pneumatic tires

Abstract

pneumatic tires, the a pair of first main grooves (1) arranged on both sides of a central position (Ce) and extending in the tire circumferential direction, a pair of second main grooves (2) arranged further to the shoulder sides than the first main grooves (1) and extending in the tire circumferential direction; a central rib (10) disposed between the pair of first main grooves (1); central ribs (20) arranged between the first main grooves (1) and the second main grooves (2); and shoulder ribs (30) arranged on outer sides of the second main grooves (2) in a tread portion (T), wherein a plurality of lug grooves (11, 21) extending from a wall surface on the shoulder sides toward center sides and blindly terminating in the rib are formed in the central rib (10) and the center ribs (20) respectively such that a groove width of the lug grooves (11, 21) gradually decreases with proximity to the center sides; a plurality of lug grooves (31) extending in the tire width direction are formed in the shoulder ribs (30) so as not to be connected to the second main grooves (2); a distance (D1) from the center position (Ce) of the tread portion (T) to the centers of the first main grooves (1) is set to be in a range of 15% to 25% of a distance (D0) from the center position (Ce) to a ground contact edge (E); a distance (D2) from the center position (Ce) of the tread portion (T) to the centers of the second main grooves (2) is set to be in a range of 60% to 80% of the distance from the center position (Ce) to the ground contact edge (E); a width of the first main grooves (1) is set to be in a range of 70% to 90% of a width of the second main grooves (2); a total area of ​​the first main grooves (1) and the second main grooves (2) is set to be in a range of 15% to 25% of an area of ​​a ground contact area (A) of the tread portion (T); and when the ground contact area (A) of the tread portion (T) is divided into a central area (Ac) and shoulder areas (As), wherein, when the shoulder areas (As) have a position at 50% of the distance from the central position (Ce) of the tread portion (T) to the ground contact edge (E) as a boundary, a groove area ratio of the central area (Ac) is configured to be less than a groove area ratio of the shoulder areas (As), wherein the ground contact area (A) is defined by a ground contact width in the tire axial direction when the tire is inflated to a maximum air pressure specified by a standard on which the pneumatic tire is based, the tire being loaded to 88% of a maximum capacity in a state where the tread portion (T) of the tire is placed perpendicular to a horizontal plane to contact the ground.

Description

Technical FieldThe present invention relates to a pneumatic tire provided with four main grooves extending in a tire circumferential direction in a tread portion, and more particularly relates to a pneumatic tire in which traveling noise can be reduced while maintaining excellent rolling resistance and excellent wet performance.Prior ArtNoise generated by pneumatic tires mounted on a vehicle is generated when the vehicle passes, and this noise is generally referred to as "traveling noise". The running noise is enhanced by a pumping action that occurs when the air in the grooves of the tread portion is compressed and released. For this reason, reducing the groove area in the tread portion is useful in reducing the running noise. For example, when a pneumatic tire in which four main grooves extending in a tire circumferential direction in a tread portion have been provided (see, e.g., Patent Document 1) is configured such that the groove area in the tread portion is reduced, the running noise can be reduced.However, when the groove area is reduced to reduce the running noise, there is a problem that the drainage property deteriorates and the wet performance is adversely affected. In addition, there is a problem that decreasing the groove area results in an increase in the rubber volume in the tread portion, resulting in adverse influence on rolling resistance. Therefore, it is difficult to reduce the running noise while maintaining excellent rolling resistance and wet performance, and at present, it is difficult to satisfy both of these characteristics simultaneously.Prior Art DocumentsDocument EP 2 108 531 A2 discloses a pneumatic tire having a tread pattern including at least three circumferential grooves having a groove width of 6 to 18 mm and a groove depth of 6.0 to 16.0 mm extending in a tire circumferential direction. Here, on groove walls on both sides constituting the at least three circumferential grooves, first inclined walls inclined by an inclination angle of 5 to 20 degrees with respect to the tire circumferential direction and second inclined walls inclined by an inclination angle of 0 to 45 degrees with respect to a tire width direction are provided so as to be connected one after another in the tire circumferential direction, so that positions of groove ends in the tire width direction vary in the tire circumferential direction. Document JP 2006-224 770 A discloses a tire having at least two main grooves, a central rib, central ribs and shoulder ribs. A plurality of lug grooves are formed in the center rib and the center ribs. Lug grooves of the shoulder portion extend in the tire width