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Gobie
Trad climber
Northern, Ca.
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Apr 15, 2008 - 02:44am PT
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I saw a post a few days ago on the triplette and Karl mentioned the same thing. (using screamer to equalize) There use to be a slow mo video around of what happens when a screamer is active. It was common practice to place lockers on screamers and It would seam practical for the belays as well. When a screamer is active it actually causes the gates of the carabiner to flutter. This is not really a problem until the last bar tack rips. At this point the gate could be open at the same time that the sling is fully loaded. The open gate strength of some biners is low and what force that was absorbed could be negated by the gate being open. I will state ahead of time that this is rare, but if your concern is a beefing up weak belay then all links in the system should be considered. I wonder how many people still use lockers on their screamers?? That would be interesting to know. I have had the displeasure of ripping one all the way open and its pretty violent.
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Karl Baba
Trad climber
Yosemite, Ca
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Apr 15, 2008 - 03:08am PT
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One of the alleged advantages of wire gate biners is less gate flutter
FWIW
PEace
Karl
PS Great minds link alike eh?
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Gobie
Trad climber
Northern, Ca.
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Apr 15, 2008 - 04:00am PT
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Im with you Karl. Most wire gates also have an I beam spline which also makes them stronger when open. It is hard to get a wire gate to open by smacking it on its backside with your hand so I would venture to say they would perform well with a screamer.
Kudos
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saho
Ice climber
Anaheim, CA
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Apr 15, 2008 - 07:56am PT
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"This is not really a problem until the last bar tack rips."
"I have had the displeasure of ripping one all the way open and its pretty violent."
How long ago was that violent bar tack rip Gobie?
Modern Screamers are not bar tacked. They are sewn in 3 rows along the length of the webbing, not across, so they tear. My experience falling on them in recent years was that they are quite smooth. I have not seen people using lockers on them, maybe I just did not notice.
-Steve
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AbeFrohman
Trad climber
new york, NY
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Apr 15, 2008 - 08:03am PT
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yea, i think ive heard the gate flutter thing is a non-issue at this point, right?
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Moof
Big Wall climber
A cube at my soul sucking job in Oregon
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Apr 15, 2008 - 11:31am PT
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Nothing worse than having gate flutter when you factor 2.5 onto your microcracked biner.
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Russ Walling
Social climber
Out on the sand.... man.....
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Apr 15, 2008 - 11:40am PT
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Couldn't read all the "higher learning" stuff above... but.... if you have 3 screamers all equalized in your anchor, wouldn't the activation force go up by that same factor? If the screamer goes off at say 600 lbs, would it Knott™ be 1800lbs now with 3 in the system?
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Gobie
Trad climber
Northern, Ca.
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Apr 15, 2008 - 11:52am PT
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Its been a while (90's) isnce I ripped one apart. It was an early yates screamer and it went the whole way. Eddie Joe mite still have it. Bar tacking could be the wrong use of words. Gate flutter is an inherient issue and should be considered even with out screamers. If you are at the point of using screamers at your belay then a lot is going on. In the cordalette thread I think Karl mentioned finding out the stats on belay failure. In the rare cases it happens (alpine and ice mostly) it is probably attributed to somethign that equalization or screamers would not of helped on. I will second my gratitude for being able to climb on Sierra granite where this si generally not a problem. Its all good data, and I like knowing it, but in the real world it just doesnt get applied all that much.
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piquaclimber
Trad climber
Durango
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Apr 15, 2008 - 12:10pm PT
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I just add a screamer to the power point with two lockers if I think there may be a factor 2 fall. (or if my anchor is suspect)
Other than that, I don't use lockers on my screamers.
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Nefarius
Big Wall climber
Fresno, CA
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Apr 15, 2008 - 12:17pm PT
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Gate flutter was only an issue in the older style Screamers. The stitching in the newer (not really so new now) Screamers doesn't have this issue. Some of the other brands still suffer from this - ie. Mammut. But, then again, why would you use another brand?
Come on guys - you know this. We've talked about this a ton here!
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rgold
Trad climber
Poughkeepsie, NY
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Apr 15, 2008 - 02:08pm PT
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Steve, your copy of Eric's mathematical analysis is missing some things that would be needed by anyone who would want to invest the time in reading it. Things start out rather badly with
How It Works
= Fc*(2*cos(theta)+1).
There is nothing on the left side of the equal sign, although further reading indicates the quantity should probably be Fa. There is also no description of what Fa, Fc and theta represent. Later on, we encounter the undefined variables Et, Er, Es, Sr, eLoad, and Is. Some of this can be inferred from the text, but frankly, few people (and I am not among them) are going to make the effort it takes to read such accounts if the author or provider hasn't put in the minimal amount of time needed to make the account readable.
Although the account as posted is basically unfit for public consumption, it is still possible to notice, at least at the very beginning, some potentially fatal errors. It appears that Fa denotes the total load applied to the cordelette and that Eric has decided that each strand has equal tension Fc. (This is not simply guesswork; if these assumptions are true, the equation relating Fa and Fc is Fa=Fc*(2*cos(theta) + 1), as contained in the post.) So an analysis that is supposed to deal with the potential inequality of strand tensions begins by assuming all strand tenstions to be equal. If this was true, we wouldn't be putting screamers in there to equalize what is already equalized!
The combination of undefined terms and a beginning assumption that is contrary to the central question suggest that it will not be profitable to try to read on unless and until Eric has a chance to fix up what's posted here.
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rgold
Trad climber
Poughkeepsie, NY
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Apr 15, 2008 - 02:20pm PT
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Russ wrote: if you have 3 screamers all equalized in your anchor, wouldn't the activation force go up by that same factor? If the screamer goes off at say 600 lbs, would it Knott™ be 1800lbs now with 3 in the system?
Well, yes, but the assumption for this contraption is that the cordelette doesn't equalize the load to each piece. The point (or one of the points) of having the screamers there is to provide equalization (and at the same time keep the loads on each piece down).
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EB
Trad climber
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Apr 15, 2008 - 02:51pm PT
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Hi, I'm the Eric referred to in saho's original post. The description in saho's post was garbled and unfortunately was missing a lot of important information. I've reposted it below. Note that it is rather long and detailed, however the detail is there only if you want it. The first few paragraphs give a reasonable description of what's going on and detail is presented to allow you to make calculations if you want to. An example caluclation is given as well. If you don't care about the details of the math and just want to see some numerical results, skip to the end. (Note added later: due to several errors, the mathematical part has been removed - a corrected version will be posted later.)
