radial pneumatic tires

Abstract

Pneumatic radial tire, comprising: a tread portion (1) extending in a tire circumferential direction and having a ring shape; a pair of sidewall portions (2) arranged on both sides of the tread portion (1); and a pair of bead portions (3) arranged on an inner side of the sidewall portions (2) in a tire radial direction, the pneumatic radial tire comprises: a carcass layer (4) mounted between a pair of bead portions (3); a plurality of belt layers (7) arranged on an outer periphery of the carcass layer (4) in the tread portion (1); and a belt cover layer (8) arranged on an outer peripheral side of the belt layers (7), wherein the belt layers (7) are formed of steel cords (7C) in an N+M structure in which the number of twisted wire strands N of an inner layer is 2 to 4 and the number of twisted wire strands M of an outer layer is 2 to 7, and wherein a twisting direction of the inner layer is different from a twisting direction of the outer layer, the steel cords (7C) being arranged inclined with respect to the tire circumferential direction to intersect each other in layers of the belt layers (7), wherein the belt cover layer (8) is formed of organic fiber cords having an elongation of 2.0% to 4.0% under a load of 2.0 cN / dtex, the organic fiber cords being spirally wound along the tire circumferential direction; where a steel cord quantity A, which is defined as the product of a cross-sectional area S [mm 2] of the steel cords (7C) and a cord count E of the steel cords (7C) per 50 mm width perpendicular to a longitudinal direction of the steel cords (7C) is within a range of 6.0 to 9.0; and wherein a ratio P2 / P1 of a twist pitch P2 of the outer layer to a twist pitch P1 of the inner layer of the belt cords is 1.0 or less.

Description

Technical field

[0001] The present invention relates to a pneumatic radial tire provided with a belt cover layer formed of organic fiber cords, and more particularly relates to a pneumatic radial tire capable of effectively reducing road noise. State of the art

[0002] In pneumatic radial tires for passenger cars or light trucks, a carcass layer is mounted between a pair of bead portions, a plurality of belt layers are arranged on an outer peripheral side of the carcass layer in a tread portion, and a belt cover layer including a plurality of organic fiber cords spirally wound along a tire circumferential direction is arranged on an outer peripheral side of the belt layer. Such a belt cover layer contributes to improving high-speed durability.

[0003] In the prior art, nylon fiber cords are mainly applied to the organic fiber cords used in the belt cover layer; however, it has been proposed to use polyethylene terephthalate fiber cords (hereinafter referred to as PET fiber cords), which are highly elastic and inexpensive compared to nylon fiber cords (see, for example, JP 2001 63312 A). Particularly, when a belt cover layer formed of such highly elastic PET fiber cords is used, the vibration frequency generated in a pneumatic tire during running tends to shift to a band less likely to vibrate with a vehicle. As a result, mid-frequency road noise can be effectively suppressed.On the other hand, it was found that the belt cover layer (high-elastic PET fiber cords) does not suppress the occurrence of vibrations generated during driving. Thus, once the vibration is generated, without sufficient damping, a driver may feel that road noise is not reduced. Consequently, there is a need for a countermeasure to not only improve road noise performance based on instrumental measurements but also provide a driver's impression (road noise performance based on sensory measurements).

[0004]

[0004] DE 112019006111 T5 discloses pneumatic radial tires, wherein the belt layers comprise steel cords arranged inclined with respect to the tire circumferential direction so as to intersect each other between the layers, each of the belt layers having a flexural rigidity S of 16500 N mm² / 50 mm or less per 50 mm of width in a direction perpendicular to a longitudinal direction of the steel cords, and wherein the belt cover layer comprises polyester fiber cords having an elongation of 2.0% to 4.0% under a load of 2.0 cN / dtex, the polyester fiber cords being spirally wound along the tire circumferential direction.

