US Patent 7219578 Flywheel

In a flywheel 2, a central part of a thin plate portion 3 having a flywheel mass provided on an outer peripheral side of the plate portion is joined to an end surface of a crankshaft 1 through a plurality of bolts 7 disposed annularly at intervals. Within a contact zone of a substantially polygon defined by linking each center of the bolts 7 with straight lines excluding the bearing surfaces of the bolts 7, the plate portion 3 has a non-contacting part S2 in a followed fashion set not to contact with the end surface of the crankshaft 1, and the area of the non-contacting part S2 is set to be 40% to 75% of the whole area of the contact zone.