|
|
|
|
JimTitt
Sep 17, 2011, 5:53 PM
Post #76 of 88
(5465 views)
Shortcut
Registered: Aug 7, 2008
Posts: 1002
|
In the dynamic equalising tests we don´t get much variation, only a few percent here or there which is normal with textile testing. The clutch effect isn´t occuring because at the moment I´m only concentrating on using cord for various reasons and anyway can be eliminated using 2 karabiners instead.
For the manual equalising some set-ups are more prone to wild outliers than others, the unequal leg and non-horizontal ones being particularly bad. Remove the visual clues (blindfold) or distort them (by specifying the direction of pull at an angle for example) and things get really bad.
I´ve pondered what to do with the outliers, on one hand safety chain theory says they are worst case and so must be the rule but on the other hand one has to accept that it is a queastion of the belay might get this huge load and one of the pieces might fail (or be likely to do so). As set-up that gives rare outliers and not too excessive could reasonably be accepted since the chance of all three combining is going to be very low. systems which consistently give wildly varying results are probably not so desirable. What I´ve done is take the bunched consistent results and given them as an average of what you´d normally expect to achieve and given the worst we saw as well. as well a description of how erratic the results are and how desirable I think each system is.
Greater accuracy than a general indication of good and bad is anyway essentially worthless since we are attatching to gear which is in itself an unknown quantity.
Personally I agree that it is better to go for the good gear rather than compromise it for a nominal (and probably non-existent) increase in safety using poor pieces in the primary set-up. The idea is to stack the odds in our favour and concentrating on the good gear with systems which give consistent results has to be better than using an erratic system on a mixture of good and bad gear.
I still use the rope and clip the rest of the rubbish with whatever´s left on my rack in hope!
Jim
|
|
|
|
|
philbox
Moderator
Sep 18, 2011, 8:36 AM
Post #77 of 88
(5439 views)
Shortcut
Registered: Jun 27, 2002
Posts: 13105
|
I've been having a play with the triple bunny ears. http://www.ehow.com/...gure-eight-knot.html
Each arm has equal amounts of rope i/e the same number of strands going to each piece in a three arm anchor. A little more fiddly to manipulate to more or less equalise than the double bunny ears.
Great discussion too by the way. I've had a play with load cells and equalisation. We came up with exactly what Jim Titt says about needing expensive pulleys to try to gain close to perfect equalisation.
With any anchor set up I do now I am looking for each piece to be bomber. Thus I am looking more for redundancy than equalisation.
|
|
|
|
|
LostinMaine
Sep 18, 2011, 11:32 AM
Post #78 of 88
(5425 views)
Shortcut
Registered: May 8, 2007
Posts: 539
|
I'm not sure I would consider known, massive unequal loading as outliers (at least not statistically to analyze a data set). Since this is a known phenomenon that can be recreated independently, it isn't "fair" to jacknife them out of analysis as it appears Long and Gains have done (though to be fair, I'm not sure they did this, it just appears that the figures do not correspond to their text).
This was the part that I was curious about - the high loading in an unequal "equalizing" system is repeatable.
Thanks!
|
|
|
|
|
JimTitt
Sep 18, 2011, 4:47 PM
Post #79 of 88
(5409 views)
Shortcut
Registered: Aug 7, 2008
Posts: 1002
|
Outliers in tests for dynamic equalisation can´t in any way be ignored since it is a purely mechanical function with fairly clearly defined errors. If outliers occur one needs to look very carefully for the cause and prove that this really is the problem. Normally you would need to show you can both eliminate the outliers and create them as well, just using one of these as proof isn´t acceptable.
Outliers in a human performance can either be incorporated in the overall results or noted seperately to show that most of the time we achieve a given result but we may sometimes not!
Finding out how well people equalise is suprisingly difficult because they know they are being tested so tend to be more fussy than normal. I tried with one person keeping equalising until they were bored when things started to get sloppy, in real life which results apply is going to depend on a lot of things like whether they are fussy beginners, is it raining, was the route easy etc so all I can do is give some vague idea of what pattern does emerge and which systems are more prone to erratic results.
Since there are other, even more vague factors in play more accuracy would anyway give a false picture of the overall effect.
Jim
|
|
|
|
|
jktinst
Sep 21, 2011, 10:08 PM
Post #80 of 88
(5339 views)
Shortcut
Registered: Jun 29, 2010
Posts: 89
|
JimTitt wrote: If the system is truly frictionless then an off-centre drop will load the pieces equally since by definition there is no opposing force on the centre point, the load difference must be infinitely small since the moment one force is greater than the other the central point will move and make them equal.
