drpw wrote:Can you explain this one? How does falling on two ropes increase the force rather then falling on one? Say two 8.5mm doubles vs one 10mm single?
To avoid comparing apples to oranges, let's focus on two strands of one type vs. one strand of the same type. This is what happens if you use doubles that are not certified as twins as if they were.
The force on the top anchor depends on how fast the falling climber is arrested by the rope. Simplifying things quite a bit, a rope is a spring, and a stiffer spring produces higher forces. If you put two springs in parallel, you get an equivalent spring which is twice as stiff. Given the same amount of energy to absorb, a spring that is twice as stiff will produce a force which is higher by a factor of square root of two.
Real ropes are not exactly ideal springs, and when all is said and done the arrest force of two strands is actually less than 1.4 times the force applied by one strand. However, it is still higher, and, at least in theory, it may exceed the maximum allowed by the standard (12 kN).
On the other hand, there are several ropes nowadays that have multiple certifications. The Beal Joker is certified as single, half, and twin. The impact force as single (8.0-8.2 kN) is lower than impact force as twin (9.1-9.3 kN), but not by much. The PMI Verglas is an excellent example of light rope that is certified both as half and as twin.
Finally, when comparing impact forces of twins and doubles, remember that they are measured with different masses (80 kg vs. 55 kg). Hence, the values cannot be directly compared.