tire

Abstract

To provide a tire that can improve the durability of the stress relaxation layer. [Solution] The tire has a tread surface and comprises a tread portion provided with tread rubber, the tread surface having a plurality of circumferential main grooves, a plurality of land portions demarcated by the plurality of circumferential main grooves, and a plurality of vent marks provided along the edges of the plurality of circumferential main grooves. At least one of the plurality of circumferential main grooves has a stress-relieving layer on the surface of the groove bottom. The stress-relieving layer mainly consists of diene-based rubber material and non-diene-based rubber material, and also contains carbon and a vulcanizing agent. The thickness Ga of the stress-relieving layer and the distance W in the tire width direction from the edge to the plurality of vent marks satisfy the relationship 50Ga ≤ W ≤ 500Ga.

Description

Technical Field

[0001] The present invention relates to a tire.

Background Art

[0002] In order to suppress groove cracks generated at the groove bottom of the main groove, a tire provided with a stress relaxation layer at the groove bottom of the main groove is known.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] During vulcanization molding, the generation of chipping on the tire surface is suppressed by discharging the gas between the molding surface of the mold and the tire surface through the vent holes provided on the molding surface. At the same time as discharging the gas, rubber flows into the vent holes, and vent marks are formed on the tire surface after vulcanization molding.

[0005] The inventors of the present invention have found that during vulcanization molding, the position of the vent hole relative to the position of the circumferential main groove may affect the dimensional stability of the stress relaxation layer provided to suppress the generation of groove cracks occurring at the groove bottom of the circumferential main groove. A decrease in the dimensional stability of the stress relaxation layer can be a cause for reducing the durability of the stress relaxation layer.

[0006] An object of the present invention is to provide a tire capable of improving the durability of the stress relaxation layer.

Means for Solving the Problems

[0007] A tire according to one aspect of the present invention comprises a tread portion having a tread surface and provided with tread rubber, the tread surface having a plurality of circumferential main grooves, a plurality of land portions partitioned by the plurality of circumferential main grooves, and a plurality of vent marks provided along the edges of the plurality of circumferential main grooves, at least one of the plurality of circumferential main grooves having a stress-relieving layer on the surface of the groove bottom, the stress-relieving layer mainly composed of diene-based rubber material and non-diene-based rubber material and also containing carbon and a vulcanizing agent, and the thickness Ga of the stress-relieving layer and the distance W in the tire width direction from the edge to the vent mark satisfy the relationship 50Ga ≤ W ≤ 500Ga. [Effects of the Invention]

[0008] According to the present invention, the durability of the stress relaxation layer can be improved. [Brief explanation of the drawing]

[0009] [Figure 1] This is an end view showing the meridional cross-sectional shape of the tire according to this embodiment. [Figure 2] This is a partially enlarged view along line II-II in Figure 1. [Figure 3] This is a partial plan view of the tire according to this embodiment. [Figure 4] This is a perspective view of a vent mark according to a modified example of this embodiment. [Figure 5A] This is a cross-sectional view of a vent mark in a modified example. [Figure 5B] This is a cross-sectional view of a vent mark related to another modified example. [Figure 5C] This is a cross-sectional view of a vent mark relating to yet another modified example. [Figure 6A] This is a partially enlarged view of a tire according to a modified example of this embodiment, along the line II-II in Figure 1. [Figure 6B] This is a partially enlarged view of a tire according to another modified example of this embodiment, along the line II-II in Figure 1. [Modes for carrying out the invention]

[0010] Embodiments of the present invention relate to the following aspects.

