Postby Duck! » Tue Aug 13, 2013 9:53 pm
The few times I rode 2x10, I found the chainrings on offer kept me out of the best part of the cassette!
I was an early adopter when Shimano introduced their 10-sp sets, which at that point didn't actually have 2x cranks on offer except in XTR, which well was out of my reach (SLX & XT both got 10-sp. as an add-on to the existing groups, rather than a full series update; I run SLX). The only choice to be made then was cassette size, 11-36 (the default) & 11-34. Conveniently, I fitted Shimano's marketing model of the average rider who spends the vast majority of the time in the middle ring. It can almost be thought of as 1x10, but with escape gears at each end.
When setting my spec, I crunched the gear numbers, thought about the majority of my riding and settled on the tighter cassette. The bigger small ring (24T vs 22T on the old set) and the smaller big ring 42T vs 44) mean I lose a small amount at each extreme from the 3x9 (the default 11-36 cassette virtually equalises the wall climber when compared to 22-34), but the mid-range is amazingly slick. When Shimano reconfigured the cassette range for 10-sp, they didn't just shove the extra cog in somewhere; they took one out & put two in its place, thereby tightening the jumps across a wider spread of the range.
I tried 2x on a a couple of different bikes, one a "trail" bike (Giant Trance 29er), with 24-36 pairing, and a Giant Anthem 29er, more race oriented, so with a 26-38 front pair. Both had the 11-36 cassette. Missing that "perfect" 32T ring, I was hunting for the right gear far more often, always in the cross-chained half of the cassette, and not really helped by the bigger wheels effectively increasing the final gear ratio over my 26er.
Sure 3x has a lot of overlapped gears (the same or very similar ratio can be found in a couple of different places in the cassette), more than 2x, but the most useable part of the range, at least for me, is found in the middle ring, where I can sweep all 10 rear gears. 2x has me hunting around to keep it in the sweet spot. The very small weight penalty of the third ring & bigger derailleurs is a small price to pay for the far greater versatility of the system.
I had a thought, but it got run over as it crossed my mind.