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  1. #31
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    Quote Originally Posted by Mouseskowitz View Post
    ... Your lark's head will be the weak point at ~60%.

    ....masters thesis it is based on....
    Quote Originally Posted by Mouseskowitz View Post
    ...I'm not understanding why a cow hitch derates the rope so much...
    I thought the info I've seen in the various reports we've been citing, stated the cow hitch maintains 85% of MBS....not the 60% you mentioned. However, I now have looked at the above report, on page 148, where it gives the lower %.

    I'll let you contact Samson this time and ask....custserv@samsonrope.com

  2. #32

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    Interesting discussion. Thanks,

    FWIW there was a comment or two about carabiner sizes. I went and pulled out and old "D" style and it is ~7/16 dia or .430 round body cross section. I also checked a newer Black Diamond "D" bent gate an it is ~25/64 (0.39 in) but the cross section is a flat with rounded cheeks yielding a tighter bend over each side of the biner instead of a continuous radius.
    YMMV

    HYOH

    Free advice worth what you paid for it. ;-)

  3. #33
    Needs more Hang time Catavarie's Avatar
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    All you people with your silly science and math. Does no one believe in magic any more these days?
    *Heaven best have trees, because I plan to lounge for eternity.

    Good judgement is the result of experience and experience the result of bad judgement. - Mark Twain

    Trail name: Radar

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  4. #34
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    Quote Originally Posted by Catavarie View Post
    All you people with your silly science and math. Does no one believe in magic any more these days?
    I figure I do. Forget all the science and math, I took one look at 7/64" amsteel and at dynaglide and knew there was no way either one could hold me without magic.

    I just play with the science for sh!ts and giggles.

  5. #35
    Senior Member Mouseskowitz's Avatar
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    Quote Originally Posted by gmcttr View Post
    I thought the info I've seen in the various reports we've been citing, stated the cow hitch maintains 85% of MBS....not the 60% you mentioned. However, I now have looked at the above report, on page 148, where it gives the lower %.

    I'll let you contact Samson this time and ask....custserv@samsonrope.com
    I will drop them a note and see what I can find out.

  6. #36
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    I have a black diamond carabiner as well and was looking at it, I'm not sure that it matters that it's not a circular shape. The part the rope hangs on is 3/8", but then tapers to the back of the biner. But if you look at an eye splice over a pin only the back of the pin sees a lot of stress. the front of the pin sees almost none. since the back of the biner would be toward the sling, no rope would be touching it. Yes it'd be ideal for it to be a perfect circle, but I think the important thing there is that the fibers aren't bent severely.

    I'm really interested in the cow hitch...if that'll maintain 80-85% that'd be enough for me and help me out a lot

  7. #37

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    The issue i have with the shape of the black diamond is that there is a flat with two small redius cheeks. The failure mode they are worrying about is the differential radius of the bend between the inside and the outside. The rope has to elongate more on the outside of the bend than the inside. With a round biner there is a larger diameter single bend to elongate over. With the flat and two cheeks approach it is two smaller "pins" set sid by side so two tight bends. That may be more stretching.
    YMMV

    HYOH

    Free advice worth what you paid for it. ;-)

  8. #38
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    I'm confident all of the climbing rated biners I've seen have a sufficient radius (even at the edges of the flattened or t shaped cross sections) to maintain MBS of amsteel when connected through an eye or loop.

    I am very comfortable hanging from Camp Nano's, one of the lightest climbing rated biners.

  9. #39
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    Quote Originally Posted by Mouseskowitz View Post
    ...Based on the info you received I'm not understanding why a cow hitch derates the rope so much. How is it any different than a pin with a 1:1?...
    Quote Originally Posted by mrjaw14 View Post
    ...I'm really interested in the cow hitch...if that'll maintain 80-85% that'd be enough for me and help me out a lot
    New info from Samson...and I no longer have concerns about the breaking strength of a "cow hitch/larks head/what I now know is a choker". The following is my email and the response from Samson (these people are very helpful).

    "Looks like I’m coming back another time for additional information.

    Having reviewed what I could find on your website, I believed a Cow Hitch in Amsteel Blue maintained 85% MBS, at least when used to connect two rope with end eyes. Then another forum member pointed to Table 54 on page 148 of this thesis paper….http://ir.library.oregonstate.edu/xm...800/Hartter%2c Joel MS.pdf?sequence=1…which indicates only ~60% of MBS. Once again I would appreciate any clarification you could provide on strength when using the Cow Hitch.

