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needtwolead
Dec 4, 2003, 4:50 AM
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I was reading somewhere about someone (can I be any vaguer!) taking in rope as his climbing partner was taking a long fall (probably passed the belay).
Here is my question, assuming that there are no ledges to hit or other obstacles, wouldn't be worst to take in rope?!
Imagine this situation. The leader has climbed 10m out of the belay without setting any pro. (yes, stupid I know, but this is a stupid leader)
He falls! He is going to be taking a 20m fall on 10 meters of rope = Factor 2....
Now let's say that during the fall the belayer is able to swallow up 2 meters of rope so now the leading is going to fall 18 meters...but now there is only 8m of rope out = Factor 2.25!
So intuitively one always feels taking in rope is better but sometimes it's not...
I ran a couple of numbers and at first sight it seems that if the fall is worst than Factor 1 than taking in rope makes the fall factor worst…if the fall is less than Factor 1 then taking in rope helps (I could be wrong but seems right right now)
What do you guys think?
Ben
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godsmybelayer
Dec 4, 2003, 5:27 AM
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check my physics, but the factor of the fall does not judge the force of the fall, but a ratio between the numbers of the amount of rope paid out and the length of the fall. in 18 m. you accumulate less velocity than you do in 20m. Force is mass times acceleration. If you reduce the distance of the fall by two meters or even two feet you've significantly reduced the amount of time the leader has to accelerate....consequently the amount of force. I would take for all I was worth!
Also taking reduces the amount of time your placements are going to have to take the load...Some placements will hold a heavier load for a short period of time, but fail under a slightly lighter one held over a longer duration, which is what would result without taking in the slack. People really get into fall factors, but they are not really that good in determining how sever a fall is. You can have a factor 1 fall of twenty feet, and take a bigger force than a theoretical factor 2 of 2ft. You have to judge wether the amount of acceleration your stopping by taking is greater than the amount of force removed from the strech of the rope your taking in. Don't have a calculator on hand, but I say take like all get out for the reason of time the placements have to bear the load.
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tim
Dec 4, 2003, 5:30 AM
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only way you're getting worse than a Factor 2 is if the rope attachment point slides down lower than it starts, eg. on a via Ferrata. It's simply not possible for an unbroken strand of rope to reach further than 100% of its outstretched length from a static anchor point, hence a factor 2 is all you're going to get unless the anchor blows and re-sets itself several meters down. ;-)
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squish
Dec 4, 2003, 6:28 AM
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This is getting purely theoretical, but I say he's right. It's possible, though very difficult and unlikely to create a fall factor greater than 2. Look at it this way:
The climber 10 metres up without pro loses contact and falls. His belayer somehow manages to take almost all of the rope. (Yeah, he's endowed with very fast hands.) Now the climber has taken an 11m fall, absorbed by only 1m of rope. The climber would essentially stop cold at the end of the fall. Strictly by definition, that's factor 11.
I know it's a ludicrous example, but it illustrates the point. You can make the fall worse by taking in rope that would otherwise help absorb the fall.
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mheyman
Dec 4, 2003, 6:47 AM
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In reply to: the factor of the fall does not judge the force of the fall
It does.
In reply to: but a ratio between the numbers of the amount of rope paid out and the length of the fall. ...Don't have a calculator on hand,
Fall factor does exactly this, and does it correctly in a theoretical world. Forces can be higher than what fall factor indicates due to friction, hence it good to think of it as best case but aa a reasonable starting point for analysis.
Squish's post is a great example of why you need learn and analyze.
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philbox
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Dec 4, 2003, 7:01 AM
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I remember some guys who were testing falls on a bridge and they winched a load up above the anchor. They inadvertently overwinched the load such that it slingshotted down past the anchor. That is a fall factor greater than 2. The upshot of the test was that they performed the same test but not overwinching and the rope held for something like 5 drops before destroying itself in a huge fuzzy sheath fluff. The rope was still sort of intact but the harness attached to the load broke. Fun times eh.
