Only one comment so far AS. these knots are never likely to see use around a 20mm fixing, so this poses the question of - what effect does the ‘bina’ diameter have?
Looking forward to the photos.
Derek
Suggestion - don't increase the diameter of the MBS test pin from 15mm, DECREASE it to the same value (i.e. 10mm) that you will be using to test the knots. If it breaks at the shackles, then so be it, it is after all the weakest point in the system. Then you will be comparing unknotted cord with knotted cord with everything else equal, so you will be comparing apples with apples.
-
This shows abandonment of the purpose of the testing: which is to determine the
relative breaking strengths (under standard, slow-pull loading) of various knots, i.p.
several variations of the bowline. -
It’s highly unlikely that an eye will break: the knot’s SPart bears 100% tension
into turns of cordage around cordage (relatively frictive material compared with
cordage & metal); whereas an eye bears 50% tension around metal. --mixed fruits.
What does Sterling mean by “MBS”? (“M” unfortunately the initial of “Min”/“Max”/“Mean”.)
SOMEwhere, I just saw some report referring to the minumum breaking strength which
was defined as 2 (3?) standard deviations below the mean. And–I’ll hope statistics-smart others
set this straight/confirm–if you were to get, say, 5.4 & 5.0 on next tests, you might have enough
variance to enlarge your Stnd.Dev. to give an MBS of this sort of, what, 4.8?
–just a remark to make sure these are all apples, and not that Sterling has a mean of say
5.4 or greater, and so … .
AGAIN: THE PRECISE FIXING OF MATERIAL STRENGTH, WHILE NICE,
IS LESS IMPORTANT than putting limited resources into the knots testing.
(And I’d be curious how a 3-wrap fixing affected things–but, yes, then you’re
deviating and in two tests have more a 1 & 1 situation. But it’s just that I cannot
figure how more wraps would make for a higher break strength, UNLESS
a few-only wraps allowed sufficient force to be transmitted to the terminal knot
to put the break there–and even one and certainly two wraps I’d think would
significantly cut the transmitted load. Adding more wraps just provides more
material to be elongated by load and thus more movement of the material
AND more time at each load (under a constant rate of pull) to reach higher
tensions–and duration at tension is a weakening condition!)
I could not undo these [Overhand stopper] knots by hand alone. They were easily untied with the aid of pointy nose pliers.
I suggest doing more finger execises! ;D (Though, seriously, I recently struggled with fingers in the
cold on Overhands set by fising useage in well-worn/-weathered 7mm marine kernmantle and was
able to get 'em all–using thumbnails was key, in forcing bits of movement (it was not quick).)
Good show!
[u][b]BOWLINE ABoK #1010 TEST SERIES:[/b][/u] Observations: The knot was cinched tight using [i]hand [/i]force only. A single 'tracer' thread of white cotton was weaved through the sheath to mark the[b] [i]exact mid point of the collar[/i] on each knot.[/b]
??? :-\ Sorry, but I cannot imagine this being a point of rupture–there’s little
tension IN the collar. But this is most interesting in terms of tightening : for there
have been reports of jammed bowlines (also of unjammed tested-to-rupture partners (Lyon)),
and the only way it can jam is if the collar snugs up, under tension and elongated/thinned
cordage such that upon the relaxation of load the swelling rope binds solid.
And if the collar showed no movement at all, well, I can’t see THIS rope jamming!
The markers need to be at Derek’s “GeeSpot”, and most importantly at the approximate
farthest-towards-EYE point of the knots “loop/hitch” --THERE is where the break will come.
Examination of the specimen after failure indicated that the break/rupture [i]did not occur at the collar[/i].
NB: THIS IS AMBIGUOUS! “at the collar” is where my worn CEO rope broke,
in one sense–but IN THE SPart, at the collar; not IN the collar.
Observations: Only one of the #1010 knots failed. On this occasion, the knot that failed was [i]closest ...
Mr. Grammar wants “closER”–there are but two.
How could it be otherwise, than “only one”?
Well, you could comment on the nature of the 2nd knot. There IS a case–w/photo(!)–
of a webbing bend tested as I suggested with TWO knots (to have a survivor, to mitigate
tension imbalance in a sling/closed-circle-of-test-material) in which the surviving knot
shows the start of some rupture–and, quite oddly, at a different place than where
the broken knot broke! Given your prior report of the rope seemingly breaking internally
and then holding to nearly the same load and breaking completely (I’ll speculate that
there must have been core material to lend support to the mantle for at likely 50% or
less of the bulk, the mantle couldn’t hold such a high force), you might have some
start of rupture in the survivor–not sure if you can feel or otherwise detect it.
to the end termination anchor point[/i] (and not the knot immediately adjacent to the load cell).
Ah, well, just as “bitter end” --despite Spydey’s use otherwise 
– means “at the bitts”,
let’s adopt a shorthand “anchor end” & “load end” here?!
Examination of the specimen after failure indicated that the BLUE tracer thread was missing. I could locate the [i]white [/i]and [i]red [/i]tracer threads, ...
EXCELLENT: for you’ll want to mail that intact piece with the red thread
to Derek so he can work it into his promised essay on dangers of the GeeSpot! 
(Note that “blue” rhymes with “true”.)
... but NOT the blue. This indicated that failure/rupture was occurring at a region on and/or around the blue tracer thread. A further test with a fourth tracer thread will be required to more accurately home in on the region of failure/rupture.The fact that the other #1010 knot remained intact enabled location and comparison of the tracer thread positions.
Good job, but you should have photos of the knots under high (50-70% of THEIR breakpoint) tension
–looking at the positions of things w/o tension won’t pinpoint the spot at crunch time,
for it DOES move, some.
[b]TEST #3:[/b] ... [ ] Peak force recorded at failure: [b]3.76 kN[/b] ...
I notice that each break is weaker than the prior one:
you’ve disproved the old adage Practice makes Perfect --let’s all break for tea! 
[b]EDIT:[/b] I would like to add the comment that this whole testing procedure is [i]very [/i]time consuming and labor intensive. In particular, weaving the tracer threads through the sheath in strategic positions is tedious and testing both my eye sight and patience... [i]I hope its worth the effort Dan![/i]
Hey, look at it as developing another job (or survival skill (i.e., sewing)) in a time of
downturned economy!
But, seriously, has anyone EVER seen any testing done like this, intelligently
answering some rather fundamental questions (which have had bogus answers
given to them and parroted, for AGES–eyes / minds closed shut tight!) ?!
