Magic X?
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The sliding x has a few things going for it on good 2 bolt anchors |
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jktinst wrote: Are you saying not only that pros that blow do not reduce the final load but that they actually increase it?Thats my understanding. The absorbs the first fall and stiffens and then can no longer absorb as much when the piece blows and acts as a rope with a larger impact force. |
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Next up: two opposite and opposed biners or one locking biner? News at 8! |
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Henryluedtke wrote:I have recently noticed a large division when it comes to people's practices while setting up anchor systems with slings. I would like to hear some sound advice from the MP community on this. Basically, the questions is whether or not tying your master point on a bite vs the magic or sliding x is safer. The argument people use for the magic x is that it can re-equalize if one point fails, maintaining the direction of pull. The argument people use for the bit (overhand or figure eight or whatever) is that it won't shock load the system by not re-equalizing and that that is preferable to shock loading your sling. I also heard some debate about dynema versus nylon in terms of shock strength, that would also be nice if someone cleared up. , personally, was trained to use multiple slings or (whatever you are using) and tie a double figure eight on a bite to make a top rope safe, but I want to know what you guys do to be safe. Please share your experience and knowledge on this question. Happy climbing.In my opinion, fwiw, the sliding x or magic x, is best reserved when you want some moveability on two absolutely bomber anchors. As an example. I used it on a far from the cliff edge tree belay on two trees. Both were decent trees but I wanted to be at the cliff edge. So I slung both with 4ft slings and then attached my cordellete to them in a sliding X. This allowed me to move around to set my belay up close to the edge. I didn't believe either anchor would fail under a distributed load but neither tree was suitable for dropping a bus off of, and thats my rule for a single tree. Thus the sliding X. Everything else, overhand or 8 after equalizing. This makes the anchor redundant and equalized, and eliminates extension. If a single piece blows, the anchor is still mostly equalized on the remaining pieces depending on the direction of pull (did a shift in pull cause the piece to fail?). There is no magic to the X, it's going ot shock load your system, and that isn't a good thing. |
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Also, especially with webbing, there is a momentary gap in equalization. |
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jktinst wrote: Are you saying not only that pros that blow do not reduce the final load but that they actually increase it?What I wrote is what I meant, the world is rarely as black and white as the above you have written would make it appear. |
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Jim Titt wrote: What I wrote is what I meant, the world is rarely as black and white as the above you have written would make it appear.Sorry. I wasnt trying to put words in your mouth. By stating things in starker contrast, I was just trying to make sure that I understood correctly and to see if I could derive some practical dos and donts. Climbing is full of B&W dos and donts that we know full-well are, for the most part, caricatures of the gray real world but theyre useful caricatures. So trying to go forward without additional clarification, I choose to interpret this as meaning that my B&W statement was not completely off the mark. This puts the equalization vs no-extension into a different light, as far as I am concerned. The PP loads each piece sequentially (another B&W caricature, I know), making it more likely that the 1st piece will blow, which in turn, makes it even more likely than I thought until recently that the next one will blow as well, and so on. By comparison, a well-built, fully SRENE anchor (taking into account the apparent strength of the available pros, the likely direction of a potential fall and the various factors that affect load distribution, such as friction, configuration of the anchor with respect to the fall direction; lengths, angles and, if applicable, hierarchy of the arms; etc.) is more likely to wring more strength out of each pro for a stronger anchor overall, ie, is less likely to have a pro blow in the first place. Of course, this leaves the question of what happens if that first pro does blow anyway. Is the dreaded shock-load going to make it much more likely that the others will follow suit? Hard data showing that this is indeed the case, even when extension is limited to, say, less than 15cm, would go a long way to change my mind about equalization but I have not seen such data. The only high shock-load figures Ive seen were those reported in the McKently paper and these tests were done from the point of view of an organized rescue, using only static materials and static loads. I tried to look up the Rock Exotica youtube clip but could not find it. Other than that, what I have is the DAV offset drop tests. These were done on an overhung gym wall. The anchors were clipped to 2 bolts set at the same level (symmetrical set-ups) and held a static load mimicking the belayer in a hanging belay. The load mimicking the leader was dropped from a spot 3m up and 1m to the side of the central point and on a dynamic rope. In the tests where they intentionally blew one of the two attachments with a low-load fuse, using a sliding X with no extension limitation, the remaining piece