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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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    Exactly my point....adding a wet rope to the equation might introduce additional factors while considering the proper friction.
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  2. Rapterman

    Rapterman

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    Finding these observations and comments to be VERY relevant at this time:
    We are in the process of evaluating two new rappel device prototypes, the CRITR3 and the RABT.
    To ensure that these devices have sufficient range they must run smoothly for both
    Desiree (115 lbs) on a worn and stiff 9.2 mm rope and
    Carl (200 lbs plus pack) on a brand new, shiny, just- outa- the- plastic- bag 8mm rope
    both single strand and double strand and
    enough variable friction to manage a 400 foot drop (four progressive friction settings).
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  3. Deagol

    Deagol too many hobbies

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    Todd,

    You can't just leave us hanging like this (no pun intended). What does RABT stand for ????
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  4. Rapterman

    Rapterman

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    Rappel
    Apparatus
    By
    Todd
    :D
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  5. ScottM

    ScottM Looking for a canyon, you got one?

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    Rappel
    And
    Belay
    Tool
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  6. Kuenn

    Kuenn

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    Which I think translates to rope consistency.
    All dry=good.
    All wet=good.
    A mixture of the two, interspersed with mud and dirt can sometimes equal a frustrating rappel. About the time you get your friction dialed in, then it ain't. [Edit: corrected exaggeration]

    That's a pretty tall order, Todd... Can't wait to try them out!
    Last edited: Mar 18, 2016
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  7. Mike Zampino

    Mike Zampino Canyon season never ends.

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    For crying out loud! I really hate these misspelled acronyms. Could you please name it RABBIT?

    {Rappel Apparatus Built By Incredible Todd}
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  8. Mike Zampino

    Mike Zampino Canyon season never ends.

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    Good point, I think this is a factor often overlooked.
    As a side note: Granted I have less than a dozen 280+ raps under my belt, but I rarely add (device) friction on any rap. I say "device" friction because friction can be added by other means. For me, dropping my brake hand below my hip usually adds just enough additional friction.
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  9. Deagol

    Deagol too many hobbies

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    there does seem to be a rodent theme lately......
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  10. ratagonia

    ratagonia

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    There is a lot of "personal preference" in how much friction people like.

    Tom
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  11. AW~

    AW~

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    Is there a lot of "personal preference" in how much friction people like when these low friction advocates lose control?
  12. Mike Zampino

    Mike Zampino Canyon season never ends.

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    I'm a "just right for me" friction advocate. I guess what I am saying is that I usually have my settings dialed in to where only minor adjustments are required.
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  13. Rapterman

    Rapterman

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    Actually Rick Demarest gets credit for coming up with the RABT acronym :).
    If it ever goes to production he gets a
    FREE RABT!
  14. AW~

    AW~

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    Thats me too...I think of it as the "hands" style. I know the rope can handle a different faster style, but I prefer to set it up where I could go glove free.
    Heat and fatigue are not going to come into play, and I understand thats in line with the device makers.They arent saying to rig the friction setting on low from the getgo. Or that a 600ft rappel with just the device is in order unless you are completely in tune with rappelling. Nor have I ever heard any instructor say to plan on severe impacting fatigue or heat building up as a proper rappel style.

    Now as far as the OP post, I would guess its how the rappel device handles the rope. I dont know about a smooth groove being created...but I do know that my Petzl Stop didnt "stop" in the last 100ft on a 9mm rope I used. Instead there was a very slowww creep. I wondered about the length of the device myself and whether the rope was still giving all the friction.

    But bottom line if you have a rappel device that is setup where you have to fatigue to feed rope through, and on the same setting its screaming eagle death 200ft later, Id get a different device to use. Im understanding "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." .....to mean that in standard mode its too much friction but then when you are 100ft away its too little and have to rope over the top to get the right friction.

    I think the DRT proponents might chime in too at some point.
  15. Rapter

    Rapter

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    This is oversimplying it, I'm sure, but essentially due to rope weight that 9mm rope you started at the top is now a 8.5mm at the bottom of that 200ft rope and perhaps even a 8mm at the bottom of a 300ft rope? Maybe it's even thinner than that.

    That makes sense to a friend I was speaking to about this. A smaller diameter rope is what you are rappelling on after it gets weighted and stretched...

    But maybe this is wrong logical thinking??
  16. Mike Zampino

    Mike Zampino Canyon season never ends.

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    If this was the case, the rope would be skinnier at the top where all the weight is. :thumbsdown:
  17. ratagonia

    ratagonia

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    Perhaps it works as an analogy: your 8mm is LIKE a 9mm at the top of a 300' rappel, LIKE an 8.5 for the middle 100 feet, and LIKE itself (8mm) for the bottom 100 feet, due to the slowing effect of the rope weight hanging below.

    Tom
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  18. gajslk

    gajslk

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    It's certainly not because of rope stretch. The tension on the rope just above the rap device decreases the farther down the drop you get. It decreases by the weight of the rope between the rap device and the anchor. So the rope feeding out of the rap device should get (very)slightly fatter as you progress.

    Gordon
  19. ratagonia

    ratagonia

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    Sorry to revive this train-wreck, but... having read it from the beginning I think we entirely missed Nathan's original point. @nathanslc

    The question as I read it is: "Why does the friction setting for the bottom 100' of a 300' rappel need to be higher than for a simple 100' rappel?"

    This is not the question (I think) most people have been answering.

    My answer is: "mostly, it does not". The point is that with all that rope weight below, the friction setting for the first 100' of that 300' rappel needs to be light to get a good level of control. After 100 feet, with about 5 lbs less rope weight, it will likely be a good idea to add some friction to the system, bringing it up to *medium*. Another 100 feet, another 5 lbs of unavoidable bottom belay removed, and it will likely be a good idea to add some more friction to the system, bringing it up to *regular* - ie, pretty much the same friction you would use on a simple 100' rappel.

    There are additional factors discussed here, that tend to have people add a bit more friction.

    A. Certainly, physical and mental fatigue play a factor. your hand, arm and abs get tired. I consider this likely the main factor.

    B. It is possible that with the device warmer and recently polished, there is less friction in the system. I consider this a very small effect.

    C. Brian mentioned dynamic vs. static friction; but I suspect that the friction does not decrease much with greater rappel speed, in the range we typically rappel, once one is moving along the rope (rather than static). Effect likely zero or very close.

    D. I KNOW that when I fail to provide enough friction on a simple 100' rappel, I can "muscle it out" for the finite experience. usually by chewing through a lot of glove. Or toss it behind my butt or push it into my leg. All these things work for a short period of time, but would not work for a LONG period of time.

    And, as BDC can offer testimony of, setting your device with too much friction such that you have to feed it rope an armful at a time can result in the bounce from each feed causing the rope to half-way cut through. And if it can cut through half way... Very dangerous, don't do this!

    :moses:
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  20. gajslk

    gajslk

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    This would be an easy thing to test with a spring scale ...

    Gordon
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