You are narrowing in on a break point by simple means (NOT by a hi-tech super-speed camera,
which in the Katherine Milne (draft) report was deemed not-quite-adequate-to-see… ) : by
needle & thread (& some needling from the cheap seats over the ocean) and a camera (yes?),
AND a 2nd-tested-but-surviving knot. All simple enough for any of the many testers to have
done decades ago, but … no. So, yes: today, 2009-01-… and counting, we are generating
a STEP FORWARD in UNDERSTANDING KNOTS. And all quite repeatable --say, of
a crude, brute-force (“shock load”) break unconcerned about calibrated force measure,
but only point-of-rupture discovery–with thread(s), or a marker. (And you are confirming
K.Milne’s observations, but with greater surety (and different material).)
I say that is quite worthwhile.
–and red-threaded bits for Derek, to boot!
Thanks for your feedback Dan…
I can indeed mail the test specimens to DerekSmith if that’s what you want… can we confirm this as postage costs to overseas destinations from Australia aren’t cheap (if its to go via airmail not snail mail).
I am only going to post via an edit… rather than creating a new post each time.
I’ve completed two more ultimate breaking strength tests on the 5mm cord (see post).
I have purchased another 20m of accessory cord. Have cut a series of 500.0mm lengths in preparation for the joining knot (bends) tests. This will include:
Rosendahl
Double fishermans
I am quite interested in testing Bowlines with 3 rope diameters fed through the nipping loop - we’ll see how they perform in comparison to ABoK #1010…
I’ll keep you updated with results… weather is bad here in ‘sunny’ Nth Queensland at the moment - lighting conditions are poor so no photography at this stage…
agent smith
THAT is a joke (a barb , even). Chisholm posited the Bowline’s break right at the
SPart’s entry point; Derek moved one point further into the knot, at the Crossing Point.
I am only going to post via an edit... rather than creating a new post each time.
I’d rather you NOT do this: perhaps what you can do is post a new, updated/full report;
then go back and edit down prior post to minimal info. But having to go back to a continually
updated post, which in the meantime has been quoted, etc., doesn’t make a good plan.
Hmmm, we can do without repeated information, such as:
Construction: Kernmantel / accessory cord
Material: Nylon
Diameter: 5.0mm
MBS: 5.2kN (1169 LBS)
Certification: EN 564
Batch coding: A050AS0100 Lot #R6-092507KT
Condition: New
Purchase date: 02 Jan 2009
Cord length: 1.0m
2 x identical #1010 Bowlines were tied - to form an eye at each end of the cord
Tails: 100mm on each knot
More helpful to know is the span between the knots and the length of the eyes
==> Anchor +> | <–5cm eye–> | nub@A-end | <–30cm span between–> | nub@L-end | <–5cm eye–> | <+ LOADcell
–something like this schema above, for what does “1m” tell us, as it is divided
into several distinct sections. Also, I think that you can be safely assured by
much use & prior testing that YOUR TAILS CAN BE QUITE SHORT,
saving you several cm per knot (2 per test (how you work this w/bends might
be by making a sling/runner tying two short lengths)).
I am quite interested in testing Bowlines with 3 rope diameters fed through the nipping loop
Agreed!
Also, note how the Bowline (#1010) moves the end out of the line of fire and hence puts
such load at the blue-threaded area (which I suspect shifts SPartwards w/force); if the
end is dressed to anticipate this “draw” by the SPart by being set leftwards under the
other bight leg (ref. your “front” perspective), then it will only come to the presented
position after considerable force, and it–the tail–isn’t under such tension as the other
bight-leg (which is an eye-leg), and so should be compressed and more yielding to
the SPart and maybe provide some cushioning/padding effect which might increase
strength!? Similarly, for the Cowboy Bowline (1034.5), the SPart’s draw should pull
the end down around rightwards behind the bight-side eye-leg, and get cushioning
in this way (one might need to dress the knot so as to ensure that this behavior occurs).
At this point, you have experienced the consequences of rupturing this material:
what happens–how explosive is it, how have your defences fared, et cetera?
I was going to also point out that it wasn’t necessarily the case that you needed to add
more marker threads, if you have photos (flash?), as with a reasonably proximate marker,
you can see that the breaks are coming at about such’n’such distance(s) from it. And I’ll
guess that the broken edges/rupture point/ends are fairly NON-sharp, some fibres
longer, others shorter, suggesting a cut & tear covering a cm or two?!
Are you sewing the thread through the mantle only, or straight through the rope in one
strand, or through, u-turn and back through? Maybe the best marking is just a brief
stitch through the mantle, more likely to stay put, and quicker to make–make a couple
about 90deg apart and cut the connecting thread and move on: more likely some
small set of so-joined sheath fibres will move in the same direction than a larger
bunch. !?
–dl*
ighting conditions are poor so no photography at this stage
Let me remind you of one hoped-for benefit of photographs: even though it
might be impossible for you to test actual rockclimbing rope (because of the
greater strength of that rope compared to the test mechanism), it could be
helpful to have images of the tested rope’s knots at some various loadings
with which to make comparisons & estimates vis-a-vis witness similar
knot geometry in climbing rope. E.g., suppose it is found that the 5mm
low-elongation cord obtains a certain geometry at 50% of tensile, but
that the climbing rope does so much sooner: that might be a good reason
to be less confident that the climbing rope will have similar knot strength.
–dl*
Would anyone care to test what i describe as a “Triple Tuck Bowline” against any other loopknot. To form the knot you simply retuck the tail through the nipping loop an additional two times to form a three-wrap coil around the “crossing part” of the nipping loop structure. The resulting knot will be bulkier, but in my very meager comparative tests with small diameter cordage I have found it to be stronger than even the Rethreaded Figure Eight (aka Figure Eight Follow Through). This particular variation appears to add both greater security and greater strength to the knot.
I am very interested if anyone else would be able to verify my test results and also be able to assess the relative efficiency of this bowline variation.
Have you looked at the pdf document associated with the secured bowlines of issue
with this thread? Several of the newly presented knots entail an additional diameter
of rope through the central loop. --and can serve you for description by reference,
if you’re lacking an image to share: words can work!
Reporting in here with some new test results… my children, holidays and life’s daily duties keep getting in the way of my little project to contribute to world knowledge.
Now that is an honorable thought - contributing to world knowledge!
Commentary:
Testing initially was carried out by forming a loop with 2 x ABoK #1415 (Double Fishermans) knots tied with 2 x 500.0mm lengths of 5.0mm accessory cord. This is the test method Dan Lehman proposed and in principle it is a brilliant idea.
However, I ran into 2 problems with this technique:
- The force required to cause knot failure was much higher - and therefore the risks also increased.