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squish
Dec 4, 2003, 7:11 AM
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In reply to: the factor of the fall does not judge the force of the fall, but a ratio between the numbers of the amount of rope paid out and the length of the fall. ... People really get into fall factors, but they are not really that good in determining how sever a fall is.
Fall factor is the single best indicator we have for how severe the fall forces are. This is because it is centered around the single most important element of the system: rope stretch.
In reply to: You can have a factor 1 fall of twenty feet, and take a bigger force than a theoretical factor 2 of 2ft.
Which do you suppose would be more jarring? I know for a fact it's the 2-foot fall. The twenty-footer takes more time to dissipate and feels like a softer catch. Just from that, I'd venture to guess that the physics forces are greater on the 2-footer. I'm not a physics nerd, but I'm sure someone can back this up.
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squish
Dec 4, 2003, 7:14 AM
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In reply to: I remember some guys who were testing falls on a bridge and they winched a load up above the anchor. They inadvertently overwinched the load such that it slingshotted down past the anchor.
So if your belayer short-ropes you and you fall while pulling against the strain, he's effectively increased your fall factor? Hmm... I'd never thought of that before... :wink:
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noodlearms
Dec 4, 2003, 7:50 AM
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The original poster, and squish, are right. If you could take in rope fast enough, you could generate insane fall factors.
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cantbuymefriends
Dec 4, 2003, 8:36 AM
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I would say that Needtwolead i absolutely right about the 2.25 fall factor.
In reply to: check my physics, but the factor of the fall does not judge the force of the fall, but a ratio between the numbers of the amount of rope paid out and the length of the fall. in 18 m.
Well, of course. If you change the defintion of "fall factor", you can say that fall factor isn't important! But the last time I checked, Fall factor was exactly the ratio between the length of the fall and the length of the rope paid out.
In reply to: in 18 m. you accumulate less velocity than you do in 20m. Force is mass times acceleration. If you reduce the distance of the fall by two meters or even two feet you've significantly reduced the amount of time the leader has to accelerate....consequently the amount of force. I would take for all I was worth!
Yes but if you take in rope, you also significantly reduce the "braking distance" (=rope stretch), and to stop in a shorter distance you have to apply a higher force, right?
In reply to: You can have a factor 1 fall of twenty feet, and take a bigger force than a theoretical factor 2 of 2ft.
No you can't. Unless you're using a static rope in the first scenario.
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psycho
Dec 4, 2003, 10:57 AM
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ok folks my physics knowledge is a bit rusty but I'll give it a try
the energy a falling climber has accumulated at the very moment the rope starts to slow him down is proprtional to the height he has fallen.
If we had a 10m factor 2 fall and the belayer takes in all but 1m the leader would still fall ~11m -> ~half the energy of a 20m fall but only 1m of rope to take all the energy -> ouch ;-(
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popol
Dec 4, 2003, 12:55 PM
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Let’s use the example that he gave himself: First fall is factor 2, 20m fall on 10m rope. Second fall is same, but belayer takes 2m rope during climber’s fall. So 18m fall on 8m rope.
Stated was that falling 2m less means a big speed difference. Not true.,Velocity after 18m drop is 18.8m/s, after 20m drop is 19.8 m/s
Kinetic energy after 18m is 90% of the kinetic energy after 20m drop.
This means that 10% less energy has to be absorbed by 25% less rope (8m rope vs 10m).
So, no good idea to take in rope while your lead climber is making a factor 2 or higher fall. Unless he’s going to make a grounder / hit something / …
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cracklover
Dec 4, 2003, 3:38 PM
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In reply to: only way you're getting worse than a Factor 2 is if the rope attachment point slides down lower than it starts, eg. on a via Ferrata.
In reply to: It's simply not possible for an unbroken strand of rope to reach further than 100% of its outstretched length from a static anchor point
Oh, by the way - that 20m fall takes only two seconds, (yes, do the math, it's two) so you're not going to have much time to take in slack anyway. IMHO, on a big fall, the little time you have is best spent locking the rope off tight, holding on, and bracing for impact.
