papounet
May 15, 2007, 10:53 PM
Views: 5933
Registered: May 28, 2003
Posts: 471
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The formula for the fall factor leads to a very simple relation, which can be graphed.
Looking at actual values, I can't help think that the value of 55kg was the single strand of a double rope was not chosen randomly.
the numbers for ropes of good performance at the time the UIAA label was conceived could well have been very close to
6,2 kN with 55kg 1 strand (maximm allowed 8kN)
10 kN with 80 kg 2 strands (maximum allowed 12kN)
which would have fit one rope of modulus 16000.
in other words; 55kg is not a magic number, it is the number that one could have found by/
1. testing an acceptable double rope in twins mode with 80kg,
2. computing for a rope of half the modulus, the equivalent weight that would give as much %safety margin vs. the standard
3. validating with a few drop tests one strand of the twin, hops wou have a standard for double
In more than a way, testing 55kg on one strand and 80 kg on two strands are tests which are very close.
YET
If ropes were springs,
Doubling up the rope AND increasing the weight from 55kg to 80kg would increase the force by 69%
using Joker values, we find that 9,3/5,8=1,6 => 60% increase
A marginal double that passes with 8Kn on 50kg one strand would most probably not pass the twins test with 80 kg two strands as it would deliver 13,5 Kn
on the contrary a marginal twin delivering 12 kN on 80kg 2 strands would deliver 7,1 kN on 55kg one strand
the general consensus is that:
double can be used as double or twins, twins only as twins.
The maths show that (under some assumptions regarding stability of rope modulus).
some double would not be certified as twins
all twins would be certifiables as doubles
The onlw difference then is abrasion and sharp edge resistance, and of course wether or not can a leader guide 2 seconds one on each strand ,
The outcome: when choosing a double, choose one with force below 7,1kN. in the beal catalogue, no double go beyond 5,8 kN =>
As a point of comparison
Beal Ice twin 7,7mm, 7,3kN(average) would probably give something like 4,6 Kn if tested as a double
Beal Ice Line 8,1 mm 4,9kN (average), would probably give something like 7,9 kN if tested as a twin
(This post was edited by papounet on May 15, 2007, 11:17 PM)
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