- The results did not directly correlate with the cord MBS of 5.2kN - and therefore it was difficult to calculate a relative % strength for the knot. Details of the initial round of ‘loop tests’ are as follows:
Test date: 12 Jan 2009
Test geometry: 2 x 500mm lengths of cord formed into a loop
Loop size: 200 ± 10mm
Tails: 30 ± 5mm
Application of force: Applied with lever action winch - continuous pumps of lever until failure occurred
Force measuring instrument: Dynafor load cell - set to record peak force achieved with sampling rate of 40hz
ABoK #1415 (Double Fishermans)
Test 1: 7.86kN (loop strength with dual knots)
Test 2: 7.76kN (loop strength with dual knots)
Test 3: 7.74kN (loop strength with dual knots)
Rosendahl (Zeppelin)
Test 1: 6.62kN (loop strength with dual knots)
Test 2: 7.22kN (loop strength with dual knots)
Test 3: 6.48kN (loop strength with dual knots)
As usual, in every test only one of the knots failed, leaving the other knot intact. The intact knot of course is maximally loaded, and its highly compressed form is worth photographing (the position of the cotton tracer threads will be interesting to compare to the surviving knot specimen). I have packaged and labeled all knot specimens.
The recoil force was violent - and the load cell was jarred by the recoil shock. There was no discernible fragmentation of the specimen knot - and I was not injured on any occasion. The test bed that I set up was intended for lower forces - so these ‘loop tests’ strained me and my equipment considerably, but still within working load limits. Note: I would not be able to perform loop tests with 6mm diameter cord as the forces would exceed 10kN…
EDIT: Another important point is that the end termination anchor pins are a crucial factor in performing ultimate break strength tests on the cord and in any test where a free (unkotted) tail end needed to be secured. Anyone trying to reproduce my results would do well to learn from this post. I initially tried 20mm diameter anchor pins with a 5 wrap tensionless hitch and then a 6 wrap hitch. I discovered that this was insufficient. Breaks were occurring at the pins which indicates that stress and strain was occurring at the pins and not isolated at the knot. Obviously, it is important to ensure that tensile force concentrates at the specimen knot, and not at the standing part end terminations.
I purchased larger Bow shackles with 30mm diameter pins. This enabled me to use an 8 wrap tensionless hitch. That did the trick! All the stress and strain was now focussed at the knots and not the anchor pins. The 30mm pin diameter provides a 6:1 ratio with respect to the cord diameter.
…
I therefore ran another series of tests, this time in linear fashion with only one knot.
ABoK #1415 (Double Fishermans)
TEST #1:
Test date: 15 Jan 2009
Cord length: 2 x 1.5m lengths of cord
Test geometry: Linear test with a single knot specimen
The knot specimen was dressed very tight by hand strength only - pulled as tight as possible with due attention to form and symmetry
Tails: 70mm ± 10mm
Tracer threads: 3 different colours inserted after knot was dressed by hand
Standing Part end terminations: 8 wrap tensionless hitches around 30mm anchor pins
Application of force: Applied with lever action winch - continuous pumps of lever until failure occurred
Force measuring instrument: Dynafor load cell - set to record peak force achieved with sampling rate of 40hz
Peak force recorded at failure: 4.16 kN
TEST #2:
Test date: 15 Jan 2009
Cord length: 2 x 1.5m lengths of cord
Test geometry: Linear test with a single knot specimen
The knot specimen was dressed very tight by hand strength only - pulled as tight as possible with due attention to form and symmetry
Tails: 70mm ± 10mm
Tracer threads: 3 different colours inserted after knot was dressed by hand
Standing Part end terminations: 8 wrap tensionless hitches around 30mm anchor pins
Application of force: Applied with lever action winch - continuous pumps of lever until failure occurred
Force measuring instrument: Dynafor load cell - set to record peak force achieved with sampling rate of 40hz
Peak force recorded at failure: 4.58 kN
TEST #3:
Test date: 15 Jan 2009
Cord length: 2 x 1.5m lengths of cord
Test geometry: Linear test with a single knot specimen
The knot specimen was dressed very tight by hand strength only - pulled as tight as possible with due attention to form and symmetry
Tails: 70mm ± 10mm
Tracer threads: 3 different colours inserted after knot was dressed by hand
Standing Part end terminations: 8 wrap tensionless hitches around 30mm anchor pins
Application of force: Applied with lever action winch - continuous pumps of lever until failure occurred
Force measuring instrument: Dynafor load cell - set to record peak force achieved with sampling rate of 40hz
Peak force recorded at failure: 4.20 kN
TEST #4: - special test to determine effects of dressing (knot loosely tied)
EDIT: The difference in this knot compared to the other 3 tests is the degree of hand force used to set and dress the specimen knot. With the other 3 knots, considerable effort was applied to cinch the knots tight (as tight as I could pull by hand). In the present case, I did not use such effort - the knot was simply tied/dressed without applying muscular force. Whereas the former knots were tightened by hand until I could no longer pull any harder. Note: Perhaps I should test my hand strength using the load cell - and then I could report that I cinched the knot tight by applying 0.5kN force…etc!