    Generally we make the Cow Hitch around a gathered end hammock, through the hammock end channel or around a ring using an end eye. I have attached three photos to show you examples (the second photo even has Zing It in it…another of our favorite products).

    Thanks again,
    "

    The 3 photos....

    IMG_3397 (Medium).JPGIMG_1920 (Medium).JPGIMG_3829 (Medium).JPG

    "Hello ____,

    Looking at the thesis you referenced, testing in Table 53 appears to have been performed on Class I fiber ropes while the 60% value referenced in Table 54 is a cow hitch between two identically sized HMPE lines. Our recommendations for larger diameter lines, in commercial marine mooring applications, is to use AmSteel Blue or other high performance lines cow hitched to polyester or nylon lines of matching strength. Testing of this configuration that has shown strengths in that specified range of 85%. Cow hitching AmSteel Blue to AmSteel Blue is not an effective, or commonly desired, connection to make. At this point, you might as well use a single, longer line rather than connect the two in this manner.

    Based on the pictures you provided however, the termination you use most is a choker. The attached document (hyperlink added), used typically for large lifting slings, shows some strength effect information of AmSteel Blue used in this configuration. The termination strength is based on the angle of the choker, caused in part by the size of the “pin” that the line is secured around.

    Even in the situation shown in your 3rd picture, this closely resembles a choker for a grommet since both legs of the eye are on the ring and contacted by the back of the eye. I would expect this termination, with both legs in the eye affected by the choker, to be as strong as the clear section of rope. (underline added for emphasis)


    Best Regards,

    ____ ______
    Application Engineer

    www.samsonrope.com"

    The Lifting Slings document took me bit of studying to understand, but looks to me like when we "larks head" (single leg choker) to the lashed end of a hammock or through the end channel, to gather and suspend a hammock, it is close to a 120* choke angle (or higher) and would derate the rope no more than to ~70% (Choker WLL / Vertical WLL). (Edit...after additional thought, I suspect this would be well above 70% due to the fact that when we use this configuration, the back of the eye contacts the buried splice rather then the single rope beyond the splice as depicted in the lifting sling document)

    However, if the end eye is long enough that both legs of the eye contact the back of the eye (in choker configuration) as shown in my third photo of the ring, it is effectively a Grommet Sling in choker configuration which maintains a breaking strength slightly greater than the single line minimum breaking strength.

    Either way has sufficient strength for me
    Last edited by gmcttr; 09-04-2013 at 19:32.

  10. #40
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    Quote Originally Posted by HomeMadeHiker View Post
    Still around 7 times the weight of the average dude! Amsteel provides
    plenty of wiggle room.
    Hey, I just wanted to point out that the tension in the rope is related to both the weight of the the load and the angle between the rope and gravity.

    Imagine a piece of rope tied to two poles that are right next to each other with the middle of the rope hanging straight down (0). If you hang a mass from the middle of the rope, the tension in the lines will be 1/2 of the weight. Thus a 100lb weight will produce 50lbs of tension. (If this seems counter-intuitive it's because there are two strands of the rope, each carrying 1/2 of the load).

    Now imagine the ropes are further apart such that the angle between the the rope and gravity is 60. Now the tension in the lines will be 1x the weight, so a 100lb weight will produce 100lbs of tension.

    At about 75, the tension is roughly 2x: a 100lb load produces 200lbs of tension.

    At 83 the tension is about 4x: a 100lb load produces 400lbs of tension.

    After 85 things start to increase really, really quickly.

    86: 7.2 x LOAD
    87: 9.5 x LOAD
    88: 14 x LOAD (Amsteel would break from a 100lb weight at this angle)
    89: 30 x LOAD

    And a perfect 90 is impossible, since it would have a theoretically infinite load under it's own weight.

    That's also just under impossibly static conditions. Dynamic weight changes from moving around or getting in and out can easily create momentary loads 2x to 3x your weight, which would then need to be multiplied by the rope angle factor.

    I weigh about 150lbs and hang my bridge hammocks at probably an 80 angle, so I could easily be seeing 450lbs on my suspension when I'm hanging there motionless, and peaking around 800lbs to 900lbs when I'm getting in and out.

    I have a dynaglide suspension, which has a breaking strength of 1000lbs without factoring in any weakening from the splice. Because I have two lines on each side, my suspension will break at a bit before 2000lbs of tension.

    This gives me about a 2x safety factor, which is much less than you would expect for a 150lb man on four 1000lb lines.

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