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alpnclmbr1
Jul 8, 2003, 3:50 AM
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In reply to: Physics of Screamers by YATES Author: John Yates/ Pro Design USA Email: prodesig@snowcrest.net Date: 1998/12/08 Forums: rec.climbing Hello all, I friend of mine rang me up this morning and told me about a discussion going on in this news group about forces involved with screamers and air voyagers < >
My name is John Yates, and I have done extensive work with various load limiting ìStitch Rippingî devices since the invention of the Screamerís almost ten years ago. I thought I could shed some light on this subject for those interested.
<>
Dan (Osman) and I had many lengthy discussions on how to limit the loads on his rope jump systems. We talked about the use of a Tyrolians, Screamers to limit loads and use of High Strength Tie-off Pulleys to terminate the rope ends. It is hard to believe that forces could be generated that would ìBRAKEî a climbing rope if Dan had the system set up as usual. Dan usually had Screamers set in the system in a tandem configuration (ie. two side by side). This configuration would limit the load and the Screamers would elongate if forces over 950-1000 lbf. were reached during a jump. If the system was configured right there was a ìWEEK LINKî ie. Screamers that would activate if forces reached any thing close to a critical level. I hope the investigation will reveal how Dan had his system set up, only then can we really speculate on what really happened.
Anyway about Screamers.
Screamers are a stitch ripping device that allows forces to be decelerated over a longer time interval than they would be if the Screamers were not in the system. Standard Screamers or ICE-Screams are configures from stitch patterns consisting of 6 rows of zigzag stitching sewn into each wing of the unit. This stitching is done by means of a computerized sewing machine. The machine can be configured to allow the Screamer to activate basically anywhere between 1 and 650 lbf. We chose to use an average activation of 550 lbf. because it seamed to be about the right force to use as a upper limit for marginal protection and Ice screws. Not every thing is a real science.
Some interesting things happen when you look at how much energy is ìabsorbedî in the system when a screamer is used. If the ìTrueî absorption is measured in a completely static system, lets say doing a drop test with a steel cable and weight we will see that about 5-600 lbf was absorbed by the stitch ripper(Screamer). When a Screamer though is put in a system which uses dynamic climbing rope instead of static steel cable the amount of energy which is absorbed is increase by 25-40%. We see that the absorption of energy increases to 800-900 lbf. I can attribute this extra energy we see being absorbed to the fact that the ìDynamicî climbing rope in the system is allowed to elongate and remain dynamic for a longer time interval than it would be, if there was no screamer in the system.
An example: A dynamometer or load cell is placed on a bolt hanger. A climber takes a fall which generates a fall with a factor of .5. This generates a force of 2000lbf as seen on the dynamometer. When a Screamer is hooked in the system below the dyno. the same fall only shows a peak force of 1200 lbf. We know from extensive testing that the Screamer can only absorb 500 lbf. So how do we account for the extra 300 lbf seen in this example??? The increased time interval(duration) of the fall allowed the climbing rope to be more absorptive!!
Thus Screamers limit loads and dissipate energy over a an increased time interval. This increase in the duration of the fall is most important in a ìDynamicî systems because it allows the rope to do its job even better than it was designed to do.
Stitch ripping devices have been used in many other areas besides just climbing. Many industrial application have been developed for the use of Stitch ripping devices. The most common is the Fall Arrest lanyard which uses a woven type of screamer device to decelerate a industrial worker if he falls in the workplace.
Over the last ten years I have worked on numerous projects with various Aerospace companies such as McDonnel Douglas and Bowing to develop various types of Stitch Ripper ìScreamerî type devices. They use them to decelerate objects which seperate from one another during controlled testing. The coolest project I worked on was a rocket fairing which was to house a communications satellite during launch on a Titan rocket. The fairing separation test was conducted in a vacuum chamber which took about 120 rippers in an elaborate configuration to decelerate the various parts of the fairing after the explosive bolts separated them. Anyway the point is that climbers are not the only ones using this type of technology to their benefit.
If any of the other physics nerds in this group would like to discuss specifics on the use of Screamers in various climbing applications, I would be more than happy to respond.
Sincerely,
JOHN YATES
Yates Climbing Equipment
prodesig@snowcrest.net |
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xanx
Jul 8, 2003, 4:04 AM
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very informative.... certainly explains what they do to limit forces, and in a way i can understand!
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dirtbag
Jul 8, 2003, 5:56 AM
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Good info -
I do have a couple questions though. The connection between screamers and improved quality of rope dynamics seems a bit fishy to me. Is this the only possible explanation for the observed difference in forces tested with and without a screamer? How do the dynamics of the system change when a longer or shorter length of rope is used in the testing? Does the screamer see greater forces when a shorter length is used (or lower forces when a longer length is used?)