Scream-o-let description:
Introduction
The proposed anchor has one Yates-style screamer (load-limiter) in each of 3 legs. We assume that the 3 legs are close to being equalized, but not quite. Since the legs themselves (not the rope) initially fairly static and have only small stretch, the load in each leg rises rapidly during a fall. One of the legs is shortest. This one will take the load first and the load will increase in this leg faster than in the others until it reaches the screamer-determined load limit and begins to deploy. Since the 3 legs were close to being equalized, almost immediately after the short leg screamer begins to deploy (at constant force), load builds up more rapidly in the other legs (their screamers haven't deployed yet) and moves towards self-equalization. Eventually all 3 screamers will be deploying and the load (by definition) is equalized. This continues until either the fall energy is absorbed, or the screamers reach the end of their fully deployed length, in which case, the anchor returns to its original, not quite equalized state. However, as will be shown below, this is an extreme case. In any case, if the fall energy is so large as to reach this point, much of the fall energy will have already been absorbed.
How It Works
The key point here is the following. In a system that is almost but not quite equalized, individual legs that obey Hooke's law (force is directly proportional to extension) will never self-equalize - the shorter leg will always take the largest force and will therefore extend further, increasing the force in this leg still more. What is needed is something that doesn't obey Hooke's law, but rather is able to extend at constant force. Such an item is readily available in the form of load limiters such as Yates Screamers. Once a load limiter reaches its deployment length, it will continue to extend (here "unzip"), but the force remains relatively constant (for example, see http://www.yatesgear.com/climbing/screamer/index.htm#1 ). Note that once the force on the rope is constant, even though the climber continues to fall the rope is no longer absorbing more energy. Using load limiters in all three legs of an anchor will allow the longer legs to "catch up" with the shorter leg once the latter's deployment begins, until the loads in all 3 legs are equal (this idea is not entirely new - it has been mentioned in one form or another by others in various web forums).
Evolution of Forces
The time evolution of the force in each leg of a 3-leg anchor system is complex and depends critically on both the relative lengths of the legs (as has been pointed out), and on whether each leg is taught or loose to start with. The only thing that is certain is that there exists one leg that will initially take the largest load. The screamer in this leg will be the first to begin deployment. Once deployment starts it will lengthen at approximately constant force (it's actually somewhat noisy as can be seen from data on the Yates web site mentioned above ) until the other legs take up the load, eventually the screamers in all legs will be deploying. By definition, at this point the force in each leg is the same (self-equilization). The important point here is that before screamer deployment, since the spring constant of the static webbing or cord is so high, force builds up with very little change in length. Thus from the time when the first screamer begins to deploy to the time when the last one starts to deploy, the change in length of all arms is small compared to their overall length and for the purpose of estimation it can be ignored. Now, once all screamers are deploying, the net upward force supplied by the anchor to balance the downward force of the rope is related to the angle between the legs. If the single screamer load limit is Fc and the angle between two adjacent legs is theta (assume for simplicity that both such angles are the same - that the anchor setup is symmetric), then the total upward force that the anchor can supply (during deployment) is:
Fa = Fc*(2*cos(theta)+1).
For example, if all legs were in a straight line (for instance, 3 anchor pieces in a vertical crack), theta would be 0 and Fa = 3*Fc. If instead the center leg points straight up and the side legs are each pointing off at 45 degrees from the vertical (theta = 45 degrees), then Fa = 2.4*Fc.
The evolution of force on the rope can be modeled in 4 stages. The first stage goes from the start of the fall to when the force in the rope has built up to the point where the first screamer begins to deploy. In this stage, the force on the rope starts from 0 and goes up to the load limiting value of one screamer.
The second stage goes from the end of the first stage to when the last screamer begins to deploy. In this stage the force on the rope increases in a complex way that depends in detail on the difference in the legs mentioned above. Stage 2 can be quite long. In fact, the fall can end in stage 2, which is good since it means that the load force never got high enough to activate all 3 screamers. However we are most interested in the case where the load rapidly progresses to stage 3 (below). In the most severe falls, stage 2 will be short. We analyze the case where stage 2 is short and argue that this is the worst-case. If the anchor was "equalized" by eye initially then the first screamer will probably have deployed less than 1 cm in stage 2 by the time the last screamer begins to deploy. If the total screamer deployment length is 33 cm (approx.), the energy absorbed by the anchor during this stage is small compared to the total energy absorbed by all of the screamers in a factor two fall (where all screamers fully deploy).
The third stage goes from the end of stage 2 to the end of screamer deployment. In this stage, the screamers in all legs are deploying, the force in all legs is the same and the force on the rope is given by the above equation for Fa.
Stage 4 goes from the end of stage 3 to the end of the fall if the fall energy exceeds the design energy absorption capacity of the screamer system. In this case, the force in the rope will start from its value at the end of stage 3 and increase (smoothly). The screamers are now static and the system is returned to its not-quite-equalized state. However by the time stage 4 is reached a lot more energy has been absorbed as compared to a static anchor just reaching the same load value so the peak load on the anchor should be lower as well.
Estimating Total Energy Absorbed In A Fall
When estimating the total energy absorbed by the system we note that when the fall is such that the system rapidly proceeds to stage 3, to a good approximation we can ignore the energy absorbed by the anchor during stage 2 and consider only the energy absorbed by the rope during this stage. To estimate the total energy absorbed by the system, we estimate the energy absorbed by the rope in stages 1 and 2 and add to that the energy absorbed by the anchor in stage 3. The design goal of the system is to absorb all of the energy in a factor-2 fall of height h above the anchor such that stage 4 is never reached. However it is important to stress that in addition to the self-equlaization property of this anchor system, it has the additional advantage that even if stage 4 is entered, the total load is spread out over a longer amount of time. Thus the maximum load on each protection piece is reduced from that which would be present in even a perfectly equalized, but non-load-limited anchor.
**
EDIT:
The mathematical analysis that was included here has been removed because of several errors that were pointed out by readers. A corrected version will be reposted later.
**
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rgold
Trad climber
Poughkeepsie, NY
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Apr 15, 2008 - 03:15pm PT
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Thanks, Eric, that's much better. Sorry if I came off as rather grouchy. And for those not interested in reading Eric's expanded account, let me just mention that the objection I raised does not occur when the equation I complained about occurs in the proper context.
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Karl Baba
Trad climber
Yosemite, Ca
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Apr 15, 2008 - 03:37pm PT
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After carefully pursuing the paragraph spaces in Eric's detailed post, I'd like to heartily concur with his eminent analysis of the pertinent factors.
carry on chaps!
Peace
Karl
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mark miller
Social climber
Reno
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Apr 15, 2008 - 06:54pm PT
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I was hoping the scream a let thread was tied into the sade De marquis thread, what's wrong with you people......
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tolman_paul
Trad climber
Anchorage, AK
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Apr 15, 2008 - 07:32pm PT
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Do you people actually climb, or just calculate theoretical anchors and loads? Do you climb with a calibrated load cell? I swear I work with some of your long lost cousins.
You can chase the idealized anchor to the nth degree, and end up with a tie in point that is at your at your toes, so if your second pulls you off, you'll place a much higher "perfectly equalized" load on your anchor than if you'd used a much simpler but real world practical anchor that you'd taken out all the slack so that a falling second wouldn't pull you off to incrase the load on the anchors.
