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JimTitt


Feb 8, 2010, 12:21 PM
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Re: [Jo_Rock] Common KNs in real world falls [In reply to]
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Before jt512 comes back and kills you, yes, you are missing something.
Take the two extremes. a) a frictionless runner. Load on the rope end is 12kN and therefore the load on the other end must also be 12kN, therefore the total load on the runner must be 24kN. b) runner with 100% friction. Load on the rope 12kN and the load on the other end is 0kN as all the 12kN force has been dissapated by friction, thus the total load on the runner is 12kN.
Tis simple really!


jt512


Feb 8, 2010, 12:25 PM
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Re: [Jo_Rock] Common KNs in real world falls [In reply to]
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Jo_Rock wrote:
Run the rope through the test stand (assume a polished surface on the redirect that simulates a biner) with a rope that passes at 12 kN.

What do you mean by "that passes"?

In reply to:
Now, sand and scuff the redirect to simulate a manky biner that increases friction and what happens to the forces? Up right?

What forces go up? The frictional force does, obviously. But the frictional force is not part of the total force on the biner. The total force on the biner is the sum of the tensions in two sides of the rope. Let F_t, F_b, and F_c be the force on the top biner, belayer, and climber, respectively, and let µ be the frictional force. Then

F_t = F_b + F_c . But

F_b = F_c – µ . Therefore

F_t = 2F_c – µ .

Clearly, then, the greater the friction µ the less the total force on the top biner F_t.

In reply to:
I think they are making the assumption that the rope must still pass the same 12 kN standard, but it either gets tested with a polished surface or a rough and passes and then it goes into the real world where circumstances change and the rope properties do not.

Okay, I think I see where you're getting confused. In the standard model of impact force, friction has no effect on the tension in the rope on the climber's side. If the maximum tension on the "climber's" side of the rope in a UIAA test is 12 kN, it will be 12 kN regardless of how much friction there is across the top biner. This is true to a first approximation. In truth, friction will increase the force in the climber's side of the rope slightly; however, this increase is more than offset by the reduction in force in the belayer's side. So the net effect of friction is still to reduce the total force on the biner.

Jay


(This post was edited by jt512 on Feb 8, 2010, 1:20 PM)


Jo_Rock


Feb 8, 2010, 12:28 PM
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Re: [JimTitt] Common KNs in real world falls [In reply to]
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Absolutely. My point is that you can't change the parameters of the test (biner friction) and assume that the force on the climber remains at 12 kN just because the rope has to meet that force maximum in the required test.

I still may be missing something and/or communicating my point poorly, but I'm with you that far anyway.


hafilax


Feb 8, 2010, 12:36 PM
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Re: [Jo_Rock] Common KNs in real world falls [In reply to]
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12kN is the maximum impact force allowable for a rope to pass the drop test. The drop test is supposed to simulate an extremely hard fall; beyond what most people will experience. Most ropes have actually have an impact force in that test of 8 or 9kN which is quite a bit below the maximum. Also, it is a maximum allowable, not a required force and a rope is not a load limiting device.


Jo_Rock


Feb 8, 2010, 12:55 PM
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Re: [hafilax] Common KNs in real world falls [In reply to]
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hafilax wrote:
12kN is the maximum impact force allowable for a rope to pass the drop test.

Yes, but if you increase the friction in your test rig with an identical rope you won't get the same result. To get the same result you would need a stretchier rope. (I think) The table seems to me to imply that increasing the friction decreases the force. Using gear with a higher friction does not change the way the rope was tested in the first place so assuming the 12 kN climber force is no longer valid over a heavily used biner as this is a change from the test parameters.

hafilax wrote:
The drop test is supposed to simulate an extremely hard fall; beyond what most people will experience. Most ropes have actually have an impact force in that test of 8 or 9kN which is quite a bit below the maximum.

Right. I realize that they acknowledge this and am not saying that their results are wrong, just that the chart is. (Or at least very misleading to me in what they are saying.)