JimTitt wrote: You´ve misread what I wrote, the difference in the DAV test was 1.3:1 which is considerably different to the tests you quoted.
JimTitt wrote: If the centre point karabiner doesn´t move for whatever reason there can be no dynamic equalisation
JimTitt wrote: With the pieces vertical it´s not only virtually impossible to equalise by hand but the slightest angular change completely unloads the upper piece immediately
|
|
|
|
|
JimTitt
Sep 22, 2011, 6:29 AM
Post #81 of 88
(5316 views)
Shortcut
Registered: Aug 7, 2008
Posts: 1002
|
Confusion reigns! Perhaps I didn´t make it very clear to start with.
There are two ways to vary the loading on an equalising system. You can offset the drop weight onto an initially equalised system but you get the pendulum effect you discuss. Or you can alter the angle of load so that straight down isn´t down any more if you see what I mean (the same as if you started with your anchor equalised at a different angle to the load angle, for example if the belayer is stood to one side).
Both scenarios can and will exist in normal life and should be considered.
Edit to add:- My thinking is that the first (offset weight) tests the ability of an anchor to re-position the weight whereas the second (changed angle) tests the anchors ability to dynamically equalise. Since the basic concept is dynamic equalisation we should be performing the second test.
With an offset weight and a fixed karabiner the belay will be equalised in the end by virtue of the weight taking up a position below the fixed point (providing you got the point equalised correctly first).
With an change in angle of load with a fixed karabiner you get a reduction on one side as I described.
With a dynamic equalising system you also get a progressive reduction on one leg until you overcome friction and this reduction will remain effectively constant so the belay is then unequalised and remains so.
Where and what the peak loads are we will find out, it´s taking a bit of time because my normal drop tower can´t cope with weights swinging around (like most it has a sliding weight) and I have to build something different.
As to the rest;- I´m not trying to duplicate anyones tests nor compare with them, someone else can do that. Using someone elses methodology is only a way to prove what they did was correct, not that their test protocol was correct in the first place. The world of science (and climbing literature) is full of errors from this approach and it is better to try to work from first principles to determine exactly what to and how to test.
Jim
(This post was edited by JimTitt on Sep 22, 2011, 9:36 AM)
|
|
|
|
|
jktinst
Sep 25, 2011, 7:56 PM
Post #82 of 88
(5233 views)
Shortcut
Registered: Jun 29, 2010
Posts: 89
|
Oooh boy! And I did know, once upon a time, that drop towers have the weight on a fixed track with no swinging possible but I was focused on the fact that ultimately, we need to model an arrest of a leader falling directly on the belay from a position up and off to the side of it, which will necessarily involve some swinging. So it was with those blinders on that I read everything you reported. Sorry.
Based on what I just said, you can probably gather that I don’t necessarily agree that it is better to do a "track drop" on a system that has been pre-equalized in a direction different from that of the track (ie different from down) rather than a "DAV-like" offset drop + swing. After all, equalization is just a means to ensure a more even spread of the force of an FF2 fall and it seems to me that the DAV tests mimic such a fall pretty realistically.
Pretty much the only result I understand from the report is that illustrated in Fig 4 showing that the ratio on a sliding X is 59:45 (the 1:1.3 ratio you had mentioned) with the higher load on the arm furthest from the axis of the drop. Of course I still wonder when, during the drop and swing, that measurement was taken but I'll assume for now that the ratio is representative of the highest load applied to each arm (for those who haven't looked at the report: this adds up to more than 100% because the fairly wide 60 deg angle results in an overall load on both anchors higher than that applied by the dropped weight).
I'm also guessing from previous posts here that the second test (with the fuse blowing one arm and leading to an additional 60 cm shock-loaded drop of both leader and belayer) is what led them to conclude that it was more important to rig for redundancy and no extension than for equalization. However, I find it quite frustrating to see an illustration of an extension-limited sliding X in the report and have no idea whether there is any result or discussion corresponding to it.