GO
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j_ung
Dec 4, 2003, 4:49 PM
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Sorry, but you're all wrong.
In this scenario, if the belayer takes in 2m of rope, leavine 8m out, then the fall is only 16m, and still the same factor of 2. To fall 18m, the amount of rope in system has to be 9m. And even in that case, 18 divided by 9 is still 2.
The answer to the question is that if no gear has been placed off the belay, then it's theoretically impossible for the belayer to affect the fall factor by pulling in rope.
j_ung
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j_ung
Dec 4, 2003, 4:52 PM
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The only exception is if the belayer pulled in rope only AFTER the falling climber passed the belay. So it's POSSIBLE, but nobody's that fast.
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curt
Dec 4, 2003, 4:53 PM
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In reply to: In reply to: the factor of the fall does not judge the force of the fall It does. In reply to: but a ratio between the numbers of the amount of rope paid out and the length of the fall. ...Don't have a calculator on hand, Fall factor does exactly this, and does it correctly in a theoretical world. Forces can be higher than what fall factor indicates due to friction, hence it good to think of it as best case but aa a reasonable starting point for analysis. Squish's post is a great example of why you need learn and analyze.
The fall factor will indeed indicate the maximum force put on the climber, anchor and top piece of gear during a fall. The fall factor was created, in fact, to make these calculations easier and is based on the premise that any additional kinetic energy to be absorbed in a fall (resulting from falling a longer distance) will be offset by a corresponding additional elongation in the rope.
You are incorrect in this statement, however...
In reply to: Forces can be higher than what fall factor indicates due to friction, hence it good to think of it as best case but as a reasonable starting point for analysis.
Curt
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tedc
Dec 4, 2003, 5:16 PM
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In reply to: Sorry, but you're all wrong. j_ung
Better check your math again. (And wipe the egg of your face). "They" are correct. Oh, and when the belayer takes in the rope is irrelevant. (Well as long as the climber is airborne.)
This is a pretty interesting thread. Has it been hashed before? I don't remember seeing it.
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jt512
Dec 4, 2003, 5:20 PM
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In reply to: You are incorrect in this statement, however... In reply to: Forces can be higher than what fall factor indicates due to friction, hence it good to think of it as best case but as a reasonable starting point for analysis. In fact any friction will decrease rather than increase any forces experienced in the fall. Curt
Actually, Curt, he is right. Friction between the rope and running protection reduces the ability of the rope to stretch, thus effectively reducing the denominator of the fall factor, and thus increasing the fall factor. Part of the braking force is transferred from the belay to device to points on the rope closer to the climber. For all intents and purposes there is less rope out. Additionally, in a severe fall, where the rope would slip through the belay device, friction adds to the total braking force, thus increasing the impact force directly.
-Jay
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iangraham
Dec 4, 2003, 5:22 PM
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But what about the situation where the belayer pulls in the rope faster than the climber is falling? The belayer would then accelerate the climbers fall and pull the poor smuck down faster. the climber would then have an instaneous stop at the anchor, turn into mush and ooze through his harness. The resulting blast force would also probably take out the belayer, if he was any where near the anchor. I am thinking here that the belayer has been replaced by a high speed winch, but then I keep getting told to stop thinking.
But actually in everything I have read there are so many other little forces, such as friction over carabineers, that the little amount of the rope the belayer could take in, while theoretically increasing the fall factor, would have almost nil practical importance.
Also the force expereinced by a climber, as long as he does not hit a ledge on the way down, is completely determined by the rope, not the fall length. Each rope is rated for its maximum fall force and this force will not be exceeded regardless of the fall length. It is the stretch characteristics (how much and how fast) of the rope that determine the force on the climber at any given time. Note that for a long fall, this maximum force will be exerted on the climber for a longer period of time, thus the climber will absorb more energy than during a small force (where energy is found from force over time). It is force that matters though. Also this only applies to ropes that are still in acceptable operating conditions (i.e. have not had too many falls on them). This fact is actually the basis of modern lead climbing and the development of most modern tools.
