Originally Posted by
sincos
I still don't see how that's possible -- the chainline will tell you to back off when you get close,
Why would chain-line necessarily tell you anything? I could easily shift into my big big combo on my old 3x9 (not that I did so intentionally very often).
Originally Posted by
sincos
so will resistance if you don't have the capacity.
Resistance in the pedaling may increase (sort of, but it will likely be too late) but not in the shifting. If you have the capacity for your 2nd biggest cog, but not your first, then you will shift into the 2nd biggest no problem. And you will most likely START to shift into the biggest cog with no problem, as you are barely using any more slack. The problem comes as the chain on the larger cog starts making its way around the cassette, slowly taking up slack as it goes. Once it reaches the point where there is no more slack, something has to give. And the problem is that at that point when you are pedaling, you have a LOT of leverage against the chain stretching.
I am less certain about what happens if the mis-shift is in the front (e.g., you are in the big cog, then shift from the middle ring to big ring). But again, you may well have the little bit if slack needed to get the chain STARTED on the big ring.... but then run out as the chain on the big cog makes its way around.
OTOH, shifting to to small a ring/cog combo usually just means that the chain goes slack. Worst case is that you drop the chain or it gets jammed somewhere.
Thus, if you need to lose combos, always lose the ones on the small cog.