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EDK Destructive Testing of Different Diameter Ropes


Original Post
20 kN · · Unknown Hometown · Joined Feb 2009 · Points: 1,346

In light of the numerous topics regarding joining different-diameter ropes via an EDK, I decided to pull-test a piece of 11mm static and 6mm cordelette joined by an EDK. The cordelette was brand new but the static rope was used. I performed two types of tests: a standard EDK end-to-end destructive test and an EDK against an A2 stainless steel Fixe rap ring, which is the same ring used on millions of rap stations. For both tests, I pretensioned all four strands, I tied the EDK with the 6mm on bottom and I tied an overhand backup knot with the 6mm. I used a clove hitch to attach the test sample to a steel carabiner, which was attached to my testing platform.

Testing Layout

Figure one depicts the first test, an end-to-end pull-test.



Figure two shows the second test, which simulates a scenario in which the EDK is only acting as a stopper knot in the belay. In this photo I pulled on the 6mm cordelette, but I also conducted another test in which I pulled on the 11mm. The results were similar for both trials. The backup knot was did not tighten in either of the two tests.


Figure four indicates where the rope failed—at the clove hitch.



Results and Observations

The first test, an end-to-end static pull-test, resulted in the 6mm failing at the hydraulic cylinder with a peak load of 1,187lbf (5.28kN). The EDK knot did not roll; however, the backup overhand knot was quite tight, which implied that the backup knot was loaded. The second test, which simulated the climber using the EDK as a stopper knot against the belay station’s rings, resulted in the knot pulling through the Fixe ring at 800lbf (3.55kN). Worth noting is that I placed a carabiner in the Fixe ring to attach it to my load cell, which restricted the knot’s ability to pull through the ring. In a real-world scenario, the knot would have more room to pull through the ring.



Figure three:



Addendum

I tested the EDK with the 6mm on bottom and with the 6mm on top, both without a backup knot. In both tests, the 6mm broke at the hydraulic cylinder without the knot rolling (although it was close to rolling).



Buff Johnson · · Unknown Hometown · Joined Dec 2005 · Points: 1,145

nice work!
Can you pull me a double sheet bend to compare?

MaraC · · Unknown Hometown · Joined May 2011 · Points: 10

I am confused about the relevance of this data.

The thread that prompted this testing was asking about using the EDK to join two different-diameter ropes for rappel. In this situation, the ropes are not going to be loaded end-to-end, as in your first test.

The second test perhaps has slightly more relevance, but I would question anyone who set up double-rope rappel relying on a "stopper" knot this way.

Am I missing something?

Ryan Nevius · · Chiang Mai, TH · Joined Dec 2010 · Points: 898
MaraC wrote:I am confused about the relevance of this data. The thread that prompted this testing was asking about using the EDK to join two different-diameter ropes for rappel. In this situation, the ropes are not going to be loaded end-to-end, as in your first test. The second test perhaps has slightly more relevance, but I would question anyone who set up double-rope rappel relying on a "stopper" knot this way. Am I missing something?
Aren't the ropes essentially being loaded "end-to-end" if the knot is to one side of a rap station / ring? Makes sense to me...
20 kN · · Unknown Hometown · Joined Feb 2009 · Points: 1,346
MaraC wrote: In this situation, the ropes are not going to be loaded end-to-end, as in your first test.
Of course they are. You rap on two strands right? Both ends of the EDK will see the same load unless the knot is flush against the belay, in which there will be some indifference between the loads on each strand.

As far as the second test goes, some people do actually use a block of some sort and rap on just one strand. I dont recommend it as there is no real advantage to doing so, only disadvantages, and most who choose to do use a carabiner block or something similar. But it does happen. Also, because the thicker rope is normally passed through the belay, and the thinner rope has less friction, often the knot is against the rings in the belay even when rapping on only one strand.
Jeff J · · Bozeman · Joined Sep 2010 · Points: 110

So to sum things up. first test: thin line broke some where about 1100 LBS of force. Second test the knot pulled through the ring at abut 800 lbs of force. . .

You will be fine rapping of that sack up and carry on.

Brian in SLC · · Sandy, Utah · Joined Oct 2003 · Points: 14,422

Neat info...

Be interesting to repeat the above without a backup knot on the 6mm, but, long tails and a nice, snug pre-tension on all four strands.

Also, how about a test where you use the same ropes, but, use a rappel ring top and bottom. Tie two short strands of ropes together through the rings. Pull to failure. You could vary the position of the knots so each was on one side of each rappel ring. I'd be interested in seeing those results. No back up to the 6mm. See which side fails, where, and what the failure load was. Easy to rig.

Great stuff. Mo' data!

20 kN · · Unknown Hometown · Joined Feb 2009 · Points: 1,346
Buff Johnson wrote:nice work! Can you pull me a double sheet bend to compare?
I supposed I can. Does anyone ever use that knot for joining raps? I dont think I would be willing to use the DSB for a life-critical application. It seems like a fairly insecure knot, but I dont use it either though.
Brian in SLC · · Sandy, Utah · Joined Oct 2003 · Points: 14,422
20 kN wrote:I dont recommend it as there is no real advantage to doing so, only disadvantages, and most who choose to do use a carabiner block or something similar.
Very common amongst the canyoneering crew (single strand over a pool in a waterfall to set the rap length so folks pop off the rope instead of get tangled in higher water flow). Also, when rappelling with a Gri Gri. Or, using a thin diameter pull cord.

