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Physics on long rappells

Discussion in 'Tech Tips and Gear' started by nathanslc, Mar 15, 2016.

  1. ratagonia

    ratagonia

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    Could you be more specific, Gordon? Which thing?

    :moses:
  2. gajslk

    gajslk

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    Whether more friction is really needed under the specified condition or not. Use the spring scale to take fatigue out of the equation and to measure the actual difference in braking force. You'll probably want to run the rope from the brake device through a leg-loop biner and pull with the spring scale in between. Might be most accurate to move the scale around to find a local minimum, probably in a direction orthogonal to the rope right at halfway.

    You could also add weight in the control condition, 100' drop with 100' to go, so that at a given friction setting a slow descent occurs with only rope weight as the braking force. Then rap with that weight down 200', stop, rerig, let go, and see what happens. Since you're so crotchety, I'll point out in advance that 'let go' implies a bit of common sense rather than a literal, pedantic, interpretation. A bottom belay could replace that common sense. The setup is going to be a bit fiddly, although you could do it in your garage with a weight added to the rope ...

    Gordon
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  3. ratagonia

    ratagonia

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    Are you suggesting that psychological factors should be removed because you are a Vulcan?

    T
  4. gajslk

    gajslk

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    I didn't suggest that other than as an implied subset of fatigue. But it's a good idea unless the test subject is a Vulcan ...

    Gordon
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  5. nathanslc

    nathanslc

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    Tom, you are interpreting the question correctly. I am curious to know if anyone else feels the same at the end of a 300'+ rappel? I always feel that I need to have a higher friction setting than I would normally have on a 100' rappel. I don't know if it is fatigue, psychological or both. I was originally thinking there was some physics that I did not know about such as rope stretch or heating of belay device. After being on a few more long rappels since this post, I have tried to be more aware of the fatigue factor and think you may be correct.
    Rapterman and ratagonia like this.
  6. Rapterman

    Rapterman

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    ...could the rope diameter be shrinking as we descend???!!!! :wideyed:
  7. ratagonia

    ratagonia

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    No.

    The rope above you will be at one diameter, because it has your weight on it.

    The rope below you will be at (or very close to) the "resting diameter".

    The difference between those diameters when using a tightly woven static rope is going to be quite small.

    There is a perceived decrease in the diameter of the rope as the rappel gets taller. There is a LARGE perceived decrease in the diameter of the rope when one needs to ascend it. The rope itself is indifferent to these activities.

    :moses:
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  8. Brian in SLC

    Brian in SLC Brian in SLC

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    Weight of the rope below you makes a difference too. You start out on a 300 footer with the full length of the rope below you which adds a bit of weight help to the brake hand. As you descend, there's less of it, so, less braking.
    Rapterman likes this.
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