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Francis
Trad climber
San Francisco
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Topic Author's Reply - Apr 20, 2009 - 01:06pm PT
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This Just in from the UIAA themselves:
Dear Francis Baker
The half ropes are for being used alternatively one strand in a runner and
the other in the next runner. Therefore when falling the climber just fall
on one strand, so the UIAA has considered that one strand of half rope must
be able to withstand once the fall of a climber of 80 kg in fall factor 2 .
But doing just a test with one fall is not very convenient and reproducible
so the UIAA has searched for which mass, 5 falls are equivalent to 1 fall
with 80 kg. The following formula has been found by experimentation :
N x M 4 = Constant
where N is the number of falls withstand by the rope when a mass M is used.
Using this formula we find that withstanding 1 fall with 80 kg is equivalent
to withstanding 5 falls with 55 kg . ( 1 x 80 4 = 5 x 55 4 )
I agree that’s need some explanations!
Best regards,
Jean-Franck Charlet UIAA SAFCOM President"
So there you have it... I could blather on more, but most of it has been covered above.
have a fun, safe, adventurous climb!
f
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tradmanclimbs
Ice climber
Pomfert VT
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Apr 20, 2009 - 01:22pm PT
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Arne, if the climb is of the nature that a fall may likly result in the zippering of a significant ammount of protection then INMOP the doubble rope system is the best way to tackle this problem. the only advantage the single rope has in this area is that it is easier to transport to the cliff;)
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dirtineye
Trad climber
the south
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Apr 20, 2009 - 01:32pm PT
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Impact force felt by climber is NOT EQUAL to total force on gear.
Stop thinking that.
the top piece feels just as much force, only spread out over more time, with possibly a lower peak value, that could be possible.
Ask Rgold.
maybe I'm wrong, I don't think too well these days.
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tradmanclimbs
Ice climber
Pomfert VT
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Apr 20, 2009 - 01:36pm PT
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I am shure that you tech guys can explaine aLL that but shouldn't a rope with a softer impact force rateing also provide a softer load at the top piece?
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tradmanclimbs
Ice climber
Pomfert VT
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Apr 20, 2009 - 02:11pm PT
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osmo is just talking out his butt.
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Osmo
Trad climber
Calgary, Alberta
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Apr 22, 2009 - 07:30pm PT
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OK guys; now we seem to be getting somewhere- I don't mind the occasional jibe as I have a sense of humor, and 'sense' that Tradman is starting to feel constructive through all our back-and-forth; and in spite of his last remark--he seems to be quite a fun guy.
So, by the way, Tradman, definitely a rope with a softer impact force rating should provide a softer load at the top piece IF THE RATING IS CORRECT for the situation. In the case of the UIAA-type tests, a rope with a lower impact force providED a lower load at the top piece IN THAT TEST, since there is only one tension in a specific section of rope between 2 contact points; but that rating is NOT CORRECT for a climbing-fall situation and can not be 'infrapolated' (if you like) to climbing falls.
To Del: I'm not just going from experience and intuition, as I'm an engineer, and familiar with the basic physics of stretchy materials, which are well-documented: climbing ropes are made of typical stretchy materials.
This will be another long one--sorry, there's no other way, as there's a lot to say, as you said, "ropes are complicated". So get out a piece of paper and a pen, read carefully, and try to draw the graph I'll describe--I wish there was a way to include a sketch in this forum, so you wouldn't have to draw it yourself.
It's possible for a thin rope to overtake or even 'out-impact' a thick rope IN THE DROP-TEST, and that can be explained because the tension curves sweep upward in the plastic zone and since the thin rope is farther into the plastic zone, its curve is steeper and so converges toward the extension of the thick rope's curve, which has been above it from the onset of tension.
