I was researching recovery rates for regenerative braking on electric vehicles, when U-tube decided it was appropriate to offer me a look at this fellow's bicycle flywheel experiment.
https://www.youtube.com/watch?v=gahKxbwUcYwIt certainly does look like it works, and could even be a quite useful gadget, if one's route consisted of lots of long, straight downhill runs, with busy cross streets at the bottom of the hill. But, the gyro effects and extra cost, complexity, and mass seem to make it impractical for the average rider.
As far as recovery rates for regenerative braking on E-vehicles, my impression is that this feature is not much more at present than a something for salesman to point out, "WE got it, and theirs ain't!" Even highly advanced Tesla isn't saying much more in print than, "it theoretically COULD be as high as, ....". The calculation involves multiplying out all the efficiency factors for the various conversions, chemical / electrical x electrical / mechanical x drive train efficiency, then squaring that, because at BEST, the same efficiency factors apply to recovering the energy. In the real world, I suspect the flywheel would recover a higher percentage of energy than regenerative braking, because motors are optimized to convert electricity to mechanical power, not visa versa, and because the two least efficient energy conversion steps don't apply to flywheels.
Reports of real world experiments with vehicles equipped with the regenerative braking feature were suggesting numbers like 15 -20% had been attained, and that the system is only really practical when one is traversing long downhill runs, where braking is required for safety or to meet legal restrictions. In general, a much better strategy to conserve energy, is to do what comes natural on our human powered vehicles, when faced with an imminent stop, start coasting.
Such a strategy would make one extremely unpopular commuting in a car in an urban setting, one is expected to drive the speed limit, regardless of what lies 100 feet down the road, but there is more freedom for the bicycle. Since I've started riding bicycles again, I find that there are long stretches on my work commute, where a 60+ year old man on a bicycle can, over a distance, easily keep up with vehicle traffic. It's classic turtle and hare, I pass them in their stoplight queues, and they pass me between the lights. Sometimes this happens 3 or 4 times in a row. I was very surprised to find that after only a week of bicycle riding, I was able to match, or beat my automobile commute time. Once I go electric, I think this will become absolutely no contest in favor of the bicycle.