Another Accident due to mis-use of the Gri-gri
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Agreed, but common sense doesn't always prevail and isn't always right. In this case, what is "obvious" to you obviously isn't obvious to some other people. And rightly so; in the math world the first place we look for errors in an argument is in claims said to be "obvious." |
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Greg, thanks for the additional evidence. The light weights might be a problem, but the trend is already clear. BillS wrote:Please first solve for the friction variable in those equations, then get back to us about how useful they are.First understand the point of the post, then get back to me with snide questions. BillS wrote:On that note, static friction is greater than kinetic friction. Kinetic friction is reduced by heat and velocity. This is the heart of the matter, as I see it.All covered in the linked article. I didn't think it necessary to mention. Your comment that "kinetic frictions is reduced by heat and velocity" makes no sense, by the way. BillS wrote:I'll let the presumption and pompus windbaggery continue.How very good of you---especially after adding to it. BillS wrote:FWIW, I'm in agreement with Mort.Me too. |
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Yes static friction is greater than kinetic friction. I did take this into account by accelerating the load to go into kinetic friction mode. If the load did not come to rest I added a bit of weight to the brake strand. Repeat until there was enough weight on the brake to arrest the load. |
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BillS wrote: A rope will glaze at the belay device and at the top biner in a hard fall. The speed of the rope also appears to do a nice job polishing gloves and skin. This changes the friction. A rope will also "vibrate" at high velocity, reducing the contact - also opening biner gates and what-not. Carry on, nothing like listening to an academic.come on - rgold's posts are consistently interesting and he summarizes his main points in a way that make sense to someone without a background in physics. Nobody claims to have a perfect theoretical understanding, but I'd far rather read a post from someone who tries to understand the key physics, and makes well-reasoned practical arguments, than from someone who provides a list a bunch of irrelevant sounding secondary concerns and rude comments. |
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BillS wrote:Carry on, nothing like listening to an academic.Oh good, then let me explain my remark: As regards kinetic friction and velocity: From hal.archives-ouvertes.fr/fi…: Almost three centuries ago Charles Coulomb (1736-1806) discovered that kinetic friction does not depend on the sliding velocity [1]. Later, more careful experiments showed that this law is only approximately valid [27]. Friction does depend on the sliding velocity, but this dependence is far from universal: some measurements find an increase when velocity increases, while others find a decay [810] or even a more complex non-monotonous behavior [2]. From stle.org/resources/lubelear… Friction is not always independent of velocity. If we exclude very low speeds and very high speeds, the friction coefficient is constant and independent of sliding velocity. But at very high speeds, the friction coefficient generally has a slightly negative slope; that is, the friction coefficient decreases gradually as the speed increases. As regards kinetic friction and heat: From scitation.aip.org/content/a… In this paper we describe a relatively simple experiment that can be done to test the effect surface temperature has on kinetic friction, and that depending on the materials being used, the coefficient of kinetic friction does indeed increase as the surface temperature of one of the materials increases. From journalamme.org/papers_vol2… Up, down, up, down... But all this is a nerdy distraction from a much more important point. So allow me to repeat myself: Lets suppose that Im wrong (hardly an unusual circumstance) and there is no such thing as situation that would physically overwhelm a belayer even though the belayer does everything correctly. Then we can still say that the problem brought to our attention by John at the beginning of this thread is one that will, at the very least, greatly increase the demands on the belayer, way past anything that they are used to dealing with when using a Grigri. Forgetting all the arguments about unconditional belayer responsibility, isnt the prospect of a sudden huge increase in the performance demands on the belayer enough, all by itself, to merit serious consideration of Johns warning? And isn't it right, in terms of the actual tragedies that have happened, to let it go at that? |
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Kedron Silsbee wrote: come on - rgold's posts are consistently interesting and he summarizes his main points in a way that make sense to someone without a background in physics. Nobody claims to have a perfect theoretical understanding, but I'd far rather read a post from someone who tries to understand the key physics, and makes well-reasoned practical arguments, than from someone who provides a list a bunch of irrelevant sounding secondary concerns and rude comments.I'm sure Bill S is just one of many who find it irritating to be confronted with facts and logic. For my part, I'd love to have seen this message thread given a more useful title: 'Another Accident due to mis-use of the belay device.' In 10 years of climbing, I've been dropped/near-dropped three times, and none of these involved a gri-gri. JL |