direction and are in contact with the outer main grooves. Document DE 600 31 465 T2 discloses a tire having a tread-bound profile defined by a plurality of circumferential grooves continuously extending in the circumferential direction of the tire and a plurality of directional slant grooves arranged at given intervals in the circumferential direction. Document DE 60 2004 001 839 T2 discloses a pneumatic tire on a tread ground contacting surface thereof three or four main grooves extending successively in the circumferential direction of the tire, wherein a non-symmetric tread of four or five block portions divided by the main grooves is formed on the tread ground contacting surface.SUMMARY OF THE INVENTIONProblem to be Solved by the Invention:An object of the present invention is to provide a pneumatic tire by which driving noise can be reduced while maintaining excellent rolling resistance and wet performance.Means for Solving the Problem:A pneumatic tire achieving the object of the present invention includes a pair of first main grooves positioned on both sides of a center position and extending in the tire circumferential direction; a pair of second main grooves positioned more toward the shoulder sides than the first main grooves and extending in the tire circumferential direction; a central rib disposed between the pair of first main grooves; center ribs disposed between the first main grooves and the second main grooves; and shoulder ribs disposed on the outer sides of the second main grooves in a tread portion. A plurality of lug grooves extending from a wall surface on the shoulder sides toward the center sides and terminating blindly in the rib are respectively formed in the center rib and the center ribs such that a groove width of the lug grooves gradually decreases with the vicinity of the center sides. A plurality of lug grooves extending in the tire width direction are formed in the shoulder ribs so as not to be connected to the second main grooves. A distance from the center position of the tread portion to the centers of the first main grooves is set to be in a range of 15% to 25% of a distance from the center position to a ground contact edge; and a distance from the center position of the tread portion to the centers of the second main grooves is set to be in a range of 60% to 80% of the distance from the center position to the ground contact edge. A width of the first main grooves is set to be in a range of 70% to 90% of a width of the second main grooves. A total area of the first main grooves and the second main grooves is set to be in a range of 15% to 25% of an area of a ground contact region of the tread portion. When the ground contact area of the tread portion is divided into a center area and shoulder areas, wherein when the shoulder areas have a position at 50% of the distance from the center position of the tread portion to the ground contact edge as a boundary, a groove area ratio of the center area is configured to be less than a groove area ratio of the shoulder areas, the ground contact area being defined by a ground contact width in the tire axial direction when the tire is inflated with a maximum air pressure set by a standard on which the pneumatic tire is based, the tire being loaded with 88% of a maximum capacity in a state where the tread portion of the tire is placed perpendicular to a horizontal plane to contact the ground.Effect of the invention:As a result of thorough investigation of the running noise of pneumatic tires provided with four main grooves extending in the tire circumferential direction in the tread portion, the inventors of the present invention have discovered that deformation in the tread portion from contact to separation from a road surface occurs most in the center region. The present inventors have also found that a loud driving noise is generated from the pumping action of the grooves disposed in the center region, and that the grooves disposed in the center region greatly contribute to the driving noise. Therefore, the present invention has been accomplished.In the present invention, a pneumatic tire specifically includes a pair of first main grooves on the center sides, a pair of second main grooves on the shoulder sides, a central rib disposed between the pair of first grooves, center ribs disposed between the first main grooves and the second main grooves, and shoulder ribs disposed on the outer sides of the second main grooves in a tread portion. A plurality of lug grooves extending from a wall surface on the shoulder sides toward the center sides and terminating blindly in the rib are respectively formed in the center rib and the center ribs such that a groove width of the lug grooves gradually decreases with proximity to the center sides. A plurality of lug grooves extending in the tire width direction are formed in the shoulder ribs so as not to be connected to the second main grooves. Positions of the first main grooves and the second main grooves are set to be in a predetermined range, and a dimensional relationship between the first main grooves and the second main grooves is set to be in a predetermined range. An overall area of the first main grooves and the second main grooves is set to be in a set range, and in the tread portion, a groove area ratio of the center region is