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Melissa
Gym climber
berkeley, ca
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Dec 12, 2007 - 06:19pm PT
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Why aren't dynamic runners (like ropes) more componplace? In order to get something to stretch enough to matter, over such a short distance, does it loose too much stregth?
Anyone every monkey around w/ draws made of tight metal springs?
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John Mac
Trad climber
Littleton, CO
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Dec 12, 2007 - 07:05pm PT
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I use Trango Superfly lockers (Great Product!) on my Yates screamers and incorporate them into my anchor when soloing and also the first few pieces to limit the impact on the anchor. Other than that I don't worry too much about it once I get a decent amount of rope since I think the rope will take care of it.
Lockers are the way to go and the Trango Locker don't weigh much more than a standard biner. I brought 20 of them one day at Bentgate and they thought I was a real looney!
http://www.trango.com/prod.php?id=112
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flamer
Trad climber
denver
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Dec 13, 2007 - 05:33pm PT
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rgold,
Thanks for your reply.
The tests I was talking about were done by Rigging for rescue.
They were done in a very controlled environment.
What they came up with was similiar to what you discussed. However they talked about a "spike" effect that occurred when the load limiter came to the "end" of it's deployment.
They were using a dynmator(sp?) attached to a computer program so they could go back and examine the force's over time. What they said was when you look at the force's over time you could see the force that activated the load limiter, then you'd see the force's descrease(or slow down?) has the limiter extended, then when it came to the end of the limiter you'd see the force "spike" back up to near the original impact force. How this all relate's in the grand scheme of physic's is beyond me. My lay man's interpretation is that instead of "impacting" the marginal piece once, you instead "impact" it twice.
The vibration "wiggling" out gear thing was fairly specific. It wasn't RP's or cams(although it could cause cams to walk), but fixed pins that apparently could be a problem. Which does make some sense in my mind. Think about a marginal pin with only a couple of limited points of contact, maybe throw in alittle leverage...wiggle, wiggle....PING???
Who knows?
josh
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Russ Walling
Social climber
Out on the sand.... man.....
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Dec 13, 2007 - 05:49pm PT
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flamer writes: My lay man's interpretation is that instead of "impacting" the marginal piece once, you instead "impact" it twice.
This is just not true. The impact force to activate the screamer (or whatever brand device) is miles below the peak force that would have been on the piece had there been no screamer.
As the screamer deploys, as I understand it, energy is absorbed by the increased "time" and the increased "distance" and a braking effect from the resistance in the stitches. In an ideal situation the screamer would be long enough to never hit the end of the ripping stitches. Usually this is not the case, and there will be a "spike" when the screamer is fully deployed and there are no more stitches to rip. This final load should be below what the piece would have experienced had there been no screamer, this due to the additional "time" and "distance" and the braking effect the stitches have had on the load, and the elongation of the rope over that time.
as an example: say you have a screamer that starts to blow stitches at 500lbs. You ping off onto this screamer, and if it starts to blow, the 500 lbs limit has been reached until you either hit the end of the screamer, or you slow down enough to put your load back under 500lbs. If you dropped a 1 million pound weight onto a 500lb screamer attached to a #2 copperhead fixed at the top of ElCap, and the screamer was 3000ft long when fully deployed, the piece would hold, and the weight would slam into the talus.
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rgold
Trad climber
Poughkeepsie, NY
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Dec 13, 2007 - 06:57pm PT
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I agree with Russ; there's nothing you'd call a double impact. I think the term "spike" is misleading too. If you graph the load on protection over time, what I believe you see is a curve that rises until the screamer activation level is reached (the rope is stretching during this portion), then basically (or ideally) a horizontal line (i.e. load to pro remains constant at screamer activation level) while screamer is deploying, and then essentially the continuation of the original loading curve up until the peak load is reached.
I really do mean the continuation of the original loading curve, because while the screamer is extending the rope tension remains constant at the activation level, after which it continues to rise just as it would have if there were no horizontal interlude. (I think calling this a "spike," considering that we would have seen essentially the same curve had their been no screamer, is what is misleading.) The difference is that, since the screamer has absorbed some fall energy (not all that much, as I have argued), the load curve for the rope peaks at a lower level. I've suggested that the eventual peak in the curve will not, except perhaps for some very short falls, be much lower than if the screamer hadn't been used, and this seems to be confirmed by the tests that have come to light so far.
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Melissa
Gym climber
berkeley, ca
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Dec 13, 2007 - 09:01pm PT
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Maybe the physics gurus and wall experts already know about this, but I did a little googling on load-limiters and learned some interesting bits about seatbelts:
First, many seatbelts use rip-away stitching too as did most of the industrial load-limiting 'I don't want to deck off this scaffolding' lanyards that I saw. Many of the latter were made of elastic material, but I recon in those cases, the main load concern is the person, not the anchor. They were rated to strengths that would be helpful in catching climbing falls, although I don't know if recoil might do more harm than good.
