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by pitaj 1565 days ago
> although the back and forth motion of agitation isn't something I'd know how to do

A smaller wheel linked to a larger wheel, where the smaller wheel is driven by the bike. As the smaller wheel spins, the linkage will move back and forth. If the second wheel is sufficiently larger, it won't be rotated enough on the push stroke for the pull stroke to complete a rotation, and will oscillate instead.

https://imgur.com/B1ykmpB