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  1. #101
    Frawg's Avatar
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    Quote Originally Posted by TeeDee View Post
    No - the experiments we performed had nothing to do with slipping under the load due to the load itself. An agent or force exterior to the system can induce the UCR constrictor section to loosen and when that happens it will not grab again - it will fail totally.
    I'm gonna respectfully disagree here. I'm pretty sure I have a handle on the heavy / light load condition implications and the mechanics of what happens. I see a positive feedback mechanism, whereby any friction in the constrictor induces a radial constriction which in turn induces more friction. The dynamic coefficient of friction applies until enough of the constrictor is engaged to counter the applied load, at which point motion will slow and ultimately cease. The static coefficient of friction, which is always greater than the dynamic one, then applies and holds the constrictor in place. Certainly, there has to be enough length of constrictor to ensure that the total friction generated can overcome the maximum dynamic force applied. If friction does get distributed exponentially, per my guess below, then that necessary length will increase only as the logarithm of the applied force.

    We even had the buried tail pulled through the constrictor section under the dynamic forces introduced. Can it happen? The probability is probably low, but definitely not zero. How low? Totally unknown and probably unknowable a priori.
    Sounds to me like your constrictor was too short, relatively speaking. Again, I will respectfully disagree with your conclusion. I need to research the physics more -- and I may well be wrong here -- but I'm inclined to think that in stasis the force distributes as a decreasing exponential for the UCR, and as a hyperbolic cosine for the whoopie sling. The transition from slipping to stasis will be mathematically interesting because the coefficient of friction will be changing dynamically as well (nonlinear DEs?) . But it really depends on the length of the constrictor section as well as the friction coefficients and the dynamic forces applied -- perhaps not precisely knowable, but certainly boundable so that one could engineer any desired margin of assurance.

    Would the rolling hitch help? unknown without a lot more experimentation.
    Well, I have been experimenting, and it does appear to hold -- I've never had it slip -- but its contribution to the UCR is not the mechanism I initially anticipated.

    My guess, based on a lot of experiment with friction knots in a suspension system, is no unless the rolling hitch is initially holding the full force exerted on the UCR, which I have found to not be possible. Once the constrictor section is induced to fail, the motion and dynamic forces will simply be more than a rolling hitch could be expected to hold. Just my guess though.
    That was my initial thinking... that the friction hitch, of whatever stripe, would have to take up the friction deficit, as I called it in an earlier post. That wasn't the case, and its contribution turns out to be different. IME, the hitch acts as a drag, ensuring that any motion induces some initial radial compression in the constrictor. Once that compression begins, the positive feedback of friction induces more compression, ultimately forcing the constrictor to grab. In stasis, the force ends up distributed exponentially, IMHO, until released by an external agency. By inducing even a small amount of drag, the rolling hitch provides insurance against the unseen hand implicitly posited in your non-zero probability estimate. Anyway, that's what my experiments to date have led me to believe. I'll certainly defer to the physicists and MEs in the group on this.

    For my part, I'd say let's keep thinking, researching, experimenting and documenting. An ounce of data is worth a pound of speculation, and I've contributed more than my share of the latter. I don't think I'm that far out in left field, but it'd be much appreciated if someone in the know would kindly point me toward a reference on the precise physics of braided rope deformation.
    Last edited by Frawg; 07-09-2009 at 21:48. Reason: fear of the spelling police
    - Frawg

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  2. #102
    Downhill Trucker's Avatar
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    I'm really interested in the whoopie sling or UCR suspension systems. I am a little lost on how they work and if it is something I want to fool around with.

    Currently, I use the ring buckle suspension with carabiners. I love the ease of initial setup and the adjustability of this system. To me the only drawback is weight as I have long straps to accomodate my trips out west (big trees) and biners cause it's just so darned easy.

    Are either of these systems lighter than the ring buckle setup?
    Are they adjustable like the ring buckle system?
    Are they easy to setup like my current system (which involves clip biner, adjust ring buckle, and throw in a half hitch)?