got a 40% higher load (summary translation provided by you on RC.com a few years ago). Im the first to admit that this is a significant increase but its not several folds greater and, within the very realistic framework of the tests, its also a worst case scenario for shock-load: full-length extension and the belayers weight goes instantly into free fall once the one pro blows and is then arrested statically. I find it simply impossible to intuitively transpose this result to a situation where extension would be limited to less than 15cm and where the belayer would provide a somewhat dynamic arrest and a) use a good stance to resist both the extension and getting pulled off, or b) be in a hanging belay but have a dynamic connexion to the anchor. Its obvious that the increase in the final load would be much less but how much less (especially by comparison with the PPs sequential failure scenario), I have no idea. |
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I have the clip favorited, I will track it down and post it. |
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Sorry, Rescue Response was the YouTube video. I also enjoyed this DMM test. |
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The difficulty with the piece blowing lowering the impact force concept is firstly to know what the work of failure is. It can be that crystal that pops off at exactly 2kN which requires virtually no energy whatsoever OR a cam ripping metres down a parallel crack which takes loads of energy out of the faller. The other aspect is the more energy the failing protection takes out of the system the longer/more the system itself has been loaded and things like knot tightening and rope stretch go to make the next impact onto a stiffer system giving a harder impactforce. No-one has tested sequential failure at such close intervals and so the rope recovery dynamics are unknown but we can make a good guess that it isn´t favourable since recovery time is minimal. |
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Thanks for the clarification, Jim. Yes, let's consider worse case scenarios for sure but not too unrealistic: achieving the optimum configuration for the belay anchor and the best possible compromise between a soft catch and a quick arrest are clearly not things that can be factored in but whether or not to add extension limitation knots or to connect to the anchor with a dynamic cow's tail or with the rope are. jktinst wrote: ...but let's stay with this scenario for the sake of comparing PP & X... Taking the same pro strength numbers with a PP anchor (3 & 6 kN) but a higher overall load, say 8.1kN...I should have taken the time to review what I know about equalization before getting into a silly numbers game. While it is not altogether unrealistic to postulate a 1:2 distribution from an X, 1:3 is decidedly less realistic. The DAV paper gives a 1:1.3 ratio (higher load on the distal arm) for their offset drop tests on the X and other tests often show ratios closer to 1:1 for more centered tests. The main point, as I said (and should have left it at that) is still that, while it is impossible to know in advance the actual strength of a pro, we can hope that the eyeball tests would raise some suspicion that 2 pros that turned out to have actual strengths of 6 & 3 kN might not make a particularly solid anchor. In that case, I might consider linking them together as a progression pro if I was in the middle of a runout but, hopefully, I would not be stuck with only these for a belay anchor. Having said that, if I was, I would hope that one of my usual tricks for eliminating the possibility of the leader falling directly on the belay anchor would be applicable. The thing is that when you play around with the actual strength of the pros and the force and horizontal displacement of the fall (even using the same not-too-unrealistic distribution ratios in each case), you can come up with all sorts of scenarios, some that show an advantage for the PP and others to the X J. Serpico wrote:Sorry, Rescue Response was the YouTube video. I also enjoyed this DMM test. Multi-Point Anchors | Rigging Physics II: goo.gl/9Ifz7N DMM Techincal Video on Slings at Anchors: goo.gl/uxNH8GThanks for the links. Yes, there's a brief mention at 4:15 of the first one that the equalization slide of the X is more staggered than completely smooth. Fair enough but are you really applying the term "shock-loading" to each time the central biner shifts along the X? Seems like a bit of a stretch of the term. In practice, as I said, there are plenty of tests that show pretty good equalization of the X, staggered sliding or not. |
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I know this has been covered, but if the anchor is super solid, I tie one but always with limiting knots. |
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The reluctance for many to accept the sliding X and the current general view that it isn´t the holy grail of anchor building is due to the simple fact that for 20 odd years it was widely proclaimed to be the solution to poor anchors whereas it sometimes is superior and sometimes inferior to conventional systems. The internet is full of "belay macrame" created apparently by spotty grad students in their bedrooms claiming the most astounding results whereas the reality is they haven´t a clue. There are even commercially available systems which claim to equalise which do absolutely nothing of the sort. |
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Any momentary transfer of 100% of the load is a shock load. If the point of the sliding X is distribution of the load amongst marginal anchors and you load any of them 100% it's a failure of the system, IMO. |
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