Although, folks rappel single strand in canyons all the time seemingly "just because".
MaraC · · Unknown Hometown · Joined May 2011 · Points: 10
20 kN wrote: Of course they are. You rap on two strands right?
You're right. My bad. I still wonder about that second test, though. It's scary to think people would set up a rap that way.
David Appelhans · · Medford, MA · Joined Nov 2007 · Points: 410
Brian in SLC wrote: repeat the above without a backup knot on the 6mm, but, long tails and a nice, snug pre-tension on all four strands.
This is what I was interested to see--test 1 with no backup knots. I want to see when/if it starts to roll and what difference the skinny rope on top or bottom of the knot made. I'd love if you did this test.
Rick Blair · · Denver · Joined Oct 2007 · Points: 266

Thanks for doing this! Would you test a flat double fisherman's a al Edelrid?

http://www.gudelius.de/spst.htm

John Byrnes · · Fort Collins, CO · Joined Dec 2007 · Points: 577
20 kN wrote: Of course they are. You rap on two strands right? Both ends of the EDK will see the same load unless the knot is flush against the belay...
Sorry, but no. With ropes of different diameters there is far more load on the fat rope, which is why the skinny rope will run through your device faster, if you let it. It's worse with a new skinny rope (slippery) and a fuzzed-out fat rope.

Because of this imbalance, the knot doesn't always tighten evenly. It can stay "soft" and have a tendency to roll. It would be nice to have some real data but I think you need to design a more realistic test.

Does the UIAA now approve of the EDK for unequal ropes?
Ryan Nevius · · Chiang Mai, TH · Joined Dec 2010 · Points: 898

^ So wait...your comments were just based on assumptions, or you have data?

Matt N · · Santa Barbara, CA · Joined Oct 2010 · Points: 287
user.xmission.com/~tmoyer/t…

Scroll down towards bottom. This knot has been used and tested before - nothing new as of two hours ago.
Derek Doucet · · Unknown Hometown · Joined Apr 2010 · Points: 64

Thanks for the work, 20Kn, but since most folks don't rap on a static rope with any regularity, I'd recommend caution in drawing any conclusions from your testing. It seems reasonable to consider the possibility that dynamic ropes might behave differently.

Leo Paik · · Westminster, Colorado · Joined Jan 2001 · Points: 22,845

20 kN, thank you for this excellent experiment!

matt c. · · Unknown Hometown · Joined Nov 2009 · Points: 155

great test. great description. awesome!

20 kN · · Unknown Hometown · Joined Feb 2009 · Points: 1,346
Rick Blair wrote:Thanks for doing this! Would you test a flat double fisherman's a al Edelrid? http://www.gudelius.de/spst.htm
Possibly, if I can find some more 6mm.

Derek Doucet wrote:Thanks for the work, 20Kn, but since most folks don't rap on a static rope with any regularity, I'd recommend caution in drawing any conclusions from your testing. It seems reasonable to consider the possibility that dynamic ropes might behave differently.
I have actually conducted this test before, and more than once. Last time I used dynamic rope and I noticed no change. I could test it again with 10.3mm dynamic though. But even if the knot did roll, previous studies show that the knot normally gets stronger each time it rolls, so with 24" tails there is virtually no chance the knot is going to roll off the end. If you are REALLY paranoid, you can tie two EDKs in series.

Brian in SLC wrote: Very common amongst the canyoneering crew (single strand over a pool in a waterfall to set the rap length so folks pop off the rope instead of get tangled in higher water flow). Also, when rappelling with a Gri Gri. Or, using a thin diameter pull cord. Although, folks rappel single strand in canyons all the time seemingly "just because".
Good point with the GriGri. Although I would argue it is kind of silly to climb a long multipitch route that requires 2-rope raps without an ATC. As far as the thin diameter goes, even if I was using 1mm cordletee as a pulldown, I would still thread it. That was kind of what I meant before when I said there are no real advantages to not running the pull strand through the ATC. I guess some would argue it could get tangled on the way down, but I havent had a problem so far and I use 6mm for a tag line.

John Byrnes wrote: Sorry, but no. With ropes of different diameters there is far more load on the fat rope, which is why the skinny rope will run through your device faster, if you let it.
Yes, I have used the EDK many times so I am aware. My comment was not in reference to the load differences between the main rope and the tag line, but rather it was about the load differences between the top strand on the EDK and the bottom. The quote was in reference to Ryan's post in which he believed the strands are not loaded end-to-end, but they are (in most scenarios).

As far as the rest of your quote, I am not following. Yes, the big 11mm fatty will see a much higher load than the 6mm noodle because of the friction differences in the ATC. But I do not see how that creates a scenario that is not analogous of the end-to-end test I performed. Regardless of the load distribution differences, the strands exiting and entering the EDK will see precisely the same load (assuming the knot is not touching anything).

If we are seeing 1200 lbf to failure with 6mm, once can likely expect upwards of 2500 lbf in the field before failure because more than half of that load will be carried by the larger strand.
Derek Doucet · · Unknown Hometown · Joined Apr 2010 · Points: 64

I wasn't suggesting there was any issue using the EDK in general, though I've heard anecdotal reports of informal tests conducted by folks I consider highly reliable that suggest it's not the best choice with ropes of substantially different diameters. In any case, I was merely wondering about the applicability of this particular test to the manner in which we actually use it in the field.

Ryan Nevius · · Chiang Mai, TH · Joined Dec 2010 · Points: 898
20 kN wrote:The quote was in reference to Ryan's post in which he believed the strands are not loaded end-to-end, but they are (in most scenarios).
I was in agreement with you. I was questioning MaraC's logic, not yours.
Guideline #1: Don't be a jerk.

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