Adding tread rubber stock. When the fabric carcass has been built to the proper number of layers and cushion rubber has been added under the tread, the thick, extra tough tread rubber is added, having been molded to the proper shape by extrusion in a previous operation. Mechanical guides help this builder to put the tread on absolutely straight, so the tire will be in perfect balance. Firestone (General Tires), Akron, Ohio