Not to mention one can outclimb the incoming thunderheads if they are actually climbing vs. building and tearing down complex anchors at the top of each pitch.
Am I missing something?
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Karl Baba
Trad climber
Yosemite, Ca
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Apr 15, 2008 - 07:44pm PT
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"Am I missing something?"
Yeah, I say the same stuff but we have to remember, geekhood is fun and the technology we develop as climbers may actually help the space program someday.
Actually, that was a joke until i remembered that screamers where used by Nasa in some testing awhile back.
Peace
Karl
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tolman_paul
Trad climber
Anchorage, AK
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Apr 15, 2008 - 08:11pm PT
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The funny thing is, I'm an engineer. Fortunately I was exposed to folks that didn't let theories and calculations get in the way of good engineering practices, i.e. you don't need to calculate whether you should use a 1/4" or 5/16" bolt for a load if experiences shows 3/8" is more than enough. I also have worked with many fine "engineers" that never received a degree, but truly were engineers in every sense of the word.
Sorry to spoil the fun, never mind me and carry on 8~)
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Al_T.Tude
Trad climber
Monterey, CA
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Apr 15, 2008 - 08:24pm PT
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It appears that after years of misinformation some climbers are beginning to realize that a standard cordelette can be set up to be EITHER equalized OR non-extending - not both.
The much publicized SERENE anchor (Secure, Equalizing, Redundant and Non-Extending) is a climbing anchor instructional book author fantasy and does not exist in the standard cordelette set-up.
Most commonly promoted is a figure 8 tied in the cordelette which achieves non-extension status, but kills the equalization feature. Adding 3 screamers is a viable solution in high risk situations (probably not worth the time and weight 99% of the time.)
A lighter and simpler solution that I frequently employ is to leave off the figure 8 knot and create a SERE anchor. In MOST situations (where I have 3 decent pieces in), sharing the load more equally between all pieces greatly decreases the chance of any of the pieces pulling. This makes for a much stronger anchor and nullifies the benefit of a non-extending design.
Worst case scenario: Unexpectedly a piece pulls and the (3 pieces of pro) system extends roughly half of the length of the loop attached to the failed piece - typically around 1.2-1.5 feet. This suddenly lowers the power point and drops the falling climber an additional 1.2-1.5 feet plus another few inches of rope stretch. As this is a small portion of the typical fall distance and highly unlikely, I see omitting the Figure 8 as a safer (and faster)choice than the commonly advertised set-up in many situations.
I will confess that I do miss the handy anchor shelf that the figure 8 provides.
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sawin
climber
Orange, CA.
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Apr 15, 2008 - 08:33pm PT
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This reminds me of air voyagers a ok idea if aware of the
vibration factors from seams breaking which open gates of
non locking carbiners. A fatality allegedly occured due to
the last broken seam which now loaded the air voyager with
a impact force greater than the seams. The gate of the
carbiner open due to vibration and now weakend dislodged
the rope of the climber as it bent open more.
What about addressing angles and weight factors from a fall
on a piece associated to the angle of crossing? Assuming not
all that read this thread are mathematicians, scientist or
engineers how about posting such a chart? I know they are
available.
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Karl Baba
Trad climber
Yosemite, Ca
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Apr 15, 2008 - 08:35pm PT
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Mr. Tude writes
"This makes for a much stronger anchor and nullifies the benefit of a non-extending design.
Worst case scenario: Unexpectedly a piece pulls and the (3 pieces of pro) system extends roughly half of the length of the loop attached to the failed piece - typically around 1.2-1.5 feet. This suddenly lowers the power point and drops the falling climber an additional 1.2-1.5 feet plus another few inches of rope stretch. As this is a small portion of the typical fall distance and highly unlikely, I see omitting the Figure 8 as a safer (and faster)choice than the commonly advertised set-up in many situations."
What I don't see noted is "HOW" much stronger such an equalized anchor is in ACTUAL practice. SO much stronger that it's worth dropping a couple feet?
Now imagine this. The falling climber might get dropped or not (depending on whether it's a factor 2 fall or not. A non-factor two fall isn't very likey to pull the anchor since the direction of pull is up. Will the extra distance allow her to hit something?
But, perhaps more importantly, what is the impact of the extending belay on the belayer? Do you get knocked off the ledge? Is it harder to escape the belay if you're dangling off somewhere now? Did you get tossed around or against a rock suddenly? These are serious are real questions.
With a regular cordalette system, it only takes one good piece to keep everything safe and sane in the event of a problem. The belayer stays put and in control. In an extending system, who know what happens to the belayer. The extra strength, does it introduce more safety or more danger. Think about it for a minute.
Peace
karl
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saho
Ice climber
Anaheim, CA
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Apr 15, 2008 - 09:48pm PT
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Hey Eric,
Thanks for fixing that!
All that math stuff is a little beyond me. I'm no physicist, just a carpenter (technically a Lather - as in metal lath and plaster).
Tolman Paul says:
"Not to mention one can outclimb the incoming thunderheads if they are actually climbing vs. building and tearing down complex anchors at the top of each pitch.
Am I missing something?"
The point of this anchor is to achieve exactly what you requesting. That is simplicity. The standardized cordelette is a great tool that is fast and easy to understand - keeping it simple. Since the second edition of Climbing Anchors by John Long and Bob Gaines pointed out that the cordellete is not safe enough in certain situations, there have been thousands of posts on the internet searching for the perfect anchor set-up. I still just use the cordelette. Most of the contraptions out there can be complex, confusing, and some are downright scary.
The Scream-o-let is a solution for the occasional situation on a multi-pitch climb where a high factor fall is a possibility. Simply add a screamer to each anchor point, then tie in your cordellete as normal. The Screamers absorb enough energy in a high factor fall to keep your already solid anchor from breaking.
If Screamers were integrated into my cordelette set-up I think I might be more likely to actually use them. Such as in the example below. Also the weight of extra carabiners would be saved.
Here is an illustration of my idea to actually have Screamers sewn into a rabbit runner. (the pic shows the webbing configuration with tape - not real stitching for illustration only)
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sawin
climber
Orange, CA.
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Apr 15, 2008 - 09:48pm PT
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Modern Screamers are not bar tacked. They are sewn in 3 rows along the length of the webbing, not across, so they tear. My experience falling on them in recent years was that they are quite smooth. I have not seen people using lockers on them, maybe I just did not notice.
-Steve
I tore 6 rows of a long air voyager with a controlled fall from
a move above a roof over non-vertical slab approximately 80%
figuring the acute angle facing into the rock which allowed me
to run down approximately 30' after the 5-6' vertical fall.
Total stitches I believe were 16 each tested at approximately
750 lbs.. Non locking carbiners were used however after this
fall I used only locking carbiners on the remaining broken Air
Voyagers. No injury occured I immediately completed the climb
without lowering.
