Abstract: An energy storage rotor (flywheel) used for kinetic energy storage has a unitary metallic hub having a plurality of spokes and a continuous rim integral with the spokes. A composite ring surrounds the rim. The rim is in a tight interference fit with the composite ring when at rest. Portions of the rim between the spokes are adapted to deflect outwardly under the influence of centrifugal force when in use to at least the same extent as the centrifugal force-induced expansion of the composite ring such that a tight fit between the rim and the ring is maintained at high rotor speeds.