Ian Graham
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tedc
Dec 4, 2003, 5:28 PM
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In reply to: In fact any friction will decrease rather than increase any forces experienced in the fall. Curt
Mostly right, however, the force that the climber feels is actually increased due to the friction of the rope in the top biner. This friction tends to issolate some of the force he/she creates from getting to the belayer's side of the rope where it could be better dissapated.
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reno
Dec 4, 2003, 5:29 PM
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In reply to: Force is mass times acceleration. If you reduce the distance of the fall by two meters or even two feet you've significantly reduced the amount of time the leader has to accelerate....consequently the amount of force.
Technically:
F = 1/2(M x V2)
Force equals one-half mass times velocity squared.
IOW, the velocity of the object has much more influence on the force applied than the mass of the object.
In a free fall, velocity is determined by gravity (9.82 m/sec, I think.) A fallking body does not INSTANTLY reach this speed, though. That falling body has to accelerate to reach the speed. Less acceleration time = less final velocity. Shorter length of rope = less acceleration time.
I don't think that taking in slack during a fall will increase the forces applied to the anchor. Sure, it may feel like it, as there is less rope to "stretch" (the dynamic properties of the rope come into play in reality, while the physics above assume static.) I'm not well-versed in physics to calculate the rope stretch.
Interesting discussion...
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dingus
Dec 4, 2003, 5:29 PM
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In reply to: only way you're getting worse than a Factor 2 is if the rope attachment point slides down lower than it starts, eg. on a via Ferrata.
A simul climb 2nd falling and pulling the leader off is another valid scenario. The leader falls to her last piece and stops dead, no absorbtion at all.
Many folks to not appreciate this inherent risk in simul-climbing or don't understand that if the leader is runnout out at all, such a fall could very well have fatal consequences.
Cheers
DMT
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tedc
Dec 4, 2003, 5:43 PM
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In reply to: ... Also the force expereinced by a climber, as long as he does not hit a ledge on the way down, is completely determined by the rope, not the fall length. Each rope is rated for its maximum fall force and this force will not be exceeded regardless of the fall length. ...
NOPE, think about it again. Ropes are rated based on the force they exert on the "climber" in a factor 1.7 fall. Thus the fall length AND rope out are both factors in the force seen by all parts of the system. If this is not clear you may need to study how a rope works.
Rope testing and certification basically assumes that factor 2 is the worst fall a belayed climber can experience. As this thread points out that is not always true.
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curt
Dec 4, 2003, 5:43 PM
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Jay,
In reply to: Actually, Cury, he is right. Friction between the rope and running protection reduces the ability of the rope to stretch, thus effectively reducing the denominator of the fall factor, and thus increasing the fall factor.
Curt
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curt
Dec 4, 2003, 5:59 PM
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In reply to: In reply to: ... Also the force expereinced by a climber, as long as he does not hit a ledge on the way down, is completely determined by the rope, not the fall length. Each rope is rated for its maximum fall force and this force will not be exceeded regardless of the fall length. ... NOPE, think about it again. Ropes are rated based on the force they exert on the "climber" in a factor 1.7 fall. Thus the fall length AND rope out are both factors in the force seen by all parts of the system. If this is not clear you may need to study how a rope works. Rope testing and certification basically assumes that factor 2 is the worst fall a belayed climber can experience. As this thread points out that is not always true.
As I posted earlier, the fall factor takes the length of the fall out of the equation--as it relates to impact force on the climber, gear, etc. Again, this is because the additional kinetic energy that accompanies a longer fall is offset by a corresponding and longer stretch of the rope.
In the case of a longer fall, with more rope out, the falling climber is brought to a halt over a longer period of time--and this is the key. The peak tension in the rope (and therfore force on the climber) is no greater than in the case of a shorter fall (in spite of higher kinetic energy) for this reason. By the way tedc, I wasn't disagreeing with your post--I merely added on my comments for further clarity.
Curt
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