FYI Below are some comments on the cow's tails test that I mentioned recently.
Mark Minton From: Ralph Hartley Sent: Fri 10/17/2008 11:58 AM To: bob.th...@md.net Cc: mmin...@nmhu.edu; nmcaver; txcaver; gvks Subject: Re: (GVKS) - Re: Cow's Tail Tests Bob Thrun wrote: > Mark Minton Wrote: >> Extensive tests on various types of cow's tails have been published >> at <http://british-caving.org.uk/rope/lanyard_tests_v6.pdf>. > > Unrealistic tests with free fall and .a rigid weight loaded in line. > Most rebelays are near a wall, so no free fall. The tests detected > the slight slippage of knots being tightened. the human body has more > give than the knots. Free fall is possible at an overhung rebelay, but you are correct that testing cows tails with a rigid weight is wrong. However there were some interesting points. The main conclusion of the study was that rope with a knot on both ends was superior to *any* manufactured product. This may be true, but as I will discuss below, the differences are actually insignificant. In the summary they say "the concept of Fall Factor does not apply to the Cow's Tails made with one or several knots," Which is about half of the truth. A more accurate statement would be: The concept of Fall Factor does not apply to Cow's Tails *Full* *Stop*. There are two desirable properties in a cows tail: (A) not generating a big shock load. (B) not breaking. They are related, in that a bigger shock load is more likely to cause a break. The big problem with all these tests is that, as actually used, the cows tail is not the main shock absorber. Almost all the energy of the fall is actually absorbed by the harness and the users body. To a very good approximation the cows tail can be considered perfectly static with no stretch at all. So (A) is no concern at all, and (B) is not as likely, since actual peak loads are much less than in the tests. The *real* only criterion is that the cows tail should not break when a person falls on it, even when worn out. Here is the *right* way to test cows tail materials: Cut 1/4 to 1/2 of the strands in the core (or equivalent if it is made of webbing). In a controlled environment, with a separate belay, go as far above the anchor as the cows tail will permit, and allow yourself to fall onto it. You might want to start with shorter falls, the longer ones may hurt. If it breaks, use thicker rope. If it doesn't break, you could use thinner rope, but beyond a certain point there is no real reason to. I suspect that 5mm Spectra/Dyneema would pass this test. It would be useful to record the shock loads during the tests, as they would also be applicable to other parts of the system (e.g. anchors). If humans produce results that are not reproducible enough, or refuse to do the repeated long falls required, a (raw) pig carcass could be used. It wouldn't give exactly the same results, but would be much better than rigid weights. One more thing to note from the report: > The tests carried out in less favourable conditions gave shock loads > well in excess of those that can be sustained by the human body. I'm fairly sure a 2 meter fall onto a completely static belay is survivable, any increase in strength beyond that is all safety margin. The same is true of any load greater than what the body can sustain. The original message was sent to some groups that I am not a member of, you may forward it to them (or anyone else) if you want. Ralph Hartley