[0005] KR 10 2003 041 072 A discloses a steel cord of a pneumatic radial tire having two core filaments and five cover filaments. The two core filaments are arranged in parallel without twisting to limit structural displacement and are surrounded by the five cover filaments. The diameter of the core filament is 0.18 to 0.30 mm, and the diameter of the cover filament is 0.22 to 0.40 mm. The ratio of the core filament diameter to the cover filament diameter is in the range of 0.50 to 0.85, and the twist length of the cover filament is in the range of 10.0 to 20.0 mm. Tire durability is improved by preventing corrosion, peeling, or seizure by tightly bonding the rubber to the cord, and cornering stability is improved by increasing lateral rigidity through the oval portion of the cord.

[0006] JP 2002 225508 A discloses a pneumatic radial tire that reduces mid-frequency road noise and maintains high speed stability even under high humidity. A belt ply of the pneumatic radial tire includes an acrylic fiber cord with a tensile strength of 4.5 cN / dtex or more and an elongation of 2.0 to 4.5% under a load of 2.0 cN / dtex, and a fiber cord made of nylon, polyester, or rayon wound at an angle of substantially 0° in the tire circumferential direction. Brief description of the inventionTechnical problem

[0007] An object of the present invention is to provide a pneumatic radial tire provided with a belt cover layer formed of organic fiber cords, the pneumatic radial tire being capable of providing road noise performance based on instrumental measurements and road noise performance based on sensory measurements in a highly compatible manner. Solution to the problem

[0008] A pneumatic radial tire according to an embodiment of the present invention for achieving the above-described object includes: a tread portion extending in the tire circumferential direction and having an annular shape; a pair of sidewall portions disposed on both sides of the tread portion; and a pair of bead portions disposed on an inner side of the sidewall portions in a tire radial direction. The pneumatic radial tire includes: a carcass layer mounted between a pair of bead portions; a plurality of belt layers disposed on an outer periphery of the carcass layer in the tread portion; and a belt cover layer disposed on an outer peripheral side of the belt layers.The belt layers are formed of steel cords in an N+M structure, wherein the number of wire strands N of an inner layer is 2 to 4 and the number of wire strands M of an outer layer is 2 to 7, and wherein a twisting direction of the inner layer is different from a twisting direction of the outer layer. The steel cords are arranged inclined with respect to the tire circumferential direction to intersect each other in layers of the belt layers. The belt cover layer is formed of organic fiber cords having an elongation of 2.0% to 4.0% under a load of 2.0 cN / dtex. The organic fiber cords are spirally wound along the tire circumferential direction. Advantageous effects of the invention

[0009] According to the present invention, by using the organic fiber cords having an elongation of 2.0% to 4.0% under a load of 2.0 cN / dtex in the belt cover layer, the vibration frequency generated on the pneumatic tire during travel can be shifted to a band less likely to vibrate with a vehicle, and the road noise performance based on instrumental measurements can be improved. On the other hand, according to the knowledge of the inventors of the present invention, steel cords having the above-described structure have characteristics that the vibration damping rate is high. Accordingly, by configuring the belt layer with such steel cords, the vibration of the tread portion can be effectively damped, and the road noise performance based on sensory measurements can also be improved.

[0010] According to the present invention, a steel cord quantity A, which is defined as the product of a cross-sectional area S (mm 2 ) of the steel cords and a cord count E of the steel cords per 50 mm width perpendicular to a longitudinal direction of the steel cord (the number of cords per 50 mm), preferably within a range of 6.0 to 9.0. Accordingly, the structure of the belt layer is appropriately adjusted, and thus advantageously road noise performance based on instrumental measurements and road noise performance based on sensory measurements are provided in a compatible manner.

[0011] According to the present invention, a ratio P2 / P1 of a twist pitch P2 of the outer layer to a twist pitch P1 of the inner layer of the belt cords is preferably 1.0 or less. Accordingly, the structure of the belt cords is appropriately determined, and thus, road noise performance based on instrumental measurements and road noise performance based on sensory measurements are advantageously provided in a compatible manner.

