Ah. I thought you were talking about flywheels, but without seeing that common word, I was confused and unsure that you were in fact talking about them.
User R.I.D.E. has talked about various forms of flywheels numerous times, such as:
http://www.gassavers.org/showthread....489#post125489
I was just thinking, what about an in-wheel flywheel? It could be extremely simple, already has a scatter shield (the wheel), there'd be no gyroscope issues (I think), and an axle is already right there. It could use the existing disc brake system as [part of] its clutch, or it could use off-the-shelf brake components kept separate from the brakes. Maybe it could use some kind of hydraulic drive instead of a clutch, but then it might as well be R.I.D.E.'s hyraulic drive instead and move the flywheel elsewhere.