Again, a day late & a dollar short I'm sure, but when discussing fall-
factors with cows tails, what scenarios were they thinking of that
could cause a a high fall-factor fall on cows tails? After thinking it
over, in nearly any scenario, (if you're doing it right) you're below,
or at best level with the anchor, when crossing a re-belay, so falling
onto that anchor would never create too much of a fall factor.
The worst-case scenario seems like it would be if you were hanging
from a cows tail attached to a re-belay and the re-belay anchor
failed, and you fell the length of whatever slack there was in the
bight of rope above the re-belay. In this case, it seems like the
possible factors for failure multiply -- what sort of knot was used at
the re-belay? How long was the bight above it? How long is the rope
overall above the rebelay, & how much will it stretch? What sort of
carabiner or quicklink was used to clip into the bolt? Seems like the
cows tail is an insignificant variable in this scenario.
Peripherally related: I know a lot of people use figure 8 knots at re-
belays, but one should consider that a figure 8 deforms when side-
loaded, as it would be if an intermittent anchor failed. There are
"those who say" (I'm one of "them") that a butterfly knot is better to
tie into re-belays, as it does not deform when side-loaded.
As usual, carry on as if I wasn't here.
Chris
On Oct 17, 2008, at 3:23 PM, Stefan Creaser wrote:
Damn, i knew that not reading the latest doc (i read an old version
a while back) would catch up with me :-(
Stefan
From: Minton, Mark [mailto:mmin...@nmhu.edu]
Sent: Friday, October 17, 2008 3:00 PM
To: nmcaver; txcaver; gvks
Cc: Ralph Hartley
Subject: [Texascavers] Re: Cow's Tail Tests
Stefan Creaser said:
>I also don't agree that the cow's tails can be considered static,
made of dynamic rope there is some stretch inherent in there, more
than a static/web harness.
According to the tests, there is indeed a dynamic component to
cow's tails, at least those made of knotted rope rather than sewn
webbing. But it came mostly from tightening and deformation of the
knots, not from stretch. On page 14 they said:
>The results of these tests are all very close to each other, the
standard deviation is only 0.25 kN, while the ropes used are very
different. It seems, therefore, that the tightening of the knots has
a lot more influence on the shock load than the type of rope.
And again on Page 31:
>In order to compare only the types and diameters of rope, we have
produced averages of the results for each type of rope.
>The results are very close (average of 6.01 kN and standard
deviation of only 0.15 kN or 2,5 %) although these ropes display
very different characteristics and have varied diameters.
It made little difference what type or diameter (8 - 11 mm) of
rope was used, although dynamic was of course better than static.
But the length is so short that one wouldn't expect a big effect.
Mark Minton
--
IMPORTANT NOTICE: The contents of this email and any attachments are
confidential and may also be privileged. If you are not the intended
recipient, please notify the sender immediately and do not disclose
the contents to any other person, use it for any purpose, or store
or copy the information in any medium. Thank you.