[0012] In one embodiment of the present invention, the organic fiber cords are preferably formed of polyester fibers. By using the polyester fibers as just described, the road noise performance (especially the road noise performance based on instrumental measurements) can be effectively enhanced due to the excellent physical properties (high elastic modulus) of the polyester fibers. Brief description of the drawings Fig.1 is a meridian cross-sectional view illustrating a pneumatic radial tire according to an embodiment of the present invention. Fig. 2 is an explanatory diagram schematically illustrating the structure of the belt cords. Description of embodiments

[0013] Configurations of embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0014] As in Fig. 1, a pneumatic tire of one embodiment of the present invention includes a tread portion 1, a pair of sidewall portions 2 disposed on both sides of the tread portion 1, and a pair of bead portions 3 disposed in the sidewall portions 2 on an inner side in the tire radial direction. Note that "CL" in Fig. 1 denotes a tire equator. Although in Fig.1 is not illustrated because Fig. 1 is a meridian cross-sectional view, the tread portion 1, the sidewall portions 2, and the bead portions 3 each extend in the tire circumferential direction to form a ring shape. Thus, a toroidal basic structure of the pneumatic tire is configured. Although the description using Fig. 1 is essentially based on the illustrated meridian cross-section, all tire components extend in the tire circumferential direction and form the ring shape.

[0015] In the illustrated example, a plurality of main grooves (four main grooves in the illustrated example) extending in the tire circumferential direction are formed in the outer surface of the tread portion 1; however, the number of main grooves is not particularly limited. Further, in addition to the main grooves, various grooves and sipes may be formed, including lug grooves extending in the tire width direction.

[0016] A carcass layer 4 including a plurality of reinforcing cords extending in the tire radial direction is mounted between a pair of left and right bead portions 3. A bead core 5 is embedded in each of the bead portions, and a bead filler 6 having a triangular cross-sectional shape is arranged on the outer periphery of the bead core 5. The carcass layer 4 is folded back around the bead core 5 from an inner side to an outer side in the tire width direction. Accordingly, the bead core 5 and the bead filler 6 are wrapped by a body portion (a portion extending from the tread portion 1 through each of the sidewall portions 2 to each of the bead portions 3) and a folded-back portion (a portion folded back around the bead core 5 of each bead portion 3 to extend to each sidewall portion 2) of the carcass layer 4.For example, polyester fiber cords are preferably used as the reinforcing cords of the carcass layer 4.

[0017] A plurality (two layers in the illustrated example) of belt layers 7 are embedded on an outer peripheral side of the carcass layer 4 in the tread portion 1. Each of the belt layers 7 includes a plurality of reinforcing cords 7C inclined with respect to the tire circumferential direction, and the belt layers 7 are arranged so that the reinforcing cords 7C intersect in the layers. In these belt layers 7, the inclination angle of the reinforcing cords 7C with respect to the tire circumferential direction is set in the range of, for example, 10° to 40°. Steel cords are used as the reinforcing cords 7C of the belt layer 7 (in the following description, "reinforcing cords 7C" may be referred to as "steel cords 7C").

[0018] In particular, in one embodiment of the present invention, as shown in Fig.2, the steel cords 7C constituting the belt layer 7 adopt an N+M structure (a 2+2 structure in the illustrated example) formed of: an inner layer 7n (core) made of N wire strands; and an outer layer 7m (sheath) made of M wire strands twisted around the inner layer 7n. The number of wire strands N of the inner layer 7n is 2 to 4, and the number of wire strands M of the outer layer 7m is 2 to 7. In particular, the illustrated 2+2 structure can be suitably employed. Furthermore, in one embodiment of the present invention, the twisting directions of the inner layer 7n and the outer layer 7m are not identical and different. In other words, if the inner layer is 7n S-twist, the outer layer is 7m Z-twist, and if the inner layer is 7n Z-twist, the outer layer is S-twist.If the inner layer 7n is untwisted, the outer layer 7m is S-twist or Z-twist.