I know that sounds like B.S. but it's important; also it's easy to understand in graph form, with tension on the vertical axis and %-stretch on the horizontal--draw and label the 2 graph axes: both graphs start from the "origin" (viz. zero tension, zero stretch) and go up and right as (essencially) straight lines, with the line for the thick rope being above that for the thin rope--draw 2 straight lines angling up and right from the junction of the graph axes at different angles. At some point along the stretch axis, both ropes leave the elastic zone and curve steeply upward, but shortly after that, the upper curve (for the thick rope) STOPS, because the big rope has absorbed all the energy of the drop--draw a horizontal line across the graph, through the end of the upper curve; however, the thin rope is 'weaker', with lower tensions all along, and has NOT yet absorbed the drop energy at that % of elongation, and so it continues to stretch, plastically, and its graph continues right and upward toward the horizontal line representing the peak or impact force of the thick rope, and so could in some cases, reach or cross it, depending on the specific polymer, comparative rope cross-sectional areas, etc.
At some %-elongation (maybe say 20, and different for all materials), a vertical line crosses the graphs for both ropes (which should be the same material, as we're comparing rope sizes, not the materials) right at the point where they both change from straight to curved lines--draw a vertical line. All climbing falls should end to the left of this vertical line, and that is arranged by a suitable combination of climber size, rope size, and fall factor--you'll notice that the tensions for the thick rope are higher than for the thin rope throughout--those are the real-world climbing-fall impact forces that we've been talking about.
Meanwhile, the UIAA drop test is necessarily to the right, as they need to break each rope within a reasonable number of drops in order to be able to document its "ultimate strength".
The drop mass of 80kg is suitable for a typical climber, but the usual ropes tested for use as single lead ropes MIGHT NEVER BREAK if tested at typical climbing fall factors and setups, so they are instead tested in an extreme and unrealistic setup, with a static belay, rigid 'protection', and a fall factor of about 1.7-1.8. Because of the curving graphs beyond the climbing-fall (elastic) range, the impact force numbers from the UIAA tests have no relevance whatever to the climbing scenario, except that impact forces in climbing falls are MUCH lower, and "falls held" could realistically be rated much higher, quite seriously 100 or more.
As for hanging a weight on the end of each (thick or thin) rope, for sure the thin rope stretches farther, and that absolutely CAN be extrapolated to climbing falls, as both cases are in the elastic range of lead-ropes for normal climbing situations. Again, results from the UIAA drop-tests can not be extended to climbing, as those tests are beyond the elastic range of lead-ropes.
One really scary trend is that climbers are using thinner and thinner ropes, meaning to use them as doubles or twins, but occasionally someone will decide to misuse the "ropes never break" concept, and lead on just one of them as a single; and if he takes a major fall, then at some point in the circumstances: maybe 250pd climber, falling directly on a locked-off belay on a 6mm rope, possibly even a 7mm or so, the rope will snap and send him to the scree. Thinner ropes are stretchier if used as intended, and so will stop a falling climber more gently, but they also have lower energy-absorbing ability than thick ropes, and so if misused, they are more likely to fail in a severe fall.
Finally, the rope physics model (as I describe it) CAN be trusted, but no doubt everyone should feel a lot better if we had some data from climbing-realistic drop tests to back it up.
Jim Ewing was only doing his bit by providing the tests specified by the UIAA, PLUS extending them to sub-single ropes being used as single ropes, so he can't be faulted for that, and even the unrealistic UIAA test setup must be understood with some sympathy on the basis that I've already stated twice.
But now I propose a few simple drop-tests toward the "serious" end of the climbing-fall range, to provide some meaningful numbers for climbers (for the first time?): 80kg weight, whatever rope might be considered for leading singly on, fine, go ahead and use a static and rigid belay (as that may be achieved in real-world climbing), and a fall factor of perhaps 0.5! I think 0.5 is pretty high as falls go: lead out 75ft., place a final piece, then climb another 25ft. and fall off--50ft. fall on 100 ft. of rope. Data from such a test could be related to the real world of climbing.
Ideally, each rope would be tested for energy absorption at the top of it's elastic range, and the results provided to buyers in a graph form relating climber weight to maximum allowable fall factor, or such. Until we have that, climbers have little to go on except their experience and intuition, unless they deign to believe well-established physics.
