis
of ruptures in gut & monofilament line tied with Blood knots--at least
for the nylon:  break came MID knot, where the SParts went around
the tucked ends, not at the endpoints/entrypoints (i.e., neither from
the severe turn at far end or the compression by this turn on entry).

I'm not sure how to interpret the particular fibre-blossom of the rupture.
It seems to show some set of evenly chopped shorter fibres, and a bit
of rougher-hewn longer ones (respectively on left/right as oriented);
this might indicate the initial break (being uniform shorts) coming on
the concave side, then a less orderly tear-off occurring?!

--dl*
====
« Last Edit: December 10, 2007, 09:18:15 PM by Dan_Lehman »

Dan_Lehman

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Re: Constrictor bend?
« Reply #12 on: November 20, 2007, 07:21:29 PM »
Quote
  [testing]'s goal is to generate information that will allow us to probe the workings of knot structure and performance,
 such that we can understand which structural parameters cause weakness and how, and beyond that, which cordage parameters
 (resistance to radial bending, stretch, compression, torsion etc..) interact with those structural aspects and how these in turn influence weakness,
 then finally, how the mode of loading impacts all of these factors.

Amen!  (and, most importantly, what are the best colors)

Quote
Out of fear of having a knot break on you, it is reasonable that folk will cast around seeking information on
which knots are 'strong' and which are notoriously 'weak' in order to make a better choice of knot.

Some might, but in nearly all cases, I think that this is a mis-guided effort:  for starters, the information
just isn't well generated; and no one should be using cordage so near to failure that the knot matters
(and, further, one can wonder at the probability of rupture forces coming in just a small enough increment
to distinguish between knots, rather than in a surge of force that would break any).

Quote
In the absence of any other information, then the results of just one test are gospel.
... to ignore the results would be foolhardy.

 ::)  Oh, come on, now!   That's like saying one should believe whatever an 8yr-old says until
one can get advice from a 15yr-old!  (I'll guess that upwards of 99% of all cordage users are
ignorant of the results at hand here, yet are not provably foolhardy.)

Your testing puts some 2nd point of info on a line (as noted above) determined w/Barnes's
data, and I believe we'll see more results to confirm.  --interesting now to consider the case
of the Grapevine, and to speculate that the Reever/Dbl.Reever will follow suit with the above.
(One testing of the Strangle Noose hitch found rupture in the structure SPart at entry, where
the Strangle SPart so tightly nipped it--and not at the u-turn around the 'biner (in 8mm nylon
kernmantle low-elongation (caving) rope).)

Compared to the Grapevine (and further multiple-Oh pull-together bends), the parallel
SParts of this C. bend (note:  discordant) lie farther APART, and there is thus a significant
curve to reach back to the parallel member; the SParts lie together in the Grapevine, and so
more force then is delivered into the u-turn wrap!?  (And I've wondered if the significantly
lesser friction in HMPE plays any role in making these two cases less distinct--i.e., that
even with the angles, the lower friction allows more load to be delivered farther into the
knot, and ... !?  And the nature of the cordage (firm & round vs. softly flexible/compressible)
could cheat angles, too.)

--dl*
====

DerekSmith

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Re: Constrictor bend?
« Reply #13 on: November 21, 2007, 12:52:25 AM »

Quote
In the absence of any other information, then the results of just one test are gospel.
... to ignore the results would be foolhardy.

 ::)  Oh, come on, now!   That's like saying one should believe whatever an 8yr-old says until
one can get advice from a 15yr-old!  (I'll guess that upwards of 99% of all cordage users are
ignorant of the results at hand here, yet are not provably foolhardy.)

--dl*
====

It's not like believing an 8 year old, unless an 8 year old had just run or read out the test results.

If 99% of cordage users were ignorant of the results at hand here, then they would not be ignoring the results, they would simply be living in blissful ignorance and hence could not be regarded as foolhardy for ignoring them.   ::)

Besides, when you do not know about strength, there is always the colour to worry about.

Derek
« Last Edit: November 21, 2007, 01:49:37 AM by DerekSmith »

myoman

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Re: Constrictor bend?
« Reply #14 on: November 25, 2007, 04:35:05 AM »
is there anywhere I can find how to tie the sliding grip hitch (SGH) as mentioned above?