Despite this conclusion that redundancy and no extension should take precedence over equalization, the lone result I understand, if anything, encourages me to stick with internally-redundant, extension-limited equalizing systems. I don't know if they tried to extrapolate from two-bolt anchors to 3 (and more) clean pro systems but that is where equalization becomes important, IMO. I feel that 59:45 was really pretty good for a drop offset by 1m laterally on a sliding X made with a tape sling (which is more prone to the clutch effect than cord). It also seems to me that this result could actually be applied to improving equalization, although I'm going out on a limb extrapolating on a single result while understanding nothing of the rest of the report.
Since stacked binary systems (2 stacked 50:50 systems giving a theoretical distribution of 50:25:25) are pretty direct extrapolations of the DAV test and since that is the kind of rig I use most frequently for 3-pro belay anchors, I'll take them as an example. In many cases, between the topo and a quick look around from the belay, it is pretty clear where the next pitch starts with respect to the belay location, ie, which side of the belay an FF2 leader fall would have to be on. Say it would be on the left. If you rig the belay with the two "25" arms to the left, according to the DAV test, a fall will load the further "50" arm even more. Back of an envelope calculations applying the DAV ratio to each part of the system suggest that you might get something like 20:25:60. If, on the other hand, you rig the belay with the two "25" arms on the right, you might get something in the order of 45:25:35, which would be much better and plenty good enough for me.
You might also make the friction work to advantage even if the next pitch starts straight above the belay by simply pre-equalizing the biner in a position shifted towards the two "25" arms. By doing so, you'd artificially create an offset drop that will, again, yield a better distribution than the 50:25:25 theoretical values. Since you get to decide how to stack and which way to shift, you should also be able to do it so the "50" arm is clipped to the stronger one of the outside pros.
(This post was edited by jktinst on Sep 26, 2011, 2:15 AM)
|
|
|
|
|
JimTitt
Sep 26, 2011, 8:18 AM
Post #83 of 88
(5206 views)
Shortcut
Registered: Aug 7, 2008
Posts: 1002
|
The drop tower thing is a problem because the weight has to pass the fixed point of whatever you are testing so it is offset, usually forwards. When the weight hits the end of the rope for example it swings backwards and when the rope breaks it goes somewhere, the health and safety guys don´t find having an 80kg weight flying uncontrolled through the air the coolest thing in the world so either you build a big tower outside to allow it to be contained or you slide the weight between two steel girders. For the belay stuff we have to allow the weight to move so need an open system which is why the DAV tests were done on their climbing wall, not the drop tower they normally use.
In reply to: After all, equalization is just a means to ensure a more even spread of the force of an FF2 fall and it seems to me that the DAV tests mimic such a fall pretty realistically.
We should test to see if dynamic equalisation occurs at all and with what limitations. Once we know whether it occurs or not we can then apply this knowledge to different scenarios, not as has previously been done, select a scenario and extrapolae the results to all cases. Especially when the scenarios chosen have never been tested or proven to initiate dynamic equalisation anyway.
The load split from the DAV will almost certainly be the highest force felt by each piece since this is the point of interest, these peak loads may have a time offset but this is normal in dynamic systems and doesn´t alter the load felt and thus the potential for failure.
The limited sliding X was not tested as the it was considered too laborious, in particular untying the knots after loading was considered impracticable (I´d agree with that!).
Their conclusion is that since in their test using a sliding X only resulted in a 3% benefit in equalising the force compared with a fixed system but had the potential for a 40% worse result if one peice failed then a fixed system is preferable. As I´ve mentioned above, this is one scenario and I wouldn´t agree with this conclusion in others for example where the load angle changed substantially more or less.
Jim
|
|
|
|
|
jktinst
Sep 28, 2011, 10:40 PM
Post #84 of 88
(5136 views)
Shortcut
Registered: Jun 29, 2010
Posts: 89
|
At first, I couldn’t figure out how the central loop of the "Kräfteverteilung mit Reihenschaltungsschlinge" was rigged but I found a more detailed method for it in here http://www.alpenverein-muenchen-oberland.de/...latzbau_juli_091.pdf(p.17). It seems that the apparently improved equalization of this system with respect to the tape-cordelette system can only come from 2 sources :
- greater stretch on the single-strand arms compared to the double-strand arms of the cordelette; and/or
- slippage of the clove hitches linking the single-strand arms to the anchor biners.
If the first effect is significant, it would support Patto’s assertion earlier in this thread that using dynamic cord for the cordelette would improve equalization.
