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rgold
Trad climber
Poughkeepsie, NY
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Aug 21, 2010 - 12:29am PT
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It is certainly old news that falls onto static materials can produce catastrophically high forces. The UIAA journal had an account a number of years ago of a factor-two fall on a quickdraw causing one of its carabiners to break; the climber decked. At that time, Petzl subjected some of its standard nylon runners to factor-2 falls and got maximum impact loads of 18 kN, well above the 12 kN so-called maximum acceptable load for humans (in a parachute harness, I think).
Of course, we tend to ignore the extensive European knowledge base, so the subject had to be revived by Mike Gibbs, at first in 2002---see
http://www.marski.org/index2.php?option=com_docman&task=doc_view&gid=74&Itemid=26
http://www.caves.org/section/vertical/nh/53/RiggingForRescue-DaisyChains-2005.pdf
These tests, in which the Metolius PAS broke in factor-1.25 falls, may have been discounted by even the climbers who knew of them because of the 100 kg rescue-appropriate weights used. (It may be of interest to note that OSHA regulations for "positioning device systems" for a single individual require them to withstand a factor-2 fall with a 250 lb (113 kg) weight.)
The Gibbs tests also indicated that a Purcell prussik tied in 6 mm or 7mm cord was a far better option. Given the moderate adjustability of the Purcell prussik tether, it remains a better solution than the single-length Beal tethers. With regard to the sewn fixed-length tethers mentioned by orsemaj, it is worth reading the French study
http://www.caves.org/section/vertical/nh/53/lanyard_tests_v6.pdf.
Their conclusion is that a tether made of the same rope material but knotted at both ends is a significantly better load limiter---the tightening of the knots has a significant energy-absorbing effect. In other words, the homemade version is more effective than the commercial version.
Getting back to HMPE tethers, a few years after the Gibbs reports, Sterling ropes replicated the breaking of the PAS, but this time with 80 kg weights and a factor-2 fall. In response, they brought out a nylon PAS clone, the Chain Reactor, which will hold several factor-2 falls without breaking (which does not mean, as in the quickdraw accident, that something else will not break).
Nobody on or off-line made a fuss about Metolius continuing, as they do to this day, marketing the PAs---and it is a dedicated tether, not a sling whose primary purpose is to extend protection. The SuperTopo site recommends the PAS without acknowledging any of the breakage results and does not even mention the Chain Reactor. The comments I posted to that review were, last time I looked, totally ignored by other posters. So I think it is fair to say that there is little to no sense of urgency in the climbing community about whether or not HMPE materials are suitable for tethers.
DMM was several years late to this particular game, and their results were already known. As a result, suddenly singling out BD makes little sense; there are many purveyors of dyneema slings and, in any case, tethering is hardly the primary function for climbing runners, which work just fine used as runners.
As to the question of whether dyneema slings or the PAS will really break with a human rather than 80kg of steel on the other end, I think the honest current answer is that we don't know. Various tests have shown that human weights do not produce as high a maximum impact load in fall situations, so the braking of dyneema slings with an 80 kg steel weight does not necessarily mean that the sling will break with an 80 kg human. Some people are already certain HMPE slings won't break with human weights, but I for one don't know what data their certitude is based on.
The explanation that humans are squishy and harnesses "give" don't come close to explaining the claimed discrepancies in impact loads. My guess is that humans are a collection of jointed weights which are not stopped at the same instant and are not all moving downwards at the moment of impact. More and more realistic test dummies are being produced, and are beginning to find their way into the testing of climbing materials, so we should expect some revised test results realtively soon.
so what to do in the meantime? I think that focusing on the hypothetical breaking of HMPE materials under human factor-2 falls is the wrong emphasis. The real point is that using either HMPE or nylon tethers in situations where they might be called on to absorb fall energy will produce dangerously high loads to the system and the body---something is likely to break, even if it isn't the tether itself. HMPE is worse, but nylon runners are still plenty bad enough. And this if for factor-1 falls, which are a potential common result of a tethered fall.
So, slngs or tethers or daisy's are a truly inferior approach if a climbing rope is available for anchoring.
If a climbing rope is not available, as in rappelling situations, then it is really important for the climber to remain below the anchor with as little slack as possible in the tether. Moving above the anchor, which of course sometimes has to be done, should be treated with the same caution and attention as if one was free-soloing, because breakage or not, you are in a serious danger zone.
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jfs
Trad climber
Upper Leftish
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Aug 21, 2010 - 12:43am PT
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^^^ rational. Thanks.
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jfs
Trad climber
Upper Leftish
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Aug 21, 2010 - 01:00am PT
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^^^ rational. :-)
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Moof
Big Wall climber
A cube at my soul sucking job in Oregon
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Aug 21, 2010 - 01:14am PT
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Yurr Gunna Dye!!!
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sethsquatch76
Trad climber
Joshua tree ca
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Aug 21, 2010 - 06:36am PT
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Excellent post Rgold!!!!!!!!!!!!!!
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Tomcat
Trad climber
Chatham N.H.
