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Does anyone know the strength of an ATC when used in a multi-pitch set up? It occurred to me that the entire weight of a climber is resting on the strength of the ATC clip and not on any carabiner. I know it is not likely, but say if the belayer did not take in slack while the climber was climbing and took a large fall, how much force would be exerted on the ATC?

I've looked at Black Diamond, and Petzl, neither of which have KN ratings on their multi-pitch belay devices. Any clue why?

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up vote 5 down vote accepted

Aha! I knew I'd read something about this somewhere, and here it is. Scroll down to the test done by Jim Titt.

He pull tested an ATC in guide mode. Quick summary: the device itself doesn't fail or suffer damage. With a 10mm rope the ATC jams severely at 4.8 kN, breaks the rope at 9 kN. I don't know how to work out how much of a fall the second would have to take to get to 4.8 kN, but probably quite a bit; s/he certainly ain't getting to nine.

With an 8mm rope it's a little more alarming; device jams at 2 kN, locking fails at 4 kN. I imagine 2 kN would be pretty easy for a dropped follower to achieve, so anybody belaying two followers simultaneously on skinny double ropes evidently needs to pay attention.

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That's a good point. I hadn't thought of two followers.. Thanks for answering my curiosity. :) – michael Nov 20 '13 at 23:42

Welcome to outdoors.SE!

It occurred to me that the entire weight of a climber is resting on the strength of the ATC clip and not on any carabiner.

This is not quite right. My first quibble is that in the situation you describe where there was slack in the rope, the load is dynamic, so it's much greater than the weight of the climber. (Sorry, I'm a physics teacher, so I get pedantic sometimes.) The second issue is that it's not true that there is no load on any carabiner. The carabiner that is clipped into the ATC is acted on by a downward force L equal to the load of the falling climber, an upward force A from the anchor, and a small force B (probably upward) from the brake strand. The ATC is designed to make the rope jam while B is still very small. So to a pretty good approximation, Newton's first law applied to the carabiner says that A=L, and the carabiner feels a stress L, i.e., it's fully loaded with the dynamic force of the falling climber.

I think it's true that in this situation, where the second falls and the belayer has left slack, the force transmitted to the anchor can be considerably greater than the force that would be transmitted to the belay station in a normal lead fall. To clarify this, let's consider a normal lead fall.

In a normal lead fall, the rope forms an inverted U around the top piece of protection, and in this U configuration, the friction in the U causes the tension in the belayer's strand to be lower than the tension in the climber's strand by a large factor. The exact factor depends on coefficients of friction, but sources I've seen for typical friction between a steel biner and a climbing rope put it at about 5 to 13. Although this is nice for the belayer because it greatly reduces the tension in the rope at the belay station, it comes at a price, because force is bring transmitted to the top piece of protection, which could pull out.

So in your situation with the second falling on a slack rope, the good news is that he's not loading some crappy, non-redundant nut or cam that was placed somewhere because there was no opportunity to do any better. He's loading the anchor at the top of the pitch, which is presumably redundant and carefully constructed.

I guess you were concerned about the stresses in the block of the ATC itself, and whether they would crack the block open. The thing is, these are forces exerted on the ATC by the rope, and by Newton's third law the ATC is exerting equal forces back on the corresponding parts of the rope. The block of the ATC is made of metal, and the rope is made of nylon. If something is going to fail in this part of the system, I think it's the rope, not the block of the ATC.

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"The block of the ATC is made of steel" .... erm, some kind of aluminium alloy. Your general point is right though, failure of the body of the ATC is simply not a concern. – AlanL Nov 20 '13 at 12:10
Well hello there Mr. Physics Teacher! :) So it sounds like the more likely scenario is that the rope will slide through the ATC can could cause a rope burn/ human failure. I really was just wondering if the companies had load tested the devices on those clips used for guide mode (carabiners all have rating on them so I was wondering why those didn't). The weight is technically passing through the ATC's guide clip on to the other anchors etc. so hopefully they designed them well so that they are not the weakest link in the chain, so to speak. Thanks. – michael Nov 20 '13 at 23:52

say if the belayer did not take in slack while the climber was climbing and took a large fall, how much force would be exerted on the ATC?

I assume we're talking about a follower being belayed with ATC in guide/autoblocking mode? Then even the most determinedly incompetent/malicious belayer cannot produce more than a factor one fall. Are you familier with the concept of fall factors? In this case, the unfortunate second climbs all the way up to the stance, without the belayer taking in any rope whatsoever, then falls all the way back down again. The distance s/he falls is equal to the total amount of rope in use: fall factor 1.

Unpleasant for the falling climber, especially if s/he hits something on the way down, but nowehere near the worst case scenario for force on the ATC. That comes with a factor two fall, where the leader falls off above the belay, without any intermediate gear, and falls twice as far as the amount of rope in use.

Now, in a factor two scenario I personally do have serious concerns about the safety of an ATC, not because of the device itself breaking but because of the rather limited braking force it provides on modern slick, skinny ropes. IMO it is seriously questionable whether many belayers would be able to hold a factor two fall on an ATC. The direction of pull is down whereas people are used to holding upwards pulls. Some rope is definitely going to run through, and most people these days don't belay with gloves, so ropeburn is pretty much a given. Etc.

For these reasons I'm seriously looking for an alternative with semi-automatic assisted braking for multipitch. The Alpine Up is big, heavy and looks dauntingly complicated; the Edelrid Mega Jul is nice but I've read about some quality control issues. Will probably buy the second generation Jul when Edelrid have ironed the bugs out.

(Otoh many people argue that the ATC is advantageous with marginal trad gear because its relatively soft braking action reduces the impact force. This is understandable, but based on some tests I've seen it seems to be exaggerated. Top Canadian ice climber Will Gadd for example is on record favouring a grigri for ice climbing these days.)

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