In reply to jimtitt:
> Hmm, I though climbing instructors were supposed to keep up with developments in climbing equipment, the Beal Joker which is triple rated has been out for years
...I mentioned I was ignoring it in my first post
Yes, the Joker has an increase in impact force of ~14% when comparing single and twin use, which is the best comparison for us to make (I just spent a few minutes wondering about the 60% discrepancy between half and twin, but of course they're tested with a different mass).
I'm not sure we can justify extrapolating that 14%, though. That assumes all ropes scale in the same manner, and they clearly don't. Take, as an example, the popular Beal Cobra II - they have an impact force of 5.1kN as halves. That's about 10% lower than the Joker as a half rope (5.7kN).
If all ropes scaled in the same way, we'd expect their impact force as twins to also be about 10% lower than the Joker, which would imply a value of about 8.3kN. But they don't qualify as twins, which presumably means that their impact force as twins is 12kN or greater (that's pretty high given a #1 wire is rated to about 10kN). Unless there is another reason the Cobra is excluded from having a twin rating*, our presumption of scaling is out.
Of course, there's also a non-linear relationship between UIAA impact force and actual forces generated in real-life situations. I suspect that, without access to a drop-test rig, the question is too complicated to figure out on paper.
re that email from Mammut: I'm genuinely surprised by it. Has any other manufacturer claimed this, I wonder? Every thread I look at on the question sooner or later comes back to a reference to that one email from 2011. If it is an urban myth, it's well-established enough that instructors far more qualified than I keep repeating it and writing it in books.
* is there? I don't know.