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I think it was either you or some internet engineering quote that stated that a chain drive should use minimum a 17 teeth sprocket. Efficiency should drop a lot below that.
svenhöek-
There's 'discussions' on a Facebook track cycling group all the time about whether big-big or small-small is better. As best anyone can work out the minuscule efficiency savings from having the chain take a smoother curve round bigger sprockets is outweighed by the chain moving faster.
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chain drive should use minimum a 17 teeth sprocket. Efficiency should drop a lot below that
There's no hard cut-off, but efficiency fall off with reducing sprocket tooth count is non-linear, so you're getting onto a steep part of the curve somewhere in the low teens. Conversely, you're on the shallow slope pretty soon, so the gain from going from 48/12 to 60/15 is much bigger than the gain from 60/15 to 72/18.
All of the numbers are in marginal gains territory, if you get your chain set up properly you're talking about something in the region of 1W between 12 and 15 and another 500mW from 15 to 18 for crank speed of 100rpm.
Sumo
gbj_tester
If we copy Team GB from the Olympics and start using big sprockets, we won't need centerlock anyway, 6-bolt goes down to 16T easily and 15T at a push.