(This post was edited by Jo_Rock on Feb 8, 2010, 12:57 PM)


JimTitt


Feb 8, 2010, 1:06 PM
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Re: [Jo_Rock] Common KNs in real world falls [In reply to]
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You are starting from the wrong end. The article is talking about the maximum possible values and in that sense the chart is perfectly valid. The 12kN force is alway sused as this is the maximum result allowed for a rope not over a karabiner. Changing the friction value changes the forces on the karabiner and the belayer when we assume the force is still 12kN.


hafilax


Feb 8, 2010, 1:10 PM
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Re: [Jo_Rock] Common KNs in real world falls [In reply to]
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With friction two things happen: the force on the climber is greater because you are isolating the section of rope between the climber and the pro which means that there is less rope in the system; the force on the gear is reduced because the force on the belay side of the biner becomes less.

If the length of rope on the belay side of the biner is small and there is a lot of friction then the force on the gear will be greatly reduced (up to nearly a factor of 2) while the force on the climber is nearly the same.


jt512


Feb 8, 2010, 1:27 PM
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Re: [Jo_Rock] Common KNs in real world falls [In reply to]
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Jo_Rock wrote:
Yes, but if you increase the friction in your test rig with an identical rope you won't get the same result.

You apparently think that the peak force on the climber's side of the rope will be greater when the friction over the top biner is increased. Under the standard model of impact force, this is false. Why do you think that friction over the top biner would affect the force felt by the climber?

Jay


Jo_Rock


Feb 8, 2010, 1:28 PM
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Re: [jt512] Common KNs in real world falls [In reply to]
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Sorry completely missed your post.

jt512 wrote:
What do you mean by "that passes"?

Produces 12 kN in the UIAA test jig and hence meets the force requirement of 12 kN or less.

jt512 wrote:
What forces go up? The frictional force does, obviously. But the frictional force is not part of the total force on the biner. The total force on the biner is the sum of the tensions in two sides of the rope. Let F_t, F_b, and F_c be the force on the top biner, belayer, and climber, respectively, and let µ be the frictional force. Then

F_t = F_b + F_c . But

F_c = F_b – µ . Therefore

F_t = 2F_b – µ .

Clearly, then, the greater the friction µ the less the total force on the top biner F_t.

Aaaahhhhh! This may be what I am missing, but...does this force that the top biner takes in friction exceed the increase to the force the climber takes because the section of rope that is between top biner and "belay does not recieve as much force and therefore does stretch as much and then does not absorb as much energy. (Don't know if my physics is technically accurate, but I hope you know what I mean.)

jt512 wrote:
Okay, I think I see where you're getting confused. In the standard model of impact force, friction has no effect on the tension in the rope on the belayer's side. If the maximum tension on the "belayer's" side of the rope in a UIAA test is 12 kN, it will be 12 kN regardless of how much friction there is across the top biner. This is true to a first approximation. In truth, friction will increase the force in the belayer's side of the rope slightly; however, this increase is more than offset by the reduction in force in the climber's side. So the net effect of friction is still to reduce the total force on the biner.

????????? Yes, i think so. I thought friction would reduce force on the belayer's side and increase on the climber's side.


Jo_Rock


Feb 8, 2010, 1:38 PM
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Re: [jt512] Common KNs in real world falls [In reply to]
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jt512 wrote:
Jo_Rock wrote:
Yes, but if you increase the friction in your test rig with an identical rope you won't get the same result.

You apparently think that the peak force on the climber's side of the rope will be greater when the friction over the top biner is increased. Under the standard model of impact force, this is false. Why do you think that friction over the top biner would affect the force felt by the climber?

Jay

Maybe the model I am using does not apply to the situation because there are other things of greater impact on the results, but if the top biner's friction is pushed to infinite then you create a fall factor of 2.

As you increase the friction from the test parameters it reduces the belayer's side of the rope's ability to stretch increasing the energy that the climber's side must absorb. If this force exceeds what is absorbed by the friction then force on the top biner goes up. (I think)

Sorry in advance for any misuse of physics terms.


Jo_Rock


Feb 8, 2010, 1:48 PM
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Re: [JimTitt] Common KNs in real world falls [In reply to]
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JimTitt wrote:
You are starting from the wrong end. The article is talking about the maximum possible values and in that sense the chart is perfectly valid. The 12kN force is alway sused as this is the maximum result allowed for a rope not over a karabiner. Changing the friction value changes the forces on the karabiner and the belayer when we assume the force is still 12kN.

This is actually what this is all about!

The table is right IF we assume that the the climber's felt force is still 12 kN.

