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 .