From: gray@rz-berlin.mpg.de
Newsgroups: rec.climbing
Subject: Dodgy belays and fall factors
Date: 20 Aug 92 11:17:14 +0100
Organization: Rechenzentrum MPG Berlin

In article <27322@sophia.inria.fr>, luong@cuba.inria.fr
  (Quang-Tuan Luong) writes:

> In article <1992Aug18.163523.8100@serval.net.wsu.edu>,
  kjones@yoda.eecs.wsu.edu (Kent Jones - EECS) writes:

> |> So my question is: Should I have used only the two points I was pretty sure
> |> to be secure, or should I have spent as long as necessary to find a third 
> |> anchor point?
 
>   If you are *really* sure of its quality, an anchor point is certainly
> enough, after all. A fortiori, if you have two good points, I see absolutely 
> no reason for placing a third point, as a single point would be enough. It
> is a waste of time, and, as illustrated by your story, doesn't add anything
> to your safety, but produces the opposite effect. As a general rule, I try
> to use two points, I sometimes use one, almost never three, even in ice and
> mixed climbing. If you really want to secure the belay, put a piece a few 
> meters above it.

  Damn, Tuan beat me to it. But do people still get taught the concept of
the fall factor?  If not...

 <sententious know-all on>

  Provided that you do not hit any projections while falling, or fall so
far that you hit the ground, the force on the anchor that holds you is
proportional to the *ratio* between how far you fall and the length of
rope between you and the belayer - the "fall factor".  Proof of this is
left as an excercise for the reader.

 <sententious know-all off>

  This has some non-intuitive results, most important of which is that
a 10 foot fall from 5 feet above the belayer (ie you fall past the belay
to hang 5 feet below, having been 5 feet above, fall factor = 2) loads
the belay far more than a 50 foot, head first plunge once you have run
out a full, 150 foot rope lead (fall factor 1/3).  Which of the two is
more likely to require the purchase of a new pair of underpants is a
different question.

  The same analysis leads to the conclusion that belays for a second
can be more slapdash than those for a leader, since a second should
only fall as far as the rope stretches (we are talking theory here).
Thus you can use up pro as you lead, make a less than bomber belay
to bring up the second, and then reinforce it with the cleaned
gear for the next lead (of course you only take this risk if you
have to).

  It also explains why Tuan's last comment is true, a piece of pro a
couple of yards above a dodgy belay immediately reduces the fall
factor.  On the sort of melt-as-you-go ice climbs I used to do when
in the UK, protection would often be logarithmically spaced so as
to minimise time spent placing it whilst keeping the fall factor
at about one.

  As for body belays, they're up there with other injury-inducing
relics of the golden age like groin-brake rapelling, the Whillans
sit harness and the Bergen rucksack.  Great if you don't really
think the belayed is going to fall, and capable of excellent
results (Americans on K2 spring to mind), but I thought the day
of bearded, macho-types comparing bodily scars over a pint of
honest ale had, thankfully, gone the way of all flesh.

  Enough drivel,

Struan  
Newsgroups: rec.climbing