I’m not sure that I would consider either of these two effects a particularly advantageous trade-off in order to achieve a distribution ratio approaching that of the sliding X while using a fixed biner. For those who do consider this an OK trade-off, I’ll mention that you should be able to achieve similar (apparent) increases in equalization with a classic cordelette. Adding a clove hitch on each pro’s biner, adjusted so that one strand of each arm is statically equalized with the others arms and the other strands are left slack would not only provide a static equalizing stretch similar to the "Kräfteverteilung mit Reihenschaltungsschlinge" but, unlike it, this cordelette variation would be adaptable to multi-pro systems and provide a separate back-up strand for each arm.
I keep going on with the "apparently" regarding the DAV measurements. That’s because I have questions about their measurements too. Having a single overall load distribution ratio representative of the highest load that each arm was subjected to is good but the time factor remains pretty critical, in my opinion, because that is what will make the difference between real dynamic equalization and successive loading of each pro in turn over the course of the swing.
|
|
|
|
|
JimTitt
Sep 29, 2011, 7:37 AM
Post #85 of 88
(5114 views)
Shortcut
Registered: Aug 7, 2008
Posts: 1002
|
The central knot is a bowline on a bight which is considered to be the best knot for forming a loop in tape, single or double. Apart from being strong and secure it has the advantage that it is easily undone after loading.
One problem with tying an overhand or fig.8 in cord is that there is so much material in the knot with considerably different length paths, unless you pre-tied a cordalette and pre-loaded the each strand to tighten it up you will never be able to control the load split. The other problem is to judge where the knot will end up and thus where you should equalise to, with unequal leg systems this is a real problem and multiple piece systems it seems impossible.
The highest loads on each piece are all that is interesting since the purpose is to find systems which reduce the potential of any piece becoming loaded to failure. That one piece recieves it´s maximum load before the other makes no difference to succes or failure of the system and is anyway inevitable unless you can eliminate friction.
|
|
|
|
|
jktinst
Sep 29, 2011, 2:52 PM
Post #86 of 88
(5096 views)
Shortcut
Registered: Jun 29, 2010
Posts: 89
|
"Bowline on a bight" rings a bell. I must have seen the name in a manual but I’d never stopped before to decipher the diagrams and try it out. Thanks for the info on it. It does seem pretty useful. I was taught the butterfly knot, which I like for its 3-way pull compatibility but it does cinch pretty tight and makes only a single-stranded loop. Thanks also for the reality check on the "successive loading" issue. I was hung up on that but, of course, what’s bad about successive - as opposed to simultaneous - loading is that it will put a disproportionate share of the load on one pro, which, as you point out, will be reflected in the overall distribution ratio.
|
|
|
|
|
JimTitt
Sep 29, 2011, 5:12 PM
Post #87 of 88
(5084 views)
Shortcut
Registered: Aug 7, 2008
Posts: 1002
|
Bowline on a bight is dead handy, it´s a common knot for tying in with in Germany in it´s re-threaded form. It has two great virtues, it can take ring loading and is easy to untie after loading.
In tape it´s handy because you get a doubled thickness for the karabiner loop, it´s probably the strongest knot around and you can still untie it after loading, we use it for the pulling end when we are textile testing for this reason.
Certainly some people seem to have not really considered how the loading occurs, with sliding systems and friction the peak loads will have a time split but with fixed systems the loads should be simoultaneous. We see the time offset in the loads clearly with drop tests on belay systems where the peak loads on one part may well occur when there is no load on another. This is where you need traces of the forces to seperate the different parts out, of course whether it makes a difference is another thing and whether we can do anything about it is yet another!
Jim
|
|
|
|
|
donwanadi
Nov 10, 2011, 6:22 PM
Post #88 of 88
(4797 views)
Shortcut
Registered: Oct 19, 2011
Posts: 170
|
Neoshade wrote: So, In the interest of "dynamic equlization" (which a knot really can't claim) I thought I'd take a whack at the hornets' nest :D Haters be hatin', so... Yo Dawg! I heard you like Sliding-X's... So put I a Sliding-X on yo' Sliding-X! [image]http://farm7.static.flickr.com/6061/6056219593_1fc4f87916_b.jpg[/image] [image]http://farm7.static.flickr.com/6188/6056788414_4f98506ffd_b.jpg[/image] Now THAT'S a handy little truly-self-equalizing leg on a cordalette (or equalette in this case). Seriously, the Double-8 is a pretty sweet and versatile knot! Enjoy. (And bring on the hate  )
Looks like you made a poor man's Trango Alpine Equalizer. 
|
|
|
|
|
|
