Self-locking descender for rappelling

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Re: Self-locking descender for rappelling

Postby hank moon » Jul 4, 2008 10:52 am

OpenTrackRacer wrote:I was actually planning on using it above my ATC but within easy reach and just short of engaging it by putting tension on the tether. Having it below would work too but I like the have the ATC close in and my hand further up on the rope (or Shunt). I guess if I used the Stop my hand would have to be low however unless I used a sling.

I think having the Shunt below the ATC would also make it harder to unload the Shunt if it's activated since you'd have less leverage with the ATC further away.


Shunt below is far easier to unlock b/c the load on it is greatly reduced by friction in the rap device. Imagination will only take one so far...just try it! :grin:

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Re: Self-locking descender for rappelling

Postby ek » Jul 4, 2008 11:30 am

Grabbing the body of the shunt (rather than the carabiner) will cause the shunt to slide, and in this way the Shunt is subject to the same panic problems as a single-stop descender.

You may find, as NZcaver suggests, that it is not so easy to unweight the shunt. Without a separate ascender, you'll probably be unable to do it. Even with an ascender, you'll need to lock off your descender. This difficulty in transferring weight off it is something you may encounter with it below the descender, but it's something you'll definitely encounter with it above the descender.

I believe Hank Moon is suggesting this technique for placing the shunt or friction hitch below the descender. This technique has the advantage that it interferes with your ability to rappel normally the least. It is also at least as safe as rappelling normally. (Though I would take issue with the climber in the picture clipping into the carabiner that it is on just one of the pitons. Not only does he take a shockload onto slingsif that piton blows out, but he probably shockloads the carabiner along the short axis. He should instead clip into the power point of the anchor system, and the only disadvantage is that he will then start his rappel slightly lower.

The disadvantage of doing it this way is that normal rappelling consists of a firm grip with the brake hand, and above that, a light grip with the prusik-minding hand. Slowing down consists of a harder grip with the brake hand and, above that, the same light grip with the prusik-minding hand. If you were to lose your brake hand, your instinctive response might not be to either let go or grab hard on the prusik above, but rather to maintain your light prusik-mind behavior. Certainly, once you realized you were falling, you would either wisely let go of it, or foolishly grab it, and either action would increase tension on the brake end of the rope and bring the rappel under control. But by that time, the friction hitch could be sliding fast enough to melt when it catches. (I don't know what a shunt does when it is released into a fast-moving rope. However, if you instinctively grab the body of the shunt in this situation, you keep falling.)

I would consider the method which (I am saying) Hank is advocating outright dangerous and extremely unlikely to succeed at saving your life when done with a shunt. With a friction hitch, I think it would be more likely to succeed...but it would be interested to know if there is real-world evidence to support that.

Anchoring a friction hitch to the leg loop has the disadvantages Hank mentions, that (1) it will be minded or munched, and consequently most likely fail to catch, if it is ever able to reach the descender, and (2) leg loops are not generally designed to be loaded in this way. But if you do succeed at getting the geometry right and you rappel by holding your brake hand on the hitch, then if you grab the hitch harder in a fall, you're doing exactly what you're supposed to do--increasing your hold on the brake end of the rope. (Which is the same effect with the Petzl method.) On the other hand, if you were to let go, it would grab by itself (also the same effect with the Petzl method). Unlike with the Petzl method though, there is no normal behavior that could extend in a panic situation to minding the friction hitch without applying a firm hold on the brake end of the rope. Nobody applies a moderately light hold with the brake hand while rappelling, as a result of panic.

On the issue of how to extend a point of attachment to a cave rope, I recommend a cowstail made from dynamic single rope (the kind climbers would use by itself without another rope, as the primary shock-absorbing component of their fall arrest system). For the most shock absorption, you should probably use a figure-eight on a bight on an end where you want a loop of fixed size, and a barrel knot on an end where you want the loop to contract around a connector and hold it in place. There's a discussion of these matters in this long thread.
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Re: Self-locking descender for rappelling

Postby hank moon » Jul 4, 2008 12:01 pm

ek wrote:Grabbing the body of the shunt (rather than the carabiner) will cause the shunt to slide, and in this way the Shunt is subject to the same panic problems as a single-stop descender.

Sorta...except simply gripping the Shunt does not defeat it.

ek wrote:You may find, as NZcaver suggests, that it is not so easy to unweight the shunt. Without a separate ascender, you'll probably be unable to do it. Even with an ascender, you'll need to lock off your descender. This difficulty in transferring weight off it is something you may encounter with it below the descender, but it's something you'll definitely encounter with it above the descender.