Some also use a deformable metal bar that twists and bends to minimize the static jolt that happens when the stitches are all deployed. It would be interesting to see a biner with a sort of false bottom that bends when loaded, but w/ a real bottom that would prevent the whole works from breaking. I'm guessing it would be a tall order to produce structurally.
I also looked into load limiting knots. The only discussions I could find talked about how it lowered the strength of the material, but I figured in a dicey aid fall, you'd worry about your peice ripping or breaking long before you'd worry about using 7 mm Perlon at 30% strength. Do any of you know of which knots are most often use this way or of any data for impact absorbed by knot slippage?
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rgold
Trad climber
Poughkeepsie, NY
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Dec 13, 2007 - 10:02pm PT
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Melissa, it seems that ordinary tie-in knots account for a significant amount of energy absorption:
"We were able to make accurate measurements of the system stiffness and show that knots play an important role in system stiffness. The figure-8 follow through knot absorbs an equivalent of nearly 1.5 m (5 feet) of rope for the first impact force. After that, the knot is "hardened" and has less absorptive ability. Some climbers have theorized this and make it a general practice to retie their knot after every fall. Although not of practical use on bolted sport routes, this could have major consequences when falling on questionable anchor points."
From Measurement of Dynamic Rope System Stiffness in a Sequential Failure for Lead Climbng Falls, J. Marc Beverly and Stephen W. Attaway.
Google gives the following link:
http://www.amga.com/resources/various/Sequential_Failure_Paper.pdf
I don't use a figure-8 myself but have gone back to a double bowline rather than a single bowline on the assumption that you'd get more tightening effect from the double bowline.
I'm also reminded of a book from my now quite distant days of youth, On Climbing by Charles Evans. There he recommended something called the Tarbuck Knot, which was meant to be an energy absorber at a time when not all ropes were nylon. This knot looked pretty much like a loop tied with a prussik; one sees similar things nowadays in knot books for tent guyline tightening. The knot contracted under the tension caused by the prussik loop part. That book had some pretty wacky stuff in it. It'd be a hoot if the Tarbuck Knot was actually worthwhile...
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Melissa
Gym climber
berkeley, ca
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Dec 14, 2007 - 08:42pm PT
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rgold...Thanks for the knot reference. I'll try to post a pic of the one that J devised.
Does anyone else have any info/input on ripstitch alternatives?
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dirtineye
Trad climber
the south
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Dec 14, 2007 - 11:13pm PT
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OK the flat spot in the curve would be something like a local minima, wouldn't it? Not a spike but almost a dip, but not quite.
Has this been covered: as the screamer extends (and once extended it's not dynamic like the rope is) you lengthen the fall (by the length of the extended screamer) but the rope out remains the same.
That load limiting tent knot thing might be scary. Rope rubbing on rope very fast, can that be a good thing?
If this doesn't make any sense I plead drain bamage
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rgold
Trad climber
Poughkeepsie, NY
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Dec 15, 2007 - 12:01am PT
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"OK the flat spot in the curve would be something like a local minima, wouldn't it? Not a spike but almost a dip, but not quite."
The "flat spot" is...flat, i.e. a section of the curve that is horizontal, meaning that during that time (the time while the screamer is deploying) the load to the protection is constant. However, the spike Josh was referring to is the portion of the curve after the level section. I argued that that portion of the curve doesn't deserve to be called a spike, since it is very little different from what would have seen without the Screamer, and in fact doesn't go up quite as high.
"Has this been covered: as the screamer extends (and once extended it's not dynamic like the rope is) you lengthen the fall (by the length of the extended screamer) but the rope out remains the same."
I did mention this. In fact, the fall is lengthened by twice the extension of the screamer. Call that extension s. The pulley point drops s units, and if you fix your attention on the pulley point and ignore the fact that it is dropping, what you see is s units of rope feeding through the pulley. So the net loss of altitude for the poor leader is 2s.
"That load limiting tent knot thing might be scary. Rope rubbing on rope very fast, can that be a good thing?"
Righto. I'm not aware of the Tarbuck Knot finding favor anywhere but on Charles Evans' waist loop. But in any case, when knot tightening happens, there must be rope rubbing against rope. Not too much, one surmises. The Tarbuck Knot might be a lot worse. Personally, I am not planning to find out.
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dirtineye
Trad climber
the south
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Dec 15, 2007 - 04:37am PT
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Crap, I ALWAYS forget about the 2s.
So, what does the area under that curve mean? Is the area under the curve with screamer equal to the area under the curve without screamer?
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flamer
Trad climber
denver
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Dec 15, 2007 - 08:30am PT
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Wow i can't believe i could have been so dumb!
As russ and rgold point out my "laymans" interpretion was way off.
I don't know what I was thinking! As I read it again I can think is DUH!
josh
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Tom
Big Wall climber
San Luis Obispo CA
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Dec 15, 2007 - 12:11pm PT
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Here is a translation of the Italian screamer and rope dissipater testing.
