The invention is directed to radially collapsible tire building drum assembly which is provided with heavy undercut bead assemblies for the fabrication of tire carcasses, particularly those of multiple bead construction as utilized in high profile tires such as required for heavy trucks, earth movers, and aircraft. The foldable bead ring assembly of the present invention is provided with a plurality of circumferential or arcuate ring segments each supported by a drum segment. In the preferred embodiment, alternately disposed ring segments are provided with pivotable end sections which are pivoted out of the way during the drum collapsing operation to provide recesses for receiving outer end portions of adjacent ring segments. Simultaneously, with this reception of the outer end portions of the adjacent ring segments in the recesses the pivoted end sections are received in receptacles behind the adjacent bead ring segments. This intermeshing or folding of the ring segments during the radial collapse of the drum enables the drum to be radially displaced into a fully collapsed state while retaining the bead rings fixedly attached to the drum assembly.