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Theory about forces in a 3-legged cordelette
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jktinst


Jul 16, 2012, 9:06 AM
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Re: [patto] Theory about forces in a 3-legged cordelette [In reply to]
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It seems to me that these predictions of doom if any extension is allowed on a blown pro are based on the premise that the belayer is attached to the belay anchor through a static link and that s/he is yanked off his/her stance and goes straight into free fall without opposing any resistance.

Nowadays, I am using as much as possible belay-setting and belaying approaches that aim to either avoid altogether the possibility of the leader falling directly on the belay or severely limit its impact. As a result, the extension vs. equalization debate affects me a lot less than it once did. However, even with these approaches, failure of the first few progression pros (however unlikely) could still result in the leader falling on the belay so the question remains relevant.

With respect to the scenario above: I use a dynamic cow’s tail and, although I am reluctant to use stancing as a primary means of limiting the impact of a leader fall on the belay anchor, I would like to think that, if one of the anchor's pros were to blow, I would offer some resistance to being pitched into free fall by the few inches of extension that this would cause on my anchor’s central point.

It seems to me that most proponents of dynamic equalization accept that it must be balanced with extension limitation. It follows that equalization cannot be expected to distribute the load of a leader's fall evenly to all the anchor’s pros regardless of where this fall may occur with respect to the belay. As a result, some anticipation of, and allowance for the likely offset should come into the planning of a dynamically equalized anchor (as it does with a static one) to try and allow optimal distribution of the load despite the extension limitation.

In addition, however much one may dislike the Sterling/Long/Gaines tests on extension, their equalization tests demonstrated very clearly that dynamic equalization is essential to allow load sharing between the anchor’s longer arms and its shorter ones. Without this, you get most, if not all of the load on the shortest arm and, if it blows, on the next shortest one, etc. in a potential cascade failure scenario. So, despite the increasingly strident warnings that I’m going to die for allowing any extension on my belay anchors, I continue to see problems at least as dire with the absence of dynamic equalization.

In building an equalized anchor, I take into account not only the potential offset of the fall, but also the length of the arms of the system, the height of the central point and the quality of the stance (as well as other things that I won't clutter this discussion with) and set my extension-limiting knots close enough together to minimize the risk of getting yanked right off while still allowing the essential long-to-short equalization to occur.

Of course there is no preventing the yanking off and freefall situation in a hanging belay so, in these situations, why not 1) have the belayer hanging well below the anchor on a long dynamic cow's tail (which is one of the approaches mentioned above), 2) clip the leader's rope into the central point, and 3) limit extension somewhat more, looking mainly to take care of the long-to-short equalization ?

(This post was edited by jktinst on Jul 17, 2012, 1:31 PM)


JimTitt


Jul 16, 2012, 12:00 PM
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Re: [jktinst] Theory about forces in a 3-legged cordelette [In reply to]
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Realistically we have to consider that the belayer can provide no resistance himself, firstly because this is the worst-case scenario and secondly because nowadays with the general rise in standards of climbing the belay stances are becoming smaller and smaller. When I began climbing belay ledges were nice big things you could have a party on, nowadays I consider a foothold half as big as my rockboot a nice spot. This is particularly noticeable as we trend to ever steeper routes on limestone which at least in Europe is the way things are going.
With the increase in standards and improvements in equipment allowing harder routes to be climbed it is inevitable that more people are exposed to stances and worse falls and belay technology has to be designed around this, at least if we are going to preach a "one size fits all" dogma.

The difficulty is that less experienced climbers have seized upon the equalisation concept as being a solution to everything without it being made clear that they are exposing themselves to enormous potential dangers to no or little benefit. From the not-inconsiderable numbers of posts about equalising top-rope anchors for example this is clearly the case.

In the hands of an expert climber who understands the limitations and the potential downsides and makes every effort to mitigate their effects then a dynamically equalising system may have some benefit (though this is generally doubtful). For the general climbing population and general use there are no benefits, only potential negatives.

Your approach of concentrating more on avoiding a catastrophic fall on the belay is the correct one, combined naturally with getting loads of the best gear you can. No matter how you package it a bag of crap is still crap.
Equalising the parts of a belay is a tacit admission that one doesn´t think any of the pieces are in fact strong enough but one is still prepared to use them which cannot be a desirable way to approach belaying or climbing in general, I would abandon the route at this stage in preference.


patto


Jul 16, 2012, 12:25 PM
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Re: [JimTitt] Theory about forces in a 3-legged cordelette [In reply to]
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For those that dislike reading big blocks of text:

JimTitt wrote:
Equalising the parts of a belay is a tacit admission that one doesn´t think any of the pieces are in fact strong enough but one is still prepared to use them which cannot be a desirable way to approach belaying or climbing in general,

As I've mentioned before I equalise RP and other micro gear that I place on lead. It makes sense in such situations.


tomcecil


Jul 16, 2012, 8:02 PM
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Re: [JimTitt] Theory about forces in a 3-legged cordelette [In reply to]
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 "No matter how you package it a bag of crap is still crap.
Equalising the parts of a belay is a tacit admission that one doesn´t think any of the pieces are in fact strong enough but one is still prepared to use them which cannot be a desirable way to approach belaying or climbing in general, I would abandon the route at this stage in preference."
That is sound advice--

There are no magic systems, it's all about the individual pieces.
The method you use to build an anchor hardly matters if the gear is good.
Practice placing gear not worrying about which jackalette to use--


blondgecko
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Jul 21, 2012, 5:31 AM
Post #55 of 56 (1091 views)
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Re: [patto] Theory about forces in a 3-legged cordelette [In reply to]
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patto wrote:
Those test results need to be retold over and over and over again. The conclusions from the Sterling have been a damaging disservice to the climbing community.

Just to play devil's advocate here, there's another way of looking at this. So limited-extension systems lead to dangerous forces if you have the belayer anchored straight into them and a piece fails. There's two possible ways to respond:

1. Limited-extension systems should not be used;
2. What can be done to counteract this and make it safe again? If you're talking about a maximum extension of 12", then about 6" of shock cord between power point and belay would provide more than enough shock absorption, with minimal increase in complexity.


avalon420


Jul 21, 2012, 7:16 AM
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Re: [patto] Theory about forces in a 3-legged cordelette [In reply to]
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patto wrote:
All this is grossly violating the K.I.S.S. principle.
No it doesn't, we all just have to start climbing with an accurate measuring device, and a calculator. OR YOU WILL DIEEEEE! Crazy

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