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rolo
climber
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Jul 22, 2014 - 12:45pm PT
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-Falls potentially generate more stretch, leading to a softer catch and less forces on gear (when used as halves, not twins)
the above is a common, albeit erroneous misconception
see here:
http://willgadd.com/single-and-half-rope-impact-forces-data/
may be someone had already posted this link in this thread but I did not take the time to look through the responses
recent tests by Mammut confirm what is posted on Will's page
half ropes have similar or higher impact force than single ropes (when the tests are done with equal weight of course, not the BS-good-for-nothing 55kg half-rope UIAA test)
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tb.in.sf
Trad climber
San Francisco
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Jul 29, 2014 - 02:09pm PT
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half ropes have similar or higher impact force than single ropes (when the tests are done with equal weight of course, not the BS-good-for-nothing 55kg half-rope UIAA test)
It makes sense to test with a lower weight because of the way that half ropes catch falls - rather than being good for nothing, it highlights one of their advantages. If you're alternating your clips, and your belayer is belaying correctly, tension from both ropes will stop a typical leader fall. And it's basic physics that if you have two ropes tugging you up, the up force in each is less than if you only had one rope doing it. One way to model that in a rope test is with a lighter weight.
Not all falls will happen that way, which is why instructions about halfs (should) emphasize practices that minimize the risk of falling on just one rope. The biggest being, don't use a half rope as a single as it wasn't designed/rated to be used as one. And alternate your clips as much as possible, so fall+stretch will tension the second rope in a harder fall (not the case if you never clip #2, effectively turning it into a tag line). And the belayer should make sure the lower (last-clipped) rope doesn't have unnecessary slack in it.
Interestingly, the second rope contributes the most towards stopping a fall (exactly half) in the worst fall-factor fall, a 2.0 right off the anchor. It contributes the least (0%) in low fall-factor fall, meaning a shorter fall way up high, right after placing a piece. The falls where you end up 100% caught by one rope are, conveniently, also the ones that are less likely to load the rope to its limits.
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tradmanclimbs
Ice climber
Pomfert VT
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Jul 29, 2014 - 04:07pm PT
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Actually I have to go with the bogus 55kg rant. the way half ropes are typicaly used the fall is caught by a single strand. I would honestly say that 99% of the falls that I have taken on halfs were caught on a single strand. Not that I fall on ice (once ever) but especially with ice you clip a single strand on ice floss and then climb about a mile or so before clipping the 2nd strand!
If the climbing is sketch and the gear sucks and it is possible a nice feature of half ropes is to place two pieces and clip one rope to each piece. I have never actually fallen when I have done this as i am usually too scared in that scenario to even consider falling. Typically I have fallen on a single strand. Typically the fall seems a lot longer than a simeler fall on both strands (do not even start with the don't clip both strands BS. Read the directions please!)or a fall on a single rated rope. I do not know if this longer fall is due to the half rope stretching more or the fact that most belayers are not nearly as good at catching falls on half ropes as they think they are.....
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rgold
Trad climber
Poughkeepsie, NY
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Jul 29, 2014 - 06:05pm PT
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It makes sense to test with a lower weight because of the way that half ropes catch falls
This is a common misconception. After both tests and calculations, it was found that holding 5 factor 1.8 falls with a 55 kg weight was a reliable way to test that the rope would hold a single factor 1.8 fall with a 80 kg weight, more reliable as test protocol than trying a single drop with the 80 kg weight.
Here's a quote on the subject from Whitehorse Jeff, http://www.supertopo.com/climbing/thread.php?topic_id=833631&tn=165
n answer to the oft repeated question:" Why do they test half ropes singly with only 55kg", I was the American delegate (of the AAC) on the UIAA Safety Commission when the Half Rope Standard was put in place. Tests done at the time by Pit Schubert of the DAV (German Alpine Club) showed that any half rope that would withstand 5 UIAA test falls with a 55 kg test mass would also withstand one UIAA test fall with the 80kg mass. The argument presented to the members of the Commission at the time was that it would be impossible to get the lighter ropes which climbers (and guides ) wanted, to pass a harsher test, and that the real goal was to insure that a half rope would sustain at least one of the worst imaginable falls in the field, that being the fall represented by the UIAA single rope test. The repeated falls caught with the 55kg mass were accepted as the minimum repeatable test standard to insure this desired result. Over the approx 20 years that we've had the half rope standard , the Standard has proved to be high enough. (before the UIAA Standard was approved, people just climbed on half ropes without UIAA approval, assuming that both ropes would never be cut at the same time in the same fall, which has proved to be true in practice).
I accept that this reason may seem less than scientific to many. It was based on many tests carried out by the different certified UIAA test labs at the time, and as I remember was passed in near, if not total, unanimity by the voting members of the Commission, National delegates and Rope Manufacturers alike. There is one thing that encourages me at present as to the wisdom of this decision at the time-- rope manufacturing has made huge progress over the past 20 years, and this test still seems to hold up. Modern half ropes are much better than the ones I first used in France and Great Britain in the 70's.
On another subject:
I do not know if this longer fall is due to the half rope stretching more or the fact that most belayers are not nearly as good at catching falls on half ropes as they think they are.....
Of course, it is hard to know whether or not the fall is really longer unless one can replicate the exact same thing with a single rope. But, there are some other possibilities: (1) The belayer has to grip a thinner strand and it is sandwiched in with a second strand, so maybe a little rope runs through the device. (2) Half ropes are celebrated for reducing rope drag. But straighter rope paths mean more of the rope is actively involved in absorbing fall energy which means more of the total potential stretch is actually achieved.
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