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requirements

I want a shifting mechanism that is: Fortunately, there is another vehicle that has requirements very similar to this:

MOTORCYCLES

OMG, you're crazy. Aren't transmissions heavy, and require a clutch?

Well, it would be smaller transmission than motorcycles have, since there's much less power being transmitted. Perhaps the size of a Rohloff Speedhub.

No, it doesn't require a clutch. No bicycle transmission requires a clutch, because the "engine" (your legs) is able to back off on the power before shifting, which accomplishes the same thing.

Where would it be located? There's no room on the chainstays.

Same place as on a motorcycle — inside of the bicycle's main triangle.

Granted, this WILL add a fair bit of weight. You need the transmission mount to be very rigid, to avoid losing power through the chain. So this requires welding two more tubes on the bike.

does it have to be a one-off design? or is there a COTS solution?