Test date: 15 Jan 2009
Cord length: 2 x 1.5m lengths of cord
Test geometry: Linear test with a single knot specimen
The knot specimen was dressed loosely, with minimal cinching by hand. However, due attention was still paid to form and symmetry
Tails: 70mm ± 10mm
Tracer threads: 3 different colours inserted after knot was dressed by hand
Standing Part end terminations: 8 wrap tensionless hitches around 30mm anchor pins
Application of force: Applied with lever action winch - continuous pumps of lever until failure occurred
Force measuring instrument: Dynafor load cell - set to record peak force achieved with sampling rate of 40hz
Peak force recorded at failure: 3.52 kN
…
ABoK #1047 (Figure 8 loop / eye knot)
TEST #1:
Test date: 15 Jan 2009
Cord length: 1 x 1.0m length of cord was used
Test geometry: 2 Fig 8 loop knots were tied in order to form a connective eye at each end. Eye to eye length = 315 ± 5mm; Eye size = 60± 5mm; Tails = 60± 5mm
The knot specimen was dressed very tight by hand strength only - pulled as tight as possible with due attention to form and symmetry
Tracer threads: 3 different colours inserted after knot was dressed by hand
Anchor pins: 10mm diameter (to simulate a carabiner)
Force measuring instrument: Dynafor load cell - set to record peak force achieved with sampling rate of 40hz
Peak force recorded at failure: 4.26 kN
TEST #2:
Test date: 15 Jan 2009
Cord length: 1 x 1.0m length of cord was used
Test geometry: 2 Fig 8 loop knots were tied in order to form a connective eye at each end. Eye to eye length = 315 ± 5mm; Eye size = 60± 5mm; Tails = 60± 5mm
The knot specimen was dressed very tight by hand strength only - pulled as tight as possible with due attention to form and symmetry
Tracer threads: 3 different colours inserted after knot was dressed by hand
Anchor pins: 10mm diameter (to simulate a carabiner)
Force measuring instrument: Dynafor load cell - set to record peak force achieved with sampling rate of 40hz
Peak force recorded at failure: 4.00 kN
TEST #3:
Test date: 15 Jan 2009
Cord length: 1 x 1.0m length of cord was used
Test geometry: 2 Fig 8 loop knots were tied in order to form a connective eye at each end. Eye to eye length = 315 ± 5mm; Eye size = 60± 5mm; Tails = 60± 5mm
The knot specimen was dressed very tight by hand strength only - pulled as tight as possible with due attention to form and symmetry
Tracer threads: 3 different colours inserted after knot was dressed by hand
Anchor pins: 10mm diameter (to simulate a carabiner)
Force measuring instrument: Dynafor load cell - set to record peak force achieved with sampling rate of 40hz
Peak force recorded at failure: 4.32 kN
…
End Bound Single Bowline - EBSB with yosemite finish (3 rope diameters placed inside nipping loop)
TEST #1:
Test date: 15 Jan 2009
Cord length: 1 x 1.0m length of cord was used
Test geometry: 2 EBSB Bowlines were tied in order to form a connective eye at each end. Eye to eye length = 300 ± 5mm; Eye size = 60± 5mm; Tails = 60± 5mm
The knot specimen was dressed very tight by hand strength only - pulled as tight as possible with due attention to form and symmetry
Tracer threads: 3 different colours inserted after knot was dressed by hand
Anchor pins: 10mm diameter (to simulate a carabiner)
Force measuring instrument: Dynafor load cell - set to record peak force achieved with sampling rate of 40hz
Peak force recorded at failure: 3.84 kN
TEST #2:
Test date: 15 Jan 2009
Cord length: 1 x 1.0m length of cord was used
Test geometry: 2 EBSB Bowlines were tied in order to form a connective eye at each end. Eye to eye length = 300 ± 5mm; Eye size = 60± 5mm; Tails = 60± 5mm
The knot specimen was dressed very tight by hand strength only - pulled as tight as possible with due attention to form and symmetry
No tracer threads
Anchor pins: 10mm diameter (to simulate a carabiner)
Force measuring instrument: Dynafor load cell - set to record peak force achieved with sampling rate of 40hz
Peak force recorded at failure: 4.02 kN
TEST #3:
Test date: 15 Jan 2009
Cord length: 1 x 1.0m length of cord was used
Test geometry: 2 EBSB Bowlines were tied in order to form a connective eye at each end. Eye to eye length = 300 ± 5mm; Eye size = 60± 5mm; Tails = 60± 5mm
The knot specimen was dressed very tight by hand strength only - pulled as tight as possible with due attention to form and symmetry
No tracer threads
Anchor pins: 10mm diameter (to simulate a carabiner)
Force measuring instrument: Dynafor load cell - set to record peak force achieved with sampling rate of 40hz
Peak force recorded at failure: 3.96 kN
…
I also performed one more test to determine the ultimate strength of the Sterling 5.0mm accessory cord. I used the 30mm diameter anchor pins with 8 wrap tensionless hitches.
Test result:
Peak force recorded at failure: 5.48 kN
Previously, I was recording lower break values using 20mm anchor pins. This proves that larger diameter anchor pins are a crucial factor in determining MBS for various cords.
MBS is taken to mean “Minimum Breaking Strength”.
…
A side note: This is a very labor intensive and tedious process. It is also self-funded (no, I am not asking for donations). Is it worth the effort? Yes, because it is contributing to our collective knowledge of knots…
agent smith
Ah, we’d been so eager for more results … !
I appreciate your concern for the closed-loop-w/2-knots doubling of force.
Conceivably, you could reduce pull force of hauler by using a pulley (and
so the hauler has 2:1 advantage, roughly; but 1:2 speed).
However, single-line specimens can have TWO knots, just as those for
the eyeknots do. (I think that one can raise some technical concerns about
the distance between knots, but that’s likely getting a tad picky for our purposes
–like pixel-peeping at camera results.) And thus you’ll have both two-knots’
results per loading, and a survivor.
One might speculate that part of the discrepancy between the trio of results
got for the two-in-a-loop Grapevines vs. the single-line ones could arise
from some imbalance of tension, which would see the even division of
force an unachieved ideal, with actually the broken side presumably
having borne more of the load, a little. I can see thus that the average
approx. 3.9kN then being 4.0 & 3.8 !? --but unlikely more. (If the knot’s
compress at different rates, one side of the loop can get relief from this
that the other lacks!?) Otherwise I’ll wonder if the rates of loading, for
whatever reason(s), differed; for the knots should be showing equal
strengths, loop’d or single-strand’d.
I have packaged and labeled all knot specimens.
Excellent. But you’re not mentioning PHOTOGRAPHY, and that worries me.
NB: even if you take a few samplings of photos, you do a LOT more than if
none is taken. And we can advance to believing your observations that
“all of the knots looked generally just like this (even though I didn’t photograph
them all–why should I, if they’re all the same?).” Of course, in the case of surviving,
highly loaded knots, you can still get a photo, well past test time.
Another important point is that the end termination anchor pins are a crucial factor in performing knot break tests. Anyone trying to reproduce my results would do will to learn from this post. I initially tried 20mm diameter anchor pins with a 5 wrap tensionless hitch and then a 6 wrap hitch. I discovered that this was [i]insufficient[/i]. Breaks were occurring at the pins which indicates that stress and strain was occurring at the pins and not isolated at the knot.
I don’t follow you here: was there any case where the break occurred at the pin
instead of in the knot? (Not for the 2-eyeknots-per-specimen, and not for the
Grapevine & Rosendahl tests, right.)
Note that I recommended that you use a FEWER-wraps Tensionless hitch,
not more!? More wraps only reduces the force transmitted to the ultimate
terminus; it in all likelihood (IMO) reduces the strength of the hitch (unless
the break occurs at the terminus).
This enabled me to use an 8 wrap tensionless hitch. That did the trick!
Again, I don’t follow this. A larger dia. pin, yes, should yield higher breaks;
but NOT the superfluous wraps, if the terminus isn’t tied back to this line.
…
this time in linear fashion with only one knot.
Although two knots in “linear fashion” should be possible, as noted above.
[u]TEST #4:[/u] - special test to determine effects of dressing (knot loosely tied)
Esp. here, we can only really appreciate this if we have photos of the actual
geometry. “loosely dressed” is highly unspecific. (And one of Tom Moyer’s
like cases for the Grapevine got I think one of his higher results.)