I'll add screamers to another rack when I buy/build it however
I'm also real about placements and equality of? Usually it's
not possible to obtain equality with load balancing on a
rock as it usually is with commercial rigging. The above photo
is close however bi-pod, tri-pod hooks with chokers on the
ground are more simple to obtain equal load balancing, and we
don't always get the best of crack formations at belays.
Thank's EB for the formulas. Appears the international test are
still being calculated with 16' falls.
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saho
Ice climber
Anaheim, CA
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Apr 15, 2008 - 10:08pm PT
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Hi sawin,
I have only ever used Yates Screamers (the regular ones)
http://www.yatesgear.com/climbing/screamer/index.htm
They activate at about 550lbs.
I have no experience with Air Voyagers. Though I understand they were the bar-tacked design.
The Yates Screamers are super smooth IMO.
"Usually
it's not possible to obtain equality with load balancing on a
rock as it usually is with commercial rigging. "
I work in the commercial construction industry. There it is no legal to use as tie off protection anything other than a single point anchor good to 5,000 lbs. So equalization is not an issue there. So you are correct in your statement. Thus the on going discussion of anchors from the sliding x, to the equalette, to the triplette, and now the Scream-o-let :)
My goal is the best of both worlds - keeping it simple and safe.
-Steve
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JAK
climber
The Souf
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Apr 15, 2008 - 10:15pm PT
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Tolman Paul, I like you. You're appropriately cranky. Kudos, brother.
That said, I like tech geekery as much as the rest of 'em.
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Clint Cummins
Trad climber
SF Bay area, CA
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Apr 15, 2008 - 10:21pm PT
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Higher priority in saho's photos would be to have at least one of the anchor pieces good for an upward pull. We are talking about someone leading above this anchor, right?
I suppose when used in a belay anchor, screamers or similar don't need the locking biner, since rope is not running through the biner on the screamer to provide the vibration for gate flutter. [Edit: see Gobie's post below - the screamer activation provides the vibration for gate flutter, so locking biners are helpful in such a belay anchor.]
(This is all a bit academic to me, since I don't use screamers, cordelettes, or locking biners except one with my ATC).
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saho
Ice climber
Anaheim, CA
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Apr 15, 2008 - 10:41pm PT
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You are absolutely right Clint,
Those illustration pics do not show the required upward pull piece.
They are simply illustrating the three piece downward pull aspect.
However, great that you pointed it out, because most of the time I see people neglecting the upward pull piece, which is even more critical in an all passive gear anchor as in those pics.
Thanks.
-Steve
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johnboy
Trad climber
Can't get here from there
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Apr 15, 2008 - 11:16pm PT
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If anyones really worried about gate flutter, Trango Superfly lockers are only 41g a piece.
If a major concern is a high FF, wouldn't a screamer on the first piece (jesus nut) right off the belay be a good place for one? Also, if screamers activate just under 600lbs of force, would'nt you have some semi ripped sceamers (he was talking sewn into slings $$) after a few modest falls?
Just asking?
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saho
Ice climber
Anaheim, CA
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Apr 15, 2008 - 11:52pm PT
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Hey johnboy,
"If a major concern is a high FF, wouldn't a screamer on the first piece (jesus nut) right off the belay be a good place for one? "
If you are in a situation where you can get in a good Jesus nut, and even if you don't have a Screamer on it, you probably don't need to bother with a Scream-o-let anchor. Just use a regular cordelette, or your regular preferred method.
"Also, if screamers activate just under 600lbs of force, would'nt you have some semi ripped sceamers (he was talking sewn into slings $$) after a few modest falls?"
As usually gets pointed out in this type of thread, the amount of times anyone actually falls directly onto an anchor with any high amount of force, is very rare. Have you ever fallen directly onto an anchor? If that is the case, just buy a new one$$$
-Steve
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johnboy
Trad climber
Can't get here from there
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Apr 15, 2008 - 11:54pm PT
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^^^^
Thanks.
I do think this may be a good "lette" for ice climbing, apart from a few of the concerns Karl stated. There might be a couple of ways to negaite them too.
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EB
Trad climber
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Apr 16, 2008 - 12:10am PT
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del cross writes:
Eric, I don't think that the angle has any bearing on the energy absorbed by a screamer. Shouldn't it simply be activation force times screamer elongation (Fc*Is)?
No. The energy absorbed by a screamer is as you say, however not all of that energy is available to slow the fall. Work is the "dot" product of the force vector and the distance vector. The dot product gives the angular dependence.
For three screamers you'd get 3*(Fc*Is). In your equation, when the pieces are in a vertical orientation (theta=0), you get 9*(Fc*Is). How's that possible?
Thank you for pointing this out. The factor of 3 in front of the equation for Es is a mistake. It should read: Es = Fa*ls*cos(theta) = Fc*(2*cos(theta)+1)*ls*cos(theta), which now reduces to 3*Fc*ls in the case where theta is 0.
What about fall energy from rope stretch and screamer elongation?
I think what you are asking here is, doesn't the fact that the rope (screamer) is stretching, increase the energy of the fall? The answer is no. The total fall energy is determined by the weight of the climber and the height of the climber above the anchor point at the time of the fall (multiplied by 2 in the case we are talking about). It is a common misconception that a stretching rope adds energy to the fall because it lengthens the fall. Rather, it is doing work on the climber, slowing his speed and decreasing his energy. Imagine instead of being attached to a rope, you fell on to a trampoline. You would not say that because you sank into the trampoline (extending the fall distance) that you increased the energy of the fall. That energy was determined by the height above the trampoline at the start of the fall.
Where's the belayer in this scenario? Does she also get pulled down and add to the fall energy as the screamers deploy?
I think if the belayer is correctly connected to the anchor, they won't add to the energy of the fall. In other words, if the connection from the belayer to the anchor is tight then the force of the fall simply gets transmitted through the belayer. If the belayer is not tight then yes, there would be an additional force of the belayer's fall, but that goes into the category of incorrect usage of the anchor, so I don't attempt to include it. In fact, when things are working correctly the presence of the belayer generally reduces the factor of the fall by introducing more dynamism into the system via stretching of the body, slipping of the rope through the belay device, movement of the harness, etc... I haven't included any of this as it simply makes things better. An exception to this is in a hanging belay where the weight of the belayer is on the anchor from the start. This is an additional force, but it is static so not nearly as large as the other forces in the system. Still I think it's correct to say that in a hanging belay, the anchor capacity is reduced.
Several people have suggested that they thought the scream-o-let idea sounded like too much bother. It seems pretty simple to me... You are carying slings anyway, so replace three of your slings with screamers. Use them in place of slings normally and then when that very unusual anchor comes along that has a chance of seeing a factor-2 fall, just clip the screamers in to each anchor piece. I often find myself extending anchor pieces anyway with a runner, so putting a screamer in instead doesn't seem like adding too much, especially if it improves the capacity of the anchor by 50% or more.
