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The NuVinci® N360 is a continuous variable transmission that is roller based. It is marketed and used mainly on hybrid and low maintenance city bikes. They must have had some market success as they have been around since 2006. I recall seeing them at Interbike around that time. More info in the internal structure
There have been designs, but cyclists tend to be picky about increases in weight or losses in efficiency. Bicycles do not have the problems of large spinning masses or rough clutching at low speed, and we seem to do OK with discrete steps in the gear ratio.
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