A flywheel assembly 10 comprises at least one flywheel mass support 14 that has a shaft 19 that extends along a rotational axis 18 about which the support 14 can rotate in use and a flywheel mass 12. The support 14 comprises a plurality of openings 24 that are each offset from the rotational axis 18. The flywheel mass 12 comprises a plurality of openings 16 that are each arranged to align with a corresponding opening in the support. The flywheel mass 12 is coupled to the support 14 so that the mass 12 can rotate with the support 14 in use. The flywheel mass 12 comprises a plurality of generally planar flywheel mass elements sandwiched together to form a stack of elements by fixing elements 23 that extend through the aligned openings in the mass elements and the support 14.