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Joining unequal diameter ropes for rappel.


Original Post
Luke · · Zürich · Joined May 2012 · Points: 0

Hey does anyone have any evidence based advice for joining ropes of uneven diameter? I've found traveling with one half rope and one regular rope a great compromise for doing both sport and multi-pitch (I usually climb with two half ropes for multi-pitch).

I've used flemish bend with stopper knots on the tails in the past; I've seen it recommended for this purpose and have no fear of it coming undone. The European death knot looks particularly scary with unequal diameter ropes, but I'm curious just how dangerous it and other common knots would be. Obviously the reepschnur method would be the safest, but it doesn't seem necessary with ropes which are really just a couple of millimeters different.

Does anyone know of any actual tests being done with different diameter/knot configurations?

divnamite · · New York, NY · Joined Aug 2007 · Points: 90
Rick Blair · · Denver · Joined Oct 2007 · Points: 266

I have been using the flat double fishermans since I saw it on Elderid's site years ago, purely psychological reasons.

If I am with someone using an EDK I have no problem with it, both their knot and my knot are proven and easy to inspect, same goes with different rope diameters.

Go here to 3:35
youtube.com/watch?v=Dzp2vlk…

bearbreeder · · Unknown Hometown · Joined Mar 2009 · Points: 3,065

for anything between 8mm-10mm ... EDK ... see the BD tests

if yr worried use a double fishermans

thats all there is to it folks

;)

John Byrnes · · Fort Collins, CO · Joined Dec 2007 · Points: 612

BITD I had an article published in Climbing on this topic.

I used to use an 11mm and a 7mm static on a regular basis. I always used a double-fisherman's knot for peace-of-mind, and that's still what I recommend for ropes of differing diameters. I use the EDK for ropes of same diameter; eg. I used twin 7.5mm ropes for a time.

Of course, there ARE significant other issues when you rap with ropes of differing diameters. I assume you've run into these or...?

Ryan N · · Bellingham, WA · Joined May 2009 · Points: 195

Yea EDK/Overhand is my go to for joining ropes. I'm pretty sure I read its the same idea as a prussick. The difference between sizes allows the thinner rope to grab the thicker. I've never had an issue when joining ropes that are 1-1.5mm difference.

Aerili · · Los Alamos, NM · Joined Mar 2007 · Points: 1,875

I've used EDK with unequal diameters many times with no problem. However, I think dressing the knot well is key, and I leave long tails in which I tie a second EDK with both ropes and/or individual single fisherman's knot on each end. Sorry, my advice is not evidence-based but anecdotal.

I think it is more important to be vigilant about the actual rappelling part. The ends can become uneven (I think most people know this). This actually happened to me once while rappelling a route in the Valley. My partner went first with no issues. But as I rapped down, the ends (which just reached the belay) started to move apart. Luckily he noticed this just before one end went out of reach and thus grabbed it and held it just enough to prevent slippage as I finished the rap. (I think the diameters were 10.2 or 10.5 and 9.2.)

I usually hand-over-hand now during such a rappelling situation in order to prevent them from moving. (Tying ends together could also work if the toss is clean.)

D Stevenson · · Escalange, UT · Joined Dec 2010 · Points: 25

EDK, 6"+ tails, backed up, or double fishermans.

Double fishermans is great, I sometimes toss in another loop for personal security reasons, and always back up. Works great tying my 6mm pull cord to 9.3mm canyoneering rope. Never had any issues with untying, even with numb hands.

If the rope pull is sketchy in terms of rope-eating cracks, or snagging on things, the EDK is the best option, as the knot will stay away from the rock as you pull. In these situations a little less security on the knot (still totally bomber) is definitely safer than getting a rope stuck when a fat knot jams in a crack.

Jason Halladay · · Los Alamos, NM · Joined Oct 2005 · Points: 12,587

I don't have any evidence-based advice, just experience-based (and backed up by guides Houston and Cosley in their book Alpine Climbing: Techniques to Take You Higher). In their book, page 185, they promote the used of the flat overhand (EDK) when rope diameter difference is less than 3mm. They also explicitly promote not using any backup knots, just long tails of 12-18 inches. And thread the skinnier rope through the anchor because it stretches more. These are the guidelines I've been using for many years. And I hate rappelling--easily my least favorite aspect of climbing.

Greg D · · Here · Joined Apr 2006 · Points: 878

The people that have had issues are not available to comment.

rgold · · Poughkeepsie, NY · Joined Feb 2008 · Points: 526

I think the EDK is not only fine for different diameter ropes, but properly tied it is actually more secure than when tied in the same diameter ropes. This is an opinion based on reasons described below, but other than being used this way by me for many years, it has not been formally tested.

I should add that I haven't used the EDK in a situation in which the rope diameters differ by more than 3 mm. The strands shown below are 6mm and 8.5mm. And please note that the ends have been tied short to fit into the picture frame; forearm-length ends are what I use in the field.

The tying method is aimed at defeating the EDK's mode of failure, in which the opposing pulls on the standing parts cause the strand most immediately loaded to roll over the other strand. I always use the "secured" method shown last.





