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Partner cracklover


Jun 29, 2012, 8:57 AM
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Re: [blondgecko] Theory about forces in a 3-legged cordelette [In reply to]
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blondgecko wrote:
rgold wrote:
and the more critical issue of arm length...

This is an interesting point that I always found myself pondering while tying rope anchors. The problem, of course, is that a shorter arm has less absolute distance to stretch than a longer arm under the same load, and so under a falling situation the shortest arm will end up taking the brunt. A fairly simple workaround is to deliberately tie the powerpoint so that the shorter arms remain loose when the anchor is unloaded. It should be relatively straightforward to work out a rule-of-thumb: for every foot of extra length, tie that arm x inches short so that under a moderate load the arms stretch to (relatively reasonable) equalisation.

Like this:



It takes a bit more thought and playing around and will never be perfect, but I'd bet money that once perfected it would outperform the standard approach.

Yeah, but if you get it only slightly wrong (by as little as a half an inch) then the long arm of your three-piece anchor gets zero force.

I think a better approach to "fixing" the standard cordelette would be to tie a large (like an extra two or three wraps) central knot, and leaving it loose. This gives you both the benefit that it will equalize itself a little bit, and also absorb some force. Unfortunately for hanging belays, you'll probably tighten it a bit already in the direction you're hanging. And, of course, as Jim Titt suggested earlier in the thread, all those strands in the knot will tighten themselves different amounts, so the force the pieces feel will never be properly equalized no matter what.

But really, the big issue with all non-self-equalizing solutions is that if you're even just a little bit off in anticipating the direction of force, you can wind up with all of the force on one piece. Not good.

This can happen very easily. Maybe you have equalized it perfectly for you, the belayer, standing on a ledge. But the leader falling past you will not have her rope run in the same direction - the ledge is in the way. It will instead pull more in an outward direction. If there is any vertical component to the pieces, the bottom one will take all or most of the load first, and if it pops, then the middle one, and if that pops, then the top one.

The same thing can happen from a horizontal perspective - if the falling leader's rope goes off to the side relative to the direction of anticipated force.

Edited to add - it is certain that leaving the knot loose, with a lot of wraps, would help solve this issue. How much? Who knows.

GO


(This post was edited by cracklover on Jun 29, 2012, 8:58 AM)


degaine


Jun 29, 2012, 9:26 AM
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Re: Theory about forces in a 3-legged cordelette [In reply to]
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All of these rigs and theories focus on getting some form of equalization between three pieces. Why three pieces?

What if the equalizing cordalette only had two arms and if one uses three pieces, two of those pieces are rigged to produce only one arm? Sure, 50% of the force/weight is on one piece, but if it's bomber...

I've been paranoid enough in one or two situations over the years to rig an anchor with more than three pieces, but when tying the cordalette or using the rope I just used sling craft or a carabiner to combine pieces so that my cordalette/rope only had two or three arms.


Partner cracklover


Jun 29, 2012, 10:02 AM
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degaine wrote:
All of these rigs and theories focus on getting some form of equalization between three pieces. Why three pieces?

I can't speak for others, but I'll say that for me, the three-piece anchor is best for discussion for several reasons:

1 - My most common trad anchor incorporates three pieces.

2 - When the anchor uses less than three pieces, those piece(s) are usually so bomb-proof that the anchoring method and importance of good equalization probably makes no difference.

3 - IMO it's very easy to get reasonably good load-sharing for two-piece anchors.

4 - My four piece anchors typically either 1 - use the same rigging methods as three piece ones (so the same theory applies) or 2 - have two weak pieces joined by a crossed sling to jointly form one "arm" of a three piece anchor.

In reply to:
What if the equalizing cordalette only had two arms and if one uses three pieces, two of those pieces are rigged to produce only one arm? Sure, 50% of the force/weight is on one piece, but if it's bomber...

That's fine too. Probably better if you have one piece that is definitely much better than the other two.

In reply to:
I've been paranoid enough in one or two situations over the years to rig an anchor with more than three pieces, but when tying the cordalette or using the rope I just used sling craft or a carabiner to combine pieces so that my cordalette/rope only had two or three arms.

