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@bobjonkman @sam @ascentale @moira I mean, that could be okay, *IF* the magic doodad is cheap enough for the target market *AND* you can just drop it in like a freewheel cassette, that's pretty cool.

A little shop could keep a pile of doodads and a pile of chains on hand and do conversions with only hand tools. The double chain thing is weird but it does allow for different ratios by fitting different chain rings, and unlike, eg: Sturmey-Archer or Shimano Nexus 3-speeds it doesn't need any torque reaction from the dropouts.

(It occurs to me, though, that any frame with dropouts wide enough for a freewheel / this thing is going to have the wrong kind of dropouts for chain tensioning, and also how do you tension both chains at once, unless you're just lucky I suppose?)

Anyway: It'd be a nice problem to solve, because there's a shitload of old bikes out there with decent frames and reasonable wheels, and there are charities already which ship these to developing countries where they can be turned into solid bikes *but* most of their derailleurs were a mess already and the rest are bent because that's what you get when you make a big pile of bikes at the municipal tip.

Bicycles are basically free. Useful bicycles are a bit more ...