John Byrnes · · Fort Collins, CO · Joined Dec 2007 · Points: 612
Jason Halladay wrote: And thread the skinnier rope through the anchor because it stretches more.


1) Your skinny rope should be a static rope for a variety of reasons. Stretch shouldn't be an issue.
2) In the above scenario, the skinny rope will run through your rappel device faster than the fat rope, the rope will "travel" through the anchor as you rappel, and you'll get to the end of the skinny rope several meters (maybe more) early. Oops.

Buy your skinny static rope 5 meters longer than your lead rope for extra margin. The extra length is nice for hauling too.

If you put the fat rope through the anchor, then the knot will catch on it, keeping the ends even. That said, be sure the knot will not JAM in some funky anchor.

On multiple rappels, I alternate which rope I thread (the usual case where you thread the rope you pull). In this case the second can pinch the ropes together with his hand to prevent the ropes from "traveling" and the first one down can hold the skinny rope to provide the same service for the second. It doesn't take much extra friction to keep them from traveling.

One of the reasons I don't use a EDK for mis-matched ropes is that if the knot catches on the anchor, a double fishermans seems less likely to jam or roll. Never had the guts to test that.
rgold · · Poughkeepsie, NY · Joined Feb 2008 · Points: 526
John Byrnes wrote: One of the reasons I don't use a EDK for mis-matched ropes is that if the knot catches on the anchor, a double fishermans seems less likely to jam or roll. Never had the guts to test that.
It actually has been tested, and you are right. See bwrs.org.au/sites/default/i…, page 23.


Starting Inertia loads:
It was observed the Overhand knot required more force than any other knot if the knot is against the sling. Graph E-5 comparing a Rethreaded Fig 8 to the Overhand Knot, indicates that the starting break free load of the overhand knot can be around three times higher if the knot is against an anchor sling. This is due to the knot clasping itself around the sling (Figure 11-a). Only the Overhand knot of the knots tested had this issue. If the knot is away from the sling this does not occur (Figure 11-b).


The language notwithstanding, the rap line in the pictures runs through a quicklink---the rappel line is not threaded directly through the sling.
Ryan N · · Bellingham, WA · Joined May 2009 · Points: 195

Nice info rgold thanks!

Gunkiemike · · Unknown Hometown · Joined Jul 2009 · Points: 2,906
Aerili wrote:I usually hand-over-hand now during such a rappelling situation in order to prevent them from moving. (Tying ends together could also work if the toss is clean.)
Tying the ends together won't prevent differential creep of different ropes. What it WILL do it generate a horrid tangle of one rope around the other.

Just tie stopper knots in each rope. If they stretch at different rates so that you hit the end of one rope, so what? Let it jam against the ATC and feed the other rope through to finish the rap.
Luke · · Zürich · Joined May 2012 · Points: 0

Hey thanks for the info everyone.

I was surprised how much info was supporting the overhand, would never have thought so. Sometimes it doesn't pay to follow your instinct.

Cheers

Bill Lawry · · Albuquerque, NM · Joined Apr 2006 · Points: 1,678
Gunkiemike wrote:Just tie stopper knots in each rope. If they stretch at different rates so that you hit the end of one rope, so what?
Generally agree, instead of tying the rope ends togther.

At the same time, some here are using 6 mm and even 5 mm tag lines. Some stopper knots in that skinny of a line can slip through an ATC.
Devin Copeland · · H-Town · Joined Jul 2013 · Points: 0

rgold, thanks for the excellent pics and description!!!

David Coley · · UK · Joined Oct 2013 · Points: 70
Rick Blair wrote:I have been using the flat double fishermans since I saw it on Elderid's site years ago, purely psychological reasons.
Rick,
Just in case you didn't know, there is a potential failure mode for this knot. See the photos near the end of: people.bath.ac.uk/dac33/hig…

This doesn't mean the knot is no good, just something to be aware of.
Chris D · · the couch · Joined Apr 2009 · Points: 2,230
John Byrnes wrote:2) In the above scenario, the skinny rope will run through your rappel device faster than the fat rope, the rope will "travel" through the anchor as you rappel, and you'll get to the end of the skinny rope several meters (maybe more) early. Oops.
One would expect this to happen, but in my experience, it does not.

Also, what's the point of discussing the strength of knots in a rappel system? Looks like even the worst knots are surviving up to 3,000 pounds. All loading in a rappel system is static, so as long as the knots aren't failing at loads under 200 pounds, it seems that strength is a moot point.
Bill Lawry · · Albuquerque, NM · Joined Apr 2006 · Points: 1,678
Chris D wrote: One would expect this to happen, but in my experience, it does not. [... skinny rope will "travel" through the anchor as you rappel ....].
That depends on the difference in the two diameters and possibly on the rap device. I've had a 6 mm cord travel when married with a 10+ mm rope on a basic ATC. And the results can be fatal if rapping near the tails.

Edit to add: I believe the "travel" is something that should not be simply accepted. Fix the system.
Guideline #1: Don't be a jerk.

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