Patent ID: 8376009

Claim:
A pneumatic radial tire comprising: a plurality of belt layers arranged in an outer periphery of a carcass layer in a tread part, an extending direction of cords in each of the belt layers intersecting that of cords in another one of the belt layers; and a belt auxiliary reinforcement layer arranged around the entire outer periphery of the belt layer, and divided into a center belt auxiliary reinforcement layer arranged in the center area of the tire and outer belt auxiliary reinforcement layers arranged in the respective two end areas of the tire, the center belt auxiliary reinforcement layer being formed by helically winding, in a tire circumferential direction, a first organic fiber cord around the outer periphery of the center area of the belt layers, the outer belt auxiliary reinforcement layers being formed by helically winding, in a tire circumferential direction, a second organic fiber cord around the outer periphery of the two end areas of the belt layers, wherein the first organic fiber cord is constituted of a hybrid fiber cord obtained by twisting together a highly-elastic fiber yarn with an elastic modulus of not less than 10000 MPa and a low-elastic fiber yarn made of nylon 46, and the second organic fiber cord differs from said hybrid fiber cord, wherein the outer belt auxiliary reinforcement layers are formed as a single layer, and the number of first twists of the low-elastic fiber yarn is smaller than the number of the first twists of the highly-elastic fiber yarn, and a ratio L/H of a first twist coefficient L of the low-elastic fiber yarn to a first twist coefficient H of the highly-elastic fiber yarn is 0.69 to 0.80, the coefficient L and the coefficient H being respectively expressed with L=T L ×D L 1/2 H=T H ×D H 1/2 where T L denotes the number of first twists (twists/10 cm) of the low-elastic fiber yarn; D L denotes the fineness (dtex) of the low-elastic fiber yarn; T H denotes the number of first twists (twists/10 cm) of the highly-elastic fiber yarn; and D H denotes the fineness (dtex) of the highly-elastic fiber yarn, and wherein the hybrid fiber cord is formed with a three-yarn twisted structure in which two aforementioned highly-elastic fiber yarns and one aforementioned low-elastic fiber yarn are twisted together.