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Perhaps someone would like to start a thread entitled "20 ways to drop the leader" (or some other number...), wherein we could categorize, by device, how to mess up. Might be helpful to have around. |
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rgold wrote:Perhaps someone would like to start a thread entitled "20 ways to drop the leader" (or some other number...), wherein we could categorize, by device, how to mess up. Might be helpful to have around.- not pay attention - have too much slack out - not have a solid brake hand (which isnt too close to the device) - not double checking the setup (also fault of climber) - not recognizing weight and strength differences (also fault of climber) - poor understanding of gear placements, zippering and forces (also fault of climber) - defeat the cam through poor/unapproved belaying techniques (in the case of assisted braking devices) other than hard to control things like rock/icefall, heart attack, getting mauled by a bear, etc ... thats about it its pretty damn simple, yet folks screw it up over and over again belaying is something that almost everyone thinks they are good at, yet in reality many arent, nor can many accept that their technique is poor psmag.com/navigation/health… theres a reason why even "experienced" folks drop others, even with grigris ... a belayer who thinks they have nothing to learn is the one you should be walking away from ... everytime a good belayer belays they should ask themselves what they could have done a bit better ... just like a climber who asks how they could have climbed better belaying is the most important thing in climbing ... folks who dont take it seriously are knowingly playing russian roulette with someone elses life |
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here is perhaps the best article ive seen on the subject ... |
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Dana Bartlett wrote:I always belay seconds with a plaquette device in guide mode. This lets me do other things while I'm belaying.That's nice. But can you tell us the failure modes in this scenario and how people have been dropped. What you need to do to avoid them?? I use baby wipes after a wet fart. But I don't go announcing that on the Internet. |
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Uh, Greg: you just announced it. |
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rgold wrote: Oh good, then let me explain my remark: As regards kinetic friction and velocity: From hal.archives-ouvertes.fr/fi…: Almost three centuries ago Charles Coulomb (1736-1806) discovered that kinetic friction does not depend on the sliding velocity [1]. Later, more careful experiments showed that this law is only approximately valid [27]. Friction does depend on the sliding velocity, but this dependence is far from universal: some measurements find an increase when velocity increases, while others find a decay [810] or even a more complex non-monotonous behavior [2]. From stle.org/resources/lubelear… Friction is not always independent of velocity. If we exclude very low speeds and very high speeds, the friction coefficient is constant and independent of sliding velocity. But at very high speeds, the friction coefficient generally has a slightly negative slope; that is, the friction coefficient decreases gradually as the speed increases. As regards kinetic friction and heat: From scitation.aip.org/content/a… In this paper we describe a relatively simple experiment that can be done to test the effect surface temperature has on kinetic friction, and that depending on the materials being used, the coefficient of kinetic friction does indeed increase as the surface temperature of one of the materials increases. From journalamme.org/papers_vol2… Up, down, up, down... But all this is a nerdy distraction from a much more important point. So allow me to repeat myself: Lets suppose that Im wrong (hardly an unusual circumstance) and there is no such thing as situation that would physically overwhelm a belayer even though the belayer does everything correctly. Then we can still say that the problem brought to our attention by John at the beginning of this thread is one that will, at the very least, greatly increase the demands on the belayer, way past anything that they are used to dealing with when using a Grigri. Forgetting all the arguments about unconditional belayer responsibility, isnt the prospect of a sudden huge increase in the performance demands on the belayer enough, all by itself, to merit serious consideration of Johns warning? And isn't it right, in terms of the actual tragedies that have happened, to let it go at that?It´s sort of interesting really, the dynamic coefficient of friction has been shown experimentally and theoretically to vary between 0 and 2. At the pressures (the important variable for us) we work with it is in the range of 0.3 to 0.16, 0.2 is a useful number to work with for most belay devices. Apropos capstan theory;- our daily climbing experience shows us this can´t be the complete answer to how belay devices work, a 3mm cord would give the same braking effect as an 11mm rope if the theory is correct in our application. Similarly when we are rapping one strand or two would give the same resistance, clearly they don´t. I still can´t really understand how Attaway overlooked this but we had another guy at Cambridge University who took best part of a year to work it out as well. The way to get drop the leader AND get rope burns on both hands with a correctly loaded GriGri is to hold the climber-side strand with your hand too low, the Grigri comes upwards and your hand prevents the cam opening. This is a known failure mode and clearly described here:- petzl.com/en/Sport/Belaying… |
