# Break testing 3 knots



## moray (Jul 21, 2009)

By now nearly everyone knows that knots weaken a rope, often severely. In those ropes that can be spliced, a splice tends to be superior to any knot, preserving 90% to 100 % of the native rope strength. A completely undistorted rope is strongest; any distortion or bend in the rope will weaken it. Knots involve very sharp bends (which get worse under load), so it is no surprise they weaken the rope. Splices, in contrast, have only very gradual bends. There is a significant disturbance to a spliced rope where the buried core penetrates the cover, but the resulting weakness is mostly or completely nullified by the fact that the disturbed part of the rope is only carrying half the load; the undisturbed buried leg of the splice is almost perfectly straight and nearly full strength.

Now that I have a rig for break-testing ropes and splices, I have not rushed to test a bunch of different knots because knots are much more complicated than splices. Whereas I can design meaningful experiments with splices, it would be much harder to do that with knots as there are so many more variables. You don't have to tie, dress, and set a splice. Also, splices keep their shape, whereas knots slip and contract and significantly change shape as the load is increased.

Nevertheless, having just received a new batch of rope for testing (5/16 in. Tenex Tec), I decided to test 3 knots while also determining a baseline for native rope strength.

For the baseline tests I made two slings: one with two normal eyes, and one with one normal eye and a second eye protected with a locked Brummel.

Three knotted slings were tested. The first had two normal eyes and an overhand knot in the middle. The next sling had one normal eye and a figure 8 on a bight aka rethreaded figure 8 aka figure 8 follow through. This is a big favorite with rock climbers for their harness attachment.
The last sling had one normal eye and a directional figure 8. This has been a favorite of mine for anchoring my SRT line to the base of the tree. It is easier to tie and easier to explain than an alpine butterfly, my other standard anchor loop.


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## moray (Jul 21, 2009)

*Results*

The picture shows all the slings prior to testing. 









Here are the breaking strengths, in pounds:

Baseline tests:
Normal eyes--- 4524
Brummel eye--- 4596

Knot tests:
Overhand------ 1676
Directional 8- 1840
8 on a bight-- 2100

The Brummel, as expected, had no effect on strength. The two baseline tests (no knots) give an average rope strength of about 4550 lbs.

The overhand knot was extremely weak, about 37% of rope strength, and the others were not much better.

The picture shows the 3 knotted ropes after breaking. 






While it is not apparent in the picture, the knots, or at least parts of them, became extremely tight and constricted under load. They all broke where the rope entered the knot, and none of them severed cleanly as a splice will do. As I said above, knots involve a lot of variables, rope type being just one. Perhaps the very same knots tested in another rope would give different results.


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## scotclayshooter (Jul 21, 2009)

Great work again!
I didnt realise that knots would make that much weakness!


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## Philbert (Jul 21, 2009)

Great post. Interesting and informative.

Tried to rep you for it, but it says that you are still in my rep holding pattern . . .

Philbert


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## Nailsbeats (Jul 21, 2009)

I agree on the 8mm Beeline, I would like to know that one with a double fishermans on it. Great work!


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## outofmytree (Jul 22, 2009)

treenoob said:


> Moray......that's just way to simple......awesome idea. How about a teaser of 5/16 beeline, inquiring minds wanna know...lolopcorn:



Moray could you please please please test a dfl termination and dfl stopper with this rig. I will gladly contribute to your costs in whatever way you request.

This work is outstanding in its benefit to climbers. It is one thing to say you "think" a knot weakens a rope, it is another to supply proven evidence using pre tested equipment.

Repped for this great effort.


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## moray (Jul 22, 2009)

A number of people have asked about double fisherman's and Beeline, which seems to be all the rage these days. I am working on an upgrade to my testing rig so I can test non-spliceable ropes and determine full rope strength. This will also allow me to test some spliceable double braids without forcing me to make TWO splices for every test.

One of the members here on AS has offered to send me some Beeline for testing, and I look forward to working with that as soon as I get it. Stay tuned...


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## Philbert (Jul 22, 2009)

Love to see photos of the test rig too, if possible.

Thanks.

Philbert


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## scotclayshooter (Jul 22, 2009)

What about rope thats been wet with petrol or other chemicals?


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## pdqdl (Jul 22, 2009)

Once more, Moray, this is great stuff.

Here is a thought: rather than testing with splices on one end, you could make comparison pulls with two different termination knots. 