[url="http://home.pacbell.net/takasper/ital_screamer_test.htm" target="new"]Italian Shock-Absorber Testing[/url]
The gist of the results is screamers are worthless for reducing the forces on an anchor.
EDIT: read at your own risk. I translated the article myself.
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rgold
Trad climber
Poughkeepsie, NY
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Dec 15, 2007 - 12:47pm PT
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"So, what does the area under that curve mean? Is the area under the curve with screamer equal to the area under the curve without screamer?"
The area under the curve, the integral of f dt, is called the impulse. Since f is the derivative of momentum, the impulse is equal, by the fundamental theorem of calculus, to the net change in momentum. Momentum at the moment the rope comes into play is mv, with v the climber's velocity at that moment. Momentum at the moment when the fall has been arrested is 0. So the net change in momentum is always mv, regardless of the way the force on the pro varies to arrest the fall. It follows from this that the area under the curve with the screamer is equal to the area under the curve without the screamer.
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Scared Silly
Trad climber
UT
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Dec 15, 2007 - 12:55pm PT
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What is difference between someone who falls 100 feet hitting the ground and someone who falls 10 feet hitting the ground?
The one who falls a 100 feet screams and then goes splat.
The one who falls 10 feet goes splat and the screams.
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rgold
Trad climber
Poughkeepsie, NY
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Dec 15, 2007 - 01:03pm PT
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Tom, thanks for the link to the translation!
It seems as if the predictions made from fundamental principles held true in the Italian tests. Note that their conclusions do indicate that Screamers may provide some value for short falls, which is again what one would expect, so their utility for aid climbing and very short leader falls within a few feet of the pro should not be dismissed. By the same token, their use on pro for moderately long runouts, a typical practice in ice climbing, I think, is probably illusory.
One does wonder why, with a live dynamic belayer, Screamers seem in some cases to perform worse than expected. A guess is that, in some cases, the Screamer activation serves to reduce the amount of rope pulled through the belay device, with the result the load at the top piece is ends up being slightly higher than without the Screamer.
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dirtineye
Trad climber
the south
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Dec 15, 2007 - 01:06pm PT
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Crap RG, I used to know that. Too much chemo, too much time passed.
I guess somewhere in the dark recesses the idea that those areas would be equal was still in there.
Thanks for the good explanation.
Impulse and Jerk (3rd Derivative) used to be two of my favorite things in classical mechanics. Maybe I'll go stare at the pages of Fowles for a while, and pretend I can still understand some of it.
Hey as far as the effect of a dynamic rope, I'm guessing that the dynamic quality widens and lowers the height that same curve, and that the dynamic properties are aiming for critical damping, ya think?
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Tom
Big Wall climber
San Luis Obispo CA
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Dec 16, 2007 - 06:12am PT
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I think rgold meant a dynamic belay in the sense that the rope slips. The Italian test data shows the extent of this slippage. They also ran "static" tests, with the belayer end of the rope securely tied off.
When the load without the rope dissipater was higher, it was probably due to operator error: the hand holding the munter hitch (1/2 barc; mezzo barcaiolo) wasn't consistent from drop to drop.
As far as critical damping goes, I probably depends on how much rock-rash the falling climber suffers.
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P.Kingsbury
Trad climber
Bozeman
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Dec 16, 2007 - 01:18pm PT
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So I'm out doing a little aid soloing the other day...up an overhanging rurp/ head seam. About 45 feet up the seam ends and my last placement is a love tapped cam hook, 3 feet above a beak, that is above a pretty good tipped baby angle.
I know that I'm going to drill soon since the features are ending, but i see one good edge that I can possibly hook off of. The edge seems good and it holds some small bounces.
I commit.
As soon as i start to drill from the hook though, I am instantly air born. Popping the hooked feature completely off and ripping the cam hook; but thankfully the beak held! The fall was only about 15 feet (eight or so feet of rope out above the beak) (all air), falling on to my cinch device. The screamer set-up at the belay only ripped about 1 stitch, and pretty much still looks new.
oh yeah none of the 16 or so pieces had screamers on them, just the belay.
My question is, me having almost 50 feet of rope out, weighing 135 plus a relatively large aid rack, and falling almost 1/3 of the pitch is not enough energy to activate a screamer?
I guess the bonus is that i still have an unactivated screamer...
(standard red yates)
The route did get finished though, 6 hours car to car in 20 degrees
edit: link to mproject route description: http://mountainproject.com/v/_in_progress/montana/gallatin_canyon/106078508
-Patrick
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rgold
Trad climber
Poughkeepsie, NY
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Dec 16, 2007 - 02:04pm PT
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"My question is, me having almost 50 feet of rope out, weighing 135 plus a relatively large aid rack, and falling almost 1/3 of the pitch is not enough energy to activate a screamer?
Not necessarily, if there is enough friction with the rope running through those 16 pieces to reduce the eventual load to the belay anchor to approximately the screamer activation level. If the screamer has been on the top piece that held you, I think you'd have seen substantial extension.
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