I'm building that very sort of thing right now. SS, not FG. There's a thread about it here: A Single Barbarian... MTB SS retrofit

Basically two chainrings and two cogs, that take the same chain. Both are magic gear ratios. No derailers or tensioners. It's called a dinglespeed. A bit easier than a flip flop because you don't have to remove and flip the wheel. Just loosen the QR unseat the wheel, move the chain, and reseat and set the QR.

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