[0019] A belt cover layer 8 is provided on an outer peripheral side of the belt layer 7 to improve high-speed durability and reduce road noise. The belt reinforcing layer 8 includes organic fiber cords oriented in the tire circumferential direction. In the belt reinforcing layer 8, the angle of the organic fiber cords with respect to the tire circumferential direction is set to, for example, 0° to 5°. In one embodiment of the present invention, the belt cover layer 8 necessarily includes a full cover layer 8a covering the entire area of ​​the belt layers 7 and may be configured to include a pair of edge cover layers 8b locally covering both end portions of the belt layers 7 as needed (in the illustrated example, the belt cover layer includes both the full cover layer 8a and the edge cover layers 8b).The belt cover layer 8 is preferably configured such that a strip material of at least one single organic fiber cord bundled and covered with coating rubber is spirally wound in the tire circumferential direction and desirably has a seamless structure in particular.

[0020] Specifically, in one embodiment of the present invention, as the organic fiber cords constituting the belt cover layer 8, organic fiber cords having an elongation of 2.0% to 4.0% under a load of 2.0 cN / dtex are used. The type of organic fibers constituting the organic fiber cords is not particularly limited, and, for example, polyester fibers, nylon fibers, aramid fibers, or the like can be used. Of the fibers, polyester fibers can be suitably used. In addition, examples of the polyester fibers include polyethylene terephthalate (PET) fibers, polyethylene naphthalate (PET) fibers, polybutylene terephthalate (PBT) fibers, and polybutylene naphthalate (PBN) fibers, and PET fibers can be suitably used.Note that in one embodiment of the present invention, the elongation under 2.0 cN / dtex load is an elongation ratio (%) of sample cords measured under 2.0 cN / dtex load by conducting a tensile test according to JIS-L1017 “Test Method for Chemical Tire Fiber Cords” and under the conditions that a sample length between grippers is 250 mm and a tensile speed is 300±20 mm / minute.

[0021] As just described, the belt layer 7 formed of the steel cords 7C having a specific structure and the belt cover layer 8 formed of organic fiber cords having specific physical properties are used in combination, and thus the road noise performance based on instrumental measurements and the road noise performance based on sensory measurements can be improved in a compatible manner. In other words, in the belt cover layer 8, due to the physical properties of the organic fiber cords, the vibration frequency generated on the pneumatic tire during driving can be shifted to a band less likely to vibrate with a vehicle, and the road noise performance based on instrumental measurements can be improved.On the other hand, in the belt layer 7, the vibration of the tread portion 1 can be effectively damped due to the characteristics attributed to the above-described structure (characteristics that the vibration damping rate is high, which is found by the inventors of the present invention), and the road noise performance based on sensory measurements can also be improved.

[0022] At this time, if the number of wire strands N of the inner layer of the steel cords 7C constituting the belt layer 7 is less than two, the effect of friction between the wire strands is reduced, and thus the road noise performance cannot be sufficiently demonstrated. If the number of wire strands N of the inner layer of the steel cords 7C constituting the belt layer 7 exceeds four, the twisted structure is unstable, and thus an initial elongation of the cords is deteriorated. If the number of wire strands M of the outer layer of the steel cords 7C constituting the belt layer 7 is less than two, the effect of friction between the wire strands is reduced, and thus the road noise performance cannot be sufficiently demonstrated.When the number of wire strands M of the outer layer of the steel cords 7C constituting the belt layer 7 exceeds seven, the twisted structure is unstable, and thus an initial elongation of the cords is deteriorated. When the twisting directions of the inner layer and the outer layer of the steel cords 7C constituting the belt layer 7 are identical, the energy loss due to rubbing between the wire strands constituting the steel cords 7C is reduced, the vibration damping rate is low, and thus the road noise performance cannot be sufficiently improved based on sensory measurements. When the elongation of the organic fiber cords constituting the belt cover layer 8 is less than 2.0% under a load of 2.0 cN / dtex, the road noise performance deteriorates based on sensory measurements.When the elongation of the organic fiber cords constituting the belt cover layer 8 exceeds 4.0% under a load of 2.0 cN / dtex, the road noise performance cannot be sufficiently improved based on instrumental measurements.

