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Ed Hartouni
Trad climber
Livermore, CA
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Feb 10, 2007 - 09:17pm PT
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sorry, the coeff. is calculated assuming static... this gives the range. Usually when I have it slipping, it is just slipping... so I take the range of static mu to be between the "yes" and the "no" values.
It can't be larger than the "no" value.
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cintune
climber
Penn's Woods
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Feb 10, 2007 - 09:21pm PT
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Petzl still makes those pulley wheels to go on ovals. And DMM has their Revolver biners that are rated to something like 10kN (the pulley part, that is.) More gear, yeah, but if you want to reduce friction.....
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rgold
Trad climber
Poughkeepsie, NY
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Feb 10, 2007 - 09:43pm PT
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"Ed, do you think the dynamic coefficient of friction will be low enough compared to the static coefficient to make a difference?
The reason I ask is that the force ratio needed to overcome static friction should occur well before the peak force on the anchor is achieved. Now wouldn't that imply that the cord/slings will be sliding when the peak is reached?"
I think this is a critical point. Somewhere I read a study of the sliding X done by static loading that concluded it was worse than the cordelette at distributing the load. Then Wootle's dynamic tests reached the opposite conclusion, due, I suspect, to precisely the effect DC mentions here.
Here's a reference to the study I mentioned
http://www.t-rescue.com/articles/self_equalising_anchors/index.pdf
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rgold
Trad climber
Poughkeepsie, NY
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Feb 10, 2007 - 10:11pm PT
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I should have mentioned that the Geekqualizer
http://www.supertopo.com/climbing/thread.html?topic_id=307091&tn=285
was subjected to testing, although separate load cells on the three anchor points were not available to quantify the presence or absence of equalization.
The rig withstood 11 falls with an 80 kg weight, at which point the climbing rope failed. the first two falls were factor 1 falls and the remaining nine were factor 1.45 falls. The same piece of rope was used, and so the peak force kept going up, from 6.1 kN on the first factor 1 fall to 8.9 kN on the third fall, which was the first factor 1.45 fall. The rest of the falls, with one exception around 10.4 kN, were all around 11 kN.
On the eighth fall, one arm of the Geekqualizer was purposely cut 50%, on the ninth fall the cut was increased to 80%, and on the tenth fall the cut was increased to 90%. The total load on the ninth and tenth falls was 11.4 kN each time. On the eleventh fall a "fuse" was inserted, but the rope began to cut and extend, ending the test.
Once the loads got up to around 10 kN, the clove hitches tended to slip about 3/4 of an inch.
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Ed Hartouni
Trad climber
Livermore, CA
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Feb 10, 2007 - 10:31pm PT
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what kind of detector does Juan use for his seismometers? maybe they could be used instead of load cells?
I also though that you could use a stress guage on a steel rod that is used as the anchors to measure the dynamic loading.
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WBraun
climber
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Feb 11, 2007 - 12:04am PT
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No those won't work Ed (Juans detectors).
Ed if you need load cells come see me next time you're here in the Valley and I'll lend you mine.
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GOclimb
Trad climber
Boston, MA
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Feb 11, 2007 - 12:41am PT
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rgold said: In my possibly mistaken reading of Gabe's diagram, the Mooselette appears to have seven bends around biners compared to the five bends around rings in the Geekqualizer. Two of the Mooselette bends are at relatively large angles, in some cases (e.g. the indoor picture posted earlier as opposed to the sketch) so large as to be insignificant, in which case the number of bends could in some cases be effectively as low as 5, the same as the Geekqualizer. However, the Mooselette has three strands interacting and possibly binding at the power point biner, whereas the Geekqualizer has no such interactions.
You're quite right that in the Mooselette, (or any configuration with multiple strands through a power-point) the strands at the power-point biner might bind in a hard fall. I've never seen it happen from bodyweight, perhaps because cord strands tend to sit next to rather than on top of each other, or perhaps because all the strands at the power-point are moving in the same direction, or because only one is moving more than millimiters. But with a harder fall, or with a piece blowing - I don't know, that's beyond the scope of my testing.
As for the number of biners with tight bends, take another look. Of the 6 biners, as you say, two of them are at angles that are probably insignificant, and one of them is completely fixed (due to the limiter knots on the middle strand). So there are really only three biner interactions at high angle. Of those three, there's only one strand/biner that moves much. Do folks think it matters how much cord must move through a biner, or is any movement equivalent?
By the way, the CharlesJMM and the Equalette look like they should be big winners on the friction front, unless the tiny-movement theory redeems the Mooselette.
GO
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climbingjones
Trad climber
grass valley,ca
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Topic Author's Reply - Feb 11, 2007 - 01:08am PT
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WOW! Thanks for all of the posts to this thread. I knew there was some discussion to be had here. Though I must admit that I dont understand all of the equations that have been posted. I still feel that the sliding "W" is the best, for a regular free climbing anchor anyway. Tying a know in the system anywhere just limits the distribution of the load. We know that the belay is always shifting if you, as the belayer, are hanging from it. I feel the slider works constantly to evenly distibute the load. The shock loading of one piece blowing is minimal, in my opinion, since we routinely rely on one piece to arrest a fall of much more distance than the resulting "fall" in the event that one piece blows in the anchor. But, I still respect the opinion of you much more knowledgeable folks in the physics of this situation. Thanks again. Keep it coming to make this the most responded to post I have seen. ;)
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murcy
climber
San Fran Cisco
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Feb 11, 2007 - 11:50am PT
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The solution is in today's SF Chronicle!
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rgold
Trad climber
Poughkeepsie, NY
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Feb 11, 2007 - 03:12pm PT
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I bet Werner's got one of those things for rescue rigging. This just shows how useful climbing gear can be when you open your mind to the full potential of uses.
Back to the Mooselette:
"As for the number of biners with tight bends, take another look. Of the 6 biners...one of them is completely fixed (due to the limiter knots on the middle strand). So there are really only three biner interactions at high angle.
It isn't the number of biners with tight bends that matters for friction, it is the number of tight bends in the system. When I look at the Mooselette diagram, I see three movable strands at the power point biner, one from the left anchor to the lower limiting knot, one from the right anchor to the lower limiting knot, and one from the left anchor to the right anchor. These three strands plus the two bends at the left and right anchor biners make for a total of five friction-inducing bends in the system, as well as whatever effects the binding of the three powerpoint strands produces. Moreover, I wouldn't completely discount the friction contribution made by the other two biners; it depends how you rig the Mooselelette.
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GOclimb
Trad climber
Boston, MA
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Feb 11, 2007 - 04:41pm PT
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Regarding the Mooselette: Yup, three moving strands through the powerpoint. What I'm just not clear on is whether or not it matters how much each strand moves. Because two of those strands move only like a millimeter per foot, and typically in the same direction as the main moving strand.
GO
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raymond phule
climber
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Feb 12, 2007 - 07:23am PT
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Nice rig rgold. It should be easy to use. Friction could definitely be on issue with this and many (all?) 3 piece setups.
"now that we are learning how bad the cordelette is, even with just two equal length arms"
The worst load distribution in the equal length arm test was 39%-61% i.e. the force in one of the arms was never less than 39% of the total force. I dont belive that this is that bad.
I think that it should be interesting to see tests performed for a 3 anchor rig, both static and equalising.
The link you posted suggested that friction playes a large roll in a setup like yours. It is hard to say what happens with a larger load and dynamic case. I think friction plays less of a roll then but I am not sure.
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