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VT Prusik: slow pull and drop testing

Discussion in 'Tech Tips and Gear' started by Sonny Lawrence, Feb 26, 2018.

  1. Sonny Lawrence

    Sonny Lawrence

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    Paul Stovall and I did some backyard testing on the different ways to tie a Valdotain Tresse; XT vs. VT method. The distinction between these two can be seen here: . Please keep in mind these tests cannot come to any scientific conclusion. They only point to patterns of behavior that may deserve more rigorous investigation.

    Our goal was to see how Bluewater VT Prusiks behaved on 8 mm canyoneering rope tied in the two configurations. For the canyoneering rope, we used pieces of new Imlay polyester 8 mm rope obtained many years ago. They had been stored in my basement having never been used. This is the smallest diameter canyoneering rope commonly used. For the prusiks, we used very old and not so old Bluewater VT Prusiks. These had seen a lot of use, admittedly mostly hanging on a harness while traversing canyons.

    Prusiks (term used generically for various hitches) have many uses. The best known is as a rope grab device to ascend a rope. They are used in rope rescue for a progress capture device, haul cam and load limiter. A prusik such as Valdotain Tresse that is releasable under load may have a great advantage in small party rescue and passing a knot. For canyoneering some critical abilities of this hitch are to function on small diameter rope, hold a heavy load and be released under load.

    Initially, we tied the prusiks and tested them by hand to see if they would “set.” One prusik could not be made to grip the rope despite two people tying it in the two configurations. The prusik on the right could not be pinched very tight and would not grab the 8 mm rope. It could not be used for further testing.

    Slow pull tests: a dynamometer was set to record the force at which the prusik released. The distance it slid before catching was measured.
    1)Tied in XT configuration: it released at 4.98 kN and slid 8.9 cm.
    2)Tied in VT configuration: it released at 4.39 kN and slid 12.7 cm.

    Drop tests: a dynamometer was set to record the peak force experienced by the anchor when the prusik released. A 0.667 kN weight was released while free hanging. An attempt was made to create between a fall factor 1.7 to 2 fall. The distance the prusik slipped was measured. Video can be seen here: .
    1)Tied in XT configuration: peak force was 5.85 kN. Prusik slipped 20.3 cm. The twist at the top of the Valdotain Tresse rearranged to become one of the crosses. Hence at rest, it appeared as though it had been tied in the VT configuration.
    2)Tied in VT configuration: peak force was 4.9 kN. Prusik slipped 52 cm.

    These few tests suggest that some VT Prusiks may function well on very skinny canyoneering ropes. The pinch test may hint at this success or not. The only way to know is to try. The condition of the prusik and rope as to; material made of, wet vs. dry, new vs. old, clean vs. dirty undoubtedly will influence the ability to grip the rope.

    With respect to VT vs. XT configurations, it appears the XT may grip more than the VT. Depending on the intended use of the prusik, this might cause the canyoneer to prefer one over the other. For the situation of ascending a rope, the XT might be better. When used in a potentially dynamic situation as a load limiter, the VT might be safer since the canyoneer would experience a lower peak force.

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    • slow pull test.
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  2. hank moon

    hank moon lovely ligatures

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    Hi Sonny

    Thanks for sharing these tests. A few comments regarding Valdotain Tresse (VT) tying methods, and cordage characteristics:

    Tying method:
    Some VT presenters (e.g. the YouTube video above by EducatedClimber) emphasize the importance of beginning the braids below the initial wraps by passing the "top arm underneath the bottom arm." This instruction is found here and there on the web; however, this detail does not appear in BlueWater's VT Prusik instructions, and few of the many "how to tie" instructions on the web emphasize this point (see image below for "top over bottom" examples). I wonder if this point is important to performance (grab, slide, release, etc.), and also whether all of the knots in your testing were tied this way?

    Cordage characteristics:
    The cords used for BW VT Prusiks have a very soft or "mushy" construction: it is easy to pinch a bight of this material so as to close any gap inside the bight (see image below). At least this is true when the product is new. I recently had the opportunity to handle an Atwood 6.8mm Tech VT and noted that the cord is much firmer than the BW product. So like you, I am curious what effect this "pinch test" might have on performance. Especially if the product becomes stiffer with age. I haven't owned one of these long enough to tell whether that happens or not.

    Do you plan to do any more slow pull or drop testing?

    valdotain-tresse-knot copy.


    8mmVTP.
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  3. Sonny Lawrence

    Sonny Lawrence

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    Hank, I think you are describing the "VT" method of tying the Valdotain Tresse. We looked at it as well as the "XT" configuration.

    I am ordering some 6.8 prusiks from Taylor to test in the future. [One advantage of a cast on my foot is a lot of home time to do this stuff.]
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