[0023] If the product of a cross-sectional area S (mm 2) of the steel cords 7C and a cord count E of the steel cords 7C per 50 mm of width perpendicular to the longitudinal direction of the steel cord 7C (the number of cords per 50 mm) is defined as a steel cord amount A, the steel cord amount A is in the range of 6.0 to 9.0. Accordingly, the structure of the belt layer is appropriately adjusted, and thus, road noise performance based on instrumental measurements and road noise performance based on sensory measurements are advantageously provided in a compatible manner. If the steel cord amount A is less than 6.0, although the road noise performance based on instrumental measurements is improved, the road noise performance based on sensory measurements cannot be sufficiently ensured.If the steel cord quantity A exceeds 9.0, the road noise performance based on instrumental measurements cannot be sufficiently improved, and the road noise performance based on sensory measurements cannot be sufficiently ensured. There are no specific restrictions on the numerical range of the cross-sectional area S or the cord count E of the 7C steel cords, but the cross-sectional area S of the 7C steel cords can be limited to, for example, 0.15 mm. 2 up to 0.8 mm 2 and the cord count E can be set to, for example, 30 cords / 50 mm to 60 cords / 50 mm.

[0024] In the belt layer 7, a ratio P2 / P1 of a twist pitch P2 of the outer layer 7m to a twist pitch P1 of the inner layer 7n of the steel cords 7C is preferably 1.0 or less. Accordingly, the structure of the steel cords 7C is appropriately adjusted, and thus the road noise performance based on instrumental measurements and the road noise performance based on sensory measurements are advantageously provided in a compatible manner. If the ratio P2 / P1 exceeds 1.0, the road noise performance based on instrumental measurements cannot be sufficiently improved, and the road noise performance based on sensory measurements cannot be sufficiently ensured. Note that when the inner layer 7n is untwisted, the twist pitch P1 is interpreted as "∞", and the ratio P2 / P1 in this case is regarded as "0".

[0025] When polyethylene terephthalate (PET) fiber cords are used as the organic fiber cords constituting the belt reinforcing layer 8, PET fiber cords having an elastic modulus in a range of 3.5 cN / (tex %) to 5.5 cN / (tex %) under a load of 44 N at 100°C are preferably used. As just described, the PET fiber cords having such specific physical properties are used, and thus, based on instrumental measurements, road noise can be effectively reduced while successfully maintaining the durability of the pneumatic radial tire. If the elastic modulus of the PET fiber cords under a load of 44 N at 100°C is less than 3.5 cN / (tex %), the mid-frequency road noise cannot be sufficiently reduced.When the elastic modulus of PET fiber cords under a load of 44 N at 100°C exceeds 5.5 cN / (tex%), the fatigue resistance of the cords decreases, and the durability of the tire decreases. Note that in one embodiment of the present invention, the elastic modulus under a load of 44 N at 100°C [N / (tex%)] is calculated by: conducting a tensile test with reference to "Test Method for Chemical Tire Fiber Cords" of JIS-L1017 and under the conditions that a sample length between grippers is 250 mm and a tensile speed is 300±20 mm / minute; and converting the inclination of the tangent at a point corresponding to the load 44 N of the load-elongation curve into a value per 1 tex.

