A flywheel comprises an annular body (13) which is arranged on a rotatably mounted metallic shaft (10). The shaft has at least one hollow shaft part (12) having an outer surface part coextending or diverging outwardly in relation to the axis of the shaft and defining a surface of revolution, which is coaxial with the shaft. The hollow shaft part (12) is interconnected with the annular body 913) so as to allow radial expansion of the latter under the influence of centrifugal forces. At least part of the outer surface part of the hollow shaft part (12) is backed up by generally peripherally extending reinforcing fibers (14) so as to allow the flywheel to rotate at an increased rotational speed.