Precisely. Which is why almost all the discussion has been around two- and three-point anchors.

GO


JimTitt


Jun 29, 2012, 10:15 AM
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Re: [rgold] Theory about forces in a 3-legged cordelette [In reply to]
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I would be climbing and belaying tomorrow but right at the moment itīs pushing into the 90īs here and I put a saw into my finger anyway so playing and gardening are on the schedule!

Thinking about how to best test this without too much work today while doing something utterly boring and was struck by some of the peculiarities of the climbing world. I was killing two birds with one stone testing a batch of bolts and at the same time calibrating the hydraulics on a new portable bolt tester, The 12mm stainless quick links kept breaking at about 68kN without bolt failure or getting to the designed pull for the tester (80kN) so every time I had to re-rig everything with my normal tester to break the bolts. So on one hand I was getting frustrated by these recalcitrant things that wouldnīt break and on the other hand thinking about how to assemble a load of weak gear to make a strong enough anchor.

Anyway the test concept is (initially) to work backwards and set up an unequal leg length belay, load it and then adjust the legs under tension until the loads are equal, release the load and see what the differences in lengths are. Whether it will be of any practical use is anyones guess!

First Iīm going to water the beans!


billl7


Jun 29, 2012, 2:23 PM
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Re: [JimTitt] Theory about forces in a 3-legged cordelette [In reply to]
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Just want to chime in that I appreciate knowledgable folks who have written down their thoughts here.

Bill L (one who has progressed from the common knotted cordalette, to John Long's equalette, to a modified equalette, back to the knotted cordalette, and now sometimes climbing with a partner who prefers to use only the rope for anchor rigging.)


ptlong2


Jun 29, 2012, 3:28 PM
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Re: [JimTitt] Theory about forces in a 3-legged cordelette [In reply to]
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JimTitt wrote:
Äs...

Umlaut?


blondgecko
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Jun 29, 2012, 4:37 PM
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Re: [cracklover] Theory about forces in a 3-legged cordelette [In reply to]
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cracklover wrote:
blondgecko wrote:
rgold wrote:
and the more critical issue of arm length...

This is an interesting point that I always found myself pondering while tying rope anchors. The problem, of course, is that a shorter arm has less absolute distance to stretch than a longer arm under the same load, and so under a falling situation the shortest arm will end up taking the brunt. A fairly simple workaround is to deliberately tie the powerpoint so that the shorter arms remain loose when the anchor is unloaded. It should be relatively straightforward to work out a rule-of-thumb: for every foot of extra length, tie that arm x inches short so that under a moderate load the arms stretch to (relatively reasonable) equalisation.

Like this:



It takes a bit more thought and playing around and will never be perfect, but I'd bet money that once perfected it would outperform the standard approach.

Yeah, but if you get it only slightly wrong (by as little as a half an inch) then the long arm of your three-piece anchor gets zero force.

Oops - I was going to make this explicit, but then forgot. All this is predicated on the use of dynamic rope for the anchor - the more stretchy, the better (within reason). As the cord becomes stiffer, the margin of error gets smaller and the range over which you get decent equalization shrinks. With standard cordelette it will be utterly impossible.

Yes, it violates KISS, but I still think it's worth thinking about - if only because understanding this leads to real understanding about exactly why the original cordelette rig is impossible to equalise. It's also fun to speculate on outside-the-box ideas - imagine making each arm mostly static, but with the last foot or so of each arm made of bungee cord. I suspect you'd get much better weight distribution that way.


JimTitt


Jun 29, 2012, 11:00 PM
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Re: [ptlong2] Theory about forces in a 3-legged cordelette [In reply to]
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ptlong2 wrote:
JimTitt wrote:
Äs...

Umlaut?

I use a German keyboard which has all sorts of wierd stuff like ÖÜÄß. And the letters are in different places compared with an English one.
Since the A and Ä are completely opposite ends of the keyboard obviously careless as well!


JimTitt


Jun 29, 2012, 11:35 PM
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Re: [cracklover] Theory about forces in a 3-legged cordelette [In reply to]
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cracklover wrote:
blondgecko wrote:
rgold wrote:
and the more critical issue of arm length...

