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im guessing thats just two chainsets with the same ratio, but has anyone ever made a double chainset where you can engage/disengage either side while riding.

you could have a low side for accleration and a big on the other for max speed.

Or... you could put several different sizes of sprocket on one side of the hub, and have some sort of sprung jockey wheel mechanism that derails the chain to move it from one sprocket to the next. You could probably fit, ooh 9 or 10 sprockets on the back, all with different ratios. It would be great for acceleration and hills.

I wonder whether anyone's ever done it?