I'm sure the connection Yates is making is valid, but I'd like to see exactly how he makes that connection.
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alpnclmbr1
Jul 8, 2003, 7:24 AM
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In reply to: I do have a couple questions though. The connection between screamers and improved quality of rope dynamics seems a bit fishy to me. Is this the only possible explanation for the observed difference in forces tested with and without a screamer?
In reply to: How do the dynamics of the system change when a longer or shorter length of rope is used in the testing? Does the screamer see greater forces when a shorter length is used (or lower forces when a longer length is used?)
I am far from being an authority on this.
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holmeslovesguinness
Jul 8, 2003, 1:29 PM
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http://www.yatesgear.com/climbing/screamer/use.htm#1
just thought i'd throw this in, has some examples and data from the yates website (which comes in a printed format with every screamer i think).
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holmeslovesguinness
Jul 8, 2003, 2:10 PM
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Although... after looking at the table on the Yates web page, I'm a little confused. They list the impact force to the anchors of a factor 2 fall as being 9kn, while a factor 1 fall is 11kn? Is that a typo?
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dirtineye
Jul 8, 2003, 2:22 PM
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Stretching the loading of the rope out over a longer time is better, and the screamer is also dissipating some of the force while this is going on.
The most important thing is probably that the popping stitches absorb some of the energy of the fall.
Realize also that while you are falling, gravity is accelerating you downward constantly, causing your velocity (speed with a direction) to increase. Every stitch that pops slows you a tiny bit. The more you slow down and the more gradually you slow down, the better off you are.
That's how dynamic rope works, it slows you down gradually, only instead of popping stitches, it stretches.
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evoltobmilc
Jul 8, 2003, 2:24 PM
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Maybe somebody can clear this up for me as I haven't had physics in a few years.
On that chart on the Yates website linked in the last post, it shows the force to the anchor (w/o screamer) as being 9kN for a Factor 2 fall, but for a Factor 1.99 fall it's listed at 15kN. Why does the impact force to the anchor increase so drastically from falling 1m less on the same amount of rope?? Is it just a typo?
Also, if the system were perfect (i.e. no friction in biners in the anchor) then the belayer feels the same impact force as the climber, no?? This means that the anchor feels both of these forces, and thus double the force on the climber. In the charts the force is markedly less than double-- What accounts for this?
I was always a little bit disappointed with my physics teachers brushing aside my climbing questions- I'm convinced they just didn't have a clue. Thanks for the help!!
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dingus
Jul 8, 2003, 2:39 PM
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I'm a little confused folks. The quoted post is 5 years old and I clearly remember John's participation in that discussion. Rec.climbing had at one time (until I ran them all off, hehe) a fairly knowledgable set of Physics Police capable of analytic discussion of the finer points of Yates physics, complete with numbers and squiggles.
Some of those people may be here now, I don't know. But I've seen no evidence to support that notion in the several physics related threads I've seen here. Most of the people who post to these things are either talking in generalities as I would, are dumbing down their answers for folks like me, are referring to anecdotal evidence or are simply quoting others.
No harm in THAT.
But there are some open questions posed in this thread and it seems odd to me. Who are you asking? Like the fall factor anchor force numbers... who do you expect to respond to this question? There is only one qualified person to answer it - Yates. Everything else would be just uninformed opinion. But Yates isn't in this discussion at present. And the quoted post is 5 years old.
I checked... send the guy an email if you're really interested. Invite him into the discussion. My past experience with the man suggests he enjoys his work mightily and will discuss the physics in a manner suitable to the Physics Police and yet understandable by the rest of us.
Or just have him answer you directly and then post a summary. Either way, you seem to be asking the wrong people at present.
Cheers,
DMT
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sspssp
Jul 8, 2003, 3:28 PM
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I've been puzzled by screamers also. I was contemplating using a pair of side by side screamers to limit the potential forces in a fall while simul-climbing. I got my physics book out and did the math (I don't have the numbers in front of me and I don't feel like redoing it). The part that I find so strange: a screamer starts out about a foot long and increases to what? Two feet long (I don't remember exactly). So regardless of what force the screamer activates at, it only happens over a foot of length. Force, work and energy are related but different terms. Work is force over a distance and energy is work over a period of time. The distance (one foot of screamer) is so short that very little work and energy can be absorbed. For my simul-climbing situation, I was assuming that the leader was "sucked" into the top piece and the rope would absorb very little. Anyway, the conclusion I came to was a fifteen foot simul-fall with the screamer was equivalent to something like a 12 foot fall without it. Just not much difference.