[0011] [Aspect 1] It has a tread surface and a tread portion provided with tread rubber, The tread surface has a plurality of circumferential main grooves, a plurality of land areas formed by the plurality of circumferential main grooves, and a plurality of vent marks provided along the edges of the plurality of circumferential main grooves. At least one of the plurality of circumferential main grooves has a stress-relieving layer on the surface of the groove bottom, The stress-relieving layer mainly consists of diene-based rubber material and non-diene-based rubber material, and also contains carbon and a vulcanizing agent. A tire in which the thickness Ga of the stress relaxation layer and the distance W in the tire width direction from the edge to the plurality of vent marks satisfy the relationship 50 ≤ W / Ga ≤ 500. [Aspect 2] The tire according to embodiment 1, wherein the thickness Ga, the maximum depth Dm of the circumferential main groove having the stress relaxation layer, and the distance W in the tire width direction from the edge to the plurality of vent marks satisfy the relationship -400 ≤ (Dm - 5W) / Ga ≤ 400. [Aspect 3] A tire according to embodiment 1 or 2, wherein the average value La of the distance between two adjacent vent marks in the tire circumferential direction among the plurality of vent marks provided along the edge, and the groove width GW of the circumferential main groove in which the plurality of vent marks are provided satisfy the relationship 0.5 ≤ La / GW ≤ 6. [Aspect 4] A tire according to any one of embodiments 1 to 3, wherein the hardness Ha of the stress relaxation layer and the hardness Hc of the tread rubber satisfy the relationship |Ha-Hc|≦5. [Aspect 5] A tire according to any one of embodiments 1 to 4, wherein the stress-relieving layer integrally comprises a bottom portion provided at the bottom of the circumferential main groove, a wall portion provided on at least one of the pair of groove walls of the circumferential main groove, and a surface portion provided on the tread surface along the edge, and the length Wa in the tire width direction of the surface portion and the thickness Ga satisfy the relationship 10 ≤ Wa / Ga ≤ 300. [Aspect 6] The tire according to any one of Aspects 1 to 5, wherein at least one of the pair of edges of the circumferential main groove having the stress relaxation layer is continuous in the tire circumferential direction. [Aspect 7] The tire according to any one of Aspects 1 to 6, wherein the diameter of the plurality of vent marks in plan view and the average value Lb of the circumferential interval of the sub-grooves penetrating the circumferential main groove having the stress relaxation layer satisfy the relationship of 3.5 ≤ Lb / d ≤ 45. [Aspect 8] The tire according to Aspect 5, wherein the area of the surface portion is 12% or less with respect to the ground contact area of the tread portion. [Aspect 9] Taking the ratio Hg / Hmax of the minimum thickness Hg of the tread portion to the maximum thickness Hmax of the tread portion as R, the ratio R, the minimum distance Gu in the tire radial direction from the groove bottom of the circumferential main groove to the reinforcing layer, and the thickness Ga of the stress relaxation layer satisfy the following formula (1). The tire according to any one of Aspects 1 to 8. [Number] [Aspect 10] The tire according to any one of Aspects 1 to 9, wherein the stress relaxation layer is provided in the circumferential main groove provided within the maximum belt width region.

[0012] [Definition] The tire radial direction means the direction orthogonal to the tire rotation axis. The tire radial direction inner side means the side facing the tire rotation axis in the tire radial direction, and the tire radial direction outer side means the side away from the tire rotation axis in the tire radial direction. The tire circumferential direction means the circumferential direction around the tire rotation axis as the central axis. The tire width direction means the direction parallel to the tire rotation axis. The tire width direction inner side means the side facing the tire equatorial plane (tire equatorial plane) in the tire width direction, and the tire width direction outer side means the side away from the tire equatorial plane in the tire width direction. The tire equatorial plane means a plane that is orthogonal to the tire rotation axis and passes through the center of the tire width of the tire. "Following" a certain standard means following a direction within a range of less than ±20°, less than ±10°, or less than ±5° relative to that standard. "Center" means the midpoint where the distance from two points is equal, and the range of ±10% of the distance between the two points from the midpoint. The circumferential main groove is a circumferential groove having a wear indicator that shows the end of wear, and generally has a groove width of 3.0 mm or more and a groove depth of 5.0 mm or more. However, the groove width and groove depth of the circumferential main groove are not limited to the above ranges. The groove width is measured as the maximum distance between opposing groove walls at the groove opening on the tread surface when the tire is mounted on a standard rim and filled to the standard internal pressure in an unloaded state. In the case of a configuration with a notch or chamfer at the groove opening, the groove width is the value measured with the endpoint being the intersection of the extension line of the tread surface and the extension line of the groove wall in a cross-sectional view parallel to the groove width direction and groove depth direction. Groove depth is measured as the maximum distance from the tread surface to the bottom of the groove when the tire is mounted on a standard rim, filled to the standard internal pressure, and under no load. If the groove in question has partial irregularities or sipes at the bottom of the groove, the groove depth shall be the value measured excluding these irregularities or sipes. The tread edge refers to the ends of the tread pattern on a tire, and is also called the design end. The contact end is the maximum position in the tire width direction at the contact surface between the tire and the flat plate when the tire is mounted on a regular rim, subjected to regular internal pressure, and placed perpendicular to the flat plate in a stationary state, with a load corresponding to the regular load (80% of the maximum load capacity) applied. In this specification, unless otherwise specified, the shape, position, and length (distance) of each component shall be based on the shape, position, and length in the meridional section of the tire. A "regular rim" refers to an "applicable rim" as defined by JATMA, a "Design Rim" as defined by TRA, or a "Measuring Rim" as defined by ETRTO. Standard internal pressure refers to the "maximum air pressure" specified by JATMA, the maximum value listed in "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES" specified by TRA, or the "INFLATION PRESSURES" specified by ETRTO. Standard load refers to the "maximum load capacity" specified by JATMA, the maximum value listed in "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES" specified by TRA, or the "LOAD CAPACITY" specified by ETRTO.