This is an interesting point that I always found myself pondering while tying rope anchors. The problem, of course, is that a shorter arm has less absolute distance to stretch than a longer arm under the same load, and so under a falling situation the shortest arm will end up taking the brunt. A fairly simple workaround is to deliberately tie the powerpoint so that the shorter arms remain loose when the anchor is unloaded. It should be relatively straightforward to work out a rule-of-thumb: for every foot of extra length, tie that arm x inches short so that under a moderate load the arms stretch to (relatively reasonable) equalisation.

Like this:

[image]http://www.rockclimbing.com/cgi-bin/forum/gforum.cgi?do=post_attachment;postatt_id=6303[/image]

It takes a bit more thought and playing around and will never be perfect, but I'd bet money that once perfected it would outperform the standard approach.

Yeah, but if you get it only slightly wrong (by as little as a half an inch) then the long arm of your three-piece anchor gets zero force.

I think a better approach to "fixing" the standard cordelette would be to tie a large (like an extra two or three wraps) central knot, and leaving it loose. This gives you both the benefit that it will equalize itself a little bit, and also absorb some force. Unfortunately for hanging belays, you'll probably tighten it a bit already in the direction you're hanging. And, of course, as Jim Titt suggested earlier in the thread, all those strands in the knot will tighten themselves different amounts, so the force the pieces feel will never be properly equalized no matter what.

But really, the big issue with all non-self-equalizing solutions is that if you're even just a little bit off in anticipating the direction of force, you can wind up with all of the force on one piece. Not good.

This can happen very easily. Maybe you have equalized it perfectly for you, the belayer, standing on a ledge. But the leader falling past you will not have her rope run in the same direction - the ledge is in the way. It will instead pull more in an outward direction. If there is any vertical component to the pieces, the bottom one will take all or most of the load first, and if it pops, then the middle one, and if that pops, then the top one.

The same thing can happen from a horizontal perspective - if the falling leader's rope goes off to the side relative to the direction of anticipated force.

Edited to add - it is certain that leaving the knot loose, with a lot of wraps, would help solve this issue. How much? Who knows.

GO

That an angular offset on the load means all the load comes on one piece isnīt in fact the case in general, only in the vertical situation and even then only if you used a non-stretch material.
If you consider a classic two point anchor with an angle of 90° between the two legs simple geometry tells us that as the load moves away from the centre axis the load on one side decreases progressively until the load offset is 45° when the one piece has no load.
Stretch and knot slip have an effect on this but generally it is beneficial, a bit complicated for here though as it would take me hours to post it all!
As a rough rule for most anchors having a dynamically equalising system has no benefit until the load offset is more than 15° which in real life is an enormous amount and relatively unlikely to occur. Friction over whatever causes the load to be offset reduces the force on the belay anyway which should be taken into consideration.
In your ledge example the advantage of a static equalised system is that the belayer can anticipate the angle of the rope over the ledge and build the belay to suit, if he used a dynamic equalising system the belay would equalise to suit him and then the friction in the system prevent equalisation occuring under load. This is the theoretical aspect, experience tells us the rope would probably get cut through on the ledge and load on the belay become irrelevant!


JimTitt


Jun 30, 2012, 3:50 AM
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blondgecko wrote:
rgold wrote:
and the more critical issue of arm length...

This is an interesting point that I always found myself pondering while tying rope anchors. The problem, of course, is that a shorter arm has less absolute distance to stretch than a longer arm under the same load, and so under a falling situation the shortest arm will end up taking the brunt. A fairly simple workaround is to deliberately tie the powerpoint so that the shorter arms remain loose when the anchor is unloaded. It should be relatively straightforward to work out a rule-of-thumb: for every foot of extra length, tie that arm x inches short so that under a moderate load the arms stretch to (relatively reasonable) equalisation.

Like this:



It takes a bit more thought and playing around and will never be perfect, but I'd bet money that once perfected it would outperform the standard approach.

Well I took the quick and easy approach and worked on a two-piece anchor (at least for now).
The setup was a fairly narrow angled anchor with 30° included angle, two lengths of 9mm rope with fig 8 on a bight at one end as one would tie-in and clove hitched into karabiners at the pieces. Basically how one would belay if you were on half-ropes.
One leg was 1m (1000mm) long and the other 50cm (500mm) so half the length and the knots pre-loaded to 10kg to tighten them about the same as by hand.

