Author Topic: Double wrap figure eight on a bight!!!  (Read 8345 times)

Mike

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Double wrap figure eight on a bight!!!
« on: February 05, 2008, 06:01:29 AM »
I was playing around with the figure eight on a bight, and figure nine, trying to come up with something different.   Well I stumbled across a variation that I think is pretty darn nice. I'm not sure if it has a name or is in use already.   When making the figure eight on a bight, take the bight around the standing part twice(instead of once) then continue as normal. It doesn't jam as easy as the standard figure eight on a bight.  I wonder if it would be more secure and what the strenght would be?  Here is a couple pictures.

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Bob Thrun

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Re: Double wrap figure eight on a bight!!!
« Reply #1 on: February 06, 2008, 10:33:59 PM »
The figure-eight loop with an extra half wrap is called a figure-nine.  With a full extra wrap it is a figure-ten.  See page 13 of the Lyon report, http://www.hse.gov.uk/research/crr_htm/2001/crr01364.htm

Mike

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Re: Double wrap figure eight on a bight!!!
« Reply #2 on: February 07, 2008, 12:01:27 AM »
My version is a little different I think.  I made the two wraps on the standing part before comming back down to make the "8" Shape.  I'm not sure if i am explaining it correctly. The way i tied it, is alot less bulky than the nine or ten, in the link you posted.

Bob Thrun

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Re: Double wrap figure eight on a bight!!!
« Reply #3 on: February 07, 2008, 01:46:38 AM »
As I read the Lyon report, it says that there is some latitude in how the wraps are done.  Only the finished knots are shown.

The First Edition of Life on a Line has a clear drawing of the figure-nine being tied.  The author has pulled the free first edition in vavor of a for-sale Second Edition.  There are still some (pirated?) copies of the First Edition on the Internet.  I just downloaded Part One from www.anthros.org/descargas/Manual%20Life%20on%20a%20line.pdf
Parts Two and Three can be found elsewhere

Dan_Lehman

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Re: Double wrap figure eight on a bight!!!
« Reply #4 on: February 07, 2008, 09:33:44 PM »
My version is a little different I think.  I made the two wraps on the standing part before comming back down to make the "8" Shape.

One might call the other forms more "cascaded", re their coil of wraps
--i.e., that there is wrapping back over the initial wraps instead of continuing
the spiral away.  That is something that happens upon loading, or might be
deliberately dressed.  In the case of this knot, I think it might help to do so
in order to avoid concentrating the load from the eye leg(s) at the entry of
the SPart.

Your particular dressing seems sloppy at the u-turn of the SPart--and, like
most presentations of such traced knots, it's unspecified which of the
twin ends is intended to be loaded.  Lyon's testing does discriminate,
but I'm not sure that they've done the best in putting each case forward
--i.e., that given THIS end qua SPart, then the knot should be set in
this way (not merely loaded on all ends for some force & duration).

There are various other similar loopknots, where the end is placed on
one or the other side of an initial SPart loop, and then the eye bight
wraps--extensions to Ashley's #1043/45, & similar.

--dl*
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turks head 54

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Re: Double wrap figure eight on a bight!!!
« Reply #5 on: February 08, 2008, 01:20:11 AM »
Bob,
Parts one tow and three of life on a line are all in that document you found.

TH54

Mike

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Re: Double wrap figure eight on a bight!!!
« Reply #6 on: February 08, 2008, 04:34:23 AM »
it's unspecified which of the twin ends is intended to be loaded.

The load end is the bottom of the picture.

So I guess this is the same as the Figure of Ten except I did not overlap the second wrap over the first one, correct?

Dan_Lehman

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Re: Double wrap figure eight on a bight!!!
« Reply #7 on: February 08, 2008, 05:32:13 PM »
it's unspecified which of the twin ends is intended to be loaded.
The load end is the bottom of the picture.
So I guess this is the same as the Figure of Ten except I did not overlap the second wrap over the first one, correct?
Your answer begs the question:  both top and bottom must be loaded (opposition),
but in one case both ends bear tension (forming an eye (or not! --but that's a definitional matter!)),
and opposite them one end is the SPart.  So, pointing to "the bottom" begs the question,
as there are two, "twin" ends exiting both top & bottom.  The answer is "left" or "right".

