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beau
Oct 8, 2012, 1:09 PM
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Someone recently showed me a picture of a kind of factory-made rabbit-ears anchor sling with sewn sections instead of limiter knots and it even had a pair of rappel rings included . . . but I don't remember the name.
It wasn't either the Mountain Tools Web-O-Lette or the Combination Cordalette/Webolette.
Anybody know what it might be?
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beau
Oct 8, 2012, 2:59 PM
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that's it, sandstone, thanks
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JimTitt
Oct 11, 2012, 6:11 PM
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Well Iīm familiar enough with them since Iīve tested both concepts. The alpine equaliser design has the worst equalisation of any system Iīve tested (82%/18%/0%) and this is inherent in itīs design and construction.
The ACR is constructionally and performance wise the same as a sliding X (58%/27%/15%) but exchanges a otherwise useful and versatile karabiner for a ring to no advantage.
Both have appalling extension if one or more points fail and no redundancy.
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paulraphael
Jun 11, 2013, 9:57 PM
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JimTitt wrote: The ACR is constructionally and performance wise the same as a sliding X (58%/27%/15%) but exchanges a otherwise useful and versatile karabiner for a ring to no advantage. Both have appalling extension if one or more points fail and no redundancy.
Could you share your methodology? I'd like to see how you arrived at these numbers. It would be helpful to see the range of scenarios tested , the number of trials, and the standard deviations.
Your comments about the ring being unadvantageous, and about the apalling extension, suggest you did not learn how to use the method.
If more than a foot of extension is possible, you haven't rigged it properly.
If the ring seems superfluous, consider the possible ways to rig it incorrectly with a carabiner, consider the added time, and consider the possibilities of loading the beaner across the weak axis or across sharp edges of the gate hook.
I am open to constructive criticism of the ACR
I'm sure anyone who uses it will be. But I want to know the evidence behind the criticism, and want to be sure it's coming from someone who's using the method properly during testing.
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JimTitt
Jun 12, 2013, 7:30 PM
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Iīve tested almost every conceivable belay set-up, well over 700 pull and drop tests and the procedure is:-
Rig the belay system horizontally orientated with a 90° included angle between the outer legs, see where the cord/tape slides, measure the wrap angles for each of these points and then use bending theory to establish a theoretical efficiency.
Add a weight to the master point and strain guages to each anchor point, pull the weight well to one side and allow it to slide toward the centre and obtain a plot for the loads on each leg.
Rig the belay as before but on a steel beam, equalise the weight using the read-outs from the strain guages then tilt the beam measuring the angle as I go until the weight slides, this tells me how far angularly one must shift the force before any dynamic equalisation occurs and what the force imbalance is on the legs. It gives the same result as the previous test but working from another direction, (the previous test is a dynamic situation to static and this test is static to dynamic).
Then I set the belay back to perfectly equalised and apply an increasing force (I go to 4kN) to obtain a plot of the load variation due to differential leg stretch.
Most of these are done 3 or 5 times depending on what Iīm seeing on the readouts and I take a straight avarage usually unless something peculiar is happening.
Then I do drop tests, the first is a straight offset drop (the weight to the side of the belay) and this is done 9 times, 3 times with the belay pre-set right, 3 times left and 3 times central which is nescessary since they give vastly different results.
Then a horizontal drop where the force of the drop weight is through a turning point to the side (imagine traversing out from a belay and then climbing up and falling off). This is the worst-case impact on a hanging belay without leg failure. These two tests are done with reduced weights as the belays can fail with the full weights, I use 30kg for the belayer, 30kg for the faller. FF1 9mm dynamic rope and repeat 3 times.
ThenI do drop release tests on the central and outer leg, these with an 30kg weight as some setups will fail with an 80kg weight.
The drop tests are normally 3 times but with the Alpine Equaliser/ACR/SlidingX a peculiar result appeared where
the peak force releasing one outer leg was variable as regards on which of the remaining legs it occured. I repeated this quite a few times, as far as I can tell it is a function of how the released karabiner jams the cord in the central ring.
Iīve done some other tests concerning load angle shift combined with leg failure but they are more general and not specific to the ACR, the results are not pretty.