[0013] (Tire configuration) One embodiment of the present invention will be described below with reference to the drawings. Figure 1 is an end view showing the meridional cross-sectional shape of the tire 10 according to this embodiment. In this figure, the tire portion is shown in a state where it is mounted on a regular rim and subjected to regular internal pressure, and in an unloaded state.

[0014] The tire 10 of this embodiment comprises, in a meridional cross-sectional view of the tire, a pair of bead portions 12, a pair of sidewall portions 14, a pair of shoulder portions 16, and a tread portion 18 arranged from the inside to the outside in the radial direction of the tire. The tire 10 comprises a carcass layer 20 and a reinforcing layer 22. The tire has an inner liner layer 23 arranged along the carcass layer 20 and exposed to the inner cavity of the tire.

[0015] Each pair of bead sections 12 has a bead core 24 and a bead filler 26. The bead core 24, although not shown, has a bead wire wound in an annular manner around the tire rotation axis. The bead wire is made of steel. The bead filler 26 is provided on the radially outer side of the bead core 24.

[0016] The carcass layer 20 has carcass plies (not shown) and, in a meridional cross-sectional view of the tire, extends from the tread portion 18 to the bead portions 12 on both outer sides in the tire width direction, and is wound back from the inside to the outside in the tire width direction so as to enclose a pair of bead cores 24 and bead fillers 26. The carcass plies are formed by covering multiple carcass cords made of steel or organic fiber material (e.g., aramid, nylon, polyester, rayon, etc.) with coating rubber and then rolling them.

[0017] The reinforcing layer 22 has a belt layer 28. The belt layer 28 has one or more belts 30. The belts 30 are formed by covering a belt cord made of steel or organic fiber material with a coating rubber and rolling it, and are positioned on the outer side of the carcass layer 20 in the tire radial direction. The reinforcing layer 22 may further have a belt cover layer 32. The belt cover layer 32 has one or more belt covers 34a and edge covers 34b. The belt covers 34a and edge covers 34b are formed by covering a belt cover cord made of steel or organic fiber material with a coating rubber and rolling it, and are positioned on the outer side of the belt layer 28 in the tire radial direction. The belt cover 34a covers the entire width of the belt layer 28, and the edge cover 34b locally covers the end of the belt layer 28 in the tire width direction.

[0018] The length between the outer edge of the belt layer 28 in the tire width direction and the tire equatorial plane CP is defined as Dc. Furthermore, the maximum belt width region WB is defined as the region from both outer ends in the tire width direction to the inner end in the tire width direction of the belt 30 or belt cover 34, which is located on the outermost side in the tire radial direction within the reinforcing layer 22.

[0019] Each of the pair of sidewall sections 14 is provided with sidewall rubber 36. The sidewall rubber 36 is positioned on the outer side in the tire width direction of the carcass layer 20 and extends from the bead section 12 to the shoulder section 16.

[0020] The tread portion 18 has a tread surface 37 and is provided with tread rubber 38. The tread rubber 38 is located on the radially outer side of the carcass layer 20, belt layer 28, and belt cover layer 32, and extends from one shoulder portion 16 in the tire width direction, through the tire equatorial plane CP, to the other shoulder portion 16 in the tire width direction.

