A flywheel rotor used in a flywheel energy storage system providing a high energy storage capacity while providing an ample volume for a high power motor-generator within its envelope includes an outer, primarily cylindrical body having conically tapered end sections, a conical hub section attached to the outer body, and a relatively short inner cylinder, which cylinder connects shaft to the inner portion of the conical hub section. In an exemplary case, the individual components are predominantly constructed of filament wound fiber composites while allowing material choices to be driven by both cost and performance. According to one aspect of the rotor, the inner portion of the inner cylinder can be a slotted aluminum cylinder.