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Aug 21, 2010 - 08:39am PT
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I jumped on the wagon when the skinny slings came out too.I still carry four as so-called "alpine draws",or more correctly"trad draws" and like them as they seem to transfer the least amount of uplift to a stopper placement,or cam that may walk.Since the Goran Kropp incident I am more circumspect about using sport draws on cams.
It's important to consider what Senor Healy has contributed,as well as rgold's always excellent input.These slings diminish in strength at a considerable pace,which for some of us conflicts with the cost.The loss of strength,coupled with the other aspects,is potentially a serious combination.
Meanwhile the quality and ruggedness of the oft used 11/16 nylon products has improved,particularly a Mammut product I love.I now carry a mix of these sewn and knotted.
After bailing from a ways up on Cannon one day,where rap anchors are seldom bolted,if they exist at all,and having to link three or four skinny slings together,since any knot is verbotten,to sling a big block,I got over these real fast.
They have a place,and a use,but it's not universal.I'm always amused how may people say they carry them to save weight,then launch up two pitch routes with six lockers, a mini traction,two rappel/belay devices and two cordelettes.
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healyje
Trad climber
Portland, Oregon
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Aug 21, 2010 - 09:46am PT
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jfs: The idea that it's not OK to fall onto a dyneema sling and yet STILL ok or somehow appreciably safer to fall on a nylon sling is...well...also silly. Both are failures of technique and systems before you ever get to the point of actual material failure.
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klk
Trad climber
cali
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Aug 21, 2010 - 11:13am PT
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The comments I posted to that review were, last time I looked, totally ignored by other posters. So I think it is fair to say that there is little to no sense of urgency in the climbing community about whether or not HMPE materials are suitable for tethers.
Yeah, no kidding.
Climber equipment preferences and practices have little to do with rationality.
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Mighty Hiker
climber
Vancouver, B.C.
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Aug 21, 2010 - 04:30pm PT
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Thanks, Rich - as always, an informative and useful post. As with so much new climbing equipment and technique, a closer examination often shows that what is promoted as the latest and greatest, isn't necessarily.
In many situations, I'll just continue to use regular nylon products, tie in with the rope, and belay off my harness. It seems to be not only more versatile, but safer.
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Hawkeye
climber
State of Mine
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Aug 21, 2010 - 10:32pm PT
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this is obviously an rc.noob thread....
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rgold
Trad climber
Poughkeepsie, NY
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Aug 22, 2010 - 12:04am PT
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That's me all right---just subtract the 52 years I've been climbing, add some dorky glasses, and voila!
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utahman912
Social climber
SLC, UT
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Aug 22, 2010 - 12:41am PT
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this is obviously an rc.noob thread....
I admire you for being able to endure 5 pages of it :-)
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labrat
Trad climber
Nevada City, CA
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Aug 23, 2010 - 09:45pm PT
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Bump for how much I learned from reading this thread!
Erik
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utahman912
Social climber
SLC, UT
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Aug 23, 2010 - 10:04pm PT
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Here, here. I'll be hanging on a purcell prusik 21 hours from now :-)
jb
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roy
Social climber
New Zealand -> Santa Barbara
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Aug 24, 2010 - 02:12am PT
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Hi,
Most of the time I'm building an anchor from the rope (double ropes) and using a sling for a backup piece if necessary. But if it's a two bolt anchor and/or we're not swinging leads I've been using a cordalette tied from dynamic prusik cord (8 mm from Yates).
I like the idea of the dynamic connection but there's an untested aspect to this. These cords have a lower breaking strength than most sling material (9.7 kN for the 6.5mm to about 12 kN for the 8mm). And if the cordalette is knotted with a figure 8 that's probably going to both increase the dynamic aspect and further reduce the strength.
So what I would really like is for someone with a DMM like test rig to drop 80 kg onto this arrangement. Any takers? Let me know what happens.
Thanks, Roy
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apogee
climber
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Aug 24, 2010 - 02:14am PT
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Weld_It hooked me up on this shite...
It's bomber!!!
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Hawkeye
climber
State of Mine
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Aug 24, 2010 - 10:30am PT
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yeah, i will buy that stuff from weld-it.
the rcnoob comment was that this stuff all SHOULD have been known by ANY CLIMBER who uses the skinny. it is downright shocking to think that many had no clue....
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Jim E
climber
away
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Aug 24, 2010 - 10:37am PT
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Roy,
Send me a six pack of Kiwi Lager and I'll do the drops for you.
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Morgan
Trad climber
East Coast
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Aug 24, 2010 - 10:58am PT
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What about slings on cams? It seems like DMM, HB, Metolius, CCH, Trango use some type of Dyneema or Spectra slings. Interestingly, it looks like Black Diamond has decided to stick with nylon on the Camalot C4s and maybe the C3s, too. And they usually do things for a reason.
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slobmonster
Trad climber
OAK (nee NH)
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Aug 24, 2010 - 11:56am PT
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What about slings on cams?
It all depends on what the manufacturer uses to cover the bare cable.
(TTBOMK) BD uses Nylon slings on its cams because it uses an Acrylic tubing, through which a Dyneema sling would quickly and easily cut.
(IIRC most other cam manufacturers use a Urethane tubing.)
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SuperTopo on the Web
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Let us know!