Originally Posted by
fietsbob
Some Electric hub motor bikes can do some, of the regenerative effect , the motor, turns generator.
anything else is approaching snake oil .. free lunch ....
Electrical systems store engergy in a battery or capacitor. Purely mechanical systems can store energy in a number of ways, but flywheels and spring are common. The question is can a practical system that works well enough to be desirable be developed, and if desirable, is the market big enough to to justify manufacturing and marketing it. .