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Physics on long rappells

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

  1. ScottM

    ScottM Looking for a canyon, you got one?

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    I concur - to some degree - but a previous statement made me question exactly when the friction is added. IE ("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'?")
  2. Rapterman

    Rapterman

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    Zach says it well:
    " 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. "
  3. nathanslc

    nathanslc

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    Atilla seems to understand the question best. I understand that at the top of the 300' rappel there is a significant amount of weight due to the rope at the bottom acting as a belay. The amount of rope at the bottom is the same if I start at 100' or start the rappel from 300' down to 100' above ground. The friction seems to be significantly less (even after stopping) on the 300' rappel. Am I the only one who has observed this? If so, why does everyone add the extra friction on longer rappels when they would normally not use it on the 100' rappel?
  4. nathanslc

    nathanslc

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    In my observation. It doesn't matter. If I have my rappel device set on low friction at the top of a 300' rappel, I will have to force feed the rope down to 100' above ground (give or take) and then add friction to control my descent. If I start at 100' above ground or rappel down to 100' above ground the amount of rope below me is the same. On the 100' rappel however, I can start with the same low friction setting and be just fine all the way down. In fact, If I add additional friction (same amount on the 300' rappel) at the top of a 100' rappel I go no where. I have been told by many people, that they add more friction on longer rappels than they would otherwise and that they speed up towards the end. Why?
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  5. Bootboy

    Bootboy Atwood Gear

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    This ^

    Three factors contributing to the need to add friction as you go in descending order of their effects

    1: Rope weight

    2: Fatigue

    3: Heat buildup

    The first two I think are easily grasped, I think. But the third one is a little less clear and probably negligible.

    Controlling your speed means countering the effects of gravity. Gravity means you have much more potential energy at the top of a 300' rappel than you do at a 100' rappel, 3 three times as much.

    All of that energy needs to go somewhere over the course of your controlled fall. Quite a bit of it goes into the rope and a gets dissipated as heat, but your device has a much smaller thermal mass than 300' of rope, therefore, it heats up much faster and can only dissipate heat so fast. The hotter it gets, the less energy it can absorb. Obviously, it can only get so hot because of the temperature gradient between the aluminum and the constant temp of the air and the incoming rope... I ramble...

    Kinda like the brakes on a car. Ever experienced brake fade on a long hill? Same concept.

    And although you are noticing rope stretch, it is not a contributing factor.
    Last edited: Mar 15, 2016
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  6. nathanslc

    nathanslc

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    So, if fatigue and rope weight are not a factor, and hypothetically I had a space age technology that eliminated the heat build up, I would never have to adjust for friction? So far, your answer makes the most sense.
  7. Bootboy

    Bootboy Atwood Gear

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    In theory, yes.

    Pro tip:

    I know it's not always possible for every big rap, but when I do Heaps, we soak the 300' rope right before starting that final rap sequence. Wet ropes keep things much cooler, including your brake hand, which I find helps reduce fatigue.
    Last edited: Mar 15, 2016
  8. ratagonia

    ratagonia

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    I realize that makes some kinda sense, but I think the evidence for it is small.

    I think the phenomenon is accounted for with hand, body and mental fatigue.

    There may be less friction at the slightly elevated temperature seen on rap devices, but they are not really that hot. As the rap device gets hotter, it radiates heat to the environment at substantially higher rate, and eventually reaches an equilibrium. The friction may be less at that temperature, hard to say. I like Kuenn's polishing theory, and that might have something to do with it, but...

    You hand gets tired, your arm gets tired, your abs get tired. The surface of your hand, with or without a glove, also gets "tired" in a sense. I think these all add up to not being as able to squeeze the rope as you are on a simple 100' rappel.

    Generally, what I teach for "long rappels" is that any rappel over 200' for most people will require an adjustment of friction. I usually adjust the friction about every 100 feet. Sqwurel! is adjustable. or Critr or Totem, I am told, are also adjustable "in combat". Otherwise, I use to use a Pirana, left leg loop biner, going to a Z-rig. But heading down a long rap without the skills to add friction tends to be, at the LEAST, uncomfortable.

    Tom

    ps: "About 3 three times as much, as acceleration as it relates to force is not linear in the real world." Uh, no. EXACTLY three times as much. (At the heights we are talking about, within 1 part in 63,000). And acceleration as it relates to force certainly is linear, in the real world. F=ma. That is a linear relationship. You worried about air resistance?, then you are rapping too fast!
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  9. Bootboy

    Bootboy Atwood Gear

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    I amended my post to fix my poorly organized thoughts.

    I should have said that the kinetic energy imparted over those distances would not be linear in free fall because acceleration alone is not linear in the real world. Since rappelling isn't (or at least shouldn't be) free falling, air resistance is not a factor and the point is moot. F = ma is always linear, a alone is not.

