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Physics on long rappells

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

  1. nathanslc

    nathanslc

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    I am trying to understand the physics of a long rappel. Why do I need to add more friction towards the bottom of the rappel? Fore example, on a 100' rappel, I can use my device in low friction mode with no problems. On a 300' rappel, the same friction setting the last 100' will not be enough. I am assuming that it has to do with elongation on the upper portion of the rope but would like to confirm without taking a Newtonian physics course.
    ScottM likes this.
  2. Kuenn

    Kuenn

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    Great question!

    I have a few thoughts on this subject and will be very interested in the responses of the group. Before I give my opinion (and it will be just that), can you offer more about your experience(s). Was it a particular rappel or something you have observed on several occasions? Describe the drop(s), sloped, free hanging, and rope condition, rap device, weather, etc. There may be answers embedded in all of these factors.
  3. nathanslc

    nathanslc

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    I typically use a totem device and use it in standard figure 8 mode for most rappels. This provides a good amount of friction for me. If I were to add friction by going over the top of the device I would be going no where on rappels 150' or less. On a 300' rappel in standard figure 8 mode at the top of the rappel is way to much friction and I have to force feed the rope through the device until I get towards the bottom. When additional friction is applied going over the top of the device, this seems to be the perfect amount of friction to control my descent. I have not done a lot of long rappels but noticed on Egypt 2 which has over hang and slope as well as another 200' sloped rappel. I eliminated speed as a factor by stopping roughly 100' above ground and then continuing. Ropes used were in new condition 8.3 mm.
    Last edited: Mar 15, 2016
  4. Zach Olson

    Zach Olson

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    I always assumed it was due to the weight of the rope. At the top of the rappel all of the weight of the rope is below you and is acting as a partial fireman's belay (for lack of a better term). As you descend the rope there is less weight below you and the effect is lessened thereby requiring more friction.

    This is my opinion based on my own observations and nothing else.
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  5. nathanslc

    nathanslc

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    I understand the weight of the rope acting as a belay. In my experience, a 100' rappel in normal friction mode is fine and if I were to add additional friction, I would go nowhere. A 300' rappel 100' from the bottom more friction is required.
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  6. Scott Patterson

    Scott Patterson

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    I always assumed it was due to the weight of the rope. At the top of the rappel all of the weight of the rope is below you and is acting as a partial fireman's belay (for lack of a better term). As you descend the rope there is less weight below you and the effect is lessened thereby requiring more friction.

    This is my opinion based on my own observations and nothing else.


    The above is actually the exact reason. The weight of the rope below the rappeller serves as a partial fireman belay.
    AW~ and Rapterman like this.
  7. nathanslc

    nathanslc

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    If that were the case, normal friction mode would be fine the entire length of a 300' rappel for me. Perhaps I am being overly paranoid and adding friction that is not required? However, in my observation, this is not the case. With additional friction on longer rappels towards the bottom (100' above ground) seems necessary; while on a 100' rappel no additional friction is necessary.
  8. Attila

    Attila

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    Just so I understand the question better, we're talking about the last 100' of both descents - the only difference is that one starts 200' feet further up - correct? Rope weight would be the same at bottom minus 100'. The only things different would be initial speed and heat of the device. If you've already descended 200', you're going to have some momentum and less static friction to deal with. In other words, at B-100, the potential energy is the same but only one descent has 0 kinetic energy whereas the other has built-up kinetic energy.

    How would the heat of the device change things?
  9. nathanslc

    nathanslc

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    Yes. You understand the question correctly. I eliminated kinetic energy as a factor by stopping roughly 100' from the ground. My device was warm but not hot. I would imagine more heat = more friction. Perhaps this is all coincidence or all in my head but everyone adds more friction on longer rappels; Why, especially when they have to force feed the rope the first 150' to 200'?
    EvergreenDean likes this.
  10. Deagol

    Deagol too many hobbies

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    I think more heat = less friction, but we will see if anyone else contradicts this.....

    another potential reason to add friction at the end of a long rappel: accumulated hand fatigue from the initial section.
  11. The Dread Pirate Roberts

