An apparatus comprising: a hollow cylindrical flywheel for a motor/generator, the flywheel being formed of a composite material including a matrix material and fibers oriented at least in part in a circumferential direction around the flywheel embedded in the matrix material, the flywheel having a longitudinal axis of rotation, a radially inner surface, a radially outer surface, and a radial thickness between the radially inner surface and the radially outer surface, rotation of the flywheel about the axis of rotation generating hoop stress in the fibers in the circumferential direction and through-thickness stress in the matrix material in the radial direction, the material properties of the fibers and the matrix material being such that rotation of the flywheel about the axis of rotation at a rotational velocity sufficiently high to produce structural failure of the flywheel produces failure of the matrix material in the radial direction and not failure of the fibers in the circumferential direction, such rotational velocity being a first rotational velocity; and a plurality of load masses distributed circumferentially around, and coupled to, the radially inner surface of the flywheel at a longitudinal segment along the axis of rotation, rotation of the flywheel causing each load mass from the plurality of load masses to produce a radially outwardly directed force on the radially inner surface of the flywheel, the force acting to reduce the maximum through-thickness stress in the matrix material such that a rotational velocity sufficiently high to produce structural failure of the flywheel in the longitudinal segment produces failure of the fibers in the circumferential direction and not failure of the matrix material in the radial direction, such rotational velocity being a second rotational velocity, greater than the first rotational velocity. a hollow cylindrical flywheel for a motor/generator, the flywheel being formed of a composite material including a matrix material and fibers oriented at least in part in a circumferential direction around the flywheel embedded in the matrix material, the flywheel having a longitudinal axis of rotation, a radially inner surface, a radially outer surface, and a radial thickness between the radially inner surface and the radially outer surface, rotation of the flywheel about the axis of rotation generating hoop stress in the fibers in the circumferential direction and through-thickness stress in the matrix material in the radial direction, the material properties of the fibers and the matrix material being such that rotation of the flywheel about the axis of rotation at a rotational velocity sufficiently high to produce structural failure of the flywheel produces failure of the matrix material in the radial direction and not failure of the fibers in the circumferential direction, such rotational velocity being a first rotational velocity; and a plurality of load masses distributed circumferentially around, and coupled to, the radially inner surface of the flywheel at a longitudinal segment along the axis of rotation, rotation of the flywheel causing each load mass from the plurality of load masses to produce a radially outwardly directed force on the radially inner surface of the flywheel, the force acting to reduce the maximum through-thickness stress in the matrix material such that a rotational velocity sufficiently high to produce structural failure of the flywheel in the longitudinal segment produces failure of the fibers in the circumferential direction and not failure of the matrix material in the radial direction, such rotational velocity being a second rotational velocity, greater than the first rotational velocity.