Osmo
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Osmo
Trad climber
Calgary, Alberta
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Apr 22, 2009 - 08:07pm PT
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Hello Dirtineye; Impact Force (say Fmax) is generally taken to mean the maximum tension in the rope--that's the peak force that the falling climber feels, and it also reaches and pulls on, the top piece of protection unless the rope touches anything in between the climber and the pro., which would slightly reduce the force reaching the pro. The rope crab on the top piece acts as a poor pulley (I usually consider it about 50% from my own crude tests), so in that case, half of the tension on the climber's side gets to the belay side of the protection. As a result, the peak force on the top piece is Fmax + 0.5Fmax, or 1.5Fmax. That's another reason that climbers should be so interested in minimizing impact force: the protection gets hit harder by a fall, than the climber or the rope.
Osmo
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dirtineye
Trad climber
the south
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Apr 22, 2009 - 08:14pm PT
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Osmo, you sould really confused. YOu ahve misuesed some terms. I'm not listnening any more LOL
Now, wait for Rgold to give you the correct explnaation, nad look for JIm ewing to show up and do the same.
Um, before I got wasted by chemo and other crap, I did indeed have a math/physics degree, and graduate school in math, so at least I know who to listen to, and that woudl be Rgold (professor of the very stuff we are discussing, and Jim ewing, rope engiineer, one of the few in the country actually working as a rope designer.
I tend to trust what they say over, um, the rest of the crowd, including me, thank you very much.
Beal has a good series of explanations of a lot of this impact force stuff if you care to look it up. I posted the links here years ago, and others also found it and liked it, so, this is not exactly new stuff you see.
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Osmo
Trad climber
Calgary, Alberta
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Apr 22, 2009 - 08:55pm PT
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Hello Francis: interesting explanation of 55kg from the UIAA--I take it the "4"'s in their equation are exponents?; that does work out about right.
I had my own theory about how they came up with 55kg.: something like 50 years ago, 11mm nylon ropes came to be accepted as a safe size for a single leading rope. Then somewhat smaller ropes were introduced, possibly for all the advantages we know of for the double/twin-rope systems: full-length rappels, greater immunity to rockfall and edge-cutting, and reduced drag through wandering protection. 9mm was accepted as a common size for such smaller ropes--who knows why?: 9mm is considerably bigger than half of an 11mm rope; possibly it was a compromise of accepting greater total weight and bulk for the stated advantages of two ropes, AND the possibility of occasionally leading on a single 9 (with caution). In any case, the UIAA deemed only the 11mm to be acceptable as a single rope, but the 9 was called a "half-rope", obviously not meaning that it is half the 'strength' of an 11, but rather that a 9 could be approved for leading on ONLY if used in a pair, to form a 'full rope'--the implication could be that both ropes would be clipped into protection, together, in "twin-rope" fashion, OR maybe the 9mm, not that much smaller than 11mm, was chosen on the basis of the double-rope system where the highest-clipped half-rope could sustain the entire shock of a fall without the second rope coming under tension at all.
At first, all the attention of testing was on the ability of a rope to withstand a fall without breaking: impact forces were not considered until later, although it must have been obvious all along, that the stretchiness of nylon and other synthetics was the key to the survival of a rope in a fall. A hemp rope can be very strong, but its lack of stretch makes it a disastrous choice for a climbing rope, as the impact forces must be extreme to make up for the static behavior, in energy absorption: the rope breaks in minor falls.
Anyway, a lead-rope was tested by dropping a standard 'climber-size' weight onto it in a standard configuration, but when it came to testing the half-ropes individually, someone must have noticed that it didn't seem fair to use the same drop-mass, since the rope was not intended to be used singly as a lead rope. It's obvious to me that if half-ropes are intended to be used in pairs for leading(twin-fashion), then an individual half-rope should be tested with a 40kg weight, so why 55kg? A bit of arithmetic reveals a possible explanation, although slightly convoluted: the cross-sectional area of a 9mm rope is 81/121 of that of an 11, and multiplying that by 80kg produces 53.55kg, which is close enough to round off to 55 for convenience. In any case, using 53.55 instead of 55 in your equation from UIAA produces an even better result, so that may be a verification of the trial-and-error method the UIAA used to decide on 55kg for 5 falls.
Keep in mind that this also demonstrates that smaller climbers can use smaller ropes, but big guys beware. 55kg would work for a 9mm rope of the same material as an 11 tested with 80, but ropes smaller than 9 could not be expected to withstand as many drops with 55kg.