The central point was lightly loaded to 20kg and the tension equalised by adjusting the piece position. A load of 200kg was applied (from a fixed direction) and the difference in the tensions was 24% more on the shorter leg.
To re-equalise the loads the shorter leg piece needed to be moved down 4mm which indicates how accurately we will have to judge things.

Clearly the rope stretches and slips in the knots, the longer leg stetched 130mm (12%) and slipped 120mm, the shorter leg stretched 80mm (16%) and slipped 130mm.
So we could correct for this at the set-up stage:- If the higher loaded leg slips 10mm more due to the increased force then we can add this to the initial length. Then we could correct the stretch for the different load which is 60mm not 80mm so we need to add in 20mm to get things equal so adding a total of 30mm into the short leg.
This actually moved the load imbalance over to the wrong side (the long leg got a higher load) so clearly too much.
After a fair number of attempts I found needed to tie the short leg 18mm slacker than it would normally be if they were initially equalised or 3.6% of the length.
Having a poorly dressed fig 8 or even using one round stock karabiner and one I beam karabiner on the pieces made more difference to the load imbalance than I could manually correct, the I beam karabiner locking the clove hitche locked up much faster, the slip on a 12mm HMS being over 50mm at these loads.

For a typical belay judging the lengths exactly enough is going to be a hopeless task but probably for something like a very long line from an anchor well back maybe a something could be gained.


blondgecko
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Jun 30, 2012, 5:28 AM
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JimTitt wrote:
blondgecko wrote:
rgold wrote:
and the more critical issue of arm length...

This is an interesting point that I always found myself pondering while tying rope anchors. The problem, of course, is that a shorter arm has less absolute distance to stretch than a longer arm under the same load, and so under a falling situation the shortest arm will end up taking the brunt. A fairly simple workaround is to deliberately tie the powerpoint so that the shorter arms remain loose when the anchor is unloaded. It should be relatively straightforward to work out a rule-of-thumb: for every foot of extra length, tie that arm x inches short so that under a moderate load the arms stretch to (relatively reasonable) equalisation.

Like this:



It takes a bit more thought and playing around and will never be perfect, but I'd bet money that once perfected it would outperform the standard approach.

Well I took the quick and easy approach and worked on a two-piece anchor (at least for now).
The setup was a fairly narrow angled anchor with 30° included angle, two lengths of 9mm rope with fig 8 on a bight at one end as one would tie-in and clove hitched into karabiners at the pieces. Basically how one would belay if you were on half-ropes.
One leg was 1m (1000mm) long and the other 50cm (500mm) so half the length and the knots pre-loaded to 10kg to tighten them about the same as by hand.

The central point was lightly loaded to 20kg and the tension equalised by adjusting the piece position. A load of 200kg was applied (from a fixed direction) and the difference in the tensions was 24% more on the shorter leg.
To re-equalise the loads the shorter leg piece needed to be moved down 4mm which indicates how accurately we will have to judge things.

Clearly the rope stretches and slips in the knots, the longer leg stetched 130mm (12%) and slipped 120mm, the shorter leg stretched 80mm (16%) and slipped 130mm.
So we could correct for this at the set-up stage:- If the higher loaded leg slips 10mm more due to the increased force then we can add this to the initial length. Then we could correct the stretch for the different load which is 60mm not 80mm so we need to add in 20mm to get things equal so adding a total of 30mm into the short leg.
This actually moved the load imbalance over to the wrong side (the long leg got a higher load) so clearly too much.
After a fair number of attempts I found needed to tie the short leg 18mm slacker than it would normally be if they were initially equalised or 3.6% of the length.
Having a poorly dressed fig 8 or even using one round stock karabiner and one I beam karabiner on the pieces made more difference to the load imbalance than I could manually correct, the I beam karabiner locking the clove hitche locked up much faster, the slip on a 12mm HMS being over 50mm at these loads.

For a typical belay judging the lengths exactly enough is going to be a hopeless task but probably for something like a very long line from an anchor well back maybe a something could be gained.