[u][b]ABoK #1047 (Figure 8 loop / eye knot)[/b][/u]
And, speaking of “unspecific”, this is a particular sore point case: there is the
dressing, and then there is the choice of which end to load. We almost never
get these things specified.
[u][b] End Bound Single Bowline - EBSB with yosemite finish (3 rope diameters placed inside nipping loop) [/b][/u] [ ] Peak force recorded at failure: [b]3.84 kN[/b] [ ] Peak force recorded at failure: [b]4.02 kN[/b] [ ] Peak force recorded at failure: [b]3.96 kN[/b]
And so, with the first of the 3-diameters-in-loop bowlines, we see a gain of about 0.2kN.
Well, that’s something. (Actual geometry at break time unknown.)
[ ] Anchor pins: 10mm diameter (to simulate a carabiner)
I very much fail to see the point of this? It was raised above and I thought
adequately rebuked: what is it you expect of this?
MBS is taken to mean "Minimum Breaking Strength".
That much I guessed; but what exactly IS “minimum breaking strength”
–how is that figured? (I have seen one figuring of it as being some
number of standard deviations below the mean (as opposed to, say,
the lowest break value obtained).) [I suppose I can ask Sterling directly.]
Also, what is the construction of the 5mm cord? The mantle is I guess
something like 24 single-strands over/under-2; but what is the kern?
–laid, or braid?
Great to have another set of numbers; l00king forward to fotos,I hope hope hope!
Sorry Dan,
Made a typo with respect to the anchor pins. I have edited my post above…
What I meant to state is that in any test which requires an unknotted standing part to be anchored, a tensionless hitch is essential and so is the correct anchor pin diameter. For example, when trying to measure the MBS of the unknotted cord, I found that 30mm diameter anchor pins were required. I reached 5.48kN in one test using 30mm diameter anchor pins with an 8 wrap tensionless hitch. The tail ends of both tensionless hitches were terminated with ABoK #515 fed through a hole in the pin of an adjoining shackle.
I can confirm that after the test was completed, the #515 stopper knots were loose and easily untied. When I used 20mm diameter pins with a 6 wrap tensionless hitch, the #515 stopper knots were cinched tight and could not be untied by hand (needed a pointy nose pliers). In my view, this is a indication that the relative distribution of force was not concentrated around the pin, but rather at the linear portion of the cord (which is what I wanted to test).
Does this make sense?
When I tested knots in tandem so that each end termination was in fact the eye of a knot, I used 10mm diameter anchor pins to simulate a carabiner. Obviously, with eye knots, the end termination is not an issue - particularly since a knot NEVER failed at the eye.
…
As for photos, I still do not have the proper lighting conditions…I want to take quality photos. Dont worry, all the knot specimens are packaged & preserved in their original condition after failure… I recognise the importance of photography!
With respect to the higher forces to break a loop, it doesn’t matter what the geometry is (ie a 2:1 pulley) - keep in mind that the actual force at the specimen knot must reach a certain value to achieve failure…therefore you still get a violent recoil at the instant of failure.
agent smith
It is expected, but does NOT make sense in terms of the objective of getting
the break strength of the rope. A larger pin, YES; more wraps, NO. Again,
all the added wraps do is consume material and ease the tension into the
terminus–but that isn’t the point of the exercise. Maybe it doesn’t much matter
for strength determination: in the few-wraps case, material moves around
the pin and builds tension at the terminal side and tightens the knot;
in the many-wraps case, material moves around the pin until it just
gets exhausted by continued friction, leaving the terminal side slack.
My hunch, though, was that the many-wraps case will give excessive
material movement, which if nothing else, will delay the rise of tension,
putting material at-tension (whatever) longer, and that tends to lead
to lesser break figures (with some balance between the allowing of
fibres to settle into an evenness of sharing load–though I think one
might need cyclic preloading at say 50% to achieve this–, and the
added weakening agent of duration). --and that few-wraps deprives
the specimen of this material contribution into the main loaded part,
and reaches higher forces sooner. In either case, it’s expected that
the break won’t come on the terminal side.
As for photos, I still do not have the proper lighting conditions...I want to take quality photos. Dont worry, all the knot specimens are packaged & preserved in their original condition after failure... I recognise the importance of photography!
This sounds like perfection being the enemy of good: SOMEthing is better than nothing;
I can’t imagine that any modern camera–from $80, 3mb P&S, to <sky’s the limit>–won’t
produce d e c e n t photos for capturing the knot geometry during test, and the
positions of the marker threads (and the photo of the broken knot showing such
markers). We’re not insisting on Medium-Format Hasselblads or a Nikon D3 w/14-24mm lens!
Post-mortem specimens have lost the transitional state (and also much of
the high-tension geometry, since tension is gone, SPart/eye-legs relaxed).
With respect to the higher forces to break a loop, it doesn't matter what the geometry is (ie a 2:1 pulley)
It matters to the force on the hauler and partner anchor, though resistance on the
opposite anchor will be doubled; recoil should be half-load to each of them,
but that pulley sure wants to be constrained!!
What steps are you aking to redress recoil?
I could see tying some anchor line, perhaps w/shock cord in its system, and that
to a 5# bag of <sand?>, to the midpoint of line between knots (or having actually
a pair of lines tied at this point, each angled to resist recoil in one direction).
Good job, this is enlightening!
(-;
Might I be allowed to indulge in a little ‘dot counting’ or ‘pixel-peeping’ as Dan calls it.
-
MBF, in my book it stands for Mean Breaking Force, not Max. There would be no value in comparing the strength of knots against the single highest figure recorded for the breaking of the cord, when the majority of the knots would likely never have been tied in that one strongest piece of cord.
-
Expecting two knots to break simultaneously is like expecting a needle to balance on its point. Both are possible, but the probability of all forces and factors being so perfectly balanced is just a little bit improbable. You should expect one to break but if both broke, then you should look carefully for a fault in your set-up to explain why the the near impossible just happened.
-
If you test two knots against one another the weaker one will always fail first. If the two knots are essentially the same (in statistical terms you would say they were from the same population), then by testing one against another, you are filtering out the weakest knots and almost never testing the strongest. This will skew your results into the lowest quadrants of results. If you do enough tests, eventually you will test a strong knot against another strong knot and get what seems to be a spuriously high result, while all the tests involving weak knots will be recorded in your data. You can compensate for this filtering effect, but it is far better not to inject it into the results in the first place.
You might argue that it is important to measure only how weak a knot is, because only the weak knots will fail. That is a logical argument but it is weak science. If a knot has a broad slew of possible strengths, then we should determine this and give ourselves the opportunity of also discovering just what it is that both imparts and takes away this strength.
