Originally Posted by
BririaBoarder
Back in the early '70s, when "ten speeds" were ubiquitous, the lore was that using the highest or lowest gears would cause the chain to fall off/get goofed up, therefore those gears weren't practical to use. (I don't remember the precise details of this bit of urban legend.)
Was that ever true?
You can have an uncommanded downshift to a smaller ring when using a larger ring with the largest cog. Modern rings with truncated teeth in down shift zones make shifting easier, and a little wear combined with short chain stays can make the chain angle sufficient for that without any front derailleur involvement. That can cause a crash if you're standing at the time. I did that twice before realizing what was going on.
I never noticed a problem when 12 speed meant 2x6 not 12 cogs, although we didn't have tweaked tooth profiles, the cog sets were narrower, and chain stays longer decreasing the angle.
Smaller ring x small cog is noisier and may rub on the larger ring or derailleur. Efficiency doesn't decrease much but the sound can be annoying.