configured to be less than a groove area ratio of the shoulder regions. Thereby, the running noise resulting from the grooves disposed in the center portion can be effectively suppressed, and the running noise of the entire tire can be reduced. Further, it is possible to set the groove area ratio of the entire tread portion to be the same as in conventional pneumatic tires, and thereby excellent rolling resistance and wet performance can be maintained.In the present invention, a total area of the lug grooves located in the center region is preferably set to be in a range of 20% to 60% of a total area of the lug grooves present in the shoulder regions. Such a setting is useful in reducing the groove area ratio of the center region of the tread portion to be lower than the groove area ratio of the shoulder regions.The groove area ratio of the center portion is preferably set to be in a range of 18% to 22%, and the groove area ratio of the shoulder portions is preferably set to be in a range of 25% to 35%. Thereby, a reduction in running noise, a reduction in rolling resistance, and an improvement in wet performance can be achieved to a higher extent.Preferably, a length in the tire width direction of the lug grooves formed in the center rib is set to be in a range of 50% to 90% of a half width of the center rib; a length in the tire width direction of the lug grooves formed in the center ribs is set to be in a range of 50% to 90% of a width of the center ribs; and a groove width at a position 3 mm toward a leading edge side from an end edge of each of the lug grooves formed in the center rib and the center ribs is set to be in a range of 50% to 70% of a groove width at a position 3 mm toward an end edge from a leading edge. Thereby, the effect of reducing the running noise can be enhanced while minimizing the increase in the groove area at the center portion of the tread portion.A pitch of the lug grooves formed in the central rib and the central ribs is preferably at least twice a pitch of the lug grooves formed in the shoulder ribs. Thereby, the area of the lug grooves existing in the shoulder regions is relatively larger, and the rolling resistance can be reduced. In addition, by complementing the smaller groove area in the center region with the lug grooves in the shoulder regions, water drainage performance can be ensured and wet performance deterioration can be prevented.In addition, in the present invention, preferably, an inclination angle with respect to the tire circumferential direction of the lug grooves formed in the center rib is set to be in a range of 25° to 40°, the lug grooves formed in the center rib are curved toward the shoulder sides, and a radius of curvature thereof is set to be in a range of 100 mm to 140 mm; an inclination angle with respect to the tire circumferential direction of the lug grooves formed in the center ribs is set to be in a range of 30° to 50°, the lug grooves formed in the center ribs are curved toward the shoulder sides, and a radius of curvature thereof is set to be in a range of 130 mm to 150 mm; and the lug grooves formed in the shoulder ribs are curved to have a plurality of radii of curvature, and the radii of curvature thereof are set to be in a range of 10 mm to 100 mm. Due to the provision of the lug grooves formed in the center rib and the center ribs having fixed inclination angles and fixed curvature radii, the effect of reducing the running noise can be enhanced by keeping the increase in the groove area at areas closer to the center position to the minimum. In addition, the groove area in the shoulder regions can be effectively increased by providing the lug grooves formed with fixed radii of curvature in the shoulder ribs, excellent wet performance can be ensured, and further, rolling resistance can be reduced due to a decrease in the rubber volume in the shoulder regions.In the present invention, "ground contact area" refers to an area defined by a ground contact width in the tire axial direction when the tire is inflated with a maximum air pressure set by the standard on which the pneumatic tire is based, the tire being loaded with 88% of a maximum load capacity in a state where the tread portion of the tire is placed perpendicular to a horizontal plane to contact the ground. In addition, "center position of the tread portion" refers to a center position in the tire width direction, and "ground contact edge" refers to a position representing the outermost side in the tire width direction of the ground contact region.Brief Description of the DrawingsFIG. 