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climbingjones
Trad climber
grass valley,ca
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Topic Author's Reply - Jan 19, 2007 - 10:26pm PT
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I agree Trusty, walls and free climbs are totally different. What I like most is carrying a roll of duct tape to "back up" anything sketchy. I prefer a triple xyz config. Maybe I should carry some silly putty too. 69 pins though? Good thing he was carrying the gear, eh? Thats alot of slings too. Did he incorporate his underwear into that deal? Cracking a St. Pauly Girl here. You?
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john hansen
climber
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Jan 19, 2007 - 10:30pm PT
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The screamer (unless I am thinking of the wrong product, or they have upgraded it ) relies on stitching ripping out to soften the impact. A bungee cordellet would load gradually without maximum static force's. The more stretch the more it would equalize.
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wootles
climber
Gamma Quadrant
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Jan 19, 2007 - 10:56pm PT
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Dynamic cord won't equalize any better than steel cable. No matter what the materials are, given identical setups, the load ratios will always be the same. Dynamic cord will, however, lower the overall peak force which probably isn't a bad thing.
8mm dynamic prussik cord is nothing more than a hunk of 8mm half or twin rope. You probably won't see much more load reduction with a dynamic 8mm vs a standard 8 or 7mm accessory cord anyway. We're talking relatively short pieces of cord here. Also the tensile strength of dynamic cord/rope is generally less than the equivalent size static due to the internal structure, not that strength would be an issue.
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TGT
Social climber
So Cal
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Jan 19, 2007 - 11:10pm PT
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My firmly braced feet are guaranteed to equalize.
And the rope burned flesh on my backside insures that there will be no shock loading.
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tito
climber
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Jan 19, 2007 - 11:16pm PT
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What do you mean by shorter leg? There is the same length of webbing on both sides..
well, yes, but the big/main sliding x in the picture looks like nylon cord, which has some dynamic properties to it. the second, smaller sliding X looks like an 8mm dyneema sling, which doesn't have much stretch. since the right hand leg of the rigging will stretch less, it will feel more force. maybe i slightly overstated the differences in the amount of force going to each leg, and the real distribution of forces lies somewhere between a setup with equal leg lengths and one with unequal leg lengths.
In general, with crossed-x situations, because the biners act as (approximate) pulleys, the force is the same on a long arm as on a short arm. Look at it this, way - if the force started to get higher on the short arm than it was on the long arm, the sling material would slip through the biner towards the short arm until those forces were equalized again.
in a hypothetical frictionless environment that would be true. but that is not the case when friction is taken into account. as the biner slides across the cord in an effort to equalize the forces, friction fights against the biner. as a result, the biner only succeeds in getting 2/3 of the way to perfect equalization. the same phenomenon occurs when a climber hangs on a top rope. the belayer only has to pull with 2/3 of the force that the hanging climber exerts on the anchor. the friction from the rope running through the anchor point helps the belayer hold the climber up. good when belaying, but bad when trying to distribute forces equally to all the pieces of an anchor.
the more times cord or webbing runs over a biner in a rigging, the more friction prevents the system from equalizing.
But as Wootles pointed out, seeing this with body-weight or two bodies-weight doesn't tell us much about what might happen from a hard fall. Only a test rig could do that.
agreed.
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Trusty Rusty
Social climber
Tahoe area
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Jan 19, 2007 - 11:21pm PT
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Clean Livin CJ. . . . J. Hanson is on to something big . .. .. Flexible American Spirit Summer Porn Squash, vagina stretched ear to ear. . something like a mother who leads 5-4 .. .Chemical Brothers Remix.
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climbingjones
Trad climber
grass valley,ca
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Topic Author's Reply - Jan 19, 2007 - 11:31pm PT
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Dude yeah. A stretchy vagina equalizes everything. Or nothing. Not sure which. But who cares? Maybe this belongs on the "Men and women are different" thread. But whatever. My mother cant lead 5.4 though, she is to into worrying about my goat porn habit. I told her to get a life. But maybe I should just get a goat. Do they eat summer porn squash? Cause I have alot. Maybe enough to equalize an anchor. But only if I use the sliding Dub-ya. No screamers though, there will be plenty of screaming when you see the anchor that I build on top of that ice formation on Sunday.
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john hansen
climber
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Jan 19, 2007 - 11:37pm PT
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With a twenty foot countinous piece of this stuff you you could make a two foot cordelette figure eight anchor could stretch to four or five feet it might be safer .Maybe this could even be applied to extreme free climbing or speed ascents with 'short roping'.
