32 is 25 [turns/10 cm] to 70 [turns/10 cm], it is easy to adjust the load at 2% elongation and the load at 5% elongation of the organic fiber cord 32 to be described later within a predetermined range.

Here, B [N/inch] is defined as the product of the load at 2% elongation per organic fiber cord 32 (LASE 2%) [N/cord] and the number of organic fiber cords implanted in the belt reinforcing ply 31 [cord/inch]. C [N/inch] is defined as the product of the load at 5% elongation per organic fiber cord 32 (LASE 5%) [N/cord] and the number of organic fiber cords implanted in the belt reinforcing ply 31 [cord/inch]. In this case, A [N/inch], B [N/inch], and C [N/inch] described above satisfy the relationships represented by the following Expression (I) and Expression (II), A [N/inch] being the bending load per unit width of a sheet of belt ply 21.

B
/
A

<
400

Expression
⁢

(
I
)

600
<

C
/
A

<
2000

Expression
⁢

(
II
)

(B/A) set to less than 400 appropriately balances the bending rigidity of the belt reinforcing ply 31 and the restraining force of the belt reinforcing layer 30, and allows for achieving both high-speed durability and high driving stability of the tire. In other words, (B/A) of 400 or more, for example, causes the restraining force of the belt reinforcing layer 30 to be too large, resulting in reduced ground contact area of the tire and deteriorated driving stability. (B/A) just needs to be less than 400, preferably less than 350, and more preferably less than 300. The lower limit of (B/A) is 50, for example. When (B/A) is 50 or less, the bending rigidity of the belt ply 21 tends to increase. This may reduce the ground contact area of the tire, and deteriorate driving stability. Therefore, the relationship may be 50<B/A<400, preferably 50<B/A<350, and more preferably 50<B/A<300.

Furthermore, (C/A) exceeding 600 ensures the restraining force of the belt reinforcing layer 30, and can ensure high-speed durability of the tire while improving driving stability. (C/A) just needs to be more than 600, preferably more than 700, and more preferably more than 800. In addition, (C/A) set to less than 2000 appropriately balances the bending rigidity of the belt reinforcing ply 31 and the restraining force of the belt reinforcing layer 30, and allows for achieving both high-speed durability and high driving stability of the tire. In other words, (C/A) of 2000 or more, for example, causes the restraining force of the belt reinforcing layer 30 to be too large, resulting in reduced ground contact area and deteriorated driving stability of the tire. (C/A) just needs to be less than 2,000, but is preferably less than 1,600, and more preferably less than 1,200. Therefore, the relationship is preferably 700<C/A<1600, and more preferably 800<C/A<1200. In addition, the relationship may be 600<C/A<1600, or 600<C/A<1200. It may also be 700<C/A<2000, or 800<C/A<2000.

In this description, the load at 2% elongation per organic fiber cord [N/cord] and the load at 5% elongation per organic fiber cord [N/cord] respectively mean the load at 2% elongation and the load at 5% elongation when a tensile test is conducted in accordance with JIS L1017 in a test room under standard conditions of a temperature of 20±2° C. and a relative humidity of 65±4%, as specified in JIS L0105.

The above B [N/inch] just needs to be within a range that satisfies the above Expression (I), but is preferably 400 or more, and more preferably 500 or more. B [N/inch] of less than 400 may make it difficult to maintain the tire shape and deteriorate driving stability. Furthermore, B [N/inch] is preferably 1000 or less, and more preferably 800 or less. B [N/inch] exceeding 1000 may cause the restraining force of the belt reinforcing layer 30 to be too large, resulting in reduced ground contact area of the tire and deteriorated driving stability. Therefore, B [N/inch] is preferably 400 or more and 1000 or less, and more preferably 500 or more and 800 or less.

The above C [N/inch] just needs to be in a range that satisfies the above Expression (II), but is preferably 1400 or more, and more preferably 1800 or more. C [N/inch] set to 1400 or more makes it easy to ensure high-speed durability. C [N/inch] is preferably 3000 or less, and more preferably 2600 or less. C [N/inch] set to 3000 or less allows for ensuring the ground contact area of the tire, thereby improving driving stability. Therefore, C [N/inch] is preferably 1400 or more and 3000 or less, and more preferably 1800 or more and 2600 or less.

The load (N) at 2% elongation and the load (N) at 5% elongation of each organic fiber cord 32 can be adjusted, for example, by selecting the type of fiber forming the organic fiber cord 32, adjusting the cord structure, the number of twists, the cord processing conditions, etc. For example, reducing the number of twists can increase the load at an elongation. Additionally, the cord processing conditions include the conditions of the dip processing in which the organic fiber cord 32 is immersed in a resin liquid for adhesion processing with rubber (resin liquid blend, processing temperature, tension, time, etc.). For example, when dip processing is performed using a resin liquid such as resorcin-formaldehyde-latex (RFL) or a blocked isocyanate aqueous solution, it is possible to increase the load at an elongation if a low-temperature bath is used and the tension applied to the organic fiber cord 32 is set high.

EXAMPLE

The following shows examples, but the present invention is not limited to these examples.

Steel cords and organic fiber cords are produced, each of which has a structure shown in Table 1 described later. The produced steel cords are used for belt ply, and the produced organic fiber cords are used for the belt reinforcing ply. Then, as shown in FIG. 1, the belt ply is formed of two sheets of ply, the belt reinforcing ply is formed of a sheet of ply, and a radial tire having a tire size of 205/50R16 87V is vulcanized and molded in the usual manner. The configuration of each tire is the same except for the belt layer and the belt reinforcing layer. The two sheets of belt ply are arranged so that the belt angles are +25° and −25°.

The tire weight, high-speed durability, and driving stability of the produced pneumatic tires are evaluated by the following methods.

Tire weight is the total weight of one tire, and is expressed as an index with the total weight of the tire of Comparative Example 1 being 100. This means that a smaller number equates to a lower tire weight.

<High-Speed Durability>