To me what the paper is implying with the chart is that if you use used gear with greater friction that the forces go down. This is not true if the rope passed the drop test on "like new biner" friction analog and then is put into service with higher friction biners(and all else remains the same). [edit: unless the whole friction absorbing more energy than the increase from the reduction in effective ff is right.]


(This post was edited by Jo_Rock on Feb 8, 2010, 1:50 PM)


Jo_Rock


Feb 8, 2010, 1:53 PM
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Re: [hafilax] Common KNs in real world falls [In reply to]
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hafilax wrote:
With friction two things happen: the force on the climber is greater because you are isolating the section of rope between the climber and the pro which means that there is less rope in the system; the force on the gear is reduced because the force on the belay side of the biner becomes less.

If the length of rope on the belay side of the biner is small and there is a lot of friction then the force on the gear will be greatly reduced (up to nearly a factor of 2) while the force on the climber is nearly the same.

This is what I was trying to say in post 187.


Jo_Rock


Feb 8, 2010, 1:58 PM
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I was never trying to dispute the validity of the information in the chart, just the context of that info and what I think they are trying to imply with it: that ropes, once tested, will never produce a greater than 12 kN force on the climber in a UIAA drop test configuration even if you change the friction of the redirect (top biner or simulation of).


Jo_Rock


Feb 8, 2010, 2:09 PM
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Re: [jt512] Common KNs in real world falls [In reply to]
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jt512 wrote:
Jo_Rock wrote:
Yes, but if you increase the friction in your test rig with an identical rope you won't get the same result.

You apparently think that the peak force on the climber's side of the rope will be greater when the friction over the top biner is increased. Under the standard model of impact force, this is false. Why do you think that friction over the top biner would affect the force felt by the climber?

Jay

Because of this:

hafilax wrote:
With friction two things happen: the force on the climber is greater because you are isolating the section of rope between the climber and the pro which means that there is less rope in the system; the force on the gear is reduced because the force on the belay side of the biner becomes less.

If the length of rope on the belay side of the biner is small and there is a lot of friction then the force on the gear will be greatly reduced (up to nearly a factor of 2) while the force on the climber is nearly the same.

I don't know that this is greater than the energy lost to friction or not, but it is the model that I have seen used and the frictional loss written off with wind resistance as insignificant.

Also, to clarify I am talking about NOT assuming that the climber's felt force stays the same. Change none of the conditions (including the elasticity of the rope) except the friction of the redirect.


Jo_Rock


Feb 8, 2010, 2:28 PM
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Even if frictional loss (better term?) is significant or even greater than reduction in the belayer's side elasticity effects on the whole system, that is clearly not what they are saying. If you look at the figure above the chart they are just adding the two leg's forces depending on which fraction you use to determine belayer side from climber side = 12 kN. If the context of the chart were that increasing friction in the test parameters required that ropes produced less impact on the redirect to pass the test, I would have no argument. That's not how I read it.


hafilax


Feb 8, 2010, 2:53 PM
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I'm still not really following what you're trying to get at.

The context of the chart is the force applied to the gear. The conclusion I get from the Maximum Force on a Running Belay section is that the frictionless scenario results in the highest impact force on the gear so that including friction isn't needed in the worst case scenario calculation and that 20kN holds as a good maximum impact force requirement.


jt512


Feb 8, 2010, 3:01 PM
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Re: [Jo_Rock] Common KNs in real world falls [In reply to]
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Jo_Rock wrote:
jt512 wrote:
Why do you think that friction over the top biner would affect the force felt by the climber?

Because of this:

hafilax wrote:
With friction two things happen: the force on the climber is greater because you are isolating the section of rope between the climber and the pro which means that there is less rope in the system; the force on the gear is reduced because the force on the belay side of the biner becomes less.

If the length of rope on the belay side of the biner is small and there is a lot of friction then the force on the gear will be greatly reduced (up to nearly a factor of 2) while the force on the climber is nearly the same.

I don't know that this is greater than the energy lost to friction or not, but it is the model that I have seen used and the frictional loss written off with wind resistance as insignificant.

Also, to clarify I am talking about NOT assuming that the climber's felt force stays the same. Change none of the conditions (including the elasticity of the rope) except the friction of the redirect.

