Self belay impact forces
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Question - I started setting routes at my local gym and they have us use a self belay with semi static ropes when we're setting routes with the grigri clipped to the belay loop and the 'climbers end' also clipped to the belay loop cause they don't believe in tying in... It just kind of freaks me out to take even 1-2 foot falls while trying moves because the catches even for a 1 ft fall are almost painful. Is the fall impact force a legitimate safety concern or am I just being a pansy |
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If you're talking about a worn out, over stretched rope when you say semi-static, plus youre doubling it up, sure that's going to result in some very abrupt stops. its not the most ideal scenario and as far as safety i would personally want some sort of backup, mainly bc of the static scenario and the single point (belay loop only). |
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If the gym has bombproof floor anchors, secure the "belayer's end" of the rope to that and run your Grigri up the "climber's end". The extra rope length in play will significantly reduce peak loads. Keep slack to a minimum and hope for the best. |
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Do what John says. Thats what I do when setting too. |
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According to Beal, semi-static rope is built to safely catch a fall of up to fall factor 0.3. This means that it is safe to take a 3 foot fall with 10 feet of rope out. Safety in this case is likely referring to force that it will put on your body, not force at which the rope will break. So a 4 foot fall on ten feet of rope will likely leave the rope in tact, but really hurt. |
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ill say it again. its not so much about specific methods. if any subject has been sufficiently beat to death on mp, its one of how to self belay (along with discussion of all elenors aliases). |
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AdamHertzberg wrote:According to Beal, semi-static rope is built to safely catch a fall of up to fall factor 0.3. This means that it is safe to take a 3 foot fall with 10 feet of rope out. Safety in this case is likely referring to force that it will put on your body, not force at which the rope will break. So a 4 foot fall on ten feet of rope will likely leave the rope in tact, but really hurt. In your situation, the rope is doubled up, so each strand is only getting half of your weight. This means that the impact forces will likely be doubled on both your body and the anchor because the rope will only stretch half as much and therefore won't absorb as much shock.No. That's not how it works. |
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Greg D wrote: No. That's not how it works.That's not very helpful unless you say why. |
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A doubled-up rope is stiffer than a single strand, but the force is proportional to the elongation (in the ideal case) which is less if you double up the rope. If you do the math, which is quite doable in the ideal case, you find that the force goes up not by a factor of two, but by a factor of square root of two. That gives you a ballpark figure for the real case, which is more complex to analyze due to damping, nonlinearities, the alignment of the planets, and what have you. |
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Thanks for the correction brenta. So really you end up with 1.4x (square root of 2) the forces in the theoretical world instead of 2x. I think it's still correct to say that the system is still no longer designed to take a 0.3 factor falls because more force than recommended is transferred to the body. |
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Also, is there a good site out there that deals with climbing math. I find it interesting, and I certainly don't want to clutter up general climbing forum with a topic that bores most. I explored going into an aerospace degree in college and some of the stuff came back to me when I started climbing, but I'm quite rusty as evidenced by my last few posts. |
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A toothed ascender such as a minitraxion strips the sheath of a rope at 4 KN |
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Thanks for all the replies guys, I guess I'm not gonna die - bearbreeder that is a good way to look at it indeed. |
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I've done it the gunkiemike way and short falls haven't been painful. |