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rgold
Trad climber
Poughkeepsie, NY
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Jun 27, 2013 - 10:35pm PT
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I've always heard that if using the EDK it helps to use the fatter rope as a block for the smaller rope.
This is what I do, with the addition of an overhand security knot as in the last picture below:
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HighTraverse
Trad climber
Bay Area
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Jun 27, 2013 - 11:46pm PT
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Some interesting test data from Salt Lake Search and Rescue comparing several knots under different conditions
http://user.xmission.com/~tmoyer/testing/EDK.html
The EDK MUST be tied properly and securely. Probably should also be checked after each re-threading on multiple rapps. Failure is always from the knot rolling through!
Lowest load failure was 200 lbs with 11mm ropes: "sloppy - crossing strands and loose"
vs 2070 lbs well dressed and pretensioned - pulled separately on all 4 strands
And more test data from Edelrid on EDK, triple overhand (new to me) and double fisherman knots. Similar conclusion about tying the EDK carefully.
http://www.gudelius.de/spst.htm
Note: 1500 DaN (deca-Newtons) is approx 1500 kg (3300 lbs)
Both sets of tests were with very small number of samples so the data cannot be very reliable.
The failures do show that long tails provide a safety margin.
Similar problem with figure eight probably led to death in Zion
http://www.supertopo.com/climbers-forum/4346/Death-in-Zion
I like rgold's demonstration of proper way to lay the two lines for the EDK.
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HighTraverse
Trad climber
Bay Area
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Jun 28, 2013 - 12:21am PT
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And here's a careful study of well tied knots.
In which case the author's favorite for joining rappelling ropes is the EDK.
http://www.bwrs.org.au/?q=research
Conclusions start on Page 27 of the main report.
All the studies I've looked up so far say
DO Leave long tails on EDK (overhand knot) and rethreaded Figure 8.
Do Not Use the overhand knot to tie two ends of tape (webbing).
DO Use the "rethreaded" figure 8 rather than the figure 8 tied like the EDK.
Drohan doesn't like the Alpine Butterfly for tying two ends together.
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grover
climber
Northern Mexico
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Jun 28, 2013 - 01:30am PT
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Has an edk with long tails ever failed?
just askin
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rgold
Trad climber
Poughkeepsie, NY
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Jun 28, 2013 - 09:49am PT
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There was a guided accident in the Tetons in which an EDK apparently failed. It had been retied for some reason and it isn't entirely clear whether or not some mistake might have been made. There is also an account somewhere on Andy Kirkpatrick's site about a very wet EDK slipping, and the second literally holding it together while the rappeller finished their rappel. Moyer's tests do suggest EDK's in wet ropes roll at lower loads. On the other hand, the knot has been in constant use, especially in Europe, for a long time now.
I use the extra securing overhand in the "roll strand" for all cases, even when the ropes are dry and the same diameter. It is much more compact than tying a second overhand, which makes a very bulky knot which I think is more likely to jam on something.
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labrat
Trad climber
Auburn, CA
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Jun 28, 2013 - 12:06pm PT
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I was taught to use EDK for same diameter ropes only with long tails (2 feet). I use two triple fishermans for ropes of different diameters.
Having to be able to tell the difference between "correct" and "incorrect" and also having to tie a back up finish to use the EDK for different diameters seem like a recipe for disaster.
Yes, the two triple fishermans will be harder to pull and might hang up but I would rather deal with that than have the knot fail when I mess it up due to darkness, tiredness, or hurrying due to weather.
My 2 cents.
Erik
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rgold
Trad climber
Poughkeepsie, NY
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Jun 28, 2013 - 12:23pm PT
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Having to be able to tell the difference between "correct" and "incorrect" and also having to tie a back up finish to use the EDK for different diameters seem like a recipe for disaster.
It is extremely simple in practice. You pull in opposite directions on the two standing part strands and look at the part of the knot they are trying to widen. You can see immediately whether you have tied the bad or good version. I don't have time now but I'll post a picture later.
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