Author Topic: Double wrap figure eight on a bight!!!  (Read 8344 times)

Mike

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Re: Double wrap figure eight on a bight!!!
« Reply #15 on: February 17, 2008, 06:26:33 PM »
I appreciate the the expert opinions.  I tried tying this knot in a piece of static Kernmantle rappeling line I have, and it was extremely difficult.  Because of that I will probably stick with the standard figure eight or figure nine.  The above pictured knot might be good in smaller more supple cordage as a " better looking" alternative, more decorative than functional. :)

Dan_Lehman

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Re: Double wrap figure eight on a bight!!!
« Reply #16 on: February 18, 2008, 09:52:22 PM »
Note:  the question of which end is to be loaded is unanswered.
(I don't think that Mike had a preference or focus re this.  Lyon Equip.'s
testing really doesn't explore this well, although at least acknowledging it
--their data seem to contradict their conclusions, and shows how limited
their testing is (e.g., several of the extreme values come in the mixed-form
case (where a loopknot is tied one  way at one end, other way at other)!)

  • 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.
I suspect one can see effects of dressing & setting variances here:  one point to inducing
some bit of cascading of the eye-bight legs from the SPart-entry point.  Also, how big
the SPart's loop (u-turn) is affects how directly those eye legs can deliver tension to that
turn around the SPart's entry.

Quote
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).
:o
Which is surprising, in that Dan made quite the opposite conjecture; rather,
he related Dave Merchant's assertion of test results:  that on rapid loading,
the extra material of the larger knots in this series 8-9-10 seems to contribute
to frictional heat which weakens the knot.  Now, weaker might still be
stronger than some other knots, but there's a good deal of uncertainty in
what knot geometries are actually of issue in the various reports of data.

Here's a quote from Mr."Life-on-a-line" himself, in a caving forum:
Quote
I agree that in a controlled tensile test the F9 is 10% stronger than the F8,
but *only* in that configuration. The increased complexity of pinch points in a F9
means that in a drop test there's no clear winner, and a loosely-tied F9 can lose
to a loosely-tied F8. I've drop-tested loads of rope with F8 one end and F9 the other,
and gave up predicting the outcome years ago.
...
One thing it seems we're getting caught up in is the idea of 'strength' based on a number for tensile pulls
-- the '70% Figure 8' or the '50% overhand' are slow tests - unless you're lifting cattle, a sport caving knot
will see the biggest force in a fall, which is a short-lived dynamic 'insult' instead of a pull. Knots all act
differently when loaded this way, and the more complex knots show wider-spaced results. An overhand
is pretty much 50% at any speed, but a F9 or F10 strength can change by a third if you load it at high speed
as there's more frictional heating going on. To say a knot is 'strong' isn't all that useful unless you know what type of force you're going to put on it.

All this is good, but I remain skeptical that Dave Merchant (LoaL) has paid
close enough attention to knot geometry--and there are so many possibilities
of variation, testing thoroughly would be prohibitively costly without some
good, clever, sampling/projecting strategy re results.  But he's clearly
paying some attention to this, and did much looking in preparing his revised
version of the e-book LoaL, 2nd (also available in hard form, I believe).

 :)

Mike

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Re: Double wrap figure eight on a bight!!!
« Reply #17 on: February 19, 2008, 02:22:34 AM »
Note:  the question of which end is to be loaded is unanswered.

Dan I did specify wich end was loaded. You must have missed it.

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.




DerekSmith

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Re: Double wrap figure eight on a bight!!!
« Reply #18 on: February 19, 2008, 02:30:02 PM »
Hi Mike,

I Understsnd that the load line going into the knot was the line without the black dot i.e. the one marked 'A' in this edit of your image.  But if not Dans question, then mine, which of the two cords around the loop lines is the loaded line? 'B' or 'C'?

Derek

Dan_Lehman

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Re: Double wrap figure eight on a bight!!!
« Reply #19 on: February 19, 2008, 06:29:47 PM »
Mike, sorry, indeed I missed it.   ::)  Okay, the end w/o dark markings.

I Understsnd that the load line going into the knot was the line without the black dot i.e. the one marked 'A' in this edit of your image.  But if not Dan's question, then mine, which of the two cords around the loop lines is the loaded line? 'B' or 'C'?

I can see the near-similar image with the twin ends reversed in terms of apparent over/under aspect
and that then the one WITH MARKINGS being loaded, and the draw of the bight legs up at that
entry point pulling the free end back around behind the SPart, and so it would be part C that is
the SPart's u-turn.  In a quick glance at what I just tied & tried, that seems to have a little less
torsion in it than the other part on loading.  And with the eye legs pulling into the END, the
SPart escapes some compressive friction at the entry (and so delivers more load into the
part C.   ... and this matters (or not) how ...

Conceivably, other actual orientations might present similar appearances (and one has to
penetrate the wraps to see what's what, or apply tension and see what tightens).

 :)