If extension limiting knots are not used the extension can be virtually the entire length of the sling or cordalette loop unless you restrict your thinking to the most simplistic horizontal setup, a foot of extension is far too much anyway in most circumstances, Iīve seen 32X force multiplication with 14" of extension on a nylon sling and others have reported similarly alarming numbers which is why we donīt use 80kg weights for testing this, 80kg being itself debatably low as there might be two or more people on a belay when it fails. McKently for example had to abandon this test as the sling broke when representing a rescue scenario with 3 persons.
I tested with both a 12mm polished stainless steel ring and a 12mm round stock karabiner with both 8mm cord and several kinds of nylon and Dyneema/Spectra tape.
Naturally Iīve done the vertical cases and some in between as well but they are more relevant to the theory of building belays, the equalisation per se doesnīt get any better.
"If the ring sems superfluous, consider the possible ways to rig it incorrectly with a carabiner, consider the added time, and consider the possibilities of loading the beaner across the weak axis or across sharp edges of the gate hook."
That bit made me smile! Youīve already lead a complete pitch clipping all its protection points, built a multi-piece belay with at least 3 karabiners and are going to use a belay device and itīs karabiner and you are worried about all that happening to one karabiner right in front of your face? You still need to clip a karabiner into the ring and cord anyway.
Since there is no measurable performance advantage in using a ring and a karabiner compared with building it all with one karabiner the ring is completely unescessary.
The ACR is not particularly bad, I tested two systems which were worse. But it isnīt any good either but whoīs is?
For two-leg systems you donīt need a whole stack of strain guages (I use 3 strain guages coupled to 2 computers) to measure the force imbalance in the legs but merely a piece of paper, a pencil and a ruler. Pull the central point (karabiner) to the side to represent a offset load, hold a sheet of paper behind the central point and trace the direction of the two legs and the applied force. Draw a simple vector parellelogram and measure the sides. Their length proportion is the load split.
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paulraphael
Jun 12, 2013, 8:12 PM
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First of all, I appreciate your taking the trouble to do these tests. For the results to be of real value, we really need to see the data. One person saying "I did these tests," followed by a brief description and a conclusion, is not enough to go on. There are so many variables,
Did you do all your tests with webbing or with cord? The recommendation for the ACR has always been 7mm nylon cord, which allows for some force reduction if the anchor extends. I would be quite surprised to see the kinds of force multiplication you describe when using this material. My own tests show webbing more likely than cord to bind under load.
My comment about the differences between the ring and the a carabiner should not make you smile. In all the instances you describe of using a carabiner, it's loaded the way its engineers intended, across the strong axis. An ACR setup, under the best of circumstances, loads a carabiner triaxially. If an arm fails and the angle of load suddenly shifts, or if friction stops it from re-orienting, who knows how the biner would end up loaded. The biner can also allow cord to slide over it under load while redistributing, and it will do this with cord stretched at wide axis, possibly over the rough edges of the gate hook.
You also haven't addressed the practical side of using the ring. It's always there, rigged properly through a half loop of cord. If using a carabiner, twisting the cord becomes a critical additional step. And if you forget to do it, or do it wrong, it's difficult see the mistake after the anchor is built. The ring adds efficiency and safety by eliminating a step and a source of human error.
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Syd
Jun 12, 2013, 9:27 PM
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JimTitt wrote: Iīve tested almost every conceivable belay set-up,..
What are your recommendations Jim ?
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JimTitt
Jun 13, 2013, 7:18 AM
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Nobody publishes the raw data, itīs just too big and generally incomprehensible unless you are using the same program as the original researcher. In the case of these tests itīs about 2,000 lines each with 99 digits in crs. format for each attempt.
Generally one produces a graph which is easier to understand and intepret for the general user, since I can also intepret the graph I can make my conclusions and give them as a simple answer understood by all. This is the one for the ACR:- 
You can draw your own conclusions which will probably be the same as mine, that is the load split is 58%/27%/15% and the equalisation is as poor as a sliding X:- 
One can do a simple analysis without bothering with testing which says the load split will be 59.5%/24%/16.5% so clearly the tests are in the right region and nothing untoward is happening.
There arenīt many variables since I use the same sets of angles and materials for all the different systems, it would make intepretation and drawing conclusions difficult if I didnīt. As I mentioned previously I used 8mm cord as well as tape of various kinds and there isnīt much between them, cord is marginally better in the impact stakes than nylon tape but the difference isnīt enough to be confident it will protect either the belay or your kidneys.