Well, as Huxley said, many a beautiful theory was killed by ugly reality! I had, of course, neglected to take into account knot slippage, which is at least on a par with stretch in those lengths and as you say will vary too much. I will point out, though, that if you were to load the rig to represent a more severe fall (say, 400 kg) then the amount of slack you'd have to add to the short leg to get equalization will grow.

... which is the whole problem, in a nutshell. No matter how you tie the rig there's only ever going to be at most one "magic" fall force at which you get perfect distribution between legs, with everything diverging on either side of that. If you deliberately tie the longer legs a little short, you're at least ensuring that they'll get a look-in. Tie them all coming to a common point at zero load, and there's a very strong chance that the longer legs will only see substantial forces in the event of the shortest leg failing. Like you say, though, it probably only really comes into play for very big differences in length.

Definitely beats a fully static rig, though.


JimTitt


Jun 30, 2012, 6:46 AM
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Well thereīs always the Shunt trick.
The Shunt has the interesting characteristic that if you have two strands in it but only one is loaded it will slip down the loaded strand until the tensions become roughly equal. If you tie the two ropes a bit slack into the pieces/bolts and fit a Shunt connected to your belay loop it actually equalises quite nicely.
With a 9mm rope the difference in load on the legs is about 20kg. And of course if the load comes from another angle it re-equalises the same.
The downside is that if one piece fails the Shunt slides down the other strand but thatīs why you only leave a bit of slack or tie an extra knot and at least it is a slide not a drop.
Some people (me included) tie into the anchor as usual with a bit of slack and then fit the Shunt on the other strands coming down from the karabiners as this removes any worries about the Shunt damaging the rope though with two strands fitted the loads have to be very high for this to occur as it will slip a bit first. Personally I donīt find this worrying but it seems easier to this way.


knudenoggin


Jul 13, 2012, 11:14 AM
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Re: [BetaRock] Theory about forces in a 3-legged cordelette [In reply to]
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BetaRock wrote:
The Sterling tests that John Long's book on Climbing Anchors summarized showed that a 2-legged cordelette with unequal legs knotted on a bight experiences an average difference of 3.5 kN between the two legs. This shows that cordelettes are miserable at equalizing even when properly "pre-equalized".

What I'm wondering is ...

Why not improve on the knotted cordelette (meaning "cord", of nylon)
with something that DOES equalize, and spare all of this long-winded
debate about forces per length & angle?

As I wrote in a nearby thread:
knudenoggin wrote:
Frankly, it strikes me
as a too-rigid and somewhat clumsy structure. Especially in contrast
to one I've sketched as an implementation of an "ELET" --2-legged
anchor ("extension-limiting equalization triangle"). It seems to me
that using the common, handy materials of 7mm (twinned) cord
(or other) and a 24"/60cm HMPE (Dyneema, mostly, these days)
tape sling, one gets simplicity, looow friction (for the *equalization*),
and greater flexibility (thinking that UNtying this will be possible,
and so the materials can do other tasks as needed; and the particular
tying --length of legs-- can be set per need, not pre-set & fixed).

[image]http://s14.postimage.org/v1w7tdw1p/knots_ELET_of_cord_sling_M888_1.jpg[/image]

[image]http://s13.postimage.org/xtxcw22df/knots_ELET_of_cord_sling_M888_2.jpg[/image]

Full images (+/- 130kb ; 888pix long side):

http://s14.postimage.org/...ord_sling_M888_1.jpg

http://s13.postimage.org/...ord_sling_M888_2.jpg


These have yet to meet a test device, but seem sound IMO.

*kN*


patto


Jul 13, 2012, 12:21 PM
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knudenoggin wrote:
Why not improve on the knotted cordelette (meaning "cord", of nylon) with something that DOES equalize
Because dynamic equalisation and no extension are an impossibility. Extension can pose significant dangers whereas the lack of in general equalisation does not.

(I'd have to be absolutely desperate to be building an anchor where I thought I needed to equalise the pieces for me to be safe)

JimTitt wrote:
Well thereīs always the Shunt trick.
Interesting... Are you actually advocating this as a useful way of constructing an anchor/belay? Or more as a novelty?