Osmo
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Osmo
Trad climber
Calgary, Alberta
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Apr 22, 2009 - 09:28pm PT
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Sorry Del; I can't imagine your problem in seeing why a thin rope is stretchier than a thick rope--it's common sense to anyone who knows the (simple) physics inside out, but to others who don't, you can rely on the physics itself, and finally all this is supported by endless experience, which is in a sense,
'data'. Even the UIAA admitted to deriving an equation by experiment.
As for the elastic range, it's this simple, almost by definition: if it was in the elastic range, it would not break (at least, not in 5 drops) because it's not being damaged. The UIAA MUST damage the rope, so that it WILL break within a realistic number of drops, so that they CAN say something about comparing the relative ultimate strengths of various ropes.
I would like to see the "actual drop test data" you're talking about, whether it was a copy of a dynamometer printout, or an artist's concept for the rope maker, based on a couple of data points, or what. I'm not being combative here, at all, but I know that I'm a lot more 'piercing' when looking at evidence, than many people are, especially when they're trying to debunk me. I don't care a bit about winning anything--but I do want to get to the facts.
Furthermore the shape of the curve beyond the elastic range is a wildcard, and it could go on for quite a ways, or it could kink upward sharply and end quickly because the impact force reaches the break strength of the rope after very little additional elongation. For example, if the elastic elongation limit of a particular rope material is 20%, and the breaking elongation is 21%, that final 1% on the curve might hardly show up, and would be easy to miss, or ignore, or not admit, etc.
Can you refer me to the data you mentioned?--I'd like to see it, I might learn something, maybe not.
Thanks, Osmo
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Dr.Sprock
Boulder climber
Sprocketville
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Apr 22, 2009 - 09:30pm PT
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Why can not a pro engineer post a chart online?
even brain dead me is always throwin down the many jpg's, word up?
not even some chicken scratch on a restaurant napkin for chrissakes.
but hey, the college was cheap, what, only 120 grand?
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TradIsGood
Chalkless climber
the Gunks end of the country
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Apr 22, 2009 - 09:34pm PT
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dirtineye, It should be quite obvious that OSMO is right about the force on climber versus force on top piece of gear.
The only way they would be the same is if the rope tied to the climber was tied to the top piece. Assuming a belay is the only thing keeping the rope from accelerating (that happens with forces) is a force in the opposite direction. If the carabiner were "ideal frictionless" the peak force on the gear would be twice the peak force on the climber.
In the real world friction reduces that force to prevent acceleration of the rope through the carabiner.
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Osmo
Trad climber
Calgary, Alberta
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Apr 22, 2009 - 09:46pm PT
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Dr.Sprock; when someone has nothing useful to contribute, he becomes abusive to hide it, but it doesn't look good. I thought I saw something worthwhile from you a ways back, why not stick to that as a pattern? I didn't even try to put in a sketch, as I expected you to be able to follow my explanation, and I've never seen one from anyone else: have you ever contributed one?
Osmo
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Osmo
Trad climber
Calgary, Alberta
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Apr 22, 2009 - 09:53pm PT
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Hello dirtineye; yes you seem to have some problems, sorry to hear it, and I will take your hint to look up what info Beal has about tests and impact forces. A person can waste a lot of time hunting for useful material, so a pointer is appreciated.
Osmo
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Osmo
Trad climber
Calgary, Alberta
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Apr 23, 2009 - 05:24pm PT
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OK Del; apparently you just want to argue: I didn't claim to present any data, anecdotal or other; I'm still looking (and waiting) for SOME regarding the drop tests; and forget about my engineering background--that's bound to bother some people, but I thought it would give me a bit more credibility than only as another climber (which I am, and have been for a long time, and learning the whole time). But it bothers you, so forget it; I don't need it, as the stuff I've been talking about is well known and established, and can readily be looked up in any number of physics texts--I didn't claim to invent it, so if you don't like it, don't blame me for it.
And I never said anything about rope damage through friction, or using screamers, etc. which other people have mentioned, since those things are off the topic of impact loading of ropes, and would only cloud it.