I, like every illustration Iīve ever seen of a sliding X use an locking HMS karabiner which is removes the concerns you describe since they are designed for this kind of loading.
If one is concerned with sharing the force between the anchor pieces then your system and the slidng X with two karabiners are as good as it gets with normal equipment. If ones concern is anchor loads if one or more piece(s) fails then your system, as with all dynamically equalising systems, is poor to disastrous. If ones expectation is that with a dramatic load shift the belay will magically distribute the force even remotely over the pieces then one is going to be dissapointed.
So what tests and methology did you use before recommending the ACR to the general public?
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Attachments:
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LoadSplitAngleACR.jpg
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LoadSplitAngleSlidingX.jpg
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theguy
Jun 13, 2013, 7:28 AM
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paulraphael wrote: For the results to be of real value, we really need to see the data. One person saying "I did these tests," followed by a brief description and a conclusion, is not enough to go on. There are so many variables,
Is the Deutscher Alpenverein report good enough for you? If not, perhaps they'll provide you with the raw data if you ask nicely.
As you say there are many variables in a climbing situation, but it's interesting that around the same time an American came up with the equalette as the answer to the shortcomings of the sliding X, DAV research had shown that reducing shock loading is generally more significant than equalization.
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theguy
Jun 13, 2013, 7:43 AM
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JimTitt wrote: So what tests and methology did you use before recommending the ACR to the general public?
This is the latest I've been able to find:
"Lab tests have so far been confined to the adjusting knots. Jim Ewing at Sterling Ropes conducted a number of pull tests on the combined shortening/limiting knot. He tied the knots loosely and untidily, as they would likely be tied in the field. His samples included 7mm nylon (which we recommend) and 6mm Technora. All samples held over 12kn.
Based on experience, Jim is confident that the knotted arm of the anchor could hold a factor 2 test fall by itself. We are working on getting drop tests performed on the whole anchor rigging, to see how efficiently it distributes loads compared with other methods."
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theguy
Jun 13, 2013, 7:51 AM
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Ignore my prior replies.
I see we at RC.com are behind the times as usual: Jim and Paul had this conversation on MP back in February.
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paulraphael
Jun 13, 2013, 5:55 PM
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Jim, thank you for posting all that. I understand it's unreasonable and unhelpful to publish raw data, but it's helpful to see these added details of your methods and the analysis.
I would like to add a synopsis of your results to the information sheet on the ACR, so people can make more informed decisions. I can only do this if I can personally vouch for the testing and the thinking behind those results. I especially need to know that the ACR method tested is substantially the same as the one we're describing. This includes, especially, kinowing it was tied to limit extension to under a foot.
I still respectfully disagree that a carabiner is a good candidate for replacing the ring. An HMS biner is not, as you suggest, designed to take loads off the strong axis. In your own testing, you said you used 90 degree angles between the outer arms. It's not hard to imagine a carabiner used in place of the ring getting cockeyed, resulting in the load going directly across the gate. The Petzl Atache I use is rated to 7kn across the weak axis; barely more than the gate-open strength. And its locking ring provides another rough surface you don't want cord running over.
Even if these weren't concerns, I'd argue for the ring. People who use the ACR overwhelmingly say they do so for the speed of rigging. The ring eliminates a piece of hardware to attach, it eliminates a critical safety step (putting a half twist in the cord) and gives you one less thing to have to check at the anchor. And it doesn't get in the way if you don't need it, the way pre-tied knots in an equalette do.
(This post was edited by paulraphael on Jun 13, 2013, 6:11 PM)
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paulraphael
Jun 13, 2013, 6:10 PM
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theguy wrote: im and Paul had this conversation on MP back in February.
Ha. The questions came up, but not the answers.
In response to what testing had been done previously on the ACR, it was all static load testing, and informal field testing. We ascertained (as Jim's drop tests confirm) that it distributed loads at least as efficiently as a sliding Xwhich has been standard practice for a long time.
Jim's testing seems to suggest that the sliding X (and everything else that dynamically equalizes) is at best highly questionable in its efficiency.
The broader, more interesting point that I'd like to see elaborated is that extension is a worse problem than inequal distribution.
Is this what's covered in theDeutscher Alpenverein report? My ability to read technical documents in German is, um, lacking.