1 is a development view showing a tread pattern of a pneumatic tire according to an embodiment of the present invention. FIG. 2 is an enlarged plan view illustrating the main constituent elements of the tread pattern of the pneumatic tire shown in FIG. 1.Detailed DescriptionReferring to the accompanying drawings, a detailed description will be given below of a configuration of the present invention. FIG. 1 illustrates a tread pattern of a pneumatic tire according to an embodiment of the present invention, and FIG. 2 illustrates the main components thereof.As illustrated in FIG. 1, in a tread portion T, a pair of main grooves 1 (first main grooves) disposed on both sides of a center position Ce and extending in the tire circumferential direction, a pair of main grooves 2 (second main grooves) disposed further to the shoulder sides than the main grooves 1 and extending in the tire circumferential direction, a center rib 10 disposed between the pair of main grooves 1, center ribs 20 disposed between the main grooves 1 and the main grooves 2, respectively, and shoulder ribs 30 disposed on outer sides of the main grooves 2 are provided. A groove width and a groove depth of the main grooves 1 and 2 are not particularly limited, but the groove width is set to be in a range of 4.0 mm to 12.0 mm, and the groove depth is set to be in a range of 7.0 mm to 10.0 mm, for example.A plurality of lug grooves 11 which extend from a wall surface on the shoulder sides toward the center sides, ending blindly in the center rib 10, and which are disposed at intervals in the tire circumferential direction are formed in the center rib 10. These lug grooves 11 are formed such that a groove width thereof gradually decreases with proximity to the center sides. The changes in the groove width of the lug grooves 11 may be continuous or stepwise.A plurality of lug grooves 21 extending from a wall surface on the shoulder sides toward the center sides, terminating in blind in the center ribs 20, and arranged at intervals in the tire circumferential direction are formed in the center ribs 20. These lug grooves 21 are formed such that a groove width thereof gradually decreases with proximity to the center sides. The changes in the groove width of the lug grooves 21 may be continuous or stepwise.A plurality of lug grooves 31 extending in the tire width direction and arranged at intervals in the tire circumferential direction are formed in the shoulder ribs 30. The lug grooves 31 are formed so as not to be connected to the main grooves 2.In the pneumatic tire described above, as illustrated in FIG. 2, a distance D 1 from the center position Ce of the tread portion T to the centers of the main grooves 1 is set to be in a range of 15% to 25% of a distance D 0 from the center position Ce to a ground contact edge E; and a distance D 2 from the center position Ce of the tread portion T to the centers of the main grooves 2 is set to be in a range of 60% to 80% of the distance D 0 from the center position Ce to the ground contact edge E. In addition, a width of the main grooves 1 is set to be in a range of 70% to 90% of a width of the main grooves 2. A total area of the main grooves 1 and the main grooves 2 is set to be in a range of 15% to 25% of an area of a ground contact area A of the tread portion T. When the ground contact area A of the tread portion T is divided into a center area Ac and shoulder areas As having a position at 50% of the distance D0 from the center position Ce of the tread portion T to the ground contact edge E as a boundary, a groove area ratio of the center area Ac is configured to be smaller than a groove area ratio of the shoulder areas As. Note that the groove width and the groove area are measured at the tread surface, and the "groove area ratio" is defined as the groove area (percentage) occupying the total area of a specific area.In the pneumatic tire described above, pluralities of lug grooves 11 and 21 extending from a wall surface on the shoulder sides toward the center sides and terminating in the center ribs 10 and 20 are respectively formed in the center rib 10 and the center ribs 20 such that a groove width of the lug grooves 11 and 21 gradually decreases with proximity to the center sides. A plurality of lug grooves 31 extending in the tire width direction are formed in the shoulder ribs 30 so as not to be connected to the main grooves 2. Positions of the main grooves 1 and the main grooves 2 are set to be in a predetermined range, and a dimensional ratio of the main grooves 1 and the main grooves 2 is set to be in a predetermined range. A total area of the main grooves 1 and the main grooves 2 is set to be in a set range, and in the tread portion T, a groove area ratio of the center region Ac is configured to be less than a groove area ratio of the shoulder regions As. Thereby, a running noise resulting from grooves such as the main grooves 1, the lug grooves 11 and 21, and the like disposed in the center region Ac can be effectively suppressed, and the running noise of the entire tire can be reduced. Further, it is possible to set the groove area ratio of the entire tread portion T to be the same as in conventional pneumatic tires, and therefore excellent rolling resistance and wet performance can be maintained.In the pneumatic tire described above, the lug grooves 11 and 21 are formed in the center rib 10 and the center ribs 20 such that the groove width thereof gradually decreases with proximity to the center sides. This configuration contributes to reduction of the running noise by keeping the increase of the groove area in the center area Ac of the tread portion T to the minimum. In addition, the lug grooves 31 formed in the shoulder ribs 30 are formed so as not to be connected to the main grooves 2. In such a configuration, the pumping noise generated in the main grooves 2 in the shoulder sides does not easily escape to the outer side in the tire width direction, and this configuration therefore contributes to reduction of the running noise.In the pneumatic tire described above, when the