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Gobie
Trad climber
Northern, Ca.
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Apr 16, 2008 - 12:14am PT
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Clint,
The flutter is not caused from the rope, its a by product of the screamer/air voyager activating. I only mentioned it because a lot of factors were being figured in and I wanted to reveal that although we can calculate things out on paper, in the real world we tend to overlook critical elements. I would never had believed the gate flutter thing myself had someone not videotaped it and showed it to me. I dont think anyone can contest that carabiners break as anyone climbing for a minimal amount of time has witnessed this happen. As far as opening one, a quick slap on the palm of your hand from less then an inch away opens most bar stock gates. If falling on the belay is the illustration then this is a reality as things are being smacked around. I agree fully that big disruptions to the belay can be prevented by adding oppositon to the anchor. I, like you, have always been a big fan of using the rope to tie everything together, even on my oppositional piece.
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Gobie
Trad climber
Northern, Ca.
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Apr 16, 2008 - 12:39am PT
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Here is alink to a video. You have to watch it through but you can see a screamer being torn apart slowly, with lockers. Notice the vibration even when being pulled slowly.
UIAA video
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Mungeclimber
Trad climber
sorry, just posting out loud.
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Apr 16, 2008 - 02:44am PT
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hrm, should all Screamers have lockers instead of wire gates?
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Al_T.Tude
Trad climber
Monterey, CA
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Apr 16, 2008 - 02:56am PT
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Karl,
In response to my suggestion that the figure 8 knot on the cordelette sometimes be left off to maintain equalization you wrote:
What I don't see noted is "HOW" much stronger such an equalized anchor is in ACTUAL practice. SO much stronger that it's worth dropping a couple feet?
But, perhaps more importantly, what is the impact of the extending belay on the belayer? Do you get knocked off the ledge? Is it harder to escape the belay if you're dangling off somewhere now? Did you get tossed around or against a rock suddenly? These are serious are real questions.
I agree that these are important issues. This is how I see it:
With 3 solid pieces approximately equalized you have an ultra bomber system that will virtually never fail. This is what I want. With the load shared fairly equally between the pieces, the odds of one failing and causing a 2' drop of the belayer's power point and an additional 2' of climber drop is virtually nill. And even if this freak occurrence does happen, it is far from catastrophic.
Now let's tie the figure 8 knot and analyze. The asymetrical act of tying the knot produces unequal length loops guaranteeing that there is NO load direction that will produce even remotely close to evenly distributed loads between the pieces of pro. Try this with spring scales (like the ones fisherman use) and you will see what I mean.
Now, lets assume the impossible and assume that you were able to tie the perfect figure 8 knot that perfectly equalizes the 3 pieces. In this perfect world the pieces would be equalized when loaded in only one direction. As soon as the load pulls even a few degrees off from the anticipated direction (which will always happen)the load distribution goes from 33-1/3%, 33-1/3%, 33-1/3% TO 10%, 60%, 30%.
Move a few more degrees off perfect (which will happen in the real world) and we quickly hit 5%, 85%, 10%.
If the piece seeing 85% of a fall factor 2 load (which is what we are planning for here) happens to be anything less than exemplary we will possibly have a failure at that point.
Now we have 50% load on each of the two remaining pieces IF they are perfectly equalized - which they will not even remotely be. In reality we are likely looking at an 80/20 split or 100/0.
With 80-100% of the load on 1 piece, it's chance of failing is tremendously magnified. If it fails, we are now guaranteed to be placing 100% of our load on one randomly selected piece. You do not get to choose which piece this will be. This whole process takes place in a fraction of a second.
Once the last piece fails we have a catastrophic failure and two dead climbers plus whomever they hit on the way down.
Comparing these two scenarios it is clear to me that in most situations equalization (not tying the figure 8 knot) is FAR more important than non-extension.
People tie the knot in their cordelettes all the time. Why haven't we heard of a rash of failures?
Primarily because:
0)Leaders very rarely pull all of their pieces and place a FF2
or near FF2 downward load on their anchor
0)Often there are multiple pieces of bomber pro at the anchor
0)Modern dynamic ropes do an amazing job of limiting peak load
by extending the load over time
0)Leaders very rarely pull all of their pieces and place a FF2
or near FF2 downward load on their anchor (so significant it bears repeating)
Does this mean that we should continue to use what's been working (If it ain't broke, don't fix it)?
No. We were given oversize brains to continually improve our lot. I think the figure 8 knot popular in cordlette use in most situations weakens our anchors and fools us into thinking that we have constructed a SERENE anchor.
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rgold
Trad climber
Poughkeepsie, NY
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Apr 16, 2008 - 10:44am PT
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Eric wrote: I think what you are asking here is, doesn't the fact that the rope (screamer) is stretching, increase the energy of the fall? The answer is no. The total fall energy is determined by the weight of the climber and the height of the climber above the anchor point at the time of the fall (multiplied by 2 in the case we are talking about). It is a common misconception that a stretching rope adds energy to the fall because it lengthens the fall. Rather, it is doing work on the climber, slowing his speed and decreasing his energy. Imagine instead of being attached to a rope, you fell on to a trampoline. You would not say that because you sank into the trampoline (extending the fall distance) that you increased the energy of the fall. That energy was determined by the height above the trampoline at the start of the fall.
Eric, this is wrong. Indeed, when you sink into the trampoline, you do increase the fall energy (the total loss of potential energy), and the "common misperception" that you mention is an integral feature of every derivation of the maximum rope tension that occurs in stopping a fall. The total fall energy is the product of the weight of the climber and the distance fallen, period. The fact that, during some of that distance, there may be an upward force acting on the climber contributes to a reduction in what might be called the net fall energy, but it does not simply cancel the portion of the fall during which an upward force acted.
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Karl Baba
Trad climber
Yosemite, Ca
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Apr 16, 2008 - 11:12am PT
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Thanks for the detailed reply AL Tude (not Al Dude, whom some of us know)
Al wrote
"With the load shared fairly equally between the pieces, the odds of one failing and causing a 2' drop of the belayer's power point and an additional 2' of climber drop is virtually nill. And even if this freak occurrence does happen, it is far from catastrophic"
Yes and no. It's not particularly unknown for one anchor piece to fail because it was placed improperly, because it had a crappy placement because the leader wanted three placements and there was only two good ones. I've seen it and even had it happen to me (cause it didn't matter cause I had a non-extending system)
This is a complex enough scenario that I think the geeks need to put their hard hats and safety glasses on to supplement the pencil protectors, and do some real world testing. Bonus points if some Sedona Sandstone is used to provide actual bad rock-placements
Peace
Karl
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Gobie
Trad climber
Northern, Ca.
