A flywheel assembly is provided which comprises a housing, a flywheel rotatably mounted in the housing and defining an inner and an outer circumferential surface, and an inner body spaced radially and inwardly from the inner circumferential surface of the flywheel. The flywheel rotates in use around and relative to the inner body, the assembly defines an engagement surface spaced radially and outwardly from the outer circumferential surface of the flywheel, and the inner body is flexibly coupled to the housing. If the flywheel mounting fails during rotation at speed leading to displacement of the flywheel, flexure of the coupling as a result of forces exerted on the inner body by the displaced flywheel allows the flywheel to contact the engagement surface.