I would love to see bowline vs 8 on a bight. Then you know _for sure_ which knot was stronger.

Perhaps double fisherman vs an anchor hitch? Oops. That would require introduction of a chunk of steel to the system...better make sure your safeties are hanging on real good.

I would REALLY, REALLY like to see a 1/2 hitch holding a timber hitch vs a running bowline, but then you would have to break the rope while it was attached to a tree. Not sure how well that would work.


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## moray (Jul 23, 2009)

pdqdl said:


> ...Here is a thought: rather than testing with splices on one end, you could make comparison pulls with two different termination knots.
> 
> I would love to see bowline vs 8 on a bight. Then you know _for sure_ which knot was stronger...



This is true, you could just compare two knots directly and skip the heavy work of making a tough splice. But here's the problem. If you were to measure two knots, in separate experiments, at 3000 and 3500 lbs. break strength, this means one thing if the actual rope breaks at 3600 lbs. but another thing altogether if the rope breaks at 10,000 lbs. The straight comparison isn't meaningless, but knowing the rope strength completes the picture. 

But there is a way out, and it needs a couple of chunks of steel. With some heavy pipe and a pair of heavy shackles I intend to make a couple of bollards I can fit to my chains. Then I can terminate ropes with several wraps around the bollards instead of a splice.


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## fsfcks (Jul 23, 2009)

The Alpine Butterfly (or Lineman's Knot) is meant to be one of the simplest and strongest knots, supposedly retaining about 80% of the rope's original strength according to some other tests I've seen on the web. The load can be safely applied: from the loop to either end of the rope; between the two ends with the loop hanging free; or to the loop with the load spread between the two ends.
http://www.animatedknots.com/alpinebutterfly/index.php 

Moray - I wonder if your tests would confirm this?


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## moray (Jul 23, 2009)

I really like the Alpine Butterfly and have used it a lot. I am about to do a batch of knot tests, and the Butterfly is high on the list, along with bowline, double fisherman's, and several others. Dollar signs swarm in front of my eyes when I think of all the rope I'm about to destroy.


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## pdqdl (Jul 24, 2009)

Well then post your mailing address, and see if some money doesn't show up!

Shucks, if everybody that read your splicing and testing threads sent you $1.00, you could probably retire.

Would the moderators step in and stop that?


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## pdqdl (Jul 24, 2009)

moray said:


> With some heavy pipe and a pair of heavy shackles I intend to make a couple of bollards I can fit to my chains. Then I can terminate ropes with several wraps around the bollards instead of a splice.



You might increase your safety and speed of testing by digging a 4' deep hole and setting a 6" steel post in it. Then it is it's own bollard, and no flying chains or other metal is involved. You might set two of them for gateposts or some other genuinely useful purpose, then use them occasionally as an anchor point for your tests.

You're up in Maine aren't you? Do you ever need a sturdy post to anchor a winch for pulling yourself out of the snow?


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## moray (Jul 24, 2009)

You guys are very funny.opcorn:

As for a bollard design for my floating cylinder pulling rig, I took some drawings and big shackles over to show them to my friend the local arborist. This guy is very old-time, but he has shown a lot of interest in my various schemes and loves to argue with me. After patiently listening to my design ideas and looking at my drawings, he went to the back room and returned with two brand new pulling bollards he had welded up for me! And painted bright red to boot. They were nearly identical to one of my own designs, and I can't wait to try them out. Tomorrow!

To get out of the snow in Maine no one ever uses a post anchor. It is impossible in Maine no matter where you go to be more than 10 feet from a tree.


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## 046 (Jul 24, 2009)

how's about posting a paypal address? much easier than mailing...



moray said:


> I really like the Alpine Butterfly and have used it a lot. I am about to do a batch of knot tests, and the Butterfly is high on the list, along with bowline, double fisherman's, and several others. Dollar signs swarm in front of my eyes when I think of all the rope I'm about to destroy.


