Maybe you could put the flywheel just aside of rear wheel (between wheel and frame, concentric with the rear wheel, supported by the frame) you could then put some kind of planetary gear system (planets on the hub face, sun gear fixed to frame) to run the flywheel faster than the rear wheel. Of course it would be much more diffucult to make but I think it would be more efficient and you could run a flywheel almost the size of the rear wheel. Just a crazy idea. Anyways good luck with your project.