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A variety of variables can effect belay applications of the "capstan theory." When using a flat, two-dimensional belay device, such as the Sticht plate, the rope will lock up more easily when using one carabiner if the slot on the device is about twice as long, plus the thickness of the carabiner, as the rope is thick. In fact, the rope will lock up too easily and this often prevents quick feeding of slack. Using two carabiners will prevent premature lock-up, yet makes catching a fall more difficult. That said, I and my usual partner, Mike Endicott, have never had difficulty catching each-other's leader falls when using two carabiners. |
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Jim Titt wrote: The way to get drop the leader AND get rope burns on both hands with a correctly loaded GriGri is to hold the climber-side strand with your hand too low, the Grigri comes upwards and your hand prevents the cam opening. This is a known failure mode and clearly described here:- petzl.com/en/Sport/Belaying…I think this could put the final piece in the puzzle in some cases:
The fact that two (known) conditions have to be met helps to explain the rarity of the phenomenon, especially in view of likelihood that many belayers do grab hard with the feeding hand, especially when lifted and nothing bad happens. If a fall happens just after the belayer is pulling in some slack, i.e. for a take, it is quite possible that the feeding hand could be right on top of the device at the moment of loading. It is too bad that Petzl chose to illustrate the second condition with a picture that also shows the climber not using the brake hand, because that suggests there is a potential danger only if the brake hand is not on the rope. Added in edit: Now that I've had to correct the second bulleted item, it has again become less clear how the rope keeps running through the system after the cam has been initially disengaged by the feeding hand pressure. |
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Mort wrote: You may be wrong in some cases, simple as that. Glad you and Slim won't be belaying me.Sure I may be, but if you seriously held the brake end and the Grigri doesn't lock, we'd have seen a lot more of these accidents. You contend somehow holding the climber's end was the problem. Well, what if you (as the belayer) didn't but somebody else did? What if it wasn't a person but friction thru the rope system (from the rope running over a small roof or what not). Can you explain to me, how besides voodoo magic, the Grigri would behave differently in these situations? There have simply been too many cases of people blaming the machine rather than the operator (how about the good ol' accelerator controversy?) And in the heat of moment, what you think you are doing may be very different from you are actually doing. There is a good reason we are taught not to use both of our feet driving an automatic (or heel/toe with one foot), even though it would reduce reaction time with an expert driver. |
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reboot wrote: Sure I may be, but if you seriously held the brake end and the Grigri doesn't lock, we'd have seen a lot more of these accidents. You contend somehow holding the climber's end was the problem. Well, what if you (as the belayer) didn't but somebody else did? What if it wasn't a person but friction thru the rope system (from the rope running over a small roof or what not). Can you explain to me, how besides voodoo magic, the Grigri would behave differently in these situations? There have simply been too many cases of people blaming the machine rather than the operator (how about the good ol' accelerator controversy?) And in the heat of moment, what you think you are doing may be very different from you are actually doing. There is a good reason we are taught not to use both of our feet driving an automatic (or heel/toe with one foot), even though it would reduce reaction time with an expert driver.Clearly you are missing the point, it is voodoo magic, and this voodoo magic only effects tradladytes from the 60's or those just starting out. The Grigri is the Devil. All those who use it are from the dark side, and should expect burned hands and broken feet. As for engineering, pssst! Devils work I say. |
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BillS wrote:My take is simple - an oscillating rope traveling at high speed over a cam will have less contact with the cam than a slow moving one, certainly less than a fixed one - less kinetic friction.The obvious flaw with this concept is that if the rope is oscillating it will vary between having more less normal force and more normal force on the cam so at some point in the wave it is more likely to lock. Since the rope is running through the belayers (burning) hand one would normally assume any oscillation is damped out by the burning flesh and the entry to the GriGri anyway. Don´t you think it is more likely the melted sub-cutaneous fat is lubricating the rope and preventing it locking? |
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Gri Gri's are like computers in that they're only as smart as the person using them... |