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## eljefe (Jul 26, 2009)

*Re: Break testing knots*

Hi moray,
Your information is very very good, very very useful. And more than a bit scary. I very much hope you continue to test various knots and ropes and publish the data here. Thank you very much for taking your time and money to do this. I agree we should all chip in for expenses. 
In all the years, in all the books, etc I've heard this knot or that knot maintains alot of the rope strength. I would not call less than 40% of the native rope strength alot. 
it does get one to wondering just how much the type of rope -3 lay, 12 strand, etc or what it is made from manila, polyester, nylon, etc plays in the strength of a particular knot. 
eljefe


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## moray (Jul 27, 2009)

eljefe said:


> ... it does get one to wondering just how much the type of rope -3 lay, 12 strand, etc or what it is made from manila, polyester, nylon, etc plays in the strength of a particular knot.
> eljefe



Absolutely! It's going to be quite a job trying to untangle all these variables to come up with some useful general rules.

When I break tested a double braid splice for the first time a few days ago, I discovered there had been fiber melting right at the tip of the bury. Everywhere else the fibers seemed normal. There must have been relative motion of the fibers at that location under very high tension, and the heat of friction was enough to cause local melting. Your comments about different rope types got me thinking about knot tests in general, and it occurred to me that knots should be far worse than splices when it comes to relative motion, and thus friction heat. The knot shrinks, changes shape, and can even roll out under heavy tension, whereas a splice pretty much just sits there.

So I went back to my bucket of broken ropes and found the test rope with the overhand knot. Sure enough, under the dissecting microscope you could see lots and lots of melting in the region of the knot. An inch or so from the knot, all the fibers seemed normal. The picture was taken through the eyepiece of the scope, and the arrow points to some molten globs at the ends of fibers. Sometimes I saw several droplets arranged along the length of a fiber like dewdrops on a spider web. Sometimes bundles of hundreds of fibers were glued together and resembled a paint brush in which the paint had dried. All this melting at only 1700 lbs. tension! Wait till we start breaking knots with 4 or 5 times as much tension.






If local melting plays a role in the actual process by which a knot fails, as seems likely, then knots in non-melting fibers like vectran or technora should behave differently than knots in the more common fibers like nylon or polyester. Knots in polyethylene (spectra, amsteel, etc.) should be especially bad because of the low melting point.


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## TheTreeSpyder (Jul 27, 2009)

Fantastic job!

i think the point should be maid that how a knot is dressed out can make a lot of differance; so someone else might not get the same results with different final "seating" of the mechanics. Also, this Tenex is flat on mount, so would have less deformed dimension/ no height at the bend on that axis(for less strength/efficiency loss there) perhaps affecting results. Global staemeants are hard to make, due to variances of dressing, materials, braid etc.

A Butterfly test should perhaps include a test of just 1 leg to eye; due to some of the ways it is used, that don't conform to leg to leg with no or less load to eye and pulled perpendicualr to line.

Thank-you very much.


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## 046 (Jul 27, 2009)

totally agree ... nice job! 

have been saying for years... how a knot is dressed out can make a big difference. 

besides the strength issues, dressing out a knot out "pretty" makes for easy and positive ID that knot is indeed tied correctly. for a life line termination knot, that could have dire consequences. 









TheTreeSpyder said:


> Fantastic job!
> 
> i think the point should be maid that how a knot is dressed out can make a lot of differance; so someone else might not get the same results with different final "seating" of the mechanics. Also, this Tenex is flat on mount, so would have less deformed dimension/ no height at the bend on that axis(for less strength/efficiency loss there) perhaps affecting results. Global staemeants are hard to make, due to variances of dressing, materials, braid etc.
> 
> ...


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## moray (Jul 28, 2009)

Good points, Spydie and 046. I am expecting this knot stuff to make splices seem like child's play.

Oh yeah, gorgeous triple fisherman's, 046. If you were going to use one all the time, but it was going to be down in a well or something where you would never be able to see it again, that's what I would want it to look like...


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## moray (Jul 28, 2009)

I have put together a few photos from today's pulling tests. The rope in all cases is 3/8 inch Samson Stable Braid, with a nominal break strength of about 5600 lbs. Since the rope is not new, and had experienced one large load of unknown magnitude a couple of years ago, I don't trust any test results with this rope. It does help me work some bugs out of my equipment, and some of the pictures came out pretty good...

First picture shows a knotless pull test with the two bollards. The awl on the right (a sharpened $.93 screwdriver) locks the final anchoring overhand knot. With one full turn around the bollard, and half a turn around the clevis pin, only about 20% of the full tension reaches the anchoring knot. Far less still reaches the tail of the overhand knot, so the locking awl may not be necessary at all.






The next pic shows the 5:1 tackle I use to pull the cylinder back out after a test. You can do it without the pulleys, but they make it easy. The butternut log on the right supports a chain anchor at each end. The ends are 21 feet apart.


