A composite disc to be made using equal masses of aluminimum and iron so that it has as high a moment of inertia as possible. This possible when. 2002 The surfaces of the discs are made of iron with aluminium inside The whole of aluminum is kept in the core and the iron at the outer rim of the disc The whole of the iron is kept in the core and the aluminium at the outer rim of the disc The whole disc is made with thin alternate sheets to iron and aluminimum.