[0021] The tread rubber 38 is formed from a rubber material with excellent contact characteristics and weather resistance, and is exposed on the tread surface 37. Preferably, the tread rubber 38 contains silica, wax, and an anti-aging agent.

[0022] The wax can be a plant-derived wax, paraffin wax, microcrystalline wax, polyethylene wax, or a mixture thereof. In particular, to ensure crack resistance at low temperatures, a low-melting-point wax that can easily precipitate and spread on the groove bottom surface even at low temperatures is preferred, for example, a wax with a melting point of 40 to 65°C is preferably selected. The tread rubber preferably contains 1.0 part by mass or more of wax when the rubber component is 100 parts by mass.

[0023] The anti-aging agent is preferably an amine-based anti-aging agent. Examples of amine-based anti-aging agents include "N-phenyl-N'-1,3-dimethylbutyl-p-phenylenediamine" and "2,2,4-trimethyl-1,2-dihydroquinoline polymer".

[0024] The tread surface 37 has multiple circumferential main grooves 40, four in the case of Figure 1. The tread surface 37 is divided into multiple, five rows of land areas 42 in the case of Figure 1 by the circumferential main grooves 40. The circumferential main grooves 40 are annular and continuous in the circumferential direction of the tire.

[0025] As shown in Figure 2, the circumferential main groove 40 has a groove bottom 44, a pair of groove walls 46, and a pair of edges 48. The groove bottom 44 is the bottom of the circumferential main groove 40 and defines the groove depth Dm of the circumferential main groove 40. The groove bottom 44 has a surface that follows the tread surface 37, with the tire width direction as the short direction and the tire circumferential direction as the long direction. The pair of groove walls 46 are continuous with the outer edge of the groove bottom 44 in the tire width direction and define the groove width of the circumferential main groove 40. The groove width of the circumferential main groove 40 gradually decreases from the tread surface 37 toward the inside in the tire radial direction. The pair of groove walls 46 have a surface that intersects the tread surface 37, with the tire radial direction as the short direction and the tire circumferential direction as the long direction. The pair of edges 48 are the boundary between the pair of groove walls 46 and the tread surface 37. The edges 48 extend in the tire circumferential direction along the circumferential main groove 40. Let Gu be the minimum distance in the radial direction of the tire from the groove bottom 44 to the belt layer 28 (or the belt cover layer 32 if one is present).

[0026] The maximum thickness of the tread portion 18, i.e., the length between the tread surface 37 and the inner surface 50 of the tire in the land portion 42, is defined as Hmax (Figure 1). The minimum thickness of the tread portion 18, i.e., the length between the groove bottom 44 of the circumferential main groove 40 and the inner surface 50 of the tire, is defined as Hg (Figure 1).

[0027] As shown in Figure 3, the tread surface 37 has four circumferential main grooves 40 in addition to a plurality of sub-grooves 41. Each sub-groove 41 extends in a direction intersecting the circumferential main grooves 40. The land portions 42 located outside the pair of outermost main grooves 40S in the tire width direction are called shoulder land portions 42S, and the three land portions 42 located inside the pair of outermost main grooves 40S in the tire width direction are called center land portions 42C. Shoulder lug grooves 41S, which serve as sub-grooves, are arranged at predetermined intervals in the tire circumferential direction in the shoulder land portions 42S. Multiple center lug grooves 41C, which serve as sub-grooves, are arranged at predetermined intervals Lb in the tire circumferential direction in the center land portions 42C. One end 41G of the center lug groove 41C is connected to the circumferential main groove 40, and the other end 41L is located within the center land portion 42C. The center lug groove 41C is located on only one side in the tire width direction relative to each circumferential main groove 40, and not on the other side in the tire width direction. That is, one edge 48N in the tire width direction of each circumferential main groove 40 is partitioned at a predetermined interval by the center lug groove 41C. On the other side of each circumferential main groove 40, the edge 48F in the tire width direction does not have a center lug groove 41C and is continuous in the tire circumferential direction.

