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baker_bc

joining dissimilar diameter ropes

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This question came up during an unanticipated descent last weekend -

What bend(s) or knot(s) are best (or worst) for joining ropes of dissimilar diameter?

 

I'm thinking about the situation where a double-rope rappel uses two very different diameters, say 8 mm & 10 mm. From reading and chatting with friends about the flemish bend, double fishermans, EDK, I've found nothing beyond anecdotes about which is the best (or worst) when the ropes are of rather different diameters.

 

Does diameter mismatch differentially affect the force at which different knots roll?

 

 

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My understanding is that all the commonly used knots will work on ropes of dissimilar diameter. Personally, I would probably avoid using the EDK to join an 8 mm with a 10 mm, unless I was really worried about difficulty in pulling the ropes.

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Normally, I uses ropes that are nearly the same diameter. If I were rapping on a 10.5 and an 8mm, I would use a double fish, or a square not backed up with double fish. I wouldn't even consider the EDK

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There doesn't seem to be any widespread data out there on this topic. instead, it just seems to be a matter of those people who look at the EDK (aka, a simple overhand knot) and get freaked out by it because it doesn't look like much, and those people (usually those who've had their ropes get stuck when pulled after a rappel.. often late in the day with the threat of nightfall approaching) who use the EDK frequently.

 

personally, i use the EDK all the time when tying ropes of different diameters together and have never had a problem. whatever you choose to use, just make sure the knot is tied cleanly and pulled snug.

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I generally use a triple fisherman's when joining two ropes of different diameters. I don't know if a double is less safe, but I feel better with that extra wrap so that's what I do.

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I managed to survive several rappels last weekend using a ~10.2mm dynamic and an ~7.8mm static joined with an EDK.

 

 

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I managed to survive several rappels last weekend using a ~10.2mm dynamic and an ~7.8mm static joined with an EDK.

 

 

Quit lying. You are sooo dead.

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The advantage of using a triple fisherman's over a double is that it is larger and hence sticks in cracks even better than the double. Therefore the rappel epic and possible unplanned bivi is more likely with triple fishy, and like they say, "no epic, no story".

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The advantage of using a triple fisherman's over a double is that it is larger and hence sticks in cracks even better than the double.

 

It also takes ~ 1/3 longer to tie, so it's got that going for it.

 

 

You are sooo dead.

 

Is that a threat!? I've been working out you know... up to 10 lb bicep curls now. I'm no pushover.

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The advantage of using a triple fisherman's over a double is that it is larger and hence sticks in cracks even better than the double. Therefore the rappel epic and possible unplanned bivi is more likely with triple fishy, and like they say, "no epic, no story".
It's a longer knot but the diameter is the same. An EDK can stick in cracks too! The reason it's a good knot is that it doesn't catch on edges the way other knots do.

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The advantage of using a triple fisherman's over a double is that it is larger and hence sticks in cracks even better than the double.

 

It also takes ~ 1/3 longer to tie, so it's got that going for it.

 

Hey MathFox, 3 is actually 1/2 longer than 2 :nurd:

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Hey MathFox, 3 is actually 1/2 longer than 2 :nurd:

 

Well that does it. I'm convinced. Anyone who uses the triple fish is just begging for an epic.

 

 

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One time, while rapping on a triple fish, I couldn't reach the next rap anchor because I had unwittingly used an extra 4 inches of rope tying my knots!

 

Oh noes!

 

I had to extend my rope with 6mm cord (tied with quadruple fishermans, of course), pass the knots and then hang off the ends of the cord with my teeth to reach the next anchor.

 

Whew! Next time, I'm tying double fishes for sure.

Edited by robmcdan

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any of the popular bends will work - a larger concern is the variance in stretch between dissimilar diameter cords - which results in knot travel during the rappel, which sometimes results in stuck ropes.

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I think what he is talking about is that more of the skinny rope passes through the belay device than the fat rope, so the knot moves, unless it happens that the skinny rope is through the anchor, then it is stopped by the sling or rap ring. This has nothing to do with a particular knot.

 

If one is aware of this slippage potential, it is possible to avoid it by the way one handles the tails on rappel, that is by pinching them together.

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Yes. I understand the knot movement issue (though your explanation has it backwards, it occurs when the skinny rope is through the anchor).

 

How does that result in stuck ropes?

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I had a stuck rope due to this behavior once. The skinny rope got pulled into the rap ring and the knot got stuck. Two people couldn't get it unstuck, I had to jug back up and fix it.

Edited by robmcdan

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Intersting stuff. Just so I got it straight, if I put the fatter rope through the anchor ("pull the skinny rope"), because the skinny rope stretches more, the knot tends to be drawn tight into the biners? So put the skinny rope through the anchor and pull the big one after the rap? Did I get that right?

 

Also, what are the properties of a double fisherman that causes it to be more prone to getting stuck over an EDK? Is it just knot size or does it also have to do with how the rope falls through certain features when the strands project from each end of the knot (like a double fishermans) vs from the same side (like an EDK). Benches vs cracks? And so on...

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The EDK has a straight profile on one edge. The idea is that when the knot catches on an edge of some kind, it will flip out of the way allowing the straight side of the knot to pass the obstruction.

 

One_Sided_Overhand_Bend_WPK.JPG

 

It is no better in cracks.

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Intersting stuff. Just so I got it straight, if I put the fatter rope through the anchor ("pull the skinny rope"), because the skinny rope stretches more, the knot tends to be drawn tight into the biners? So put the skinny rope through the anchor and pull the big one after the rap? Did I get that right?

 

Err, I think you have it backwards? You want the thicker rope through the rap rings. Anyway, that's how we ended up doing it after getting it stuck, and it worked for another 8 or 9 raps or so. I don't think it's so much due to rope stretch, but due to the ropes being different diameters and passing through your device at different rates -- i.e. the rope didn't "Stretch tight" -- it actually slid and moved until the knot got caught.

 

When we placed the knot on the other side of the rings (i.e. pull skinny), the knot continued to move, but it moved away from the rap rings, instead of into them.

 

 

Edited by robmcdan

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Yes. I understand the knot movement issue (though your explanation has it backwards, it occurs when the skinny rope is through the anchor).

 

How does that result in stuck ropes?

Yes you are right, I had it backwards. If the knot is moving while the knot is under load and there happens to be a convenient crack waiting, it could jam. But if the knot moves very much you've got other things to worry about, like rapping off the end of your rope.

 

Anyway, if you are doing multiple rappels, it is most convenient to alternate which rope is through the anchor. To always have to put the fat one through, would be a pain in the neck.

Edited by catbirdseat

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