What is their final recommendation? Looking simplistically at the proposition that extension is worse than unequal loading, it would follow that we should go back to static methods, like the cordelette. Or do they propose a more interesting solution?
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JimTitt
Jun 13, 2013, 8:08 PM
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The simple sliding X was done to death about 25 years ago by someone from the cavers when it was shown to be considerably less sliding than hoped which we in fact already knew, that the force on one side of a rope over a karabiner is ca 1.6 greater than the other was published in 1965 and a belay system is no different just generally more complex and with yet more friction.
The whole matter of multi-point anchors of both the fixed and sliding type including the human ability to tie them correctly and the consequences of leg failure was covered extensively by McKently, Parker and Smith and in another paper by Marc Beverly mostly on fixed systems with the points at various angles to the pull axis to investigate the effect of different leg lengths. All came to the conclusions one would nowadays expect and the use of sliding X is certainly not allowed for instructors in the UK, not recommened if not banned in Germany and Iīm lead to believe in the USA as well.
The Germans naturally also did their bit, drop tests and all and didnīt like what they saw:- "Conclusion: The classic sliding X is not recommended any more as failure of one anchor point can lead to a considerably higher total impact force (on the remaining one).
To prevent failure of the remaining point the fixed triangle (cordalette) or the inline triangle sling (effectively tie from the climber to the first point, into the next and back to the climber) are better.
The two systems give in belays with questionable points the best compromise between an even load sharing and the additional impact force in case one point fails."
The previous studies left a lot of gaps, partly because they were looking at particular scenarios to suit their own needs, from time restraints or because the testers hadnīt looked at all the possibilities.
For example the DAV did drop tests with the weight to one side to represent a climber falling from above and to the side and to see how equally a sliding X managed to equalise. Curiously they got a different result to previous tests of this type so I looked further into this and found that by adjusting the amount of pre-equalisation (where the master point was in relation to the anchor points) you can actually achieve whatever result you want. That is, I know where to position the karabiner on a sliding X in an offset weight drop test to give whatever result you desire.
More interesting, since McKently et al had already done the leg failure tests with the load central was to test the much vaunted aspects of the sliding X (or its relatives) combined. That is the ability to correct for a load angle shift and correct for leg failure. If the load shifts considerably we know how the load sharing works but then add in point failure and the picture looks exceedingly grim. For example in the diagram in your website of extension if the load moved 30° to the right
(your limiter knot may prevent this actually happening which is another issue altogether) then the extension on the furthest left leg is probably nearly doubled. I built an ACR with a 120cm sling, 90° angle and was unable to tie a limiter knot and still get 30° load shift so the extension when the outer leg failed was 14". So you need a very good idea of the expected load angle shift before you tie limiter knots as the choice is the knot stops any equalisation and removes one piece from the belay OR there is excessive extension. For load shift angles of less than 13-15° both fixed and sliding systems share the load equally as well so small shifts are of no interest.
There have been comments on the internet that extension is not a real problem, this is naiive rubbish at best and often reflects simplistic thinking, in a top-roping anchor situation extension is the least of ones worries (one should worry that in a top-rope situation anything fails since you have no compulsion to use anything but a bomber belay), with 3 people all hanging on a belay when one point fails it is a massive problem very likely to kill all three. Once you start testing with this in mind the forces rapidly get over 20kN and things break, as I mentioned before I have seen 34X the drop weight and others report similarly high figures, while a squashy climber will undoubtedly give lower results several squashy climbers probably wont!
There is also a previously unexamined aspect which has the potential to be the worst scenario of all and that is a belayer hanging on an equalised belay and then subject to a sideways force. In our tests the belay itself re-equalised perfectly since the karabiner didnīt actually slide but flew to the up and to the side, the impact forces however are extreme since the belay now has to take the impact of the faller (at least they are on a stretchy rope) and the belayer who has been accelerated sideways at considerable speed and is directly connected. We broke all our tests until we reduced the weights to more reasonable levels (think anorexic 10yr olds) and still donīt know how high the forces can be though we are seeing 17 X the combined weights with an extremely small fall.
The concencus is extension is potentially dangerous and I agree.
The evidence confirms that no sliding system gives adequate equalisation BUT fixed systems are equally as poor at load sharing if not worse, however fixed systems have no extension and redundancy and thus are preferable.
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