[0028] The tread surface 37 further has a plurality of vent marks 43. The vent marks 43 are provided along the edges 48 of the circumferential main grooves 40. The vent marks 43 are cylindrical in shape and integral with the tread portion 18, and protrude outward from the tread surface 37 in the tire radial direction (Figure 2). Let d be the diameter of the vent mark 43 in plan view. Let W be the distance in the tire width direction between the center of the vent mark 43 in the tire width direction and the edge 48.

[0029] Multiple vent marks 43 are provided along the circumferential main groove 40 at predetermined intervals La in the tire circumferential direction. The interval La is determined by focusing on a vent mark 43 located on one or the other side in the tire width direction of a single circumferential main groove, and dividing the tire circumferential length of the tread surface 37 at the position where the vent mark 43 is formed by the number of vent marks 43 arranged in the tire circumferential direction for one full tire circumference.

[0030] (Stress relaxation layer) The tire 10 is provided with a stress-relieving layer 52 on the surface of the groove bottom 44 of the circumferential main groove 40 (Figure 2). The stress-relieving layer 52 has a bottom portion 54 provided on the groove bottom 44 of the circumferential main groove 40. The bottom portion 54 is formed over the entire groove bottom 44. The bottom portion 54 is annular and continuous in the circumferential direction of the tire along the circumferential main groove 40.

[0031] The stress relaxation layer 52 may further have a wall portion 56 and a surface portion 58. The wall portion 56 is provided on each of the pair of groove walls 46 of the circumferential main groove 40. One end 56U of the wall portion 56 on the inner side in the tire radial direction is connected to the bottom portion 54, and the other end 56T on the outer side in the tire radial direction reaches the edge 48. The surface portion 58 is provided on the tread surface 37 along the edge 48. One end 58G of the surface portion 58 in the tire width direction is connected to the other end 56T of the wall portion 56 on the outer side in the tire radial direction via the edge 48, and the other end 58L in the tire width direction is positioned at a predetermined length, for example, 0.5 mm to 5 mm, in the tire width direction from the edge 48 toward the land portion 42. Here, the length in the tire width direction between one end 58G and the other end 58L of the surface portion 58 is Wa. The thickness Ga of the stress relaxation layer 52 is preferably 5 μm or more and 100 μm or less, more preferably 8 μm or more and 80 μm or less, and even more preferably 10 μm or more and 50 μm or less. The thickness Ga of the stress relaxation layer 52 is the value measured at the center of the groove width of the circumferential main groove in the meridional cross-section of the tire. The thickness Ga of the stress relaxation layer 52 being within the above range makes it easier to follow the deformation of the tread portion 18.

[0032] The stress relaxation layer 52 mainly consists of diene-based rubber material and non-diene-based rubber material, and also contains carbon and a vulcanizing agent. The diene-based rubber is selected from the group consisting of diene polymers including natural rubber and synthetic diene-based rubber (isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene rubber (SBR), butadiene-isoprene rubber (BIR), styrene-isoprene rubber (SIR), styrene-isoprene-butadiene rubber (SBIR), chloroprene rubber (CR), etc.). The non-diene-based rubber is selected from the group consisting of non-diene polymers including synthetic non-diene-based rubber (butyl rubber (IIR), ethylene-propylene rubber (EPDM, EPM), urethane rubber, silicone rubber, etc.). Furthermore, it is preferable that the stress relaxation layer 52 does not contain resin components in order to ensure weather resistance.

[0033] The stress relaxation layer 52 does not necessarily have to contain an anti-aging agent. Because the stress relaxation layer 52 has a thin thickness Ga, the anti-aging agent contained in the tread rubber is migrated to the stress relaxation layer 52 to compensate for this. The stress relaxation layer 52 can further suppress the occurrence of cracks by containing an anti-aging agent. If the stress relaxation layer 52 contains an anti-aging agent, it is preferable to use other anti-aging agents (e.g., phenolic, phosphite, organic thioacid, benzimidazole, etc.) in an amount of 0.1 parts by mass to 5 parts by mass per 100 parts by mass of the rubber component, rather than using amine-based anti-aging agents.