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Apr 16, 2008 - 11:54am PT
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Al,
Where is your redundancy when you take the fig 8 out of the cordalette? How strong is your cordalette? Even though you have equalized your protection you have limited yourself to one piece in that if the cordalette (which is loaded 100%) fails you now are left with this equation
100%-100%=nothin'
Im with Karl, I want real world data. If we are going to base it all on speculation then why not just use one carabiner for the belay, its says on the side of it it can hold 5000 lbs?
(I was being sarcastic, I know why).
I like to call it the act of God equation. A little humility goes a long way in accepting you cant account for everything on paper.
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sawin
climber
So., CA.
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Apr 16, 2008 - 01:19pm PT
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Q.
I like to call it the act of God equation.
R.
Physics are the laws that govern the universe!
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sawin
climber
So., CA.
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Apr 16, 2008 - 04:00pm PT
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saho this looks fairly nice except possibly 1?
Grey, Blue and Purple I can see being replaced by Air Voyagers
reducing load factor. Pink I can see being replaced by perlon
or equivalent. I've never used screamers?
I suppose it's possible to use Camelots, Hex's, Friends, Nuts,
Stoppers etc. very close to each other with per_se a very small
Micro catching a fall and not taking a 180 degree angle crossing
over the carbiner hence possibly further load reduction upon the
micro and other piece or pieces are occuring. However that pro
has a con being the rope drag.
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clustiere
Trad climber
berkeley ca
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Apr 16, 2008 - 04:30pm PT
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interesting idea. Great for desert aid routes like in the fishers ehh.
What about fabric burn??
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JohnRoe
Trad climber
State College, PA
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Apr 16, 2008 - 05:39pm PT
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Al wrote
"A lighter and simpler solution that I frequently employ is to leave off the figure 8 knot and create a SERE anchor. In MOST situations (where I have 3 decent pieces in), sharing the load more equally between all pieces greatly decreases the chance of any of the pieces pulling. This makes for a much stronger anchor and nullifies the benefit of a non-extending design.
Worst case scenario: Unexpectedly a piece pulls and the (3 pieces of pro) system extends roughly half of the length of the loop attached to the failed piece - typically around 1.2-1.5 feet. This suddenly lowers the power point and drops the falling climber an additional 1.2-1.5 feet plus another few inches of rope stretch. As this is a small portion of the typical fall distance and highly unlikely, I see omitting the Figure 8 as a safer (and faster)choice than the commonly advertised set-up in many situations."
The "worst case scenario" above is correct provided you set things up so that each cordelette loop from a piece of gear goes into *both* sides of the power point locker. If the three pieces are "left", "middle", and "right" that means you pull down the three loops and then put a half-twist into the loop from "left" to "right" (like when you make the sliding X).
If in a hurry one forgets or bungles this, the worst case scenario seems to be that one piece pulls and you end up hanging on a very long American Triangle on the other two, falling almost twice the length of the attached loop rather than half.
Sure you know this, just thought it might be good to mention...
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EB
Trad climber
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Apr 17, 2008 - 01:32am PT
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Thank you for the comments on the analysis of this anchor system. Comments from del cross and rgold were particularly insightful and have uncovered some errors. Rather than contribute to further confusion, I have decided to remove the math portion of my post. I will reenter a corrected version when I have it put together. I left the text portion in rather than delete the entire post because I think that part still presents a useful description of the anchor system.
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Karl Baba
Trad climber
Yosemite, Ca
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Apr 17, 2008 - 08:58am PT
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"Al,
Where is your redundancy when you take the fig 8 out of the cordalette? How strong is your cordalette? Even though you have equalized your protection you have limited yourself to one piece in that if the cordalette (which is loaded 100%) fails you now are left with this equation
100%-100%=nothin' "
Let's not forget about this real issue. A rock could fall, or be pulled off by a falling climbing, and impact the belay, cutting the cordalette.
If it had an eight, there's hope.
No Eight, Sorry Charlie
Peace
Karl
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rgold
Trad climber
Poughkeepsie, NY
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Apr 17, 2008 - 02:04pm PT
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Eric,
Thanks for your efforts so far, which as we all know are done in your abundant spare time. Looking forward to the revised version.
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EB
Trad climber
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Apr 18, 2008 - 12:54am PT
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del cross:
I'd bet that your 7.5 feet is probably not too far off, especially if you let the angle between legs get smaller.
Once we have some more reliable equations, two questions I would like to answer are:
1) What is the comparison between max loads in each leg with and without the screamers (all other things being equal), and
2) How gracefully does the system degrade: that is, if you are above the maximum height allowed for complete screamer deployment, what does the max force in each leg go up to at the end of the fall when screamers have reverted to regular (static) slings.
With respect to (2), you have brought up something else that I think is very important: you suggested that not all legs will finish their screamer extension at the same time (almost certainly true for legs with different angles relative to the vertical). This leaves open the possibility that one or two legs will see a disproportionately higher force at the very end of the fall (for falls greater than a certain height). Any analysis needs to look as carefully at what happens after one or more screamers finish extending as during extension.
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TradIsGood
Chalkless climber
the Gunks end of the country
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Apr 18, 2008 - 12:59am PT
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This analysis screams for experimental evidence.
:-)
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Karl Baba
Trad climber
Yosemite, Ca
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Apr 18, 2008 - 02:11am PT
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"Three fully ripped screamers absorb 1.8 kJ of energy. That much energy corresponds to a fall of 7.5 feet for a "standard" leader. That's not very far and I think suggests that screamers in the anchor are most effective for a relatively short fall. "
Seems to me that there are lots of factors weighing in that we don't have numbers for.
I took a 20+ footer on the Muir on a screamer and it didn't even fully deploy.
Peace
Karl
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TradIsGood
Chalkless climber
the Gunks end of the country
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Apr 18, 2008 - 11:53am PT
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navblk, The load is not doubled unless the protection is a "frictionless" pulley.
To see that most readily, imagine that the rope is knotted into the protection. Then the load is climber only, the line to belayer can be slack.
This was covered to some extent in Hartouni's post in the Fall Factor thread.
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TradIsGood
Chalkless climber
the Gunks end of the country
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Apr 18, 2008 - 01:20pm PT
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sawin, nav.
What I am pointing out is that friction reduces the force on the belayer side (unless you have a frictionless pulley). The force on the protection is the vector sum of the forces, but the two legs are not identical. The other obvious way to notice this is to see that the 100 pound belayer is lifted off the ground by the hanging 150 pound climber.
Empirical evidence - measurement of the actual performance of the legs, versus predicted would be a good idea.
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sawin
climber
So., CA.
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Apr 18, 2008 - 04:16pm PT
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delcross,
A 7/8" metal cable is very static, though I would hate to
fall 20' on it.
I'm looking at this of course with equipment in mind.
< Link >
Do you recommend the link above or the Muir Wall for
a 1'st grade V1?
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Ed Bannister
Mountain climber
Riverside, CA
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Apr 18, 2008 - 06:45pm PT
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Geez,
Russ was right, if the thing has three screamers, no deployment till 1800 lbs if the rig is equalized.
