A flywheel energy storage system includes a rotating assembly having a plurality of magnets and a longitudinal axis about which the rotating assembly rotates and static assembly having a stator configured to magnetically interact with the plurality of magnets of the rotating assembly. The rotating assembly includes a rotor back iron supporting the plurality of magnets and disposed further from the longitudinal axis in a radial direction than the plurality of magnets. The back iron being formed of a material having a first stiffness, relative permeability of at least 10, and an electrical conductivity 10% or less than the electrical conductivity of magnetic steel. There is composite structure supporting the rotor back iron and disposed further from the longitudinal axis in a radial direction than the rotor back iron. The composite structure comprises a composite material having a second stiffness, which is greater than the first stiffness.