    The Dread Pirate Roberts

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    It sounds like the problem has been worked out to me. The only thing I would add is the human factor. A 300' rap takes a decent amount of time and by the time you are on the last 100' you are more fatigued than would be on a 150' rap. In my experience being tired can make it feel like you need more friction to be safe and under control.
  12. nathanslc

    nathanslc

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    This may be possible but doubtful. I literally stopped and hung out; could have drunk a red bull and smoked a cigarette then continued. Does anyone else add more friction on long rappels and have the same problem? I know that my entire group added more friction and had to force feed until at the bottom. I also know that they do not add more friction on shorter rappels.
  13. Kuenn

    Kuenn

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    That was my understanding of the question you are posing. Absolutely, the rope weight is the difference at the top of a long rappel, but I don't think that is what is at question here.

    Restating the question, why does the same device friction setting, same rope, same body, same conditions, result in varying speeds of descent. Shouldn't you be able to stop on rope, 100' above the ground (on a long rappel) and experience a similar or equivalent descent speed, as you would a 100' rappel from start to finish? Logic suggests that it should be the same.

    I've experienced this phenomenon many times, and this is my conclusion. So, put on your tin-foils hats and hear me out.

    Let's remove any other friction variables and compare apples to apples. Comparing a 300' free hanging rap with a 100' free hanging rap. The final 100' should be the same...but they're often not. My opinion: The difference is in the first 200' of the long rap. And I submit as evidence the sport of Curling (o_O). No no no no - YES, I'm serious.

    On most rappel devices, once the rope has found its happy spot, it rarely moves from it for the duration of THAT descent. In this case, the previous 200' feet of polishing (over what can translate to be several mm of friction surface area) has now created a slick, even glass-like surface. Temperature, possibly playing a small role, but my guess is the speed is faster because the friction is less. Bottom line...if the friction was the same, the rate of descent would be the same.

    Yeah, that's out there...but it's my logic and it makes sense to me.

    (Embedded references to Signs and The Vicar of Dibley.)
    Last edited: Mar 15, 2016
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  14. nathanslc

    nathanslc

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    I believe it has to do with the tension of the rope and the additional 200' at the top and the forces pulling up on the rope. I just have no evidence to back this and was hoping someone did.

    Really the only difference is the additional 200' at the top. I believe the further down the rope you get the more you have an opposite force up top which would require more friction to stop/slow your descent.
  15. Deagol

    Deagol too many hobbies

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    I think we agree about higher heat = less friction. Kind of like when snow skiing, they say that due to pressure, you are actually skiing on a layer of water on the microscopic level..
  16. Rapterman

    Rapterman

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    Good thread Nathan!
    I think the biggest factor by far, is the rope weight.
    To test rappel devices for long drops (when we only have a 100 foot drop to practice on)
    we will keep the remaining rope in the rope bag (the extra 200-300 or so feet) and
    tie a knot just above the ground, and suspend the bag.
    This creates the effective rope weight when starting a long drop (requiring less friction)
    Then we have the fireman take the bag off the line (look out!) and the friction
    required (more friction) reverts to the level of the actual practice rappel height.
    You can get real fancy and group the extra rope in coils of say, 50 feet, then have your fireman
    un-hang them as you go to simulate a more incremental loss of friction.
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  17. qedcook

    qedcook

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    I think rope buoyancy also plays a factor. Even static ropes have some buoyancy. The farther away from the anchors your are, the more buoyancy the rope will have. In my personal experience, braking and slowing down is more difficult with more buoyancy.
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  18. ScottM

    ScottM Looking for a canyon, you got one?

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    Just so that we're all clear; when you state "add friction on longer rappels", do you mean the additional friction is added at the top (prior to going on rappel), or mid rappel when your judgment tells you that additional friction is needed?
    Last edited: Mar 15, 2016
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  19. ScottM

    ScottM Looking for a canyon, you got one?

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    Clever!
  20. Rapterman

    Rapterman

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    Mid-rappel, when your smokin' glove tells you it is time!
    (the rope weight acts as a firemans when you are near the top)
    :D
    qedcook-
    are you referring to rope stretch?
    That does SEEM to make a difference....as though the rope diameter seems to decrease...
    have to bring some calipers for the Heaps drop...;)
    Last edited: Mar 15, 2016
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