Let's get back to your original objection from your first post in the thread:

Jo_Rock wrote:
Did you notice that in the document you linked to they have force on the runner dropping as friction in the pro biner increases? (table 1 p7) They seem to be assuming that the 12 kN in the UIAA drop test is a law of physics.

Your interpretation of Table 1 is incorrect. They are not assuming anything at all. They are presenting results of calculations done by Petzl along side empirical results from two different labs. The table shows that as the friction coefficient increases, the force on the top biner decreases. This is not a well-written paper, and the details of the experiments are not specified, but, as far as we can tell, Table 1 supports the author's contention that the greater the friction over the top biner, the lower the force on the top biner, without regard to any assumption about how friction affects the force on the climber.

Now, as I have stated, you are technically correct that friction over the top biner increases the force on the climber. But as I have also stated, this effect is small—especially when the fall factor is high, as it is in the UIAA test—and, in terms of the force on the top anchor, is more than compensated for by a reduction in force on the belayer's side of the rope.

To hopefully hammer this home once and for all, let's put some numbers to it.

The typical assumption is that the frictional force is one-third the force felt by the climber; that is, the "friction factor" u = 1/3. Assuming that this friction factor is correct for a UIAA test rig, then a standard UIAA drop test using a rope with an impact force rating of 12 kN will generate forces on the climber, belayer and anchor of 12, 8, and 20 kN, respectively. (These numbers agree with those of Petzl in Table 1 of the paper you referenced, suggesting that Petzl used u = 1/3 in their own calculation.) In comparison, if we do the UIAA test under identical conditions, except that we increase the friction factor to 1/2, then the forces on the climber, belayer, and anchor will be 12.11, 6.06, and 18.17 kN, respectively. Thus, for a 50% increase in friction over the top biner, the impact force on the climber has increased less than 1%. Moreover, owing to the reduction in force on the belayer, the impact force on the top anchor has decreased from 20 kN to just over 18 kN.

The methodology for these calculations is explained here.

Jay


(This post was edited by jt512 on Feb 8, 2010, 3:18 PM)


Jo_Rock


Feb 8, 2010, 3:14 PM
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jt512 wrote:
Okay, I think I see where you're getting confused. In the standard model of impact force, friction has no effect on the tension in the rope on the climber's side. If the maximum tension on the "climber's" side of the rope in a UIAA test is 12 kN, it will be 12 kN regardless of how much friction there is across the top biner. This is true to a first approximation. In truth, friction will increase the force in the climber's side of the rope slightly; however, this increase is more than offset by the reduction in force in the belayer's side. So the net effect of friction is still to reduce the total force on the biner.

Jay

You must have edited while I was responding. Now this makes perfect sense. You are agreeing that the force felt by the anchor would go up from friction if you ignored the energy loss from that friction because you are effectively reducing the ropes ability to stretch (over part of it's length) and absorb energy over a longer time period, right?

BUT, if I read you right, you are saying that this frictional loss is greater than the increase.


Sorry, I think I might have caught all of that if I had seen your post right away and weren't already trying to reply to other posts.


Jo_Rock


Feb 8, 2010, 3:32 PM
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I got you now. (last post was while you were posting)

That makes sense I was not aware of that. Good info, and thank you.

So their conclusion from the chart is correct, but aren't they actually using the incorrect model that I was using where friction is only figure into the proportion each leg takes of total force and frictional loss is ignored? That's what it seems from the numbers they get. They just assume that climber force is 12 kN and apearently that gives similar numbers to the correct (or closer to reality) model that you are using.

The only way I could see that they actually are exactly right is if the effects of the increased force on the climber's leg were exactly equal to the effects of loss to friction and they just did not show any explanation of that.


jt512


Feb 8, 2010, 3:41 PM
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Jo_Rock wrote:
jt512 wrote:
Okay, I think I see where you're getting confused. In the standard model of impact force, friction has no effect on the tension in the rope on the climber's side. If the maximum tension on the "climber's" side of the rope in a UIAA test is 12 kN, it will be 12 kN regardless of how much friction there is across the top biner. This is true to a first approximation. In truth, friction will increase the force in the climber's side of the rope slightly; however, this increase is more than offset by the reduction in force in the belayer's side. So the net effect of friction is still to reduce the total force on the biner.

Jay

You must have edited while I was responding.