[0026] In addition, when polyethylene terephthalate (PET) fiber cords are used as the organic fiber cords constituting the belt reinforcing layer 8, the heat shrinkage stress of the PET fiber cords at 100°C is preferably 0.6 cN / tex or more. The heat shrinkage stress at 100°C is adjusted as just described, and thus, based on instrumental measurements, road noise can be effectively reduced while successfully maintaining the durability of the pneumatic radial tire. If the heat shrinkage stress of the PET fiber cords at 100°C is less than 0.6 cN / tex, the tire performance during travel cannot be sufficiently improved, and it is difficult to sufficiently maintain high-speed durability. The upper limit of the heat shrinkage stress of the PET fiber cords at 100°C is not specifically limited, but is preferably, for example, 2.0 cN / tex.Note that in one embodiment of the present invention, the heat shrinkage stress (cN / tex) at 100°C is the heat shrinkage stress of a sample cord measured with reference to “Test Method for Chemical Tire Fiber Cord” of JIS-L1017 and when heated under the conditions of the sample length of 500 mm and the heating condition at 100°C for 5 minutes.

[0027] For example, in order to obtain the PET fiber cords having the above-mentioned physical properties, it is preferable to optimize the dip processing. In other words, dip processing with adhesive is performed on the PET fiber cords before a calendering process; however, in a normalization process after a two-bath treatment, it is preferable that an ambient temperature is set in the range of 210°C to 250°C and the cord tension is set in the range of 2.2×10 -2 N / tex up to 6.7× 10 -2N / tex. Accordingly, the PET fiber cords can be given desired physical properties as described above. If the cord tension in the normalization process is less than 2.2×10 -2 N / tex, the cord elastic modulus is low, and thus the medium-frequency road noise cannot be sufficiently reduced. However, if the cord tension is greater than 6.7×10 2 N / tex, the cord elastic modulus is high, and thus the fatigue resistance of the cords is low. Examples

[0028] Tires were manufactured according to Prior Art Example 1, Comparative Examples 1 to 5 and Examples 1 to 11. For the tires with a tire size of 225 / 60R18 and with the basic structure as in Fig.1 illustrates the structure of steel cords constituting the belt layers; the twisting direction of the inner layer; the twisting direction of the outer layer; where the steel cord amount A is calculated as the product of the cross-sectional area S of the steel cords and the cord number E of the steel cords per 50 mm of width perpendicular to the longitudinal direction of the steel cord; the twist pitch P1 of the inner layer; the twist pitch P2 of the outer layer; the ratio P1 / P2 thereof, the type of organic fibers used in the organic fiber cords constituting the belt cover layers; and the elongation of the organic fiber cords under a load of 2.0 cN / dtex are differentiated as in Tables 1 and 2.

[0029] In each example, the belt cover layer includes a seamless structure in which a strip of at least one single organic fiber cord (nylon 66 fiber cord or PET fiber cord) bundled and covered with coating rubber is spirally wound in the tire circumferential direction. The cord count density in the strip is 50 cords / 50 mm. Furthermore, each organic fiber cord (nylon 66 fiber cord or PET fiber cord) has a structure of 1100 dtex / 2.

[0030] As an example of Prior Art 1 and Comparative Example 1, in these examples, since the wire structure of the steel cords constituting the belt layers is a 1×3 structure, the twist direction and twist pitch are described in the "inner layer" column for convenience. The twist pitch when the twist direction of the steel cords is "untwisted" is regarded as "∞." For the organic fiber type column, nylon 66 fiber cords are indicated as "N66," and PET fiber cords are indicated as "PET."

[0031] Regarding these test tires, the road noise performance based on instrumental measurements and the road noise performance based on sensory measurements were evaluated by the following evaluation methods, and the results are also shown in Tables 1 and 2. Road noise performance (instrument measurements)

[0032] Each of the test tires was mounted on a wheel with a rim size of 18×7J, mounted as the front and rear wheels of a passenger car (front-wheel drive vehicle) with a displacement of 2500 cc, and inflated to an air pressure of 230 kPa. A sound collection microphone was placed on an inner side of the driver's seat window. A sound pressure level near a frequency of 315 Hz was measured when the vehicle was driven at an average speed of 50 km / h on a test track of an asphalt road surface. The evaluation results were based on a prior art example as a reference and indicated the change amount (dB) of the reference. Road noise performance (sensory measurements)