-
Photography is critical just before failure in the loaded situation, because an unloaded knot cannot be expected to display the same geometry as it would under load. Images also have to be captured from different perspectives because one shot cannot capture the 3D nature of the directions, angles and vectors involved in a knot. This is especially important when you consider the time effort and cost going into creating a single breaking strain figure and the fact that photography today is ridiculously inexpensive per picture (ca 10p per gigabyte), takes but a few seconds to record, and offers a means of sharing the information of the event via the internet.
-
The suggestion that anchor point diameter influences MBL and knot strength. The ‘Weak Point’ of a cord depends on two factors a) the physical strength of the cord at any given point and b) the focus or magnitude of force at that point. A physically strong point can become a weak point if there is a concentration of load at that same point.
In testing a straight piece of cord, the weakest point can be anywhere along it. The chance that the physical weak point being at the anchor point goes down proportional to the length of the test cord (increasing the MBL) and the longer the cord the greater the chance it will contain a a rare weak point (decreasing the MBL).
Increasing the anchor point diameter will reduce the load concentration so it will lessen the effect of stress concentration upon happenstance location of a physical weak spot at the anchor ‘bend’. However, this should not translate into the cord in such a way as to alter knot characteristics unless core / sheath characteristics have a propagation ability and this reaches a closely tied knot.
Happy knotting
Derek
Clean your glasses before peeping at anything: nOne has said “max”, but, rather,
“MIN”; and the question is What does “Minimum Breaking Strength” mean
–it might be defined statistically re standard deviations, and not as a lowest datum.
2. Expecting two knots to break simultaneously is ...
…, again, not part of any discussion here: so, what’s the point of discussion?
(What we expect is that two knots enduring tension X are better than having
one do so, re data/stat.s, and also yield the benefit of a survivor.)
3. If you test two knots against one another the weaker one will always fail first.
–a tautology.
4. Photography is critical just before failure in the loaded situation, because an unloaded knot cannot be expected to display the same geometry as it would under load. Images also have to be captured from different perspectives because one shot cannot capture the 3D nature of the directions, angles and vectors involved in a knot.
I’m thinking of a mirror involved, like the extra knot in a test specimen: two things
for the effort for one!
–dl*
Thanks for the kind words Dan, but I think it must be the clot behind the glasses that is at fault here - either I am going to have to lay off that 30 yo Glenfarclas or there has been a sneaky Edit ? ?
Anyway, we probably need to bottom this one out before moving on to any strength calculations.
In terms of defining a ropes usable strength, manufacturers declare the rating that fewer than one rope in a thousand will fail to reach. It is called the 3-Sigma Minimum Breaking Strength, and you are right that it is not the lowest datum, it is in fact much lower than the lowest recorded value. The manufacturer measures a number of break strengths, then calculates the average (Mean) and the Standard Deviation (Sigma). They then step down from the Mean by three Standard Deviations which (for a ‘Normal Distribution’) gives the once in a thousand safety limit. (see http://www.sterlingrope.com/faq.asp )
However, for the purpose of declaring knot strength, this figure is nearly as meaningless as a Max BS because you are comparing the knot breakage with rope you are almost never going to come into contact with.
For the purpose of assessing the effect of the knot on the rope, we should compare like with like, i.e. the strength of the rope unknotted with the strength of the rope knotted, and for this comparison when there are not many test values, the mean is the most meaningful comparison (calculating Sigma on two or three values is as silly as reading max when the text says minimum). We should be careful not to mix up rope safety strength claims with what is being performed here - a comparative test.
Derek
AKA ‘Dot Counter’
“so, what’s the point of discussion?” The point is that if AS is intending to publish his results he should not expose himself to criticism that he failed High School Stats. Even the claim that the method generates a valuable surviving sample is questionable because the shockwave caused by failure might have altered the structure in some way impossible to predict.
Luckily the problem is very easy to demonstrate.
Assume we have a length of reasonably uniform cord, we divide it up and make ten test samples. If we test each one to destruction we might for example get the following set of results (modelled roughly on a Normal distribution) –
47, 53, 44, 56, 59, 47, 41, 51, 49, 53.
The average strength of these samples is 50.
Now let’s do a paired test, you can do it easily for yourselves. Write the values on scraps of paper and put them into a hat, pull out two at random and write down the lowest of the two, then throw them both away (tested). Repeat this four more times till you have the five double knot test results.
Here are a set I drew out.
53 vs 47 result 47
44 vs 53 result 44
47 vs 59 result 47
51 vs 56 result 51
49 vs 41 result 41
Average 46
@A Fluke of the draw’ you cry - OK do it again several times and you will see that you are getting a result which is focussed towards the average of the low values, or roughly 10% lower than the true average for that knot in that cord. The reason is that the method filters out the high values and discards them, biasing the result away from the real value.
If you counter argue that you want to declare the statistically weakest strength, then again you have to measure all ten individually to be able to determine the mean and a value for Sigma, then take the 3-Sigma value, but agian you must not skew the data before calculating the mean and sigma.
Derek
Just some comments here…
Photography: I have preserved all the knot specimens - the surviving knots (from the dual knot test series) have been cinched very tight - and their shape and structure has not altered with the passage of a few weeks. There are a few very good reasons why i haven’t taken photos; i) I ran out of time - I am currently at work and away from home; ii) I want to take photos in the right conditions (using combination of flash + natural sunlight) but North Queensland (Australia) has had a deluge of rain/storms the past month. There are a few perfectionists on this forum and I for one dont want to send out sub-standard images of my work!
I’m on to it guys, just be patient a bit longer - I will be back home in 3 days and it will be the weekend…
The lever action winch: I purchased a $AUD 100.00 winch and it consists of a 2:1 pulley configuration. I always mount the load cell at the knot specimen side - so it measures what is going on right there at the knot. I cant be specific about the speed of increasing load - I simply crank on the lever and keep cranking away. I cant for example state a speed of say 300mm per minute or whatever. So there is always going to be some degree of lack of measurable quantities in my tests… The 2:1 configuration of the winch doesn’t alter the force bearing down on the specimen knots…and there is still cable length run out from anchor point to anchor point…whichever way you look at it, there is force in the system and it has to be treated with respect.
Recoil at instant of knot failure: Linear type knot tests with 5.0mm diameter accessory cord are easy to manage - the anti-recoil system I use is simple. I have an old punch bag (boxing) which I lay down on the driveway to which is attached an old (retired) dynamic climbing rope. The punch bag acts as a ‘dead weight’ - and absorbs the shock. I also lay down foam padding underneath the load cell - because it jumps a bit - I don’t like it jarring on a concrete surface. The recoil force in the loop style tests a little more scary due to the force hovering around 7-8kN. I use the same system, only with more caution thrown in. Hearing protection is not required. I don’t bother to wear eye protection in the linear tests, but I do wear them in the loop tests.