    Obviously fatigue plays a much bigger role

    I'm sure the effect of heat build up is negligible, but a factor nonetheless
    Last edited: Mar 15, 2016
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  10. CRNPRES

    CRNPRES

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    Rap out of a bag hanging on your daisy and you can easily understand rope weight below you. The others are hard to show (fatigue and heat) but that does not mean they are not there.
  11. gajslk

    gajslk

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    Me too. And the mental thing isn't necessarily all fatigue. On the first part of the long rap, you get used to the extra friction and when you get to the bottom, it feels too fast in comparison. Add in a little fatigue, and you feel the need to add friction.

    Gordon
  12. Kuenn

    Kuenn

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    Maybe, maybe not. On long rappels, more often than not, I use stainless steel devices. SS does not dissipate heat as well as aluminum. The trade off is, it's "slicker" and doesn't generate friction heat as quickly. With that said, I have experienced this speed phenomenon on both types of material.

    I can buy-in on the fatigue-times-3 theory, with some reservations. I think it does apply, however, I've experienced this so called phenomenon when I'm positive the fatigue factor was not in play.

    So, maybe the take away is, on really long rappels (and I'm talking 300' plus), you better get your stuff together. If hand fatigue is and issue for you at 300', you're going to be toast at 5, 7, 10! (which I realize is probably foreign in canyoneering). Just saying, hand and mind fatigue reduction is not going to be the results of some sort of clever calisthenics.

    The remedy will, however, be found here:
    Ding, ding, ding! Get your friction right, get it where you can adjust it in-flight, and you will eliminate or greatly reduce the fatigues.
    Last edited: Mar 17, 2016
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  13. ScottM

    ScottM Looking for a canyon, you got one?

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    Question: doesn't a wet rope also increase the speed of the descend? Thus, possibly, requiring additional friction?
  14. Brian in SLC

    Brian in SLC Brian in SLC

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    Nope...I don't think so. I think a wet rope has more weight, more mass, more stuff to bend around your rappel device.

    Maybe I missed something above, but, you start out on a 300 foot rappel, and, you have the weight of 300 feet of rope hanging below...which, adds weight and hence, doesn't require as much to hang onto. When you only have 100 feet left of that rappel, the weight below your device is only 100 feet of rope. Less weight, less force on the device, which means you need to change something up to maintain your speed of descent.
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  15. Brian in SLC

    Brian in SLC Brian in SLC

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    And...as Roger used to say...analogue to digital...er something.

    I think one of the differences, besides the rope weight below the device, is, when you're on rappel, you're at a speed, probably a fairly constant rate. So, 200 feet into a 300 foot rappel, to maintain your speed, you need to change your friction. When starting at 100 feet, you have to accelerate to get to speed. Kinetic friction versus static friction. Acceleration, constant speed, deceleration.

    Wonder how much a player in the heat of the device is on friction...? Some, I'd imagine.

    Interesting to optimize device(s), length of drop, size/type of rope. Perfection is a smooth, constant speed. Smooth, long transition to constant speed...then smooth deceleration to a stop at the bottom. If you can change hand position just a little, rope position just a little, can seem sorta seamless. Maybe its similar to making the perfect turn on a ski. Subtle.
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  16. Tom Collins

    Tom Collins

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    Yeah, I think the point the OP was trying to make is that the friction you need 100' from the bottom of a 300' rappel is different from the friction you need at the top of a 100' rappel. In this case the rope weight is actually the same so why is the friction setting different, I think you nailed it with your second post, you're already moving when you hit the 100' point on a 300' so you're trying to keep the acceleration to zero, but on 100' rap you aren't moving at the top so you're friction needed is less since you want to allow some accel.
  17. Rapterman

    Rapterman

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    We have found that having the rope WET on new-ish ropes WILL make them run faster.
    On worn ropes, not so much.
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  18. Bootboy

    Bootboy Atwood Gear

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    I would think that whether or not a certain rope is faster when wet depends on other variables.

    Is it dirty/sandy?
    How worn or fuzzy is the rope?
    How big is the rope?
    Sheath material?

    Sometimes I notice a difference, other times I don't.
    I will tell you that I'd rather have a wet rope 99% of the time.

    I would imagine that being wet effects new, skinny ropes the most.
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  19. ratagonia

    ratagonia

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    Maybe I'm obscurely pulling from a different thread but - what do you mean? YOU determine the speed of the descent, by how you SET your tools. The tools do not determine the speed of the descent.* (and yes, Scott, I know that is not what you meant).

    One thing I like about the Sqwurel is that usually I can set a perfect friction setting, such that I have to use very little "hand" (or "glove") to control the speed. Thus, no hand fatigue.

    :moses:

    *
    “When I use a word,’ Humpty Dumpty said in rather a scornful tone, ‘it means just what I choose it to mean — neither more nor less.’

    ’The question is,’ said Alice, ‘whether you can make words mean so many different things.’

    ’The question is,’ said Humpty Dumpty, ‘which is to be master — that’s all.”


    ― Lewis Carroll, Through the Looking Glass


    (probably a little early in the morning to expect people to make the linkage)
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  20. ScottM

    ScottM Looking for a canyon, you got one?

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    Newer-ish ropes (specifically Imlay) is the experience I'm speaking from.
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