    Thanks for all of the info here. It's just a bit hard to understand because it's so much different than the other systems I've learned about. I do want to learn (as others do) because it seems this is the system that is on the forefront of hammock technology (whoopie and UCR).

  3. #103
    Senior Member amac's Avatar
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    Downhill Trucker,
    Yes, they are lighter than the ring buckle system, adjustable, and easy to setup, but I think the ring buckle is a little easier. I just recently switched from the standard ring buckle system to a UCR. My ring buckle system consisted of 2 Neutrino carabiners, 4 SMC rings, and 2 lengths of the 14' webbing that comes with a WBBB. The complete hammock + suspension system weighed ~40 oz. I just converted to a UCR using 2 14' lengths 7/64 Amsteel, 2 5' lengths of 1" webbing, and 2 tent stakes (for marlin hitch). My complete system (hammock + suspension) now weighs ~36 oz. Saving roughly 4 oz. I could probably save another ounce or two by using lighter webbing (I did not cut my WBBB webbing) and lighter pins for the marlin hitch.
    Last edited by amac; 07-10-2009 at 07:40.
    "Every minute outside ... is a good minute!" -> Calvin & Hobbes, 8/1/1993

  4. #104
    Frawg's Avatar
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    Quote Originally Posted by Frawg View Post
    ... I'm inclined to think that in stasis the force distributes as a decreasing exponential for the UCR ...
    It does indeed. It's analogous to the 'rope around a post' problem in which the Euler friction from multiple turns of a rope around a post allows a small force at one end of a rope to counter a larger force on the other end. The solution to that problem is well known and the smaller force, f, is of the general form f = F exp(-ks) where F is the larger force, k is a friction constant and s is the arc length of the multiple turns around the pole.

    If you (mentally) unwind the skin of the post & rope together, and then wrap the skin around the rope, you can see that it's the UCR scenario (with a different value for k).

    If the constriction section is long enough, a securing friction hitch would be unnecessary -- the simple force of gravity acting on the free end of the constrictor section would suffice to hold the force at the other end of the constrictor.

    ... and as a hyperbolic cosine for the whoopie sling.
    ... with the minimum point offset from the standing end, in the direction of the adjustable eye. It will be the sum of two exponentials (having the same decay constant!) oriented in opposite directions, which is the generic form of a hyperbolic cosine: cosh (ax) = [exp(-ax) + exp(ax)]/2.

    IMHO.

    Sorry for the digression; had to get it out of my system.
    - Frawg

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  5. #105
    GrizzlyAdams's Avatar
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    Suspension math

    Now we're talking! Bring it on...

    Grizz

  6. #106
    Frawg's Avatar
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    Quote Originally Posted by GrizzlyAdams View Post
    Now we're talking! Bring it on...

    Grizz
    God said:



    ...... and there was light!

    But I digress yet further!
    - Frawg

    {generic tagline}

  7. #107
    GrizzlyAdams's Avatar
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    Quote Originally Posted by Frawg View Post
    God said:



    ...... and there was light!

    But I digress yet further!
    had that T-shirt in college
    you can bet I was a hit with the girls

    Grizz

  8. #108
    Senior Member NCPatrick's Avatar
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    Can you explain it for the math morons in the group? (speaking for myself) Or is it too complicated?


    "Civilization is the limitless multiplication of unnecessary necessities."
    - Mark Twain
    I go to nature to be soothed and healed, and to have my senses put in order.
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  9. #109
    GrizzlyAdams's Avatar
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    Maxwell's Equations

    you don't want to know the details, but these are known as "Maxwell's Equations", which describe properties of electricity and magnetism. They show that light can be viewed as an electromagnetic wave. Hence, "and there was light!"

    and here's the T-shirt!

    Grizz

  10. #110
    Frawg's Avatar
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    Quote Originally Posted by GrizzlyAdams View Post
    ... and here's the T-shirt!
    The image in my post also links to a song parody.
    - Frawg

    {generic tagline}

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