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## moray (Jul 28, 2009)

This experiment shows a test on two double fisherman's loops. The small clevises have a working load of 4.5 tons. The first and second photos show the test underway. Tension is 420 lbs.











The last photo shows the result. The rope broke where it entered the knot, or just inside the knot. The scorpion tail saw a lot of heat, and as it pulled through the fibers melted and fused together. The knot was squeezing the rope so hard that once the rope was out of there, the knot clamped shut. It is very hard to see where the rope had passed through the knot.


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## moray (Jul 28, 2009)

In this test an alpine butterfly is compared to a bowline. In the first photo we see the two knots under 480 lbs. tension.






The rope broke where it enters the butterfly, as shown in the next photo.






The last photo is a closeup of the break area. Knots are very irregular creatures: some parts are under far more stress than others.


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## pdqdl (Jul 29, 2009)

I am a bit confused by the pictures. The alpine butterfly is tied as a midline knot, right? Did you attach the clevis to the loop formed, leaving the tail unattached, or is something else at work there?

Can we presume that the Bowline outperformed the loop portion of the alpine butterfly? Was there any damage to the bowline? I would expect to see fused rope if it was close to breaking strength. Can the knot still be untied?


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## moray (Jul 29, 2009)

pdqdl said:


> ...The alpine butterfly is tied as a midline knot, right? Did you attach the clevis to the loop formed, leaving the tail unattached, or is something else at work there?...



Yes, the alpine butterfly is a "midline" knot, but I think that just means you can tie it in the middle of the line without access to either end. The standard bowline cannot be tied without access to an end. So I tied it in the middle, the middle being 4 inches from the end. Yes, the tail was left unattached. 
I searched my break bucket for the bowline; here it is:






The arrow shows how the cover of the loaded rope accordioned after the break. I never thought to try untying it. The double fisherman's were clearly way too tight to untie--I ended up cutting them off. To my surprise, the bowline untied easily! It had 2392 lbs. tension on it when the butterfly broke, and even though it had been visibly stressed, I didn't see anything that looked like actual damage.


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## pdqdl (Jul 29, 2009)

Cool! 

So we can safely conclude (on a single test) that bowline beats alpine butterfly for strength and ease of untying?

Another question: you did not report the "load at break point" for the double fisherman's knot. It would be interesting to see how it compared to the alpine butterfly.


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## blewgrass (Jul 29, 2009)

Funny story between the butterfly and the bowline. I used to do a bunch of boat mooring work, which in maine entails attaching chain to huge pieces of granite. To move these moorings, we would hook onto them with a boat and drag them off a shoreline until they hung from the bow of the boat. We had a set way of lowering them to the bottom in a controlled fashion which included the use of a bowline on a bight. Thinking I was going to improve on the technique and impress all who cared, I tied a butterfly instead of a bowline. After that ton and a half of maine granite hung on the butterfly it was impossible to untie. I remember my boss handing me the rope and saying "here you go, get that untied." The three strand nylon was fused and probably pretty close to it's breaking point. Learned my lesson.

I've definitely seen many knots/ropes fail in my years with marine work, but it's never been in such a controlled setting as moray. Great to be able to see a number put to a picture and have hard data from an expected rope failure.


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## moray (Jul 30, 2009)

pdqdl said:


> ...Another question: you did not report the "load at break point" for the double fisherman's knot. It would be interesting to see how it compared to the alpine butterfly.



The DFL broke at about 3300#.



blewgrass said:


> ...but it's never been in such a controlled setting as moray. Great to be able to see a number put to a picture and have hard data from an expected rope failure.



I wouldn't take any of this too seriously at this point; as I mentioned earlier the stable braid is in an unknown condition. However, the general picture is starting to emerge. 

The knotless pull with two bollards gave a reading of 4374 lbs., well above any of the knots. Even though we may dress and set the knot until it is perfect enough to be a museum piece, the knot immediately starts changing shape under load for the simple reason that some parts are loaded more heavily than others. The actual configuration when the knot breaks may be rather different from the unloaded configuration. This clearly varies from knot to knot; the DFL seemed little changed, but the butterfly changed a great deal. Finally, melting seems to be present in every break. Even for the non-melting fibers, we can safely assume great heat would be present at every break. Perhaps a knotted rope would be stronger if the tension increased very slowly, allowing the heat to dissipate? Or, to say it the other way, perhaps a knotted rope would be much weaker under a fast (shock) load than what a slow-load test would indicate.