So, if you want the effect of a screamer, clip it to the cord, and, you then are hanging off a single piece and have no reedduunnddancy, geez.
take your pick,
the truth is if you only tie in with a cordellette, you are constantly dependant on a single piece of gear. If you are talking belay stance, generally a bad habit for anything besides a rope, at least in old timers terms, and the journal.
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sawin
climber
So., CA.
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Apr 18, 2008 - 08:33pm PT
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del cross,
Maybe with a CAT 977, D-8, D-9 or train pulling it or it pulling
a train with a crane.
Are you saying either climb asked about has that potential?
Iv'e been there and beyond free climbing and soloing.
Are you referring to apogee? What idea? Of course not and
that many years back when he allegedly stated what he did
I doubt he knew velocity of orbits.
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rgold
Trad climber
Poughkeepsie, NY
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Apr 18, 2008 - 11:06pm PT
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Navblk4 wrote:"Maybe this will answer your question:
http://www.myoan.net/climbart/climbforcecal.html"[/i]
Petzl has a fall calculator that is reliable---go to http://en.petzl.com/petzl/SportConseils?Activite=14&Conseil=56. I'd be extremely wary of others, especially if they don't tell you what formulas are being used. I think that sites whose language is imprecise or just poorly written, and sites that reveal fundamental misunderstandings of the physical principals involved should be considered suspect and not used. The MyOAN Rock Climbers (sic) Shock Force Calculator seems to be an example of such a suspect site, for more reasons than the failure to use an apostrophe when it is required.
Infelicitous or imprecise language: "the force of your shock is dependent on..." (Beyond repair, but worth a chuckle) and "The fall factor is the ratio of the distance you fall to the length of the rope." (Not "the length of the rope," but rather "the amount of rope from belayer to leader at the instant the leader falls.")
Misunderstanding basic principles (combined with amusing language): "Dynamic rope considerably decreases the shock a climber feels because it has the ability to stretch and absorb more of the force."
These just beg to be made fun of---I'm going to restrain myself. But with these telltale clues suggesting possible incompetence, note that in spite of the description of the fall factor (yes, faulty, but we know what they mean), when you enter "Length of rope"=10 meters and "Distance from last anchor"=5 meters, the Rock climbers Shock Force Calculator calculates the fall factor to be 1.5 instead of 1. Further experimentation reveals that the calculator is incapable of finding any fall factor to be less than one, apparently because it is performing the calculation
(Not the) Fall factor = 1 + (dist from anchor)/(length of rope)
This happens to get fall factors of 2 right, but nothing else of course.
I haven't bothered to check the accuracy of the remaining shocking calculations; who could have any faith in those results given what has come before?
Del X wrote:"I've also read hints that systems with screamers might not behave quite the way that simple theory predicts."
As far as I know, the "simple theory" predicts that a single screamer will, in general, have little effect in reducing the maximum impact to its anchor, and experiments confirm this.
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Karl Baba
Trad climber
Yosemite, Ca
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Apr 18, 2008 - 11:49pm PT
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"no deployment till 1800 lbs if the rig is equalized. "
I thought having an equalized anchor was the goal. I idea of a scream0lette is for the screamer to deploy in a tough situation until equalization is reached. Then you have a system that is serene. The other equalization systems extend or aren't redundant (and don't absorb energy like a screamer system either.)
Peace
Karl
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Al_T.Tude
Trad climber
Monterey, CA
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Apr 19, 2008 - 04:55am PT
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John - Yes, when using a cordelette w/o figure 8 knot, a sliding x must be used as with a sling equalizing 2 pieces.
When I need greater extension to reach the pieces that I am equalizing I retie my 5.5mm spectra cord cordelette from a loop into a rabbit runner (a cord with a small loop at each end). I clip the two loops into the power point biner at the bottom and run the resulting loop up to one piece, down to the power point and back up to the next piece etc. This does not require the sliding x as the ends are captive.
Karl - My opinions about leaving the fig 8 off are based on 3 solid pieces and it being appropriate MOST of the time, not in all situations and not with the "crappy" placements that you postulate.
Gobie wrote "Where is your redundancy when you take the fig 8 out of the cordelette? How strong is your cordalette? Even though you have equalized your protection you have limited yourself to one piece in that if the cordalette (which is loaded 100%) fails you now are left with this equation
100%-100%=nothin'"
Good catch, Gobie. I missed that very significant benefit of the figure 8. We often do trust our lives to one element; one harness, one rope, one biner. However, it behooves us to avoid that when practicable. That alone makes a good case for putting the figure 8 knot into the cordelette.
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Moof
Big Wall climber
A cube at my soul sucking job in Oregon
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Apr 21, 2008 - 01:08am PT
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"Rope slippage is another issue. I've read that a tube style belay device will slip at 2-3 kN (Petzl's fall simulator assumes 2 kN). Is this realistic? If it is and if the rope isn't redirected through the power point, then in a factor 2 fall the force on the entire anchor is limited to 2-3 kN. So why bother with the questionable virtue of screamers in the anchor? Why not just wear a belay glove instead?"
Most common use for screamers is aid climbing, where a gri-gri is also commonly used. No slippage.
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TradIsGood
Chalkless climber
the Gunks end of the country
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Apr 21, 2008 - 07:31am PT
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Karl, in the screamolette, the only time you will have equalization is if/when all three screamers are actually tearing - assuming that they really work as advertised and tear at a fixed force regardless of extension. Once they have stopped, if the anchor holds, you are back into static analyis with slightly more perpendicular angles - i.e. at that point they have just become ropes of likely unequal length but similar stretch characteristics to each other.
(Of course, the more parallel legs should be more equalized.)
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sawin
climber
So., CA.
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Apr 21, 2008 - 12:39pm PT
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_c_
a_b
In the real working static ground world, the 3'rd middle leg
is usually not feasible to achieve and is not applied such
as with tilt up building construction. Assuming a+b = 300,
then c=300 and each leg carries 100. It's easier to acheive
2 leg balance.
However working above the ground with tons of mass over critical
objects a tri-pod or quad pod equalizing the load is of the
upmost necessity as crane charts read static weight hence a
shock load (caused by dynamic mass) of a max static load could
roll $100,000.00's and destroy $1,000,000.00's below. Of note
when I rigged these, equality between the legs of the load I
admit is not instantaneous though equals out over a very short
distance and in fractions of a second. I have rigged 100's if
not 1000's of these with 0 roll overs or a failed load.
The secret as mentioned is a power point for the screamers
to activate. Now that I have thought about this it should
not be to difficult to build a balanced screamer system.
Opinion:
I only have experience with Air Voyagers. Applying knowlege
of missed-distance at super-sonic mulitple mach speeds we know
the vibration from acelleration and also decelleration causes
tracking signal disturbance (same as gate flutter).
It appears the vertical screamer design is feasible for test
as the horizontal Air Voyager design allows more acceleration
and deceleration causing (gate flutter) as they are not a
constant brake as are the screamers.