I don't have any reason to doubt Yates' data, but it sure seems puzzling. In a fall over 15 feet or so, you would be traveling so fast that I would think the screamer would extend the full foot in such a short length of time that not much energy would be absorbed.
But Dingus has it right. This thread has more questions than answers.
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alpnclmbr1
Jul 8, 2003, 4:31 PM
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Are questions a bad thing? In order to get an answer you need to form a question first. I re-posted the Yates post in order to start a thread with a reasonably informed post, that is better then the way most threads start. As far as Fall factor force on an anchor Yates is not the only person to ask. The petzl site has a fall factor numbers generator (it is down for the next couple days) and the math is not that complicated until you bring the screamers into it. Any thread has a lot of uninformed opinions, the trick on the internet is being able to tell the difference. There is more data available to work with here then is usually the case. So is anybody saying this post was a bad idea?
As far as the Yates chart it is a typo.
It would be great if we could get Yates involved in this thread.
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dingus
Jul 8, 2003, 4:43 PM
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In reply to: Are questions a bad thing?
No. So maybe you can help me out here. What is the correct spark gap on my 96 Jeep Cherokee (4 liter, inline 6) if I want to maximize uphill acceleration power?
In reply to: As far as Fall factor force on an anchor Yates is not the only person to ask.
It's HIS CHART!!! Do you think Petzl or anyone on this site can offer first hand experience of the Yates tests behind Yates numbers?
In reply to: So is anybody saying this post was a bad idea?
I'm not. I AM saying the technical questions pertaining to test results posted by Yates are best and perhaps can only be answered by Yates. That's why I suggested emailing him.
DMT
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flamer
Jul 8, 2003, 4:45 PM
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Dude! John Yates is cool! He spent close to an hour discussing this stuff with me at the Ice fest last year. If you want to ask him something just e-mail him- He's responded to me more than once!
josh
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dsafanda
Jul 8, 2003, 4:47 PM
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In reply to: Are questions a bad thing? In order to get an answer you need to form a question first.
Agreed, but where exactly is the question in the post that started this thread? There is no question.
I'm with Dingus in thinking that simply taking an old letter written by John Yates out of context and posting it here as the topic of new conversation is a bit odd.
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gunkiemike
Jul 8, 2003, 4:59 PM
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In reply to: As far as the Yates chart it is a typo.
How do you know it is a typo? Are you saying that because you can't get your mind around how there could be such a dramatic difference between 1.99 FF and 2 FF? Awwww....
It is not a typo. I have been around long enough (thumps chest) to remember the last time this chart was discussed. Picture a FF 2 fall: the climber lands directly on the anchor. Bang! Hard landing.
Now what about 1.99? The rope runs through a piece just above the anchor. Now - pay attention here - what do we know about the force on the top piece in a fall? That's right, it is HIGHER than the force on the climber. It would be 2x as high in the absence of friction, but most folks assume it's 60% higher. Well Yates' measurements (15 kN vs 9 kN as an approximation for the climber force) are pretty close to that.
Class dismissed.
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fitz
Jul 8, 2003, 5:04 PM
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In reply to: Anyway, the conclusion I came to was a fifteen foot simul-fall with the screamer was equivalent to something like a 12 foot fall without it. Just not much difference.
Usually the physics crossover from my working life to my climbing live is limited entirely to a healthy respect for the impressive forces that can be generated through load multiplication in anchors. This isn't at all my area of expertise, and I am notoriously bad at explaining physics, but there are some things you need to consider when examining the benefits of screamers.
Probably the best place to start appreciating the benefits of screamers is to revisit the benefits of dynamic rope. A 10' fall on 30' of static rope would mean major trauma for your body and tremendous stresses on a top piece of protection. A 10' fall on 30' of dynamic line could be a bit wrenching, but the stresses both on your body and the system are dramatically lower.
There is plenty written on the physics of this, and I won't confuse everyone by adding my own muddled explanation here, but an important point to consider is that the dynamic rope probably 'stretched' a whopping 1' or so in the second case. Most lines have more elasticity (say 1.8 to 2.5'), but friction, particularly in the top biner generally stops the rope from reaching maximum efficiency.
Understanding how such a relatively small change can make such a big difference will give you a lot of insight into how a screamer works. But, even without a lot of study, you can relate some of the device's effects to practical experience.
First, think about shock loading. We don't have to understand the physics to intuitively understand that easing weight onto a gear placement generally results in better holding power than suddenly loading it. Although it is not elastic in the traditional sense, a screamer does help a gear placement by spreading loading over time.