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## pdqdl (Jul 30, 2009)

I have often thought about the melting or fusing of rope under load, and I have come to a conclusion, based mostly on my limited understanding of thermodynamics: it requires a great deal of energy to break a rope, and that energy must be physically expended somewhere on the rope. Since breaking the rope requires deformation, it makes perfect sense that the energy released while deforming the rope might generate enough heat to cause melting. Heat from deformation predisposes adjacent fibers to fail, while simultaneously reducing the available fibers to hold the load. PRESTO! You have an accelerating localized rupture of rope fibers; very shortly, a broken rope.

My initial thoughts upon seeing fused rope in knots had me thinking that it was the massive friction caused by tightening the knot, but I think otherwise now.


************************************************

Has anyone ever seen wire rope break? It does it a bit differently, in my experience. The twisted fibers begin to unravel, and they all pop individually in a big frayed-out mess of sharp steel wires. The energy released from the individual wires seems to go into each strand, bending the broken ends away from the core of the rope, leaving a splayed out end.

They make a metal singing sort of "rrrip" sound, and then you had better duck behind something big.


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## treemandan (Jul 30, 2009)

I have to say I am one for tying the rope to the truck and all the ropes I broke did not break at the knot.


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## Philbert (Jul 30, 2009)

pdqdl said:


> Has anyone ever seen wire rope break? It does it a bit differently, in my experience. The twisted fibers begin to unravel, and they all pop individually in a big frayed-out mess of sharp steel wires. The energy released from the individual wires seems to go into each strand, bending the broken ends away from the core of the rope, leaving a splayed out end.
> 
> They make a metal singing sort of "rrrip" sound, and then you had better duck behind something big.



When I was 16 I had the privilege of watching a tow truck operator try to pull a stuck fork lift out of a rut with brute force. As you note, the cable splayed out into a giant fan/funnel shape in a split-second. Very impressive.

Philbert


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## treemandan (Jul 31, 2009)

See what I mean about the rope breaking in the middle rather than the knot? Granted these are old bull ropes and such I am using but the " pop" usually starts about there for me.


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## moray (Jul 31, 2009)

treemandan said:


> See what I mean about the rope breaking in the middle rather than the knot?



Ah, the picture tells the whole story!

In various tests I have done to measure the friction effect of a rope wrapped around a wooden post or limb, as a rough rule of thumb I came up with this: for every half wrap the tension in the rope diminishes by at least 50%. A second half wrap would knock it down another 50%, so a full wrap reduces tension to 25% of the original. 

Applying this rough rule to your photo, and noting that you have about 3/4 wrap around the trunk, the knot at the upper arrow would see something like 35% of the tension on the main line. Since your knot can easily handle 35% of the rope breaking strength, it isn't the weakest part of the system.

But why isn't it breaking right where the main line enters the loop? That would seem to be the weakest spot to me.


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## pdqdl (Jul 31, 2009)

I have an answer for that:

Knots weaken the rope by putting a bend in the rope that has a smaller radius than the rope can handle. Then the outside of the curve carries most of the load, and it tears the rope fibers most at the outside curvature.

Since the bend of the loaded leg of the rope as it passes through the bight is not even a full 90° turn, the rope is not as significantly weakened there as it would be if it were in a full turn as part of a knot. The loop end of the rope only holds enough force to balance the line deflection of the loaded leg and whatever line pull is present after it makes the 3/4 wrap around the log. 

So...less likely to break at the hitch than a casual observation reveals.


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## TheTreeSpyder (Aug 1, 2009)

i think the answer(s) are more multi-dimensional. We have the inner and outer effects as stated, the outer arc will stretch further and carry most of the load.

But also, the rope only resists distance on the inline axis; and then also only in the tension direction. Because the volume of force is power X distance; only by resisting distance can we raise force from the volume of available force. Thus, any compressed fibers on the inside of a curve are not supporting at all; and other fibers then do this work/carry the inner fiber's share of the load. Now, flat line has less bent dimension on this axis, and less resistance to bend (typically); with much less / if any 'strenght'/efficiency loss on bend. But, also a bent leg of support holding 100# isn't inline all the way. So only the sine of it's angle is supporting the load. So, line tension at that point must be higher until the line tension X the sine (percentage of tension on inline axis to support load)=100# needed to be supported. Or 1/sine (cosecant) x load to be supported. With 2 angled lines supporting, that would be load /2 X cosecant to show line tension to support the given load at angle.

