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iltripp
Dec 5, 2005, 7:25 PM
Post #26 of 30
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Registered: Oct 6, 2003
Posts: 1607
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In reply to: That the guy in iltripp's pics walked with bruised ribs would also seem to argue in favor of failing pieces reducing impact force.
Those actually weren't my pics. I dragged them up from another thread. I don't really know any details about the fall or what happened to the climber.
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antiqued
Dec 6, 2005, 3:09 AM
Post #27 of 30
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Registered: Aug 18, 2005
Posts: 243
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By physics / engineering, it is better
Unless it pulls off a chunk and knocks your belayer out...
Unless it was out of line and feeds more slack into the fall...
Unless.....
The rope will recover partly. Let me ignore friction and protection pulling, and concentrate on the rope.
Imagine the bad piece inifinitesimally above the good one. The rope stretches, absorbs X kJ, and then the bad piece pulls. Even though the rope has no time to recover, the leader is moving slower, and the catch would be the same for distance and impact force as if the bad piece was never there - regardless of how big 'X' is.
Now if the distance is measurable, the rope has a chance to partially recover, as misanthropic nihilist and others have been arguing.
We could do all sorts of elementary math demonstrations, with various assumptions, but it's kind of pointless. It's all in the assumptions. I think that the key one is that the impact force is not a function of the velocity - that is that the additional speed the leader picks up after the bad piece pulls does not make the catch harder than it would be absorbing the same energy at the slower speed.
I do have some marginal data to back this up. PMI had a list of impact forces for different weights and FF. I'm not up to posting graphs tonite, but if I compare falls of equal energy - light weight vs heavy weight, the heavy climber causes more impact force despite moving slower.
Examples
mass distance FF energy speed impact force
80kg 4.8m drop 1.7FF 3763J 9.70 m/s 8.3kN
114kg 3.3m 1.2FF 3687J 8 .04m/s 9.1kN
137kg 3m 1.1FF 4028J 7.67m/s 10.3kN
Unfortunately, the table doesn't include the maximum extension.
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sausalito
Dec 6, 2005, 3:27 AM
Post #28 of 30
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Registered: Jul 20, 2005
Posts: 155
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no straight answer. The math done is guess work at best because it assumes that the peice puts a certain amount of force on the rope. This would suggest that it held to a point.
You can go round and round on this one. I would look at it less scientifically though. If you have good gear between you and your belay that will catch than run it out. If I feel I have to place something marginal I usually try and find passive gear...
I agree with statements about getting pumped if you mess around too long. Place. Evaluate and go.
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thegreytradster
Dec 6, 2005, 3:35 AM
Post #29 of 30
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Registered: Jul 7, 2003
Posts: 2151
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In reply to: Imagine the bad piece inifinitesimally above the good one. The rope stretches, absorbs X kJ, and then the bad piece pulls. Even though the rope has no time to recover, the leader is moving slower, and the catch would be the same for distance and impact force as if the bad piece was never there - regardless of how big 'X' is. Now if the distance is measurable, the rope has a chance to partially recover, as misanthropic nihilist and others have been arguing. We could do all sorts of elementary math demonstrations, with various assumptions, but it's kind of pointless. It's all in the assumptions. I think that the key one is that the impact force is not a function of the velocity - that is that the additional speed the leader picks up after the bad piece pulls does not make the catch harder than it would be absorbing the same energy at the slower speed.
Anybody that has zippered part of an aid pitch has empirical experience that this is how it works. Bad pieces pulling do not add to the forces at the end of the fall, but they also do not reduce the force nearly as much as you would expect them to. Even if the pieces don't hold, the momentary impulse can tend to keep you in an upright position with the hard to break, easier to mend body parts pointed in the right direction.
Sometimes that is enough justification for a marginal piece.
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stymingersfink
Dec 6, 2005, 9:48 PM
Post #30 of 30
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Registered: Aug 12, 2003
Posts: 7250
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In reply to: enough justification for a marginal piece.
^exactly^
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