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When rappelling and using an autoblock consisting of a french prusik knot it seems there may be some options on how and where to attach the autoblock to my harness.

Is there a correct placement on the leg loop to attach the autoblock?

Is there a correct orientation for the carabiner?

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for clarity, can you confirm that "autoblock" and "french prussik" are the same knot, and are distinct from a regular prussik? (I have used an autoblock for years, but didn't hear the term "french prussik" until now). –  DavidR Apr 30 '13 at 18:04
    
I think there are three types of knots that can be used: Prusik, klemheist, and French Prusik. The French Prusik is described here: climber.co.uk/skills/skills/how-to-prusik.html –  Ben Crowell Apr 30 '13 at 21:38
    
@DavidR I've seen all three knots mentioned by Ben Crowell called an "autoblock" knot. I think in that sense it's referring to the purpose rather than a specific knot. –  STW May 1 '13 at 11:57
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No! There's an important distinction. :) An "ordinary Prussik" can catch the entire weight of your body directly on itself, without even loading the belay device. And it can't be released unless you can get your weight off the knot. If this happened during a free-hanging rappel, you'd have to perform a self-rescue or you'd be stuck. A "French Prussik" can't grip that tightly, and can only be used to help the main belay device engage. And I actually had a regular prussik jam on me once (in a rescue practice), it was very hard to fix. –  DavidR May 1 '13 at 20:40
    
there are two systems for backing up a rappel - regular prussik above the rappel device, or french prussik below the belay device. When the regular prussik is above the device, and it catches, its straightforward (but difficult) to unload it. If you put a regular prussik below the device, and it manages to catch your whole bodyweight, there isn't a direct way to fix it without breaking out more complex techniques. People argue about which of the 2 systems to use, but you can't mix and match them. :) –  DavidR May 1 '13 at 20:43

2 Answers 2

up vote 5 down vote accepted

Leg loop verses main belay loop

You'll find recommendations for placing an autoblock on either the leg loop or the main belay loop. Some concerns about either placement are:

Leg Loops

  • If you're placing an autoblock on your leg loop AND you're using a "quick release" buckle, the carabiner that's holding the autoblock can catch on the buckle and release the tension on that leg loop. It probably wouldn't make you fall out of your harness, but would be uncomfortable, and you'd hang at an odd angle.
  • The leg loops of a harness aren't strength-tested independently of the harness as a unit. Also, if the autoblock did manage to catch the entire weight of the climber, it would place a different stress on the leg loop than a person normally weighting the harness would. If you had a harness with stitched leg loops, like the Black Diamond Chaos, the weight of the autoblock would be pulling the stitches apart (this is not the direction of force you have when you weight a harness normally).
  • I no longer put my autoblock on my leg loop for the reasons I mention above. I originally did, when I had a different harness, but my 2 current harnesses are the new Misty Mountain Cadallac, and the Black Diamond Chaos, one has quick release buckes, and the other has stiched leg loops.

Belay Loop

  • If you're placing an autoblock on your belay loop, you have to make sure and extend your belay device when rappeling (using a sling or something like the Metolius PAS). This issue is that if the loops of the autoblock are too close to the belay device, they can get caught in it, and jam the rappel system (probably making you unable to decend the rope, but possibly forcing the rope to "fail open"). If you got stuck in this fashion, it would be pretty difficult to free yourself, esp. if you were in a free-hanging rappel.

Sorry for not giving a single recommendation, as in many situations in climbing, there are multiple options, and its important to know what the tradeoffs are.

Carabiner

As per usual, you want to make sure your carabiner is loaded along its spine. That is, longways. So, one end of the carabiner will be running through the leg loop or belay loop, and the other end will have both strands of the autoblock coord.

What I do

I place my autoblock on my main belay loop, then extend my belay device with a PAS. The rope runs down between my legs, and I can use either hand to control my descent. My autoblock is made out of coord that I measured and cut specifically to be too short to catch up in my extended belay device.

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"they can get caught in it, and jam the rappel system" More often than not, I find most belay devices to be "prussik minding." That is, they will push the prussik down the rope and prevent it from locking, which is even worse than jamming in that situation, I think. –  Nisan.H May 1 '13 at 8:25
    
Do leg loops have any standard strength rating? Would there be any concern about them catching a slip/short fall--even though they only need to catch enough weight to apply the brake and not the full weight of the fall? –  STW May 1 '13 at 12:38
    
@STW - I don't think that leg loops are independently rated, so your concern is valid... and actually, I think the strength of leg loops may vary greatly if the autoblock caught the full weight of the fall (I'm thinking of stitched leg loops now, where that loading would pull at the stitching.) –  DavidR May 1 '13 at 13:45
    
@STW: This is why it's better to attach the Prusik below the belay device rather than above it. The belay device amplifies the braking force by a large factor, which is why you can brake yourself without burning your hands on the rope. Only a small force is needed from the leg loop in order to stop you. –  Ben Crowell May 3 '13 at 16:47

I recently switched to the autoblock-on-belay-loop, ATC extended on a sling system. Partly because of the risk of an autoblock krab on a leg loop releasing the leg loop buckle, but mainly because I find the extended system generally more conventient and controllable.

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