Second, think about braking. If you fall 50' off a cliff, you fall 50' rather there are trees at the base or not, but falling through tree branches might save your life. To risk over simplifying, some of the energy from the fall is transferred between you and the trees, lowering the amount of energy transferred between you and the ground. The ripping stitching in a screamer has the same type of effect. Energy is expended on the stitching that would, otherwise, be transferred to the anchor.
The last effect, which Yates explained in his original post, may seem a bit non-intuitive, but is easily demonstrated with a rubber band and a pencil. Yates proposes that some of the energy 'missing' from direct experiment is due to 'time' allowing the rope to be more dynamic.
If you pull a rubber bands from the ends, you get a lot of stretch. If you put your fingers close together and pull you get a lot less stretch (duh). What Yates was trying to explain (I think) is related to something I pointed out above, ropes seldom reach maximum dynamic potential in falls. You can see this effect (exagerated) by simulating a 'fall' on your rubber band 'rope' using a pencil 'anchor'. You can see (and feel) that friction causes 'belayer' side of the rubber band to absorb less impact than the 'climber' side.
It takes time for the friction to be overcome, allowing the load to spread, and it takes time for the rope to stretch. Yates is saying that the screamer gives the system time to do this before impact peaks on the anchor.
Personally, I think that it is slightly more complicated. I think that the screamer not only provides time, I think it actually allows energy to spread 'down' the line more easily. Think about pulling on something until it breaks. I won't try to muddle the physics, but I suspect that as each stitch breaks, friction between the rope and the top biner in the system is briefly lower, which helps more energy dissipate down the length of the line before loading peaks on the placement.
-jjf
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sspssp
Jul 8, 2003, 5:56 PM
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I agree that my simul-climbing example is similar to a fall on a static rope and isn't directly applicable to a normal fall. However, it still seems odd that such a short extension is going to make a significant difference.
Let's use your tree analogy. Falling through branches could save your life. Now, suppose you fell fifty feet and then fell through 1 foot of branches. Is it going to make any difference. Hardly.
The screamer activates around ~500 ftlbs. So for a 150 lb climber, that represents around 3 gs (that is, around three times the force of gravity). This 3gs is applied over the one measly foot that the screamer can extend. This just isn't much energy dissipated. Without getting my physics book back out, 3gs over 1 foot is going to be around 1 g over no more than 5 or 6 feet (1 g being the force of gravity acting on a falling climber). So it seems the screamer would might reduce the "equivalent" fall by 5 or 6 feet plus the added benefit of additional rope stretch. When I was doing my calculations before (on falls in the 15 to 30 foot range), I came out that the screamer would activate in a few tenths of a second. Not a lot of extra time for the rope.
I don't believe that Yates is lying with his charts. So I'll assume that I am missing something. But is sure seems strange to this engineer (my life outside of climbing).
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ljthawk
Jul 8, 2003, 6:18 PM
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Though Yates didn't go into detail, his explanation makes sense to me. Crack open a high school physics book, or just read some of Leepers old articles on rope forces, and then draw out a force vs deflection diagram for a spring system with a force limiting coupling. The force limiting coupling will limit force until full ripping at which the spring then further deflects and forces start to rise again. The area under the curve must equal the dissipated energy, or the climber's weight multiplied the fallen distance. The area under the curve equals the energy absorbed by the rope and the energy absorbed by the screamer. The energy absorbed by the screamer is the ripping force times the distance it rips, the energy absorbed by the rope is roughly 1/2 kx^2 where k is an approximate stiffness (based on rope modules, length of rope, and friction in the system). Max force on the climber is equal to kx. As long as the screamer rip force is less the climbers body weight, balancing the energy equations will show that the more energy the screamer dissipates the less the rope has to absorb, the less the rope will stretch, and the less the max force will be; it's not a linear relationship.
Also inspecting the equations will show that the higher the rip force is the more energy it will absorb, but you must make sure you don't make it so high it doesn't start to rip. The longer it rips the more force it will absorb as well.
Yates description is a little confusing because he refers to energy as lbf (pound force) when it really is lbf-distance (pound force x distance). His reference to increased stopping time is true, but considering force vs increased stopping distance is easier to use when calculating energy dissipation becasue the integral of that plot is energy, not the integral of force vs time.
Thanks for posting his comments, made me think about it.
L.J.
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curt
Jul 8, 2003, 6:25 PM
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Dirtineye posted
In reply to: Stretching the loading of the rope out over a longer time is better...
1) Climber falls on a static line.
2) Climber falls on a dynamic rope, with or without screamers.
Assuming the falls for these scenarios are of equal distance, the kinetic energy generated in the falls are equal.