JimTitt


Jul 14, 2012, 12:19 AM
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Your system does nothing better than any of the other common ones, merely adding some more complication and using more gear. And it will equalise just as badly as any of them as well.
And as Patto says, you canīt have equalisation without potential extension which we prefer to avoid to put it mildly.


JimTitt


Jul 14, 2012, 12:32 AM
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patto wrote:
knudenoggin wrote:
Why not improve on the knotted cordelette (meaning "cord", of nylon) with something that DOES equalize
Because dynamic equalisation and no extension are an impossibility. Extension can pose significant dangers whereas the lack of in general equalisation does not.

(I'd have to be absolutely desperate to be building an anchor where I thought I needed to equalise the pieces for me to be safe)

JimTitt wrote:
Well thereīs always the Shunt trick.
Interesting... Are you actually advocating this as a useful way of constructing an anchor/belay? Or more as a novelty?

Well Iīd advocate bolting the belays anyway! And certainly wouldnīt advise anyone to use a piece of equipment outside the instructions from the manufacturer.
However it is a good way of achieving exactly what we want (in fact by far the best way) and I certainly know a couple of people who use this system. Itīs actually quite cool the way it releases the higher loaded rope until the tensions become equal. All at your own risk of course and back it up!


knudenoggin


Jul 14, 2012, 8:10 AM
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JimTitt wrote:
Your system does nothing better than any of the other common ones, merely adding some more complication and using more gear. And it will equalise just as badly as any of them as well.
And as Patto says, you canīt have equalisation without potential extension which we prefer to avoid to put it mildly.

Not so fast (and noting that we've not seen test restuls):

1) Re "complication", I submit that it's far simpler to tie
cord (twinned 7mm nylon, perhaps preferably --or 6mm)
to the bight ends of an HMPE sling than to fashion appropriate
knots per cordelette or other, more complex structure;
1.b) ... and, so, morEasily adjusted

2) The "extra gear" is but one 60cm/24" HMPE sling
--hardly going exotic, with this. (No extra hardward,
no clever fashioning of a single bit of cordage into
multiple functional parts.)

3) And given that that particular material --low-friction HMPE--
is at the point where 'biners must slide to equalize,
yes, it does equalize better than others
(this, I'll note, is Craig Connally's observation).

4) Finally, re extension, note the full extent of that :
about a half foot (with 60cm/24" sling; and not counting
pendulum effects of a blown anchor, of course).
What I took from Sterling's testing was that extension
was far over-rated (and, Patto, where's the testing
that proves otherwise, body in system?).

*kN*


JimTitt


Jul 14, 2012, 8:50 AM
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Dyneema hybrid slings arenīt much better than pure nylon in the friction stakes and adding the second karabiner you show negates almost all of the difference.
Tell me exactly what you want to test and how and I will test it for you, if I designed the test protocol I already know the answers from other tests and can assure you it is no better than any of the conventional systems.
Your system is already more complex by using two slings to do the job of one. Knotting cord and Dyneema together is already dubious and you will have problems untieing the knots afterwards
You are making the basic beginners error of assuming the extension is only half of the length between the limiter knots/length of sling. If one is introducing an equalising element you are by definition saying that the direction of load is unknown as if it is known you could use a static system. Therefore the possiblity exists that the load can come from one side and if the other piece fails the extension is the full length you allowed for equalisation.
With the belayer directly attatched to the belay even a limited extension can be disastrous and will anyway ALWAYS give a higher load on the remaining piece than if no extension occured, therefore any system allowing extension is worse.


patto


Jul 14, 2012, 12:11 PM
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knudenoggin wrote:
What I took from Sterling's testing was that extension was far over-rated (and, Patto, where's the testing that proves otherwise, body in system?).

Those Sterling tests have been the worst thing to happen to this debate as time and time again they are brought up to suggest extension barely matters. The conclusions were wrong, extension matters.

It matters when there is a mass at the belay such as when the belayer is hanging on the belay. As many other tests have shown a body weigh mass falling onto static slings produces very high forces.