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rgold
Trad climber
Poughkeepsie, NY
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Apr 21, 2008 - 03:35pm PT
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Del X wrote "There have been a few of what I called "hints" that there may be more to the story. John Yates wrote about extra energy absorption, although his description was muddled. John Middendorf attested to this extra absorption as well in an earlier ST thread. And Craig Luebben described tests in his ice climbing book where measurements did not match those predicted by simple theory. Did these guys all get it wrong?
I'd be prepared to say that Yates got the explanations for extra energy absorbtion wrong, and I have neither seen or heard informally about any test that comes anywhere near confirming the numbers he has posted. Moreover, his observation that more energy was absorbed than predicted was made in a vacuum devoid of any account of the predictive theory, the protocols for obtaining data, and the data itself, so there is no way to form an independent judgement about the existence of this extra absorbtion from anything Yates said. I don't know about the other two; are they in part or wholly repeating Yates? Do they have data other than what Yates has on his website, which does not even appear to be experimental data? When Luebben mentions "the simple theory" that is not matched, does he also actually give an account of that theory? Does the failure to match whatever simple theory Leubben is using occur in the direction of additional unexplained energy absorbtion?
Del X wrote: "Rope slippage is another issue. I've read that a tube style belay device will slip at 2-3 kN (Petzl's fall simulator assumes 2 kN). Is this realistic?
As what? An average over all climbers? If so, I'm betting on a very large standard deviation.
"If it is and if the rope isn't redirected through the power point, then in a factor 2 fall the force on the entire anchor is limited to 2-3 kN. So why bother with the questionable virtue of screamers in the anchor? Why not just wear a belay glove instead?"
Indeed! But remember that the screamers were, at least originally, proposed by Karl Baba as much for their equalization potential as for their load-limiting contribution.
And if rope slippage really does keep the anchor load down to 2 kN, then one does have to wonder how, for example in the Middle Cathedral accident, a 2-3 kN force managed to extract a four-piece anchor constructed by experienced climbers.
---------------------------------------
Footnote added in edit:
Theoretical calculations suggesting that screamers will be of marginal utility for reducing maximum impact to protection are confirmed by tests performed by the Italian Alpine Club. A translated page can be found at http://home.pacbell.net/takasper/ital_screamer_test.htm.
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rgold
Trad climber
Poughkeepsie, NY
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Apr 23, 2008 - 02:02pm PT
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Del, thanks for reminding me about the thread entitled "help with some math please." I have indeed read Middendorf's comments and replied to them there. He too seems to refer to tests he did a long time ago that indicated that screamers did absorb some "significant" amount of fall energy. He grants that this isn't actually possible, and then searches for explanations, like arresting the fall over a longer period of time, that don't agree with basic physics.
Luebben's account is interesting. He reports taking factor-1 falls on a climbing wall, which should have resulted in him hitting the floor every time. Maybe the belayer was up on a balcony? He says the belayer was anchored to "the ground." Even if they did get the belayer up off the ground, was Leubben really willing to lend his body to science by taking that big a fall? I'm impressed.
He also reports on drop tests with weights. His findings appear to say that screamers performed as predicted (i.e. negligible advantage) in the drop tests with weights but, when real climbing falls are taken by real people with real belayers, peak loads were reduced more than any theoretical analysis of screamers load-absorbing capacities would predict. He takes this extra load reduction to be the "real" reality and does not discuss reasons for the discrepancies between theory and his observations.
So we seem to be left with at least some of the original confusion (or, as you said, "interest. The most obvious explanation is that the deploying screamer changed the belayer behavior in some way. Other side-effects are possible, such as the rope stiffening after multiple falls and then causing the belayer to give a much more dynamic belay, resulting in a lowering of the anchor impacts.
The amount of the reported reductions, near or beyond 50%, seems really too good to be true. On the other hand, if memory serves, it is consistent with Yates' claims (which were, however, made, as far as one can tell, with weights not humans).
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deuce4
climber
Hobart, Australia
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Apr 26, 2008 - 03:37pm PT
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rgold-
Haven't really studied this entire thread, just noted that I was being quoted from the old days of slide rules ;) I'll be happy to hear when someone does the full analysis, and finally solves the mystery, of screamers.
My engineering guess on bar-tack screamer type devices is NOT that they themselves absorb a significant amount of energy, but that they allow the rope (and other energy absorbing elements in the system, ie. body, knots, friction, etc.) to do so.
Thus, my hypothesis:
(1) The use of screamers with a dynamic rope system can result in a longer absorption period of the fall energy.
(2) With a longer absorption period, the peak forces can be reduced (energy under the curve type-of-thing).
(3) Therefore, the use of screamers can reduce peak forces.
Now, you state: "arresting the fall over a longer period of time, that don't agree with basic physics." and of course you may be right, but I do recall seeing John Bouchard's tests on bartacked screamer type devices way back when and it did seem that the force loading was extended in time when the screamer was utilized. The force loading looked like this:
(lower peak, multiple peaks, longer time)
---00----------00----------00
-00---00---00---00---00---00
00-------00--------00----------00
rather than this:
(higher peak, single peak, shorter time)
---------0000--
-----000-------000
---00--------------00
00-------------------00
In both cases the area (energy) under the Force/Time curves is equal.
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rgold
Trad climber
Poughkeepsie, NY
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Apr 30, 2008 - 03:56pm PT
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John wrote: "In both cases the area (energy) under the Force/Time curves is equal."
The area under the force-time graph is often called the impulse. It represents the net change in momentum, not energy absorbed.
The mechanism for energy absorbtion in an idealized rope is just the work done in stretching it, and this quantity depends on elongation but not on the time required to realize the elongation. To the extent that a screamer reduces anchor loads, the rope stretches less, so it is difficult to see how giving the rope "extra time" increases the energy it absorbs when the net result is less energy-absorbing behavior.
Of course, the answer to all this is that a rope is not an idealized rope, and Del has mentioned some hypotheses that could conceivably explain the results of those tests that do not find a negligible screamer effect. But there is also the possibility that the tests are either flawed in some way or, because of the small number of trials, have mistaken some aspect of systemic variation for an actual effect.
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deuce4
climber
Hobart, Australia
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My bad-
I was thinking force x time = work, thus...
It's been too long since I did any real engineering.
Ask me about Geodetic Datums, I think I could probably be more reliable.
Hope you guys figure it out.
cheers
http://www.lynxgeos.com
ps: still, my design intuition, which often serves me well, tells me that it has something to do with the extended TIME of absorbing the fall energy with a screamer doohickie in the system. Perhaps it has something to do with a variation of the role of other energy absorbing components of the system...?
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Karl Baba
Trad climber
Yosemite, Ca
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May 22, 2008 - 07:58pm PT
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I wish a got a massage!
Are you saying she's a screamer?
I don't get it
PEace
karl
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