[0033] Each of the test tires was mounted on a wheel with a rim size of 18×7J, which was mounted as the front and rear wheels of a passenger car (front-wheel drive vehicle) with a displacement of 2500 cc and inflated to an air pressure of 230 kPa. Sensory evaluations for road noise were conducted by five test drivers when the vehicle was driven at an average speed of 50 km / h on a test track of an asphalt road surface. The evaluation results were evaluated using a 5-point method, with 3 points (reference) assigned to the results of the prior art example 1, and an average value of the evaluations of the three test drivers, excluding the highest point and the lowest point, was given. Larger points indicate superior road noise performance (sensory measurements). [Table 1-I] Example of the prior art 1 Comparison example 1 Comparison example 2 Example 1 Comparison example 3 Belt layer Steel cord structure 1 × 3 1 × 3 2 + 2 2 + 2 2 + 2 Twist direction of the inner layer S-twist S-twist Untwisted Untwisted S-twist Twist direction of the outer layer - - S-twist S-twist S-twist Steel cord quantity A 6,5 6,5 7,3 7,3 7,3 Twist pitch P1 of the inner layer mm 14 14 ∞ ∞ 15 Twist pitch P2 of the outer layer mm - - 15 15 15 Ratio P1 / P2 - - - 0 1,00 Belt cover layer Type of organic fibers N66 PET N66 PET PET Elongation under a load of 2.0 cN / dtex % 7,5 2,8 7,5 2,8 2,8 Road noise performance (instrument measurements) dB 0,0 -2,0 0,0 -2,0 -2,0 Road noise performance (sensory measurements) 3,0 2,8 3,0 3,3 3,0 [Table 1-II] Example 2 Comparison example 4 Example 3 Example 4 Comparison example 5 Belt layer Steel cord structure 2 + 2 2 + 2 2 + 2 2 + 2 2 + 2 Twist direction of the inner layer Z-twist Untwisted Untwisted Untwisted Untwisted Twist direction of the outer layer S-twist S-twist S-twist S-twist S-twist Steel cord quantity A 7,3 7,3 7,3 7,3 7,3 Twist pitch P1 of the inner layer mm 15 ∞ ∞ ∞ ∞ Twist pitch P2 of the outer layer mm 15 15 15 15 15 Ratio P1 / P2 1,00 0 0 0 0 Belt cover layer Type of organic fibers PET PET PET PET PET Elongation under a load of 2.0 cN / dtex % 2,8 1,8 2,2 a3,8 4,3 Road noise performance (instrument measurements) dB -2,0 -2,5 -2,3 -1,5 0,8 Road noise performance (sensory measurements) 3,2 3,0 3,2 3,2 3,0 [Table 2-I] Example 5 Example 6 Example 7 Example 8 Belt layer Steel cord structure 2 + 2 2 + 2 2 + 2 2 + 2 Twist direction of the inner layer Untwisted Untwisted Untwisted Untwisted Twist direction of the outer layer S-twist S-twist S-twist S-twist Steel cord quantity A 5,5 6,5 8,5 9,5 Twist pitch P1 of the inner layer mm ∞ ∞ ∞ ∞ Twist pitch P2 of the inner layer mm 15 15 15 15 Ratio P1 / P2 0 0 0 0 Belt cover layer Type of organic fibers PET PET PET PET Elongation under a load of 2.0 cN / dtex % 3,0 3,0 3,0 3,0 Road noise performance (instrument measurements) dB -2,0 -1,8 -1,6 -1,4 Road noise performance (sensory measurements) 3,2 3,3 3,3 3,2 [Table 2-II] Example 9 Example 10 Example 11 Belt layer Steel cord structure 2 + 2 2 + 2 2 + 2 Twist direction of the inner layer Untwisted Z-twist Z-twist Twist direction of the outer layer S-twist S-twist S-twist Steel cord quantity A 7,3 7,3 7,3 Twist pitch P1 of the inner layer mm ∞ 20 15 Twist pitch P2 of the inner layer mm 15 15 20 Ratio P1 / P2 0 0,75 1,33 Belt cover layer Type of organic fibers PET PET PET Elongation under a load of 2.0 cN / dtex % 3,0 3,0 3,0 Road noise performance (instrument measurements) dB -1,8 -1,7 -1,6 Road noise performance (sensory measurements) 3,3 3,3 3,2

