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majid_sabet
Sep 30, 2009, 11:51 PM
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According to page 30 of this report, jumping up more than 20 cm (@ 8 inches) while belaying only increases fall distance and does not decrease peak force.
only 8 inches my man
http://www.xmission.com/...nd_Rescue_Belays.pdf
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hafilax
Oct 1, 2009, 2:28 AM
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I don't see any discussion or data that leads to that conclusion. There isn't any data that looks at the belayer moving. The data that is there seems to be based on belaying off of the wall which would be the same as a belayer of infinite mass. I have never seen anyone let as much rope slip through as in slides 25-27.
They may have more data somewhere that leads to that conclusion but it isn't on any of the slides as far as I can tell.
There is another figure of merit other than just the impact force. Longer falls are useful for protecting the leader from swinging hard into the wall and to clear a roof.
Finally, if the belayer far outweighs the leader then they will have to jump in order to achieve that 20cm limit. If 20cm is the optimum and anything more doesn't hurt but less does then it makes more sense to err on the side of jumping a bit too much since you never really know how hard a fall is going to be on the first go.
Having watched my friend sprain his ankle due to a hard catch I'm quite confident that there is good reason to provide dynamic belays.
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el_layclimber
Oct 1, 2009, 2:41 AM
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Numbers.. numbers.. picture... graph... math.. more numbers.. another picture. I don't understand this. Better just to use my head-brain.
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shockabuku
Oct 1, 2009, 3:39 AM
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In most scientific analyses simulations from a model are only that until verified by experiment. Hence the comment "In theory, there is no difference between theory and practice. But, in practice, there is."
- Jan L.A. Van de Snepscheut
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crazy_fingers84
Oct 2, 2009, 12:12 AM
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majid, did you help make this presentation? it sure looks a lot like some of your other handiwork...
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adatesman
Oct 2, 2009, 12:44 AM
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crazy_fingers84
Oct 2, 2009, 5:33 AM
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holy shit.
thank you for letting me know that exists.
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king_rat
Oct 2, 2009, 10:54 AM
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I’m to fat and lazy to read through all of that, and work out what it means. Can some one just read it and sum it up in one or two concise sentences.
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xtrmecat
Oct 2, 2009, 1:47 PM
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1 Ok, one or two sentences.
2 If in a rescue situation, be very diligent with the belay(brake bar rack), and if climbing, use yer noggin, and a little slip is gooder, but more to save the hard hits and on the overhangs and such.
Bob
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adatesman
Oct 2, 2009, 3:21 PM
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ClimbClimb
Oct 3, 2009, 6:36 PM
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That was an interesting read, although very focused towards towing, rather than climbing belay, applications. It certainly has math & physics thare are more credible than the "this is how non-nubes do it" mystical approach.
Incidentally, it's never been all that clear (and it's not really explained in this presentation) the exact mechanism of jumping up providing a soft catch, beyond the effect of extra slack and/or slippage.
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jt512
Oct 3, 2009, 10:08 PM
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ClimbClimb wrote: Incidentally, it's never been all that clear (and it's not really explained in this presentation) the exact mechanism of jumping up providing a soft catch, beyond the effect of extra slack and/or slippage.
What's there to know? A properly timed jump decreases the deceleration, and hence the impact force.
Jay
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ClimbClimb
Oct 3, 2009, 10:31 PM
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jt512 wrote: What's there to know? A properly timed jump decreases the deceleration, and hence the impact force.
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jt512
Oct 3, 2009, 11:26 PM
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ClimbClimb wrote: jt512 wrote: What's there to know? A properly timed jump decreases the deceleration, and hence the impact force. That general objective is clear enough... Does the benefit come from adding slack to the system (rope not yet under tension), attempting to match the speed of the fall, or merely "giving way" more gently (rope coming under tension, pulling a lighter belayer into the air w/o a jump, and a heavier belayer into the air with a jump). To my mind, all of these could result in "softer catch", but through different mechanisms. Indeed, this presentation seems to suggest that beneficial rope slipping through belay device may even be more important than jumping or other whole-body activities... .
It is easy to see that adding slack to the system normally increases the fall factor, and hence increases the impact force. Say h is the length of the fall and L the amount of rope out, each with no slack in the rope, then the no-slack fall factor
r_0 = h / L .
Now, add an amount of slack s to the rope. Then the new fall factor
r_1 = (h + s) / (L + s) ,
and note that, as s increases, r_1 approaches s / s = 1.
That is, the more slack there is in the rope, the closer the fall factor is to 1. Therefore, assuming that r_0 < 1, as it usually is, then adding slack increases the fall factor, and hence the impact force. So, the benefit of jumping cannot be due to increasing slack in the rope. Therefore, the belayer should not jump before the rope begins to stretch. Instead, the belayer should jump just as he feels tension in the rope. This increases the distance, and hence the time, over which the climber's velocity is decelerated, reducing the impact force.
Jay
(This post was edited by jt512 on Oct 4, 2009, 12:15 AM)
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olderic
Oct 4, 2009, 12:03 AM
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That would be a shame when the edge that is going to break his nose when he slams in needs 21 cm to clear...
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skelterjohn
Oct 4, 2009, 12:39 AM
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jt512 wrote: and note that, as s increases, r_1 approaches s / s = 1.
Consider the case where FF>1 initially.
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jbrown2
Oct 12, 2009, 9:06 PM
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Do any of you actully climb. If a climber falls ust catch them. I am sure you wont be able to jump just 8 CM intead of 20cm. Get over it. Ropes strech that is what they do. Lock off your belay device and stay on the ground. that is your job. Catch, no jumping. If it is a hard fall you'll be yanked up to where you need to be any way. Just Lock off.
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1080climber
Oct 12, 2009, 9:07 PM
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It may not decrease peak force but it will increase fall distance which mean the climber is less likely to slam into the wall.
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jt512
Oct 12, 2009, 9:49 PM
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jbrown2 wrote: Do any of you actully climb. If a climber falls ust catch them. I am sure you wont be able to jump just 8 CM intead of 20cm. Get over it. Ropes strech that is what they do. Lock off your belay device and stay on the ground. that is your job. Catch, no jumping. If it is a hard fall you'll be yanked up to where you need to be any way. Just Lock off.
Please never belay me.
Jay
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