Dual knot testing: I still support the concept of testing dual knots - because there is always a surviving specimen. With tracer threads woven in both knots, it makes it easy to draw comparisons and determine the region of knot failure. Its just that I HATE inserting the cotton threads. I now know that my calling in life wasn’t sewing! I wish that I could mail all my knots to Dan Lehman (by the bag full) and let him do the weaving (and then mail them all back to me)…
i find it hard to understand the test results of loop style tests - eg 2 x ABoK #1415 Double Fishermans joined from 2 x 500mm lengths of cord. Sterling report an MBL of 5.2 for their 5.0mm diameter cord…but the test results are up around 7-8kN… I would have to get a harness manufacturer in Australia to form a loop out of the cord by using a stitched join. Then I’d have to break the darn loop (3 times) to get some figures…and then compare it to the looped knot tests… too expensive and too time consuming.
Knot security/stability Vs raw strength - Although its interesting to go through this exercise, at the end of the day I think security and stability are more important factors with life support knots. I don’t think any knot is ever going to break in the field…
MBS: Minimum Breaking Strength.. have not researched into how Sterling derive their published figures (ie 5.2kN for 5.0mm diameter cord). I dont know about the technical detail of the composition & weave pattern of the sheath either. Perhaps Dan/Derek could write to Sterling and aks for more detail???
Anchor pins / tensionless hitch: Can we ask how Sterling anchor the standing Parts of their cordage for linear pull tests? I used 30mm diameter anchor pins with the 5.0mm accessory cord, and I found this worked for me…I got an immediate increase to 5.48kN using an 8 wrap tensionless hitch around a 30mm diameter pin… again, what do Sterling use?
agent smith
Ah, well, implying that we expected simultaneous/equal breaks was an odd
way of stating this concern. But I don’t give it much value, anyway.
Luckily the problem is very easy to demonstrate. ... the following set of results (modelled roughly on a Normal distribution)
Holy haphazard hypothesis, Batman!!
You are spewing results that differ in extremes of the lowest being 75% of the highest!!
That is quite some diversity, which makes your lament seem worthwhile.
But even so, the conclusions you want to draw are simply not those that should
be drawn. Knowing that the surviving knot is stronger, one can (as I suggested
above) toss in some bit of statistical hocus-pocus to adjust for that; it is of some
comfort to realize that one’s data is in this sense a bit of “worst-case” values.
Given a more reasonable set of values, there isn’t all that much difference to
have to worry about, anyway. (Perhaps one could take Dave Richards’s
results from kernmantle-rope breaking and figure a reasonably likely maximum
difference–it will be well shy of 25%!!–, and then use that to guesstimate some
2- or 3-stnd-dev. below Mean value (by bumping Mean; or, by going just 2 vs. 3
stnd. dev. below.)
What you miss is that this project requires EFFORT of breaking, COST or MATERIALS:
PER UNIT TEST SPECIMEN WE FIND A VALUE THAT TWO KNOTS ENDURE
rather than just one.
Now, taking my logic to extreme, I guess we should urge a stringing together
of some 10-20-50-100 knots in a row, and … . That goes beyond practical,
but might give a more believable Min value than statistical figuring from merely
5 knots.
you must not skew the data before calculating the mean and sigma.
Sad to think that adding information degrades the result; I don’t.
And I also don’t lose sight of the purpose of this plan,
which is to:
compare various knots (this consistent method does so);
learn about knot behavior (statistics aren’t invited here);
and gain confidence in some seldom-tested, “new” knots.
Considering that other results to which these arguably improperly figured
values might be compared come without the basic information to assess
the tested items, I don’t see any loss of value here.
As for the value of the surviving knot, photos can show whether there
is any deformation of structure from some shock wave–I doubt it.
–dl*
A moderately good photo is infinitely better than none. I reiterate prior urgings.
As for knot change, while I don’t think so much of the post-rupture geometry will
be lost, I do not believe that in-tension geometry survives rupture–especially for
bowlines, whose collar won’t hold the full tension of the SPart.
I have an old punch bag (boxing) ...
This is good to know: I will henceforth let Derek argue w/you about results. ;D
to which is attached an old (retired) dynamic climbing rope. The punch bag acts as a 'dead weight' - and absorbs the shock.
I don’t follow. There’s (lots of!) mass, and a climbing rope: but what’s tied to what?
I HATE inserting the cotton threads. I now know that my calling in life wasn't sewing! I wish that I could mail all my knots to Dan Lehman (by the bag full) and [i]let him[/i] do the weaving (and then mail them all back to me)...
Well, maybe it’s the insertion at in-tension time rather than prior, making an
estimate of position (and a photo to document where they happen to end
up). Again, I’m curious as to what sort of sewing you do. At first, I thought
one would just pull a thread through once or twice; but then it occurred to
me that it might be better to just try to make a nipping loop of thread around
a few of the mantle fibres (thinking it less likely then that the thread would
be pulled in two directions–odds being that those few fibres would all go
in the same direction).
i find it hard to understand the test results of loop style tests - eg 2 x ABoK #1415 Double Fishermans joined from 2 x 500mm lengths of cord. Sterling report an MBL of 5.2 for their 5.0mm diameter cord...but the test results are up around 7-8kN... I would have to get a harness manufacturer in Australia to form a loop out of the cord by using a stitched join. Then I'd have to break the darn loop (3 times) to get some figures...and then compare it to the looped knot tests... too expensive and too time consuming.
??? Um, the knots broke under a load of the loop of 8kn, which implies a load
on the knot (i.e., on each side of the loop) of 4kN. And we can wonder at the difference
between this derived value and that of the individual tests (which were higher). It’s not
huge, and I gave some reasoning that might explain. --no reason to create a loop of
original material: that makes no sense, unless your break is at the pin.
MBS: Minimum Breaking Strength.. have not researched into how Sterling derive their published figures (ie 5.2kN for 5.0mm diameter cord). I dont know about the technical detail of the composition & weave pattern of the sheath either. Perhaps Dan/Derek could write to Sterling and aks for more detail???
Re “MBS”, yes, we (I) can. Re weave: you have busted (and cut) bits right in
your hands! Can’t you take a peek at this–esp. in the ruptures, the kern should
be plainly visible. --and a collection of twisted (or braided?) strands, or maybe
a braided unit (sort of a rope within a rope (which would break my definition
of “kernmantle”, but that’s just me)).