I know about sheath damage in rappelling, and the value of screamers, and some other issues that are all topics for other threads, but when the UIAA specified their drop tests, they didn't specify screamers (for example), because they WANT the rope to break within a few falls, because it HAS to, otherwise there's little they could say about what the rope could REALLY take. And when those ropes fail, it's not because of sheath damage from rappelling, it's from excessive elongation resulting from progressive plastic damage from one drop to the next. That's what climbers want to avoid--if we have to fall, we'd like to keep our rope in the elastic range, ideally so it will rebound fully and be ready for the next fall, and the next, and the next.
You want a complex model?--and yet you're having trouble grasping the simple idea of elasticity; you're better off to tackle one concept at a time, and you know what?--as it happens, simple models generally work extremely well for representing real-world systems in spite of their apparent complexity.
Then you say, "But this is not what is observed." FINE, if that's true, let's have it: talk about "anecdotal"!--I have been looking, and waiting, and still have not seen any real 'observations' or data, except point-shots called impact forces--so what should we do? draw a straight line from the origin to the impact force and call it good for a representation of the rope's behavior during the test?! Sorry, that's no good: 2 points does not make a graph, and further, such a graph would clearly suggest that the small rope absorbed more energy than the big rope, while in fact the absorbed energies are much the same.
I'm still looking elsewhere, and also eagerly waiting for your claimed 'observations', including Jim Ewing's test results, which you say I can't explain--I've seen some of his numbers and was surprised of course, but after a little thinking, offered a viable explanation, unless some new information appears, which contradicts it.
So Del, what do you know? Have you seen a curve?: preferably drawn by a plotter immediately following a drop test? Maybe a bright-colored graph from a tag included with a rope? Anything? Let's have it. Please share it with me--all of us who are interested in ropes. Surely you wouldn't want to see all your friends continue to clip in only one of the ropes on manky protection, knowing that 'both clipped in' is easier on it?
Thanks in advance, at this point I'd even welcome being proven wrong about one thing--at least I'd be learning something new, but so far I'm getting mostly jibes and bickering from guys who don't like to be told about 'their' sport.
Osmo
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Jim E
climber
away
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Apr 23, 2009 - 10:01pm PT
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What was/is the question? Seriously.
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Dr.Sprock
Boulder climber
Sprocketville
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Apr 23, 2009 - 10:22pm PT
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One experiment is worth a million theories, so the best way is to go out back and hook up an anvil, and have at it.
and i do have a chart up on this thread, i forget what page.
did you see it?
it was two small circles and one
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Jim E
climber
away
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Apr 23, 2009 - 10:29pm PT
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I use one of these...
and a drop tower.
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Osmo
Trad climber
Calgary, Alberta
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Apr 23, 2009 - 11:15pm PT
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Hello Jim; good to hear from you; I sent a note to Sterling the other day, to get in touch with you.
We've been going at it for a few days here after another guy mentioned your 80kg half-rope impact force findings and started advising everyone to start clipping in both ropes or a big rope, to their questionable protection, to give it the best chance of surviving a fall. I could just feel the eyebrows snapping up throughout the forum at that advice, and said, now just hold on, we all know from long experience that thin ropes stretch more and give a softer catch, so what's going on? I went into the physics of stretchy materials to explain the high impact forces near breaking and how they do not relate to the real world of climbing in the elastic range: --sorry.
Anyway a couple of the guys have been very sensitive about being preached to, so I'm saying what we really need is some drop tests in the realistic fall-factor range. Several climbers in the various forums are agreeing that 1.0 is pretty extreme, never mind 1.78, and I suggested that even 0.5 is serious in terms of "common" climbing falls: say 25 ft. above a 75ft. final piece: 50ft fall on 100ft. of rope.
Have you done, or could you do, some such tests? This is the kind of data that climbers need. Just think, you could test the ropes and still sell them as new.
Thanks, Osmo
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Osmo
Trad climber
Calgary, Alberta
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Apr 23, 2009 - 11:25pm PT
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Dr.Sprock; yes I saw your diagrams and that's what I was referring to as something worthwhile--definitely a cut above your last entry previous to this one. But no fear--I'm getting a bit tired of this too, and Sterling Jim may come to the rescue--let's see what he says about some real-world impact tests.
Osmo
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