distance D 1 from the center position Ce of the tread portion T to the centers of the main grooves 1 is less than 15% of the distance D 0, the running noise increases and the dry performance is adversely affected. On the other hand, when the distance D 1 exceeds 25% of the distance D 0, the wet performance is adversely affected. When the distance D 2 from the center position Ce of the tread portion T to the centers of the main grooves 2 is less than 60% of the distance D 0, the running noise increases and the dry performance is adversely affected. On the other hand, when the distance D 2 exceeds 80% of the distance D 0, the wet performance is adversely affected.In addition, when the width of the main grooves 1 is less than 70% of the width of the main grooves 2, the wet performance is adversely affected. On the other hand, if the width exceeds 90%, the running noise is increased and the drying performance is adversely affected. When the total area of the main grooves 1 and the main grooves 2 is less than 15% of the area of the ground contact region A of the tread portion T, the wet performance is adversely affected and the rolling resistance increases. On the other hand, when the total area exceeds 25%, the running noise increases and the dry performance is adversely affected.In the pneumatic tire described above, a total area of the lug grooves 11 and 21 present in the center region Ac is preferably set to be in a range of 20% to 60% of a total area of the lug grooves 21 and 31 present in the shoulder regions As. When the total area of the lug grooves 11 and 21 present in the center region Ac is less than 20% of the total area of the lug grooves 21 and 31 present in the shoulder regions As, the wet performance is adversely affected. On the other hand, when the total area exceeds 60%, it is difficult to reduce the groove area ratio of the center region Ac of the tread portion T to be sufficiently lower than the groove area ratio of the shoulder regions As.In addition, the groove area ratio of the center region Ac is preferably set to be in a range of 18% to 22%, and the groove area ratio of the shoulder regions As is preferably set to be in a range of 25% to 35%. Thereby, a reduction in running noise, a reduction in rolling resistance, and an improvement in wet performance can be achieved to a higher extent. When the groove area ratio of the center region Ac is less than 18%, the wet performance is adversely affected and the rolling resistance increases. On the other hand, when the groove area ratio exceeds 22%, the running noise increases and the dry performance is adversely affected. Further, when the groove area ratio of the shoulder portions As is less than 25%, the wet performance is adversely affected and the rolling resistance increases. On the other hand, when the groove area ratio exceeds 35%, the running noise is increased and the dry performance is adversely affected.Preferably, a length L 1 in the tire width direction of the lug grooves 11 formed in the center rib 10 is set to be in a range of 50% to 90% of a half width W 1 of the center rib 10, and a length L 2 in the tire width direction of the lug grooves 21 formed in the center ribs 20 is set to be in a range of 50% to 90% of a width W 2 of the center ribs 20. Thereby, the effect of the running noise reduction can be enhanced by keeping the increase of the groove area in the center area Ac of the tread portion T to the minimum. When the length L 1 in the tire width direction of the lug grooves 11 formed in the center rib 10 is less than 50% of the half width W 1 of the center rib 10, the wet performance is adversely affected. On the other hand, when the length L1 exceeds 90%, the running noise is increased and the drying performance is adversely affected. When the length L 2 in the tire width direction of the lug grooves 21 formed in the center ribs 20 is less than 50% of the width W 2 of the center ribs 20, the wet performance is adversely affected. On the other hand, when the length L2 exceeds 90%, the running noise is increased and the drying performance is adversely affected.A groove width at a position 3 mm toward a leading edge side (open edge) from an end edge (closed edge) of each of the lug grooves 11 and 21 formed in the central rib 10 and the center ribs 20 is preferably set to be in a range of 50% to 70% of a groove width at a position 3 mm toward an end edge from a leading edge. Thereby, the effect of the running noise reduction can be enhanced by keeping the increase in the groove area in the center area Ac of the tread portion T to the minimum. If the end edge side of each of the lug grooves 11 and 21 is excessively narrow, it causes a negative influence on the wet performance. On the other hand, if the end edge side is excessively wide, it causes a negative influence on the running noise and the drying performance.Pitch distances P 1 and P 2 (arrangement distances in the tire circumferential direction) of the lug grooves 11 and 21 formed in the central rib 10 and the center ribs 20 are preferably at least twice a pitch distance P 3 (arrangement distance in the tire circumferential direction) of the lug grooves 31 formed in the shoulder ribs 30. Thereby, the area of the lug grooves 21 and 31 existing in the shoulder regions As is relatively larger, and the rolling