[0034] The thickness Ga of the stress relaxation layer 52 and the distance W in the tire width direction between the center of the vent mark 43 in the tire width direction and the edge 48 satisfy the relationship 50 ≤ W / Ga ≤ 500. This is not limited to the case where all of the multiple vent marks 43 provided on the tire 10 satisfy the above relationship. When defining the distance W, the edge 48 is the edge 48 of the circumferential main groove 40 and does not include the edge of the sub-groove 41. It is more preferable that the thickness Ga and distance W satisfy the relationship 60 ≤ W / Ga ≤ 400. It is preferable that 75% or more of the vent marks 43 satisfy the above relationship, and it is more preferable that 80% or more of the vent marks 43 satisfy the above relationship.

[0035] It is preferable that the depth Dm of the circumferential main groove 40, in which the stress relaxation layer 52 is provided, and the distance W in the tire width direction between the vent mark 43 and the edge 48 satisfy the relationship -400 ≤ (Dm - 5W) / Ga ≤ 400. It is more preferable that the relationship between the depth Dm and the distance W satisfies -350 ≤ (Dm - 5W) / Ga ≤ 350. It is preferable that 75% of the vent marks 43 satisfy the above relationship, and it is more preferable that 80% or more of the vent marks 43 satisfy the above relationship.

[0036] It is preferable that the interval La between adjacent vent marks 43 in the circumferential direction of the tire and the groove width GW of the circumferential main groove 40 in which the vent marks 43 are provided satisfy the relationship 0.5 ≤ La / GW ≤ 6. It is more preferable that the interval La and groove width GW satisfy the relationship 0.6 ≤ La / GW ≤ 5, and even more preferable that they satisfy the relationship 0.8 ≤ La / GW ≤ 3.

[0037] It is preferable that the hardness Ha of the stress relaxation layer 52 and the hardness Hc of the tread rubber 38 satisfy the relationship |Ha-Hc|≦5. More preferably, the relationship |Ha-Hc|≦4.5 is satisfied between hardness Ha and hardness Hc. Hardnesses Ha and Hc are hardnesses measured in accordance with JIS K6253 using a durometer type A under temperature conditions of 20℃±2℃. The hardness Hc of the tread rubber 38 is measured as the rubber hardness of the rubber material that is in surface contact with the stress relaxation layer 52 among the rubber materials constituting the tread rubber 38.

[0038] It is preferable that the thickness Ga of the stress relaxation layer 52 and the length Wa of the surface portion 58 in the tire width direction satisfy the relationship 10 ≤ Wa / Ga ≤ 300. More preferably, the thickness Ga and length Wa satisfy the relationship 12 ≤ Wa / Ga ≤ 250.

[0039] For a given circumferential main groove 40 of interest, it is preferable that the diameter d of the vent mark 43 and the circumferential spacing Lb of the center lug groove 41C connected to the circumferential main groove 40 where the stress relaxation layer 52 is provided satisfy the relationship 3.5 ≤ Lb / d ≤ 45 on at least one side in the tire width direction. It is more preferable that the diameter d and spacing Lb satisfy the relationship 4.0 ≤ Lb / d ≤ 35, and even more preferable that they satisfy the relationship 4.5 ≤ Lb / d ≤ 30. The spacing Lb is preferably 15 mm or more and 50 mm or less, more preferably 18 mm or more and 45 mm or less, and even more preferably 20 mm or more and 40 mm or less.

[0040] It is preferable that the area of ​​the surface portion 58 is 12% or less of the contact area of ​​the tread portion 18. The contact area refers to the area of ​​contact between the tire and the contact surface. The contact area is measured at the contact surface between the tire 10 and the flat plate when the tire 10 is mounted on a regular rim, subjected to regular internal pressure, and placed perpendicular to a flat plate in a stationary state, and a load corresponding to a regular load (80% of the maximum load capacity) is applied. It is more preferable that the area of ​​the surface portion 58 relative to the contact area of ​​the tread portion 18 is 10% or less.

[0041] The ratio R of the maximum thickness Hmax of the tread portion 18 to the minimum thickness Hg of the tread portion 18 is defined as Hg / Hmax. Preferably, the above ratio R, the minimum distance Gu in the tire radial direction from the groove bottom 44 to the belt layer 28 (or the belt cover layer 32 if a belt cover layer 32 is present), and the thickness Ga of the stress relaxation layer 52 satisfy the following formula (1). By satisfying the following formula (1), the tire 10 can achieve both durability of the stress relaxation layer 52 and rolling resistance of the tire.

