Sheldon Brown wrote:

See: http://sheldonbrown.com/nanodrive

Sheldon, brilliant idea! We are all growing old waiting for CVT
(continuously variable transmission), and this is almost as good but
built on proven technology.
My question is, I have an old non-indexed 3x7 speed bike. Will I
be able to use my existing shifters and derailers, and just buy
your new chainrings, cassette and chain?
Will there be a cassette to fit narrow dropouts of 6/7 speed frames?