The Shunt is easily unweighted w/o an ascender - that's what the little hole is for! Rope Access Technicians use the Shunt as a backup device on a second rope by attaching a small cord to this hole and gripping it loosely between the fingers of the non-brake hand. Key is to avoid using a thick cord and/or tying a knot or making any other grab-able point along the cord. When done correctly, the thin cord makes It easy enough to tow the Shunt down the rope, but the hold on the cord is tenuous enough so that releasing it is likely in case of a fall. This is the only method that should be used if the Shunt is to be positioned above the device. Please feel free to call me or contact me at Petzl for more info on this method (which is not officially sanctioned by Petzl, btw, but used worldwide by thousands of RATs).

ek wrote:I believe Hank Moon is suggesting this technique for placing the shunt or friction hitch below the descender.

More or less, but not necessarily with the knotted shoulder-length sling or clip-in method. The take-away is the extended device - all else in the Petzl diagram should be ignored for focus.

ek wrote:The disadvantage of doing it this way is that normal rappelling consists of a firm grip with the brake hand, and above that, a light grip with the prusik-minding hand. Slowing down consists of a harder grip with the brake hand and, above that, the same light grip with the prusik-minding hand. If you were to lose your brake hand, your instinctive response might not be to either let go or grab hard on the prusik above, but rather to maintain your light prusik-mind behavior. Certainly, once you realized you were falling, you would either wisely let go of it, or foolishly grab it, and either action would increase tension on the brake end of the rope and bring the rappel under control. But by that time, the friction hitch could be sliding fast enough to melt when it catches. (I don't know what a shunt does when it is released into a fast-moving rope. However, if you instinctively grab the body of the shunt in this situation, you keep falling.)


It is useful to imagine scenarios, but not to draw conclusions from them. Some real-world testing is required to make reasonable conclusions.

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Re: Self-locking descender for rappelling

Postby OpenTrackRacer » Jul 4, 2008 1:12 pm

I understand what you're talking about Hank but I still think I'd prefer it to be over. I have to rig it up again and mess around. While I haven't gotten a chance to test the Shunt "on rope" yet I did rig everything up at the REI store to a tied off rope and experimented with the motions. Towing the Shunt with a small cord is a good idea but I wonder if trying to hold it loosely and still hold onto the rope above the ATC will be workable for me. My intention was indeed to hold onto the body of the Shunt while descending. In a uncontrolled descent (with me not being incapacitated) from losing the rope or an ATC/carabiner failure all you have to to is extend your arm to load the tether and lock the shunt. Letting go is not required if your tether is the right length.

I believe it'll be pretty easy to unweight the Shunt in either location if the tether is the right length.

Obviously, I need to go experiment some more. I'm trying to find a place I can do that right now. At this point I need to decide between the Stop or an ATC and Shunt.

Speaking of the ATC... what do you think about slow 100-300' descents on an ATC? Too much heat to get rid of?

Again, thank you guys VERY much for all the feedback and comments!
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Re: Self-locking descender for rappelling

Postby OpenTrackRacer » Jul 4, 2008 1:34 pm

Hank, I have a few questions for you as a "Petzl person" if you don't mind...

For 11mm PMI Pit Rope (typically dusty but not muddy), would you recommend an ATC and Shunt or a Stop? Again, the application is not big long rappels but shorter vertical descents slow speeds. We're talking 50 to 300 feet max. For sloping descents (decline shafts) I'd use just the ATC.

If the Shunt is used, do you think that the Spelegyca cows tail (from my Frog system) is an suitable tether?

Thanks!
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Re: Self-locking descender for rappelling

Postby hank moon » Jul 4, 2008 1:41 pm

Hi Mike

It takes a lot of training to be able to use any "let go to stop" system as a voluntary backup system. The natural reaction when out of control is to "panic" and grab stuff - most often defeating the very device you were counting on to catch you. One way to help reduce the effects of this reaction is to get one of the "panic brake" devices already discussed (Anthron, SRT Double Stop, etc.). A lower-tech way to mitigate the problem is the friction hitch on the leg loop that ek described. However, that introduces other problems already discussed. Pros and cons...and then the final decision which only you can make, based on your own personal safety considerations..

Good luck with whatever system you settle on. I have done many 200-300' descents with an ATC XP, but not with a regular ATC (I normally use 8 or 9 mm rope). The XP works fine, but wears out relatively quickly. I haven't tried this device, but you might want to check it out:

http://www.dmmclimbing.com/productsDeta ... =6&id2=100

I'm getting one this summer with the hope that it will last longer than AL devices (it ought to).

