Abstract: Rubber compositions are disclosed which exhibit enhanced dynamic modulus and tensile strength characteristics. A selected type of carbon black, resorcinsol and/or its precondensate and hexamethoxymethyl melamine are combined in specified amounts with a selected class of starting rubbers. The carbon black has specified nitrogen adsorption surface area, dibutyl phthalate adsorptivity and Dst mode of diameter.

Abstract: Pneumatic radial tires are disclosed which are comprised of two or more belt plies reinforced respectively with an aramid fiber cord and a steel wire cord. Both cords are coated with one and the same rubber composition made up of a selected class of starting rubbers, sulfer, resorcinol and/or its precondensate, hexamethoxymethylmelamine and a selected class of cobalt compounds derived from organic acids. The belt plies excel in water-resistant adhesion and heat resistance and thus have freedom from ply separation, contributing to a radial tire of improved durability.