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TGT
Social climber
So Cal
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Jan 19, 2007 - 11:40pm PT
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The answer to all your equalization problems
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Ed Hartouni
Trad climber
Livermore, CA
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Jan 19, 2007 - 11:54pm PT
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I did an analysis of the Cordelette and the Sliding X which end up being extremes in the physical model of a 2-anchor system. I'd be happy to send it anyone...
...in this model I correctly calculated the different performance of the test results.
You all may think this is obvious, but, the way the Cordelette works, each arm works in parallel. This means that when you try to extend the "master point" the stiffness will be determined by the stiffest arm, which is the shortest arm. That means that much more force will be on the short arm then the long one.
The Sliding-X works with both arms effectively in series, so the entire sling extends, reducing the force and equalizing it, except for the friction at the "master point" (the 'biner "pulley"). This is also possible to calculate to get the differential force on the arms.
Here are the details of the calculation... rgold might like to comment on whether or not this makes any sense... I am notorious for goofing up on this stuff...
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GOclimb
Trad climber
Boston, MA
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Jan 20, 2007 - 12:29pm PT
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Tito - that's right - that's why Wootles' test results typically show a 25% difference in peak load between the two arms of the uneven crossed sling.
I simply don't know how large a factor the friction over biners would be for the mooselette in the case of a hard fall. Yes, the cord has to run around three biners - the power point and two out of the three protection biners. But at least it has the potential to equalize pretty well over all three (or more) points.
Compare that to the CharlesJMM anchor, which only has full (potential) equalization within a narrow range of angles, or if all three pieces are within the same plane, or the equalette, which pretty much only equalizes on any two pieces at a time, except within a narrow range of angles where it can equalize over three or more strands.
GO
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ghand
Sport climber
Golden,Colorado
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Jan 20, 2007 - 01:04pm PT
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This guy seems to have forgotten his mooselette!
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Largo
Sport climber
Venice, Ca
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Jan 20, 2007 - 05:42pm PT
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Hey, Ed, if you were to try and break those figures down for us who never got past Cal.2 (and didn't want to even go that far), what would it all mean?
Thanks,
JL
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Ed Hartouni
Trad climber
Livermore, CA
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Jan 20, 2007 - 06:07pm PT
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boy I guess I take up the mantel of "buzz kill" from Roger...
OK, simple...
The stiffness of the arms depends on their length and their "Young's Modulus" which is a constant property of the material (so it is the same independent of what the size of a sling, or the diameter of the rope, etc).
If an arm is shorter, it is stiffer, which means that you need more force to pull it a distance x.
When you tie a cordelette with arms at two different lengths, you are making the shorter one stiffer than the longer one.
When you apply force at the "master point", you are pulling both arms the same distance x, but the force on the short arm is higher than the force on the long arm, thus, the forces are not equalized. This is what the test found.
For the sliding-x, you are applying the force at the 'biner, which is free to slide. The entire sling extends, it is less stiff than either of the cordelette arms because it is longer. When you extend the master point it takes less force to move it x than in the case of the cordelette, and the force is equal in each arm, the 'biner allows the sling to slip around and equalize the forces as long as you neglect the friction of the 'biner sliding at the master point. The test saw this too.
The analysis of these two systems also agree quantitatively with the test, which gives me confidence that the analysis.
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Mighty Hiker
Social climber
Vancouver, B.C.
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Jan 20, 2007 - 08:12pm PT
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Another angle on this subject, so to speak, is the subjective side. Over half of mountain and climbing fatalities in western Canada are due to snow avalanches. A few years ago, 29 died in one winter. (The average is something like ten.) Partly due to that, the various governments decided to put some funding into looking at the subject.
There were already lots of introductory and refresher and intermediate and advanced and professional avalanche courses. Most backcountry skiers were taking them and practising the various skills they learned. Less so snowmobilers, and those being guided - though the latter usually got at least some introductory training.
I'm no expert, but used to teach introductory avalanche (and rock) courses, have been backcountry skiing for decades, and have lost ten or more friends. I've attended various programs on what has been learned, and it may be relevant to this discussion, as far as I understand it.
They've done a great deal of analysis, and looked hard at what's done in Europe and elsewhere. Some of what they've learned, which is being translated into tools for the recreational user, is objective. That is, create tools that can be easily learned and applied to assess risk and make decisions.
Some is what we call subjective. Crudely, that people sometimes do stupid things, often for environmental reasons. They include things like fatigue, poor conditions, lack of nourishment, group dynamics, and even composition of the party. (A party comprised of both males and females is more likely to get into trouble, especially if they're in the 15 - 30 age range. The males almost always end up in the decision making roles, and make poorer decisions.)
