Abstract: A tire building drum for assembling and shaping green tire carcasses toroidal includes a central drum and side drums with membranes capable of being blown up to shape the tire carcass. Supporting rings are located on opposite sides of the central drum. The invention is characterized in that the core supporting ring consists of an elastomeric material with a high E-modulus and is provided on the inner side with a disk-shaped flange supported in a groove, the flange having concentric collapsible grooves and being capable of extending radially under air pressure.