Then, also; the rope deforms around the mount, as tension on the outer curve pulls around as the friction on the inner part of the curve is pushed backwierds by the passive/ responding force of the spar. As tension decreases (because of the friction reducing force) so, therefore the rope is thicker at this stage. All of these distortions of form and force happen in a small area; so they 'impact' with their cumulative changes. Note, if we form a bend by a Dbl.Noose/ Scaffold Hitch. the bend, size etc. distortion is spread out over a longer distance,so is less impacting. This effect is also seen in Buntline and Lobster Buoy etc. But, is essentially lost, if a Round Turn rather than a simple Turn around the mount; for the line tension is reduced after the Turns, so the finishing choke around Standing is less forceful, and doesn't grip line as hard to buffer the forces of distortions etc. Taking away most of the 'strength' increase of Dbl.Noose, Buntline and Lobster Buoy over singleNoose, Turn + 2 Halfs, or Turn + 2 halfs opposing (respectively). Also finding, that Hitches opposing as a finish (Cow, Turn + 2 Halfs, Lobster etc. ) are typically easier to untie.

The last knot lacing should break where the knot forms, any of these things should get many permutations before we really drew hard data to forecast by.

As to the Bfly; i've never h-eared it said it was maid to be the strongest etc. To me it has a typical primary distortion point of a Half-Hitch; like a Bowline to each side; but then this primary distortion is longer like a dblBowline, suggesting more strength/by same change over longer distance, for less impacting change. But, the Bfly doesn't have the 2nd tier level to possibly help leverage inversion like a dbl.Bowline. Bfly though does have other distortions pushing to side that Bowline doesn't. And also distortions just past that , that could place 'back pressure'(?) of distortion, in the imagery of the fibers getting stiffened inside the lacing and this stiffening giving further distortion/ weakness to the primary(?) if close enough.


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## brnchbrkr (Aug 1, 2009)

:jawdrop:



rep's on the way!


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## moray (Aug 1, 2009)

pdqdl said:


> ...Since the bend of the loaded leg of the rope as it passes through the bight is not even a full 90° turn, the rope is not as significantly weakened there as it would be if it were in a full turn as part of a knot. The loop end of the rope only holds enough force to balance the line deflection of the loaded leg and whatever line pull is present after it makes the 3/4 wrap around the log.



Since I would NEVER nitpick, I agree with this as far as it goes. An engineer would look at the junction where the main rope passes through the loop and say something like this: "Since we don't have a system in motion, the vector sum of the tensions in the 3 legs at the loop must be zero, that is, they must be in balance (as you noted). Any set of 3 angles and 3 tensions that is balanced is a legitimate solution to the problem."

Take this one (very common) example. The 3 angles are each 120 degrees. That means the 3 tensions are exactly the same. Which means the main line, where it makes the 60-degree bend through the loop, is under full tension all the way. Surely then, it is weaker where it bends than where it is straight!

Now we don't expect this scenario because as load was applied to the rope and it was tightening and stretching around the trunk, the trunk friction was applying significant counterforce. The result would be much less tension at the knot and loop than in the main line. Then the angles cannot be equal (120degrees).

But what we care about is the main line before and after it passes through the loop, because that is where the big tension is and where we expect the break. Call the main line before the loop "B" and the line after the loop "A". In order for the tension in A to be different from B, the angle from the loop to A must be different from the angle from the loop to B, that is, not balanced. This also means, equivalently, that there is a frictional force where the loop holds the rope that is pushing either towards A or B. Let's say we have 200 lbs. pushing towards A. That means the tension in A is 200 lbs less than in B. If the loop were a steel ring, I would say sure, it can easily apply a sideways force to the rope. But it is a 3-strand rope, and I think it would try to roll to relieve any sideways force. Conclusion: with the setup shown in the picture, the tension in the rope would change very little where it passes through the loop, and therefore that bent passage should be the weakest spot in the entire system.


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## moray (Aug 1, 2009)

TheTreeSpyder said:


> ...The last knot lacing should break where the knot forms, any of these things should get many permutations before we really drew hard data to forecast by...