[0042]

number

[0043] When the maximum thickness Hmax is large and the minimum thickness Hg is small, i.e., when R is small, the amount of deformation of the tread rubber 38 at the bottom of the groove 44 is large, making it difficult to obtain durability of the stress relaxation layer 52, and the rolling resistance of the tire 10 tends to increase. On the other hand, when R is large, the amount of deformation at the bottom of the groove 44 is kept small, making it easier to obtain durability of the stress relaxation layer 52, and the rolling resistance of the tire 10 tends to decrease.

[0044] Furthermore, when the minimum distance Gu is large and the thickness Ga is small (when Ga / Gu is small), the thickness Ga of the stress relaxation layer 52 becomes small relative to the thickness of the tread rubber 38 at the minimum distance Gu, i.e., the groove bottom 44, so the durability of the stress relaxation layer 52 becomes difficult to obtain. On the other hand, when Ga / Gu is large, the thickness Ga of the stress relaxation layer 52 becomes thicker relative to the thickness of the tread rubber 38 at the groove bottom 44, so the durability of the stress relaxation layer 52 becomes easier to obtain.

[0045] By being above the lower limit of equation (1) above, the stress relaxation layer 52 has sufficient thickness relative to the thickness of the tread rubber 38 at the groove bottom 44, thus providing an effect of suppressing the occurrence of groove cracks. Furthermore, because the tread rubber 38 at the groove bottom 44 has sufficient thickness, the amount of deformation at the groove bottom 44 can be suppressed, thus suppressing an increase in the rolling resistance of the tire 10.

[0046] By keeping the value below the upper limit of equation (1) above, the stress relaxation layer 52 does not become too thick relative to the thickness of the tread rubber 38 at the groove bottom 44, and can follow the deformation of the tread portion 18, thereby suppressing the occurrence of groove cracks. Furthermore, by maintaining an appropriate thickness of the tread rubber 38 at the groove bottom 44, an unnecessary increase in tire mass can be suppressed, thereby suppressing an increase in the rolling resistance of the tire 10.

[0047] The stress relaxation layer 52 is preferably provided in the circumferential main groove 40 located within the maximum belt width region WB. The maximum belt width region WB has high rigidity and low strain during operation because the belt 30 or belt cover 34 is provided in that region. Therefore, by providing the stress relaxation layer 52 in the circumferential main groove 40 located within the maximum belt width region WB, strain can be reduced and durability can be improved. The stress relaxation layer 52 is preferably provided in the circumferential main groove 40 located in a region within the maximum belt width region WB where two or more belt covers 34 are provided.

[0048] Of the circumferential main grooves 40 provided with the stress relaxation layer 52, the circumferential main groove 40 located on the outermost side in the tire width direction is specifically called the outermost main groove 40S. The distance between the center of the groove width of the outermost main groove 40S and the tire equatorial plane CP is defined as Dg. The distance between the outer edge of the belt 30 or belt cover 34 located on the outermost side in the tire width direction of the reinforcing layer 22 and the tire equatorial plane CP is defined as Dc. The ratio of Dg to Dc (Dg / Dc) is preferably 0.3 or more and 0.7 or less.

[0049] Generally, during the manufacturing process, when inflating the green tire, when mounting the tire 10 onto the rim, and when the tire 10 makes contact with the ground, the tread portion 18 experiences greater stress closer to the tread edge. As a result, the strain generated at the groove bottom 44 of the outermost main groove 40S, which is located near the tread edge, also increases. Therefore, by keeping the above ratio (Dg / Dc) above the lower limit, the strain generated at the groove bottom 44 can be suppressed, thereby improving the durability of the stress relaxation layer 52. By keeping the above ratio (Dg / Dc) below the upper limit, appropriate rigidity of the tread portion 18 can be obtained, resulting in excellent handling stability.

[0050] The tire 10 of this embodiment described above is obtained through the usual manufacturing processes, namely the mixing process of tire materials, the processing process of tire materials, the molding process of green tires, the vulcanization process, and the inspection process after vulcanization. When manufacturing the tire 10 of this embodiment, a coating agent that forms a stress-relieving layer 52 is applied to the green tire