Here is the crucial point--if you plot peak tension in the rope vs. time (with tension on the vertical axis and time on the horizontal axis) you will get quite different looking graphs for the above scenarios. For scenario (1) the fall is stopped very quickly due to lack of stretch in the rope. Hence, the graph will have a very high peak tension in the rope--for a short period of time.
For scenario (2) the fall is stopped more slowly because of the stretch in the rope. Hence the peak tension in the rope will be much lower--as the energy is absorbed over a longer period of time.
Although integrated area under these two graphed curves will be identical, the peak tension and therefore peak force on the climber is much higher in scenario 1.
Curt
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alpnclmbr1
Jul 8, 2003, 6:39 PM
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As far as taking a post out of context, I copied this post from Fish's tech weenies thread and have not seen the original context.
As far as a factor 2 fall being preferable to a factor 1.99 fall I don't buy it and that is what the chart implies. (I will get back to you on that one)
As far as how this thread is working out, I am happily entertained and may even learn a thing or two.
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alpnclmbr1
Jul 8, 2003, 6:50 PM
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In reply to: It is not a typo. I have been around long enough (thumps chest) to remember the last time this chart was discussed. Picture a FF 2 fall: the climber lands directly on the anchor. Bang! Hard landing. Now what about 1.99? The rope runs through a piece just above the anchor. Now - pay attention here - what do we know about the force on the top piece in a fall? That's right, it is HIGHER than the force on the climber. It would be 2x as high in the absence of friction, but most folks assume it's 60% higher. Well Yates' measurements (15 kN vs 9 kN as an approximation for the climber force) are pretty close to that. Class dismissed.
Instructor challenged.
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dirtineye
Jul 8, 2003, 7:02 PM
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In reply to: I've been puzzled by screamers also. I was contemplating using a pair of side by side screamers to limit the potential forces in a fall while simul-climbing.
Yates suggests puting them in series. Side by side would double the activation force. Think about it a minute and you'll see how that works.
YOu guys that think one foot is not enough distance to dissipate force in a meaningful way, have you ever fallen on a crash pad from say 20 feet? People routinely stack several pads to a height of about a foot and fall 20 feet or so onto them. THe give in a foot of those sheets of open and closed cell foam is much less than a foot, as one can plainly observe, but hte impact absorbed is greatly appreciated by the faller.
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gunkiemike
Jul 8, 2003, 7:04 PM
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In reply to: The higher force would be on your first piece, not the anchor. Instructor challenged.
Bring it on!
Where does one typically use a Screamer? On the anchor or on the first (and maybe subsequent) piece(s) of gear? On gear, of course. Now, get this...there is NO gear involved in a FF2 fall so the peak force is on the anchor. 9kN in Yates chart.
Using a Screamer (ergo placing GEAR) generates the peak force on said top piece of gear. This force is greater than the force the anchor sees in the same situation, by roughly 60% as discussed before.
Go climb more, and maybe you will understand.
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ljthawk
Jul 8, 2003, 7:30 PM
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dirt in eye
Yates did suggest putting them in parallel, as that is what he had discussed with Osman per his comments posted by alpnclmbr1.
Your a math major and understand physics. Putting them in parallel will raise the activation force, but can decrease the total force because the integral of force over rip distance is now greater. For example, assuming a perfect world and the energy equations I scribbled out are correct, if you assume a 150 lb climber and a screamer that rips at 550lbf, then for every foot the screamer rips it should have the same effect in decreasing fall forces as would decreasing the fall distance by 2.66 ft with the same amount of rope out. If your screamer rips at 1100lb (two screamers in parallel) it it should have the same effect in decreasing fall forces as would decreasing the fall distance by 6.3 ft per foot of rip with the same amount of rope out; assuming you reach 1100 lbf in the fall.
Putting two screamers in series you increase the rip distance by two, playing back into scenario one; this also means the climber falls little further.
Of course this is just an arm chair ball park analysis and the physics behind Yate's data, which incorporates real world non linear rope modulas and friction is probably more complicated.
L.J.
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deadfish
Jul 8, 2003, 7:32 PM
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In reply to: The higher force would be on your first piece, not the anchor. Instructor challenged.
Of course it will be on your first piece (or first protection anchor, as Yates calls them on the page). I don't know what you are challenging, as this is exactly what GunkieMike said. This applies to the whole chart, not just to the 1.99 FF line. The chart is describing force on the top piece of pro or "anchor" in every cited FF situation, not the force where the belayer is standing.
While many of the items you dredged up recently make interesting reading and spawn useful threads, it is odd to me also that you are often citing past references that you clearly don't completely understand.
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