JimTitt


Jul 14, 2012, 12:43 PM
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Certainly worth noting that even with a simplistic system doing extension tests McKently had to abandon testing when the sling broke at over 20kN though with a heavier weight than we would normally use but which would reasonably refelcts the forces we would encounter. He was using 9" of extension.
With the same set-up but more extension (12") Iīm seeing 32 times the suspended weight alone (the belayer) on one piece and 25 times on the other without even considering the falling climber.
And there are worse scenarios out there!


patto


Jul 14, 2012, 3:48 PM
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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.


knudenoggin


Jul 15, 2012, 10:21 AM
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JimTitt wrote:
Certainly worth noting that even with a simplistic system doing extension tests McKently ...

Thanks, URLink/citation for Mckently (among his many testings)?

*kN*


patto


Jul 15, 2012, 11:18 AM
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knudenoggin wrote:
JimTitt wrote:
Certainly worth noting that even with a simplistic system doing extension tests McKently ...

Thanks, URLink/citation for Mckently (among his many testings)?

*kN*

Google worked for me, this was the third link. I think this is what Jim was meaning.
http://files.meetup.com/1324053/A_Look_at_Load-Distributing_and_Load-Sharing_Anchor_Systems.pdf

But even without test results simple high school physics analysis leads to the same conclusions. The Sterling tests didn't have any mass attached to the anchor so there was nothing to cause the shock load. (Their mass was only the falling climber which is attached with a DYNAMIC rope.)


JimTitt


Jul 15, 2012, 12:22 PM
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Dropping a weight on a rope and allowing extension is naturally a very selective scenario and in fact it represents a situation which I have never encountered in my entire climbing career which is someone using a direct belay for a lead climber off a trad anchor. It reflects neither the worst case nor the usual situation where the belayer is directly in the system.

Regrettably when the belayer is attatched to the belay and one piece fails under the impact of the falling leader (which is the most likely event) the belayer is accelerated downwards not by gravity but the force imposed by the leader through the belay device.
Traditionally one was "plucked from the stance" to oneīs doom but nowadays we can see that the acceleration is probably around 4g looking at the typical forces we can generate through a belay plate. So simple FF1 thinking is incorrect when reviewing extension because really you would get a belayer taking a FF4 equivalent onto the sling plus of course the still regrettably falling leader so we need to add maybe 4 to 6 kN as well. The smart and fast reacting belayer will of course drop the leader and grab the belay to reduce the combined impact!

All in all a somewhat worrying prospect which leads me to think that potential extension has no place in a belay, especially if you involve low-stretch materials.


patto


Jul 15, 2012, 11:44 PM
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JimTitt wrote:
Dropping a weight on a rope and allowing extension is naturally a very selective scenario and in fact it represents a situation which I have never encountered in my entire climbing career which is someone using a direct belay for a lead climber off a trad anchor. It reflects neither the worst case nor the usual situation where the belayer is directly in the system.

Regrettably when the belayer is attatched to the belay and one piece fails under the impact of the falling leader (which is the most likely event) the belayer is accelerated downwards not by gravity but the force imposed by the leader through the belay device.
Traditionally one was "plucked from the stance" to oneīs doom but nowadays we can see that the acceleration is probably around 4g looking at the typical forces we can generate through a belay plate. So simple FF1 thinking is incorrect when reviewing extension because really you would get a belayer taking a FF4 equivalent onto the sling plus of course the still regrettably falling leader so we need to add maybe 4 to 6 kN as well. The smart and fast reacting belayer will of course drop the leader and grab the belay to reduce the combined impact!

All in all a somewhat worrying prospect which leads me to think that potential extension has no place in a belay, especially if you involve low-stretch materials.

This needs to be made a sticky in the LAB if not the front page. For half a decade I have been arguing this but the notion that extension doesn't matter has been entrenched thanks to John Longs book.

Talk about 4G acceleration an equivalent FF4 fall is down right scary. (That said the physics can be comparable to seat belts, its about ensuring even deceleration.)

Thankfully all this discussion is mostly academic because belay failures are extremely uncommon. However if we are going to start trying to reinvent things like some people are, we better damn improve things!


(This post was edited by patto on Jul 15, 2012, 11:47 PM)

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