[0034] As can be seen from Tables 1 and 2, in contrast to Prior Art Example 1 as a reference, the tires of Examples 1 to 11 provide both improved road noise performance based on instrumental measurements and improved road noise performance based on sensory measurements. On the other hand, although Comparative Example 1 uses PET fiber cords with appropriate elongation under a load of 2.0 cN / dtex as the belt cover layer, the wire structure of the steel cords constituting the belt layer is a 1×3 structure, and thus the road noise performance based on sensory measurements deteriorates.In Comparative Example 2, although the steel cords constituting the belt layer are suitable, the elongation of the belt cover layer at a load of 2.0 cN / dtex is too large, and thus the effect of improving both the noise performance based on instrumental measurements and the road noise performance based on sensory measurements is not achieved. In Comparative Example 3, the twisting directions of the inner layer and the outer layer of the steel cords are identical, and thus the effect of improving the road noise performance based on sensory measurements is not achieved. In Comparative Example 4, the elongation of the belt cover layer under a load of 2.0 cN / dtex is too small, and thus the effect of improving the road noise performance based on sensory measurements is not achieved.In Comparative Example 5, the elongation of the belt cover layer under a load of 2.0 cN / dtex is too large, and thus the effect of improving both the road noise performance based on instrumental measurements and the road noise performance based on sensory measurements is not achieved. List of reference symbols 1 tread section 2 side wall section 3 bead section 4 carcass layers 5 Bead core 6 bead fillers 7 belt layer 7C Reinforcing Cord (Steel Cord) 7n inner layer (core) 7m outer layer (shell) 8 Belt cover layer CL Tire Equator

Claims

[1] Pneumatic radial tire, comprising: a tread portion (1) extending in a tire circumferential direction and having a ring shape; a pair of sidewall portions (2) arranged on both sides of the tread portion (1); and a pair of bead portions (3) arranged on an inner side of the sidewall portions (2) in a tire radial direction, the pneumatic radial tire comprises: a carcass layer (4) mounted between a pair of bead portions (3); a plurality of belt layers (7) arranged on an outer periphery of the carcass layer (4) in the tread portion (1); and a belt cover layer (8) arranged on an outer peripheral side of the belt layers (7), wherein the belt layers (7) are formed of steel cords (7C) in an N+M structure in which the number of twisted wire strands N of an inner layer is 2 to 4 and the number of twisted wire strands M of an outer layer is 2 to 7, and wherein a twisting direction of the inner layer is different from a twisting direction of the outer layer, the steel cords (7C) being arranged inclined with respect to the tire circumferential direction to intersect each other in layers of the belt layers (7), wherein the belt cover layer (8) is formed of organic fiber cords having an elongation of 2.0% to 4.0% under a load of 2.0 cN / dtex, the organic fiber cords being spirally wound along the tire circumferential direction; where a steel cord quantity A, which is defined as the product of a cross-sectional area S [mm 2] of the steel cords (7C) and a cord count E of the steel cords (7C) per 50 mm width perpendicular to a longitudinal direction of the steel cords (7C) is within a range of 6.0 to 9.0; and wherein a ratio P2 / P1 of a twist pitch P2 of the outer layer to a twist pitch P1 of the inner layer of the belt cords is 1.0 or less. [2] A pneumatic radial tire according to claim 1, wherein the organic fiber cords are formed of polyester fibers.

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