Abstract: A high energy storage flywheel having a rotor and annular disk which are of a composite material. A metal hub is secured to a shaft and joined to the disk at a hoop. The hub has opposite sections which are of a generally double conical shaft. A concentric rim extends around each hub section and upon rotation the hub sections expansivity deform about the rim to apply a compressive force to the disk to maintain a substantially stress free condition at the rotor and disk interface.