That paragraph was in my original post, but I mixed up "climber" and "belayer," which I quickly corrected in an edit.

In reply to:
Now this makes perfect sense. You are agreeing that the force felt by the anchor would go up from friction if you ignored the energy loss from that friction because you are effectively reducing the ropes ability to stretch (over part of it's length) and absorb energy over a longer time period, right?

That's a rather convoluted way of putting it. It is simply, that when you increase friction over the top anchor, the increase in force on the climbers side of the rope is quite small, especially when the fall factor is high, and is much less than the decrease in force on the belayer's side; so the net effect is to decrease the force on the top anchor.

Jay


Jo_Rock


Feb 8, 2010, 3:51 PM
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jt512 wrote:
The typical assumption is that the frictional force is one-third the force felt by the climber; that is, the "friction factor" u = 1/3. Assuming that this friction factor is correct for a UIAA test rig, then a standard UIAA drop test using a rope with an impact force rating of 12 kN will generate forces on the climber, belayer and anchor of 12, 8, and 20 kN, respectively. (These numbers agree with those of Petzl in Table 1 of the paper you referenced, suggesting that Petzl used u = 1/3 in their own calculation.) In comparison, if we do the UIAA test under identical conditions, except that we increase the friction factor to 1/2, then the forces on the climber, belayer, and anchor will be 12.11, 6.06, and 18.17 kN, respectively. Thus, for a 50% increase in friction over the top biner, the impact force on the climber has increased less than 1%. Moreover, owing to the reduction in force on the belayer, the impact force on the top anchor has decreased from 20 kN to just over 18 kN.

The methodology for these calculations is explained here.

Jay

Maybe just bad assumption on my part: I ASSUMED that they were not doing complicated math and rounding the results because if you run the numbers in fig 2 at 12kN listed in the left of table 1 you get exactly the result on the chart, not the close number that your model produces.

I think how I got this model stuck in my head was from a discussion about further out in the pitch at lower FF. Friction in the rope system can increase fall forces significantly when you are further out on the pitch, right?


Jo_Rock


Feb 8, 2010, 3:56 PM
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jt512 wrote:
That paragraph was in my original post, but I mixed up "climber" and "belayer," which I quickly corrected in an edit.

That's why it just confused my poor little brain when I was trying to respond. (At least that's the excuse I'm gonna use!)


jt512


Feb 8, 2010, 4:06 PM
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Jo_Rock wrote:
So their conclusion from the chart is correct, but aren't they actually using the incorrect model that I was using where friction is only figure into the proportion each leg takes of total force and frictional loss is ignored? That's what it seems from the numbers they get. They just assume that climber force is 12 kN and apearently that gives similar numbers to the correct (or closer to reality) model that you are using.

As far as I can tell, Table 1 of the paper you referenced does not present the results of a model per se. Like I said before, it appears to compare the results of a standard calculation, done by Petzl, with results from two experiments. The table implies that in all three cases the force on the "climber" was 12 kN, but for the two experiments (if that's what they were), it is not clear what this really means; that is, was it a measured quantity, an assumed value based on the conditions of the experiment, or what. Like I said, the paper is not a paragon of technical writing.

Jay


Jo_Rock


Feb 8, 2010, 4:09 PM
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hafilax wrote:
I'm still not really following what you're trying to get at.

The context of the chart is the force applied to the gear. The conclusion I get from the Maximum Force on a Running Belay section is that the frictionless scenario results in the highest impact force on the gear so that including friction isn't needed in the worst case scenario calculation and that 20kN holds as a good maximum impact force requirement.

What I was getting at must have gotten garbled by me trying to articulate things I don't really understand while my head is shoved into my rectum!


jt512


Feb 8, 2010, 4:19 PM
Post #200 of 211 (2184 views)
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Re: [Jo_Rock] Common KNs in real world falls [In reply to]
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Jo_Rock wrote:
jt512 wrote:
That paragraph was in my original post, but I mixed up "climber" and "belayer," which I quickly corrected in an edit.

That's why it just confused my poor little brain when I was trying to respond. (At least that's the excuse I'm gonna use!)

Looking back through the thread, it is clear that you were responding to my post before I had a chance to correct it, so I apologize for having misled you.

Jay


(This post was edited by jt512 on Feb 8, 2010, 4:24 PM)

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