Anchor pins / tensionless hitch: Can we ask how Sterling anchor the standing Parts of their cordage for linear pull tests? I used 30mm diameter anchor pins with the 5.0mm accessory cord, and I found this worked for me...I got an immediate increase to 5.48kN using an 8 wrap tensionless hitch around a 30mm diameter pin... again, what do Sterling use?
Okay. Again, I question the value, to strength, of extra wraps;
they have value in absorbing force transmitted to the end,
but that isn’t a great issue here. (I suspect Sterling use an
even larger bollard–maybe 6" diameter?)
Cheers,
(-;
Dan, even in jest I am not here to ‘argue’ with anyone. It is Agent Smith’s project and he has posted here to attract input and comment from other minds. From past contact with him I have found that he demands a very high standard of himself and for his publications, and to achieve this he willingly subjects his work to open review, then makes his own balanced judgement as to what his final publications will contain.
I have also found your good self to be a perfectionist, so I am more than a little surprised to see you advocating “Guestimation” and “hocus pocus” rather than the simple application of a little bit of rigour to eliminate an effect documented in studies which you have referenced before on this forum.
Just because a piece of work is not being carried out in an accredited laboratory does not mean that it has to be sloppy or that it cannot present the very highest standard of professionalism. Often in the Amateur world, cutting edge work has been produced well in advance of any standard industry could afford to achieve.
This trial uses a concrete drive as its testbed and a hand winch as it force source, but what does that matter so long as these factors can be shown not to distort the results? In my test rig, I use a 50 KN hydraulic car jack as the force source, but it does not detract from the quality of work that is possible, provided knowledge, ingenuity and peer input are fully utilised.
Your comment about stacking multiple knots to find the weakest in a single test is completely valid, but it does not tell us how weak it was against the other knots and because it was destroyed in testing, we cannot infer anything from the structure of the remaining knots, because they were all stronger than it was. Their structure may well have been different in some significant aspect, such as to bestow the additional strength on them. We just finish up with an unknown situation which weakens a knot to a known low value.
You also state that “hocus pocus” can be utilised to take the biased ‘pair results’ back to full population results - fine, how about pointing us then to how this should be done or to someone who can do it as I would not be able to advise Agent Smith how to do it.
How many times have we seen reports from professional labs or Universities where the test set up and statistical analysis has been exemplary but all detail of knot structure, dressing etc has been ignored - virtually every time. So when the job is approached from the other direction - from the knot tyers perspective, should we fall into the same trap of ignoring aspects which are not particularly interesting to us or not within our sphere of expertise, resorting to a little bit of “Guestimation” and “hocus pocus”? I would suggest not, particularly when the solution is so easy to achieve.
How large is the effect? I have no idea. The hypothetical data set I used in the example was simply to demonstrate the existence and mechanism of the effect for you. We cannot know the significance of this effect unless we have access to unbiased data and we cannot begin to postulate reasons for strength variations unless we have access to images of the knots from several perspectives under near terminal load. Whether Agent Smith decides if any of this is relevant to his final report, is up to him, and I have no intention of arguing with him or yourself over the issue. The facts are more than capable of arguing for themselves.
Derek
There is a simple matter of making X number of loadings of X test specimens:
I want to increase the information got from that, and believe that including two
knots per does that. By your insistence on that pristine-for-statistics 1-per-1
work you have X broken knots & data; by the current test plan we have that
AND add X more load-enduring knots, which hold some form that can be
analyzed. Statistics, as you point out, can point out that the latter sample
of 2X knots got data from the weaker of each pairing; well, then, we can
be happy that our results are more solid in finding minimum values (as
one tends to have a bigger complaint for things weaker than expected).
I’ll surmise–w/o knowledge–that there is some statistical analysis that
can accommodate the unknown strengths of the surviving knots; and
that especially in a population where differences (stnDev) are small,
ultimate values are not going to change much: and, again, one could
use the X-values-only and proceed with a confidence of conservatism.
As this method can be applied I think for all knots (bends–2 in line–,
hitches), they all can be compared.
And you would prefer to have half the number of knots tested?
(And, yes, yet, I have not urged FOUR knots per test.)
AND, again, we are not seeking to make huge inferences or promotions
with these values vis-a-vis other tested values, about which a great
deal is unknown (in some cases, e.g., I wonder at transposition of
the digits!). And my Perfectionism relaxes there in realization that
such fine-tuned scrutinizing just cannot have much sway in anything
that matters (for one, conditions of material and loading in practical
cases will vary greatly).
I do urge the jettison of wanting photographer-studio conditions and
losing all capture of images in its absence–much is lost to us, now.
very highest standard of professionalism.I've never thought that this was an issue, so I don't understand your words on this point.
Their structure may well have been different in some significant aspect, such as to bestow the additional strength on them.
But this has nothing to do with examination of a survivor, but everything to do
with photographing knots during testing!
You also state that "hocus pocus" can be utilised to take the biased 'pair results' back to full population results - fine, how about pointing us then to how this should be done or to someone who can do it as I would not be able to advise Agent Smith how to do it.
I’ll see if some statistically adept person has a definite answer,
beyond my remarks above about at least simply noting that the
data comes from a selectively weaker set than the survivor set.
"Guestimation" and "hocus pocus"? I would suggest not, particularly when the solution is so easy to achieve.
You’re making more of the words than is intended–I’m happy enough to use
them for statistics in general, after all. The “solution” of losing results doesn’t
appeal; I will hope that statistics shows some way(s) to deal.
The hypothetical data set I used in the example was simply to demonstrate the existence and mechanism of the effect for you. We cannot know the significance of this effect unless we have access to unbiased data ...
Oh, but you can. You have unbiased weakest-knot data–i.e., the range of values
of randomly selected pairings. You also have Richards’s results in some similar
cordage (i.p., 7mm accessory nylon kernmantle).
here: www.caves.org/section/vertical/nh/50/knotrope.html
The largest stnDev in the 7mm (and about largest throughout, ignoring those
knots that required back-up knots to arrest slippage) is 3.2%. The smallest
value is 93% of the largest; in your set, it was 75%. Absent the exagerated
difference, there really is little to fuss about, other than the purity of the statistics
for statistics sake. Again, I’m happy to put a footnote “* NB: these values are
probably slightly understated.” (or simply “understated”). We’re not hiding
anything, anyway.
and we cannot begin to postulate reasons for strength variations ...
Oh?! I distinctly recall someone’s red-painting a “GeeSpot” and vowing to get
water from rock on that! --even lacking some obvious aspects of the issue.
:
unless we have access to images of the knots from several perspectives under near terminal load.
It might well turn out that for some knots the strength-determining geometry
is set well shy of near-terminal load; but it will take some scrutiny of knots
preferably with sharable photos in order to understand this.