Spydie, I have never tried to forecast knot performance beyond the extremely general idea that a really sharp bend under heavy load (overhand knot) is going to perform worse than a more complicated knot that presents less load to the first sharp bend. 

The butterly illustrates the problem of forecasting. First off, there are two ways to load it, and it seems plausible that they could behave differently. Second, the one I tested changed shape significantly even under very modest loads. Obviously the knot breaks in its heavily loaded shape, not the configuration it had when you tied it. If you can't predict the final shape then any forecast of performance is going to be somewhere between weak and worthless.

Though I have not yet tested this, it seems likely that rope material in the guise of rope on rope friction is going to be a factor affecting the shape-shifting knots like the butterfly. It won't be able to change shape as much if there is a lot of friction, and so it will break in a different configuration than a slippery rope of the same construction. One might be able to test this just by squirting oil all over one knot while testing another one dry.


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## outofmytree (Aug 3, 2009)

It seems logical to me that used rope would have sections already stretched and thus any test performed would be moot. This would also explain why Dans rope broke in the middle rather than at the knot. I wonder if the middle of a length of rope actually stretches further than the ends which are tied to rigid objects or is that stretching uniform over length....?

Waiting patiently for more data... opcorn:


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## pdqdl (Aug 3, 2009)

moray said:


> Since I would NEVER nitpick, ....
> 
> ....Conclusion: with the setup shown in the picture, the tension in the rope would change very little where it passes through the loop, and therefore that bent passage should be the weakest spot in the entire system.



I'll take those comments as mostly true, with pretty good analysis on the loaded legs. I don't think that at 120° angles, that all the legs must be equally loaded. It is only essential that the "bight end" have a force on it equal to the force applied by deflecting the loaded side of the rope, and that the sum of the vector forces of the "after bight" and the "bight end" equal the loaded leg. Some of the load is absorbed by friction going around the log, which would be converted to torque on the log. Not all logs are free floating. Some are rigidly mounted to the ground. The torque on this log might very well be hidden by the log not wanting turn; as in say, a tree being pulled over?

Sure, the main line MUST be weakened where bent, even if only a little. OK! Technically, the weakest point.

But how much does that little bend weaken the rope? Not much, I'll bet. And beyond that bend, the load is carried by two legs of rope. So any imperfections in the rope closer to the log than the bight become moot.

Then you must compare that bend-point against the other possibilities along the entire length of the rope. Given that most ropes are used when we load them to breaking (after all, when they were new, they were strong enough to not break!), then there must be other imperfections along their length. So the problem now becomes a statistical analysis: what is the probability that there is another imperfection along the length of loaded rope that exceeds the weakening that occurs at the loop where the loaded line is tied to the log? 

That probability on a well-used rope is pretty good, hence The Dan's observation that they usually break in the middle.

My own personal observation is that they ALWAYS break where they bend most sharply around the strongest piece of rigging in the system. Bumper hitch, figure-8, or the half-hitch holding the timber hitch securing the giant log that really shouldn't have been cut off at that size. I don't think I have ever broken a rope "at the knot", since I never use a knot to carry a rope breaking load (they are such a b**** to untie afterward!). I always try to isolate the loads on the rope around structural items that are stronger than knots.

Really though, I can't remember breaking enough ropes to qualify as an expert on the topic. We usually end up cutting them with a chainsaw, or destroying them in some other fashion than breaking them.


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## grizzly2 (Aug 4, 2009)

When a load is put onto a rope, the rope would continually constrict to a point and then it would expand back out beyond that point. The apex of that constriction is where the rope has failed Dan. At that apex, the most energy is being forced onto the fibers causing heat and failure. Now, with that being said, general use of our ropes would cause degradation and could skew the location of the break. Also, pulling from only one end (dragging a tree) would shift that apex closer to the weighted end. If you have one of those exercise bands, you'll see how the middle constricts. Rope, technically and theoretically, stretches under the same principles.


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## TheTreeSpyder (Aug 4, 2009)

If the teepee is flat; the line is generally leveraged hard; giving also tighter grip; but uses more of the potential tensile strength to do so.

Leveraged hard around a tight bight, that excludes all but the outer strands is more trouble than same around on larger bight. Stiffer line will show this more. It is the resistance to bend that gives the leveraging, that is why you can leverage a tensioned, but not a slack line with perpendicular force. Once again this (d)effect is much less compromising in flat lines, that give less dimension on this bent axis; so can't be leveraged against you as much(for they don't stand as tall at bend, and are more flexible generally.