One last point: the Shunt makes a pretty good ascender and if used above the device, is already positioned for easy changeover. The Stop *can* be used to ascend, but it can be very hard to pull the rope back through and tends to creep. Not the greatest for ascending...

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Re: Self-locking descender for rappelling

Postby NZcaver » Jul 4, 2008 2:32 pm

hank moon wrote:The Shunt is easily unweighted w/o an ascender - that's what the little hole is for! Rope Access Technicians use the Shunt as a backup device on a second rope by attaching a small cord to this hole and gripping it loosely between the fingers of the non-brake hand. Key is to avoid using a thick cord and/or tying a knot or making any other grab-able point along the cord. When done correctly, the thin cord makes It easy enough to tow the Shunt down the rope, but the hold on the cord is tenuous enough so that releasing it is likely in case of a fall. This is the only method that should be used if the Shunt is to be positioned above the device.

Using a Shunt, I was shown a method similar to what you are describing but with the addition of a small stopper knot. I rigged my Shunt this way after having it explained by a certain RAT/NCRC guru named Ken a few years ago.

I don't have my Shunt with me, but I recall it has about a 10cm tail of approximately 4mm cord with a barrel knot (no bight) in the free end. The idea is that you pinch the end of the cord between the base of two of your fingers, and make a fist. The stopper knot sits comfortably concealed in your first, and pulls the Shunt down the rope. However if you descend too fast and outpace your safety, the small stopper will pop through your fingers and the Shunt will lock. It doesn't matter how tight your grip is.

All that said, I still feel Mike's needs would be better suited to his using an autostop descender. Where are you located, Mike? You might consider trying to contact local cavers. Some may let you try out their gear under supervision. Click on this grotto finder to locate local chapters of the NSS. You might even get to like caving too.
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Re: Self-locking descender for rappelling

Postby OpenTrackRacer » Jul 4, 2008 3:06 pm

I'm trying to get in touch with the San Diego Grotto. Two of the three e-mail addresses I've found for them bounce and there has been no reply from the third. I think we'd like caving a lot!

I'm working on finding a place to test out at least the Stop with my rope. The local climbing gyms aren't really helpful for that but there is a climbing store I just found out about (closed today of course) that I'm going to check into as well.

I agree completely about the required training for taking counterintuative action. The original idea with this whole thing was to have some sort of backup if the person descending was knocked out or stunned. In that case, no action is needed at all. Having a second safety for rappelling in other cases is secondary. We certainly don't have it now using a figure 8 or ATC. With all that in mind, either solution puts us ahead of where we are now.

All in all, I believe losing the rope is unlikely. You don't see this happen very often.

The Stop would really seem to be a good choice. It offers the safety and is undoubtably a better choice as a descender over an ATC or figure 8. I think the only reason I'm hesitating is because I'm worried about how it'll feed our 11mm Pit Rope.

There in lies the problem...
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Re: Self-locking descender for rappelling

Postby OpenTrackRacer » Jul 4, 2008 3:33 pm

By the way... I'd like to know what you think about the I'D for this application Hank.

Thanks!
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Re: Self-locking descender for rappelling

Postby ek » Jul 4, 2008 11:27 pm

hank moon wrote:The Shunt is easily unweighted w/o an ascender - that's what the little hole is for! Rope Access Technicians use the Shunt as a backup device on a second rope by attaching a small cord to this hole and gripping it loosely between the fingers of the non-brake hand. Key is to avoid using a thick cord and/or tying a knot or making any other grab-able point along the cord. When done correctly, the thin cord makes It easy enough to tow the Shunt down the rope, but the hold on the cord is tenuous enough so that releasing it is likely in case of a fall. This is the only method that should be used if the Shunt is to be positioned above the device. Please feel free to call me or contact me at Petzl for more info on this method (which is not officially sanctioned by Petzl, btw, but used worldwide by thousands of RATs).

Hmm...this rings a bell. I believe I will be contacting you for more information about this technique. Did you want me to use your Petzl contact info?

So...given this, why do you still counsel that the shunt should be placed below the descender?

hank moon wrote:It is useful to imagine scenarios, but not to draw conclusions from them. Some real-world testing is required to make reasonable conclusions.

My point exactly. Attachment of the friction hitch to the leg loop, with everything measured out right and tested to make sure it can't contact the descender, is shown to be effective in cases where someone loses control on rappel. Is similar evidence available for the method where the descender is elevated above the hitch and the hitch is minded by the non-brake hand?
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Re: Self-locking descender for rappelling

Postby OpenTrackRacer » Jul 5, 2008 6:03 pm

Afternoon all! Just got back from the climbing store with some observations.