resistance can be reduced due to a reduction in the rubber volume. In addition, by complementing the smaller groove area in the center region Ac with the lug grooves 21 and 31 in the shoulder regions As, the water drainage performance can be ensured and wet performance deterioration can be prevented.In addition, preferably, an inclination angle θ 1 with respect to the tire circumferential direction of the lug grooves 11 formed in the center rib 10 is set to be in a range of 25° to 40°, the lug grooves 11 formed in the center rib 10 are curved toward the shoulder sides, and a radius of curvature R 1 thereof is set to be in a range of 100 mm to 140 mm; and an inclination angle θ 2 with respect to the tire circumferential direction of the lug grooves 21 formed in the center ribs 20 is set to be in a range of 30° to 50°, the lug grooves 21 formed in the center ribs 20 are curved toward the shoulder sides, and a radius of curvature R 2 thereof is set to be in a range of 130 mm to 150 mm. Due to the provision of the lug grooves 11 and 21 formed in the center rib 10 and the center ribs 20 having the set inclination angles θ 1 and θ 2 and set curvature radii R 1 and R 2, the effect of reducing the running noise can be enhanced by keeping the increase in the groove area at areas closer to the center position Ce to the minimum. Note that the inclination angles θ 1 and θ 2 are angles that form straight lines connecting the center positions at the start edges and the center positions at the end edges of the lug grooves 11 and 21 with respect to the tire circumferential direction. In FIG. 2, these settings may be configured, for example, as follows: θ129°, θ2=44°, R1=130 mm, and R2=140 mm.In addition, the lug grooves 31 formed in the shoulder ribs 30 are curved to have a plurality of radii of curvature R 3- 1 and R 3- 2, and the radii of curvature R 3- 1 and R 3- 2 thereof are set to be in a range of 10 mm to 100 mm. By providing the lug grooves 31 formed in the shoulder ribs 30 having the predetermined radii of curvature R 3- 1 and R 3- 2, the groove area in the shoulder regions As can be effectively increased, excellent wet performance can be ensured, and further, rolling resistance can be reduced due to a decrease in the rubber volume in the shoulder regions As. Note that the lug grooves 31 have a structure having another radius of curvature R 3- 3 besides the radii of curvature R 3- 1 and R 3- 2. In FIG. 2, these settings may be configured, for example, as follows: R3-1=60 mm, R3-2=20 mm, and R3-3=180 mm.ExamplesTires were manufactured for Comparative Examples 1 to 5 and Working Examples 1 to 4. A tire size for each of these tires was 195 / 65R15 91H. A pair of first main grooves disposed on both sides of a center position and extending in the tire circumferential direction; a pair of second main grooves disposed further to the shoulder sides than the first main grooves and extending in the tire circumferential direction; a central rib disposed between the pair of first main grooves; center ribs disposed between the first main grooves and the second main grooves; and shoulder ribs disposed on the outer sides of the second main grooves were provided in a tread portion. Pluralities of lug grooves were formed in the central rib, the central ribs, and the shoulder ribs, respectively, the lug grooves having different structures for each tire.The tires of Comparative Examples 1 to 4 and Working Examples 1 to 4 are tires in which, as illustrated in FIG. 1, a plurality of lug grooves extending from a wall surface on the shoulder sides toward center sides and terminating blindly in the rib are respectively formed in the center rib and the center ribs such that a groove width of the lug grooves gradually decreases with proximity to the center sides. In addition, a plurality of lug grooves extending in the tire width direction are formed in the shoulder ribs so as not to be connected to the second main grooves. The tire of Comparative Example 5 has the same structure as that of Embodiment 1, except that each of the lug grooves crosses its corresponding rib, thus dividing a plurality of blocks.In Comparative Examples 1 to 5 and Working Examples 1 to 4, the width of the first main grooves, the width of the second main grooves, the ratio of the total area of the first main grooves and the second main grooves with respect to the area of the ground contact area of the tread portion (indicated as "main groove area ratio" in the table), the ratio of the distance D 1 from the center position of the tread portion to the centers of the first main grooves with respect to the distance D 0 from the center position to the ground contact edge of the tread portion (D 1 / D 0×100) were determined, the ratio of the distance D 2 from the center position of the tread portion to the centers of the second main grooves with respect to the distance D 0 from the center position to the ground contact edge of the tread portion (D 2 / D 0×10%), the groove area ratio of the center region, the groove area ratio of the shoulder regions, and the ratio of the total area of the lug grooves present in the center region with respect to the total area of the lug grooves present in the shoulder regions (indicated as "lug groove area ratio" in the table) are set as indicated in Table 1.In addition, in Comparative