Some of the tweaks yield less; but i due believe the art is in the practice of putting out the most maximum security/strength in an equitable amount of time, with as little wear as possible. And developing an eye that can immediately spot a problem; and 'polish out' any imperfections quickly and correctly. If these things take 'too long' perhaps more practice is needed; is what i'd always tell myself. Also, the more intently you try to maximize these things to their pure/unadulterated state, the more truer relationships and commonalities your eye can catch- in time. Anything else can go wrong, and the more chips ya got stacked on your side the better over all.


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## scotclayshooter (Aug 5, 2009)

Slowly and in English please!

Hey Moray the results from the tests were way more interesting than the theory.


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## moray (Aug 5, 2009)

pdqdl said:


> I'll take those comments as mostly true, with pretty good analysis on the loaded legs. I don't think that at 120° angles, that all the legs must be equally loaded...



Yes, and your discussion is mostly right on, but I must complain about this 120° business, even though we are straying far into new territory and we have jointly nitpicked this thing to the bone. The 120° angle between 3 loaded legs leads to a *mathematical *truth that really has nothing to do with physics or mechanics per se. If three forces act on a central point, and no others, and they are respectively 120° apart, then they are equal, or they are not equal and the central point is in accelerated motion. 120° (how do you make that degree sign?) is not special. If you have 3 forces on a central point and no motion, then the size of one of the forces unconditionally and unambigously determines the sizes of the other two. This can be a very good reality check on your reasoning. There may be good reasons for believing something is very large or very small, but the angles tell the story.

But I agree; the loop does not apply much force to the main line, it is not bent much nor weakened much. The visible breaking point is surely due to some existing weakness or damage in the rope.


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## moray (Aug 5, 2009)

scotclayshooter said:


> Slowly and in English please!
> 
> Hey Moray the results from the tests were way more interesting than the theory.



I'm glad you liked at least some of it. Some serious tests are yet to come; I am ordering a bunch of new rope that should be nice and uniform and give meaningful results...


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## pdqdl (Aug 5, 2009)

moray said:


> ... (how do you make that degree sign?) ...



Read ropensaddle's signature line! That is where I got it.

http://www.arboristsite.com/showpost.php?p=1658592&postcount=1096


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## TheTreeSpyder (Aug 6, 2009)

2 Vertical poles supporting a clothesline with a 120°(thanx°) spread, and load in the middle; will exert 1xLoad on each hitch point on pole. While the same with poles next to each other, and Zer0° deflection betwixt the support lines; will exert .5xLoad on each hitch point on pole.

To my weigh of thinking...(OOOOooooooooooooPs....) -

This is because; there are 2 legs of support, so if self equalizing (theoretical Zer0 % friction pulley on load point/ center bight of line); each leg would have to support half the load everytime, regardless of angle/ deflection from inline.

But, rope is not like wood, steel etc.; it will not support on any of the cross axises( i know bad grammar, but somehow less confusing than calling this plural "axes" on a tree site...), only on the inline axis. 

Anyway; we can't get support from both the leveraged and inline axises like a non flexible device; only on the inline axis. So, the cosine of Zer0 (no spread angle of lines)is 1; so each leg can support it's half share of the load; with cos(1 or rather 1/1) x Load/2(supports)=line tension of .5xLoad.

Now, with a 120°spread; there are still 2 support lines sharing the load; so whar does the 'extra' tension come from?? Well, with a 120° spread; each leg is 60°from inline (for a 120° spread). The cosine of 60 is .5; so each half loaded support leg can only support that cosine (.5) x the line tension; or 1/cosine(.5) x load supported on that support leg (Load/2) gives 2 (1/.5) x 1/2 (half of load supported on each leg); for a total line tension = 1xLoad (2x 1/2 x Load); thus that pull on each supporting leg.

Now whippin'out (y)our windows calculator and placing it's view in scientific mode; we can see that 170°spread gives 85°deflection from inline on each leg. Place 85°on the calc and press cos key and we get ~0.0871; press the 1/x key; and we get ~11.4737. A 1000# Load would need to get 500# support from each of the 2 legs; but each load support leg can only give ~11.4737 inline support to each leg. 500(support needed per leg to support the 1000#Load ) X ~11.4737 = ~5736.8566 line tension. Which is a consistent pattern to other calculations(?).


Oooops gotta run, but i hope this is close.


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