Now you've raised another question with "at the bottom":  if THAT is the case, your knot
is upside-down re what Lyon shows; that you have a more difficult loopknot to tie, in
needing to form wraps around *air* (or finger(s), ...), and then bring the eye-bight through
these wraps
!?
NB:  these all are knots worth considering (with the Fig.8 & Overhand, there is no difference
with the reversal; with these particular forms of *higher* members in the knot series
(nominally ascending from the name "8" as "Fig.9", "Fig.10", and so on) there is a difference,
as the forms are asymmetric;
there are, howerever, two quite different forms of such knots that are symmetric
(i.e., one can transform/re-dress a canonical Fig.9/10... asymmetric form into either
of two symmetric forms (and vice versa)--though figuring how to do so can be a pain)!
An example of a symmetric Fig.9 (in single strand) is Ashley's #525 stopper (though
Ashley doesn't present it in exactly that form, I think; but he gets close to it, esp. with
the lefthand tying diagram, from which one can snug down the knot into symmetry.

But back to your "at the bottom" vis-a-vis:
Quote
My version is a little different I think [from what is shown by the Lyon Equip. document].
I made the two wraps on the standing part before coming back down to make the "8" Shape.
The difference (Fig.9/10 vs. Rev'd Fig.9/10) is how to interpret the highlighted expression above.
I took "on the SPart" to mean that you wrapped the bight around its earlier part, which is
the usual form.  "of" vice "on" would better describe tying the structure in reverse.
And for the usual form to be the case at hand, then the SPart is one of the TOP ends,
eye below.  (And the issue re slop in the particular dressing is at the SPart's u-turn
into the wraps.)

*kN*

Mike

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Re: Double wrap figure eight on a bight!!!
« Reply #8 on: February 09, 2008, 03:42:30 AM »
Dan, you have lost me with all of your technical knot knowledge.  To make things easier, I am going to make a short(approx 30 seconds) video of how I tied it. I'm not sure how to host a video on here, so if you dont mind I would like to send it to you via email.

Dan_Lehman

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Re: Double wrap figure eight on a bight!!!
« Reply #9 on: February 09, 2008, 07:40:32 AM »
Whoa, not to discourage the video,
but this isn't so tricky a matter, at least on the basic question.
You presented an image of a knot--in a way, a sort of cookie-cutter
image showing just the "nub" (a Dick Chisholm term) of the knot.
Two parts run off the top of the photo, two off the bottom.
So, what is it?
Well, you told us it is a loopknot.  Fine, where is the eye--top or bottom?
And, opposite the eye, which of the two ("twin") ends should be loaded?
(Either might be--the usual case with such things going unspecified,
and maybe well enough, with no great difference, but ...)

So, it's a simple top/bottom & left/right question, and that's hardly technical.

- - - - - - - - - - - -

I did touch on a philosophical/conceptual issue regarding how one might
define "loopknot", in remarking "or not" about there being an eye at all.
From the perspective of this "nub", there is--for a loaded such "loopknot"--
a set of relative tensions on certain ends.  For a bend it would be 100% & 0%
opposite the same (2 SParts loaded, two corresponding ends not); for a
loopknot it would be 50% & 50% opposing 100% & 0%.  And my remark
was an allusion to the thought that this would be the essential definition
of a loopknot--i.p., nothing about there being an acutal eye!!  That of course
seems odd, but there is no effect on the nub at least--it behaves according
to the various tensions.

An example I like to mull over is tying a tow line to one side of a barge,
and then bringing a second, similar (short) line from the other side and
tying into the tow line in the manner of a Bowline.  From the same sort of
cookie-cutter perspective looking at the nub, but for perhaps distinctively
different ropes involved, one would see a bowline's body.

In your case, one can imagine the eye being either top or bottom, and
I so discussed that a little.  And, with the particular general shape (i.e.,
the "Fig.9", I revealed that with some playing around one can put it
into either of two symmetric forms.  Ashley's #1425 (not -a) can be
seen as an abbreviated bend in this form, where the ends each stop
short of completing its trace (they trace part way then stop).  There are
some nice loopknots in which the SPart makes the full form and the
end makes the 1425-like partial return:  apparently strong, secure
when slack, fairly easy to untie after loading, and TIB (tiable in-the bight).

But, again, top/bottom, left/right are simple questions/answers, for starters.

 ;)

Mike

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Re: Double wrap figure eight on a bight!!!
« Reply #10 on: February 12, 2008, 05:28:09 AM »
Dan, here is a couple more Pics. Hope this helps clarifyany questions you had.  I am really interested to know what the strength of this not is compared to the figure 8.  I was able to put some serious load on this knot and it never lost its shape and was still easy to untie.  In this example, the "twin" without the black dots is the load end.


« Last Edit: February 12, 2008, 05:31:07 AM by Mike »

Bob Thrun

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Re: Double wrap figure eight on a bight!!!
« Reply #11 on: February 12, 2008, 06:47:24 PM »
Dan, here is a couple more Pics. Hope this helps clarifyany questions you had.  I am really interested to know what the strength of this not is compared to the figure 8.
I've already mentioned the Lyon report.  As you say, Lyon did not have the knot dressed as neatly as you did.