This seems to apply to anchors also. A lot depends on the objective factors - how strong and reliable is the gear, how strong is the rock, how well is the gear placed, is it well connected? And the SERENE stuff. And, how well people have learned about both the technical side, and the judgment and experience needed to apply it.
Failing belays aren't exactly common - there was one on the DNB of Middle Cathedral a few years ago, but fatalities are more common for other reasons. Some of it is the influx of new climbers, who're sometimes perplexed by a stance that doesn't have a fixed belay. (I've seen some who couldn't figure out that a healthy large tree in a forest abutting the top of the cliff was a perfectly good anchor.) Perhaps some is how they learn, given that so many now learn in courses.
In terms of constructing belays, we shouldn't forget the non-technical side. That is, are other subjective factors such as those mentioned relevant, and should be climbers learn to be aware of them? After all, how often do we simply think "We're in a hurry/it's getting dark/I'm hungry or thirsty/I need to look like I know what I'm doing so that chick will be impressed/I know what I'm doing even if that old fart says I don't"? Worse still, be affected by these things, but not be aware that we're being affected?
An example is commonly referred to on ST. The novice, setting up a toprope belay. Who may not be receptive to help or suggestion, for pride or similar reasons. (Interestingly, the typical adolescent male may be more receptive to such suggestions from females than males, AOTBE.)
Anyway, just some thoughts.
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TGT
Social climber
So Cal
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Jan 20, 2007 - 10:17pm PT
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All my preceeding posts have been jokes, but have kind of obtusely been generated by this point of view.
1. You can't teach mechanical aptitude.
2. The abillity to visualize the forces involved is not all that common.
3. No rule set can cover every or even a small fraction of belay anchor possibilities, or lack thereof.
Sometimes a good bucket to place your butt in really is good enough.
Sometimes the most complex multi piece anchor isn't.
knowing the difference, is more of a talent and or product of experience than a science project.
Now go forth and analyze the physics for another 170 posts. I'm not doubting the value of the analysis or that we all might learn something, just its practicality,
If you don't have the judgement and eye for the practical application all the theory in the world won't do you any good.
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knudeNoggin
climber
Falls Church, VA
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Jan 21, 2007 - 12:36am PT
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> How much wottle
> Would a Wootler wot
> If a Wootler would wot wottles?
ootles!
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Ed Hartouni
Trad climber
Livermore, CA
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Jan 21, 2007 - 01:59am PT
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TGT - working out the physics taught me something important about setting up ALL anchors. I am probably still not explaining this very well, or maybe it isn't as important to someone else as to me...
...but the trying to understand the tests that were done allows me to generalize the lessons contained in the results.
The lesson? only the sliding-x equalizes a two point anchor. The problem is the extension on a blown anchor. The tests also show that in some situations, the extension of the sliding-x onto one anchor doesn't load as much as feared.
I sometimes use a sliding-x and tie in with the rope on the third anchor. Seems redundant and secure.
A cordelette has real problems when the arm lengths are very different, especially if you have poor anchors. There may be a situation where a cordelette is fine to use, however, especially with nearly equal arm lengths (I know wootles doesn't think so, but here I would probably believe the analysis).
The physical analysis of the two rigs, compared to the test results, allows for a general understanding which is useful, at least to me.
I have to admit, I was completely surprised that the cordelette was so poor in unequal arm anchors. That is worth a lot.
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Chiloe
Trad climber
Lee, NH
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Jan 21, 2007 - 08:51am PT
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The lesson? only the sliding-x equalizes a two point anchor.
I'd qualify that by saying that the sliding-X equalizes better than a cordelette. In the tests, friction proved to be a significant complication for the sliding-X, resulting in a median absolute difference of 1kN between arms in the unequal-length setup (compared with 3kN for the cordelette, but only 0.3kN for the "equalette"). That was the second unexpected result of the tests.
These comparisons, not all of them included in Wootles' data table, are visualized in the graphic that he posted earlier (from John's book):
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GOclimb
Trad climber
Boston, MA
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Jan 21, 2007 - 02:33pm PT
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Chiloe, I have a question about methodology. It relates the the chart in your post, and I'll get to that later.
1 - In the chart Wootles posted (the one where I circled several of the data points) each item got three drop tests, each with increasing values. The average absolute values vary a good bit from test to test.
2 - Wootles mentioned that in later tests, he used a rope that gave pretty consistent force values, and mentioned that the equalette drops were done on that rope.
So my question - were the numbers in your chart taken from those two different sets of drops? The reason why I ask is that for the percentage difference between arms, it seems to make very little difference how much force the anchor felt, so the methodology doesn't matter, but your chart shows delta in absolute value, not percentage value. So the different methodology could produce very different results.
If my point isn't clear, I could give examples.
GO
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