Examples 1 to 4 and Working Examples 1 to 4, the length L 1 in the tire width direction of the lug grooves formed in the center rib is set to be 50% of the half width W 1 of the center rib; a length L 2 in the tire width direction of the lug grooves formed in the center ribs is set to be 70% of a width W 2 of the center ribs; and a groove width at a position 3 mm toward a leading edge side from an end edge of each of the lug grooves formed in the center rib and the center ribs is set to be 70% of a groove width at a position 3 mm toward an end edge side from a leading edge. A pitch of the lug grooves formed in the central rib and the central ribs was set to be twice a pitch of the lug grooves formed in the shoulder ribs. The inclination angle θ 1 of the lug grooves formed in the central rib with respect to the tire circumferential direction was set to 29°, and the radius of curvature R 1 thereof was set to 130 mm. The inclination angle θ 2 of the lug grooves formed in the center ribs with respect to the tire circumferential direction was set to 44°, and the radius of curvature R 2 thereof was set to 140 mm. The radii of curvature R3-1, R3-2 and R3-3 of the lug grooves formed in the shoulder ribs were set to 60 mm, 20 mm and 180 mm, respectively.These test tires were evaluated for running noise, rolling resistance, dry road surface steering stability and wet road surface steering stability according to the following evaluation methods. The results thereof are shown in Table 1.Driving noise: Each test tire was mounted on a wheel having a rim size of 15×6JJ, inflated to an air pressure of 230 kPa, and mounted on a test vehicle. The running noise (dB) was measured according to the measurement methods based on the requirements for the measurement of EEC / ECE tire noises found in the European regulations for "running noise".Rolling resistance: Each test tire was mounted on a wheel having a rim size of 15×6JJ and inflated to an air pressure of 230 kPa. Rolling resistance at a speed of 80 km / h was measured. The evaluation results are shown as an index, with Comparative Example 1 being 100. Smaller index values indicate less rolling resistance.Steering stability on dry road surfaces: Each test tire was mounted on a wheel having a rim size of 15×6JJ, inflated to an air pressure of 230 kPa, and mounted on a test vehicle. Sensory evaluation by a test driver was performed on a dry road surface. The results were evaluated by a 5-point method, with Comparative Example 1 as 3 (reference result). Higher results indicate superior steering stability on dry road surfaces.Steering stability on wet road surfaces: Each test tire was mounted on a wheel having a rim size of 15×6JJ, inflated to an air pressure of 230 kPa, and mounted on a test vehicle. Sensory evaluation by a test driver was performed on a wet road surface. The results were evaluated by a 5-point method, with Comparative Example 1 as 3 (reference result). Higher results indicate superior steering stability on wet road surfaces. Table 1 Table 1Width of the first main grooves (mm)6,254,57,05,55,5Width of the second main grooves (mm)6,258,05,57,07,0Main groove area ratio (%)1818181818D1 / D0×100%1919191419D2 / D0×10%6060606060Groove Area Ratio of Center Area (%)2218242523Groove Area Ratio of Shoulder Portions (%)2732262532Lug Groove Area Ratio (%)4040404240Driving Noise (dB)66,065,066,566,566,5Rolling resistance is rolling resistance100961019895Steering stability on dry road surfaces33,252,752,752,675Steering stability on wet road surfaces32,53,253,1253,375Table 1 (continuation)Table 1 (continuation)Width of the first main grooves (mm)5,56,05,55,5Width of the second main grooves (mm)7,07,07,57,0Main groove area ratio (%)18191918D1 / D0×100%19191923D2 / D0×100%62626275Groove Area Ratio of Center Area (%)21182119Groove Area Ratio of Shoulder Portions (%)31313329Lug Groove Area Ratio (%)40404038Driving Noise (dB)65,565,865,565,3Rolling resistance is rolling resistance97969697Steering stability on dry road surfaces3,253,1253,1253,375Steering stability on wet road surfaces3,253,3753,3753,125It is clearly understood from Table 1 that the tires of Working Examples 1 to 4 were each capable of reducing the running noise while maintaining excellent rolling resistance and wet performance as compared with Comparative Example 1. In the tire of Comparative Example 2, on the other hand, the first main grooves of the center sides were excessively narrow as compared with the second main grooves of the shoulder sides, and thereby the steering stability on wet road surfaces was adversely affected. In the tire of Comparative Example 3, the first main grooves of the center sides were wider than the second main grooves of the shoulder sides, and thereby the running noise was enhanced, and the steering stability on dry road surfaces was adversely affected. In the tire of Comparative Example 4, the first main grooves of the center sides were excessively close to the center position of the tread portion, and thereby the running noise was enhanced, and the steering stability on dry road surfaces was adversely affected. In the tire of Comparative Example 5, the ribs were divided by the lug grooves, and thereby the running noise was enhanced and the steering stability on dry road surfaces was adversely affected.REFERENCE NUMERALS:1 Main groove (first main groove) 2 Main groove (second main groove) 10 Central rib 11 Lug groove 20 Center rib 21 Lug groove 30 Shoulder rib 31 Lug groove A Ground contact region Ac Center region As Shoulder region Ce Center position E Ground contact edge T Tread portion