Techniques de la Speleologie Alpine, second edition by Marbach and Rocourt, gives an efficiency of 70% for the figure-nine, and 55% for the figure-8.  The figure-ten was not mentioned.  These numbers were quoted in Vertical by Alan Warild.  Warild's book may be found on the web.

Mike

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Re: Double wrap figure eight on a bight!!!
« Reply #12 on: February 13, 2008, 04:07:34 AM »
I am curious if the way its dressed can effect its strength. Since the two wraps are not crossing each other and lay symmetrical, I feel it would be stronger, but I have been known to be wrong every now and then. ;)

Dan_Lehman

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Re: Double wrap figure eight on a bight!!!
« Reply #13 on: February 15, 2008, 07:00:58 AM »
Mike, my questions are simple, and can be answered simply, verbally.
But you posted more photos; they answer one question--viz., Where's
the eye.  They don't answer Which end is loaded?  This is a question
left unanswered by most people for such knot-in-a-bight loopknots.
Lyon recognizes the difference, and makes some effort to determine
it effect (if any), with mixed (per knot) results.  (Actually, as limited as
their testing was--in test cases, e.g.--, one shouldn't make any con-
clusions based upon it, other than that there is something worth
pursuing in further testing.)

Bob says that Lyon didn't dress their Fig.10 LK so neatly, but I see
it as they worked in stiffer rope (for what is pictured, at least), which
shows evidence of apparent torsion in the eye, e.g.; I think that they
got it pretty well dressed, and the same, re crossing strands (none).
But they might have not forced the wrapping as far as could be done,
letting it come over the earlier turn more than is necessary.

The effects of dressing (and even orientation vis-a-vis loading of
which end) might matter and matter differently in different materials.
I think that it's conceivable that in a material with greater friction
that one might see some friction-gripping of the wraps, whereas
in a slicker rope this wouldn't happen, and so for it, another dressing
might work as well or better.

Then there is the question of How to measure strength?
--with slow-pull loading?
--with sudden ("shock") loading?
--over time of useage & battering and sustained loading?

I know that Bob likes to dismiss "shock loading" as mythical, but
Dave Merchant asserts (if not in Life on a Line then on the NSS's
forums.caves.org OnRope forum) that it makes a significant difference
in these very loopknots, with the more complex ones (Fig.9, 10?)
losing strength from presumed greater frictional heat (perhaps from
there being more material to be drawn out of the knot on compression
at the high loads).  Re the third mused strength measure above, what
I'm wondering at is there being a chafing penalty for some knots that
don't jam and hence see movement of material on loading/unloading,
and which though maybe testing stronger on a device in a lab would
fail sooner in the rigors of repetitive practice vs. a knot that gets tight
and holds its position, without such movement.  (OTOH, one might
see the latter as preserving tension that weakens over time, whereas
the former, loosening, enables some recovery!?)

March's 70% & 55% seem somewhat out of line w/other soundings
(i.p., Lyon's), especially compared to the big difference between 8 & 9
(i.e.,the difference of the relative strengths of the two knots is too great).
The Fig.8's strength is just too low, here.

I'll recall that one person doing crude break tests with a truck and
small stuff (quarter to three-eighths inches PP (mostly) rope) of different
bends, used Fig.8 loopknots for the connections to the anchors
and they NEVER broke (Blood knot, fyi, won vs. other bends)!
(A perplexing aspect of this sort of testing is that the technocrats
will decry it for not being done w/laboratory controls; and yet it
might well be more like the actual use--and so more relevant,
as real life tends to occur outside of laboratories.)

--dl*
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DerekSmith

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Re: Double wrap figure eight on a bight!!!
« Reply #14 on: February 17, 2008, 04:44:51 PM »
Mike,

Following Dans excellent analysis, I think it would be hard to add anything other than detail to the discussion.

I have not put this knot onto the test rig yet, but based on what I have learned so far in my testing (AKA destruction) of knots, I would offer two predictions.

The first is that in the test cord I am using and for very slow load application til failure, I would predict that this knot will not be significantly different from the single wrap variant based on the following arguements.
  • As the loop legs share the load at ca 50% each and as neither has any chronically weak features which might take its strength to below 50%, it is unlikely that the loop will fail
  • Failure will occure at one of two points, either through compression (strangulation) as the loaded line enters the first loop twists or through tight radius as the load line makes its first tight turn around the load loops.  As both of these structures are identical in both the single and double turn variants, then both knots should perform comparably.

My second prediction based on experience to date is that my first prediction will almost certainly be found to be lacking and is highly likely to be wrong.

For the real world though, I have to agree with Dans conjecture that the extra 'meat' in this knot would make the knot more forgiving of falls or jerks (of both types).

Happy knotting
Derek