I was able to hook my dirty PMI Pit Rope up to a (low) overhead anchor and test the Stop and GriGri (they didn't have any Shunts). I tried descending (all of three feet) with the excess rope on the ground and also wound up so it's full weight was hanging. So...

The Stop seemed to work fine. Descents were easy to control with the brake hand on the rope and it reliably stopped when the handle was released. Descent was fairly slow but not unreasonable.

The GriGri worked too but descents were harder to control with the brake hand on the rope. With the level pulled all the way back descent was very fast and difficult to brake. It seemed like you'd have to use the handle on the GriGri to control your descent which breaks the rules. Still, you could really zip down the rope if you wanted and it definitely stopped when released.

As I mentioned, I wasn't able to try the Shunt but I have been thinking about it more based on some of the recent posts. I like the idea of tying a piece of thin accessory cord to the release eyelet. You can just put that between the fingers of your upper rope hand and grip the rope (below the Shunt and above the ATC) like usual. If you start to fall, either the cord will pull out and you'll stop or you'll grip the rope hard and as your arm extends the Shunt will hit your hand and then lock and you'll stop. All in all a very nice system.

Based on my tests, I'm thinking the Stop is probably the best way to go since it seems to feed okay on the rope. I still like the idea of two attachment points using the Shunt and ATC but I can always try a Prussick (or French Wrap) with the Stop and experiment.

Thoughts or comments?
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Re: Self-locking descender for rappelling

Postby ek » Jul 5, 2008 7:45 pm

OpenTrackRacer wrote:I was able to hook my dirty PMI Pit Rope up to a (low) overhead anchor and test the Stop and GriGri (they didn't have any Shunts). I tried descending (all of three feet) with the excess rope on the ground and also wound up so it's full weight was hanging.

I believe this does not quite simulate the tension you might have below you on your rope in real-world conditions. If there is rope drag below you while rappelling (i.e. if the rope is rubbing on the walls and is difficult to lift) then that might produce more apparent weight on the rope under some conditions.

OpenTrackRacer wrote:The GriGri worked too but descents were harder to control with the brake hand on the rope. With the level pulled all the way back descent was very fast and difficult to brake. It seemed like you'd have to use the handle on the GriGri to control your descent which breaks the rules.

The alternative would be to do something that any rappeller using any device under any circumstances should know well how to do (for emergencies at least)--pull the rope behind your back with your brake hand and use your body to add more friction to the brake end of the rope.

OpenTrackRacer wrote:Based on my tests, I'm thinking the Stop is probably the best way to go since it seems to feed okay on the rope. I still like the idea of two attachment points using the Shunt and ATC but I can always try a Prussick (or French Wrap) with the Stop and experiment.

If you're going to use a friction hitch (the prusik is perhaps not the best option because it is very difficult to release under load, even when placed below the descender--the autobloc, a.k.a. "French wrap" may be a better choice, but then if you want it to be a separate point of attachment to the rope, the autobloc by itself has a much more tenuous hold than a prusik) or the shunt then perhaps you should also consider other descenders that don't implement automatic braking, like a long-frame BMS micro-rack. Such a device is not unreasonably unwieldy (whereas a full-size J-rack is a boatanchor when considered alongside the ATC or Petzl Stop), has good thermal characteristics, and provides for very fine-grained control of rappelling speed.
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Re: Self-locking descender for rappelling

Postby OpenTrackRacer » Jul 5, 2008 11:26 pm

Yep, that's exactly what I had to do with the GriGri to be able to control the rate of descent without using the handle.

ek wrote:The alternative would be to do something that any rappeller using any device under any circumstances should know well how to do (for emergencies at least)--pull the rope behind your back with your brake hand and use your body to add more friction to the brake end of the rope.
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Re: Self-locking descender for rappelling

Postby ek » Jul 5, 2008 11:30 pm

You could also--though the geometry of the situation might be a little strange and awkward--try using a braking carabiner with the Grigri, as you would with a bobbin descender such as a Stop or a Simple. Petzl appears to indicate their approval of this practice by specifying the Grigri as one of the devices with which you may use their Freino carabiner.
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Re: Self-locking descender for rappelling

Postby OpenTrackRacer » Jul 6, 2008 1:23 am

Yeah, I noticed the Freino when I was looking at Petzl's site a while back. Not a bad idea at all. It would reduce the chance of gear conflicting or hitting the gate when using the Stop.

I think the Stop is the way to go but I'm curious to hear Hank's (and anyone else's) thoughts on my testing.
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