Claims

A pneumatic tire comprising a pair of first main grooves (1) disposed on both sides of a center position (Ce) and extending in the tire circumferential direction, a pair of second main grooves (2) disposed more toward shoulder sides than the first main grooves (1) and extending in the tire circumferential direction; a central rib (10) disposed between the pair of first main grooves (1); center ribs (20) disposed between the first main grooves (1) and the second main grooves (2); A shoulder ribs (30) disposed on outer sides of the second main grooves (2) in a tread portion (T), wherein a plurality of lug grooves (11, 21) extending from a wall surface on the shoulder sides toward center sides and terminating blindly in the rib are formed in the center rib (10) and the center ribs (20), respectively, such that a groove width of the lug grooves (11, 21) gradually decreases with proximity to the center sides; a plurality of lug grooves (31) extending in the tire width direction are formed in the shoulder ribs (30) so as not to be connected to the second main grooves (2); a distance (D1) from the center position (Ce) of the tread portion (T) to the centers of the first main grooves (1) is set to be in a range of 15% to 25% of a distance (D0) from the center position (Ce) to a ground contact edge (E); a distance (D2) from the center position (Ce) of the tread portion (T) to the centers of the second main grooves (2) is set to be in a range of 60% to 80% of the distance from the center position (Ce) to the ground contact edge (E); a width of the first main grooves (1) is set to be in a range of 70% to 90% of a width of the second main grooves (2); a total area of the first main grooves (1) and the second main grooves (2) is set to be in a range of 15% to 25% of an area of a ground contact area (A) of the tread portion (T); and when the ground contact area (A) of the tread portion (T) is divided into a center area (Ac) and shoulder areas (As), wherein when the shoulder areas (As) have a position at 50% of the distance from the center position (Ce) of the tread portion (T) to the ground contact edge (E) as a boundary, a groove area ratio of the center area (Ac) is configured to be less than a groove area ratio of the shoulder areas (As), wherein the ground contact area (A) is defined by a ground contact width in the tire axial direction when the tire is inflated with a maximum air pressure set by a standard on which the pneumatic tire is based, the tire being loaded with 88% of a maximum capacity in a state where the tread portion (T) of the tire is placed perpendicular to a horizontal plane to contact the ground.The pneumatic tire according to claim 1, wherein a total area of the lug grooves (21) present in the center region (Ac) is set to be in a range of 20% to 60% of a total area of the lug grooves (31) present in the shoulder regions (As).The pneumatic tire according to claim 1 or 2, wherein the groove area ratio of the center region (Ac) is set to be in a range of 18% to 22%, and the groove area ratio of the shoulder regions (As) is set to be in a range of 25% to 35%.The pneumatic tire according to any one of claims 1 to 3, wherein: a length (L1) in the tire width direction of the lug grooves (11) formed in the center rib (10) is set to be in a range of 50% to 90% of a half width (W1) of the center rib (10); a length (L2) in the tire width direction of the lug grooves (21) formed in the center ribs (20) is set to be in a range of 50% to 90% of a width (W2) of the center ribs (20); and a groove width at a position 3 mm toward a leading edge side from an end edge of each of the lug grooves (11, 21) formed in the central rib (10) and the central ribs (20) is set to be in a range of 50% to 70% of a groove width at a position 3 mm toward an end edge side from a leading edge.The pneumatic tire according to any one of claims 1 to 4, wherein a pitch (P1, P2) of the lug grooves (11, 21) formed in the central rib (10) and the center ribs (20) is at least twice a pitch (P3) of the lug grooves (31) formed in the shoulder ribs (30).The pneumatic tire according to any one of claims 1 to 5, wherein: an inclination angle (θ1) with respect to the tire circumferential direction of the lug grooves (11) formed in the central rib (10) is set to be in a range of 25° to 40°, the lug grooves (11) formed in the central rib (10) are curved toward the shoulder sides, and a radius of curvature (R1) thereof is set to be in a range of 100 mm to 140 mm; an inclination angle (θ2) with respect to the tire circumferential direction of the lug grooves (21) formed in the center ribs (20) is set to be in a range of 30° to 50°, the lug grooves (21) formed in the center ribs (20) are curved toward the shoulder sides, and a curvature radius (R2) thereof is set to be in a range of 130 mm to 150 mm; and the lug grooves (31) formed in the shoulder ribs (30) are curved to have a plurality of curvature radii (R3-1, R3-2, R3-3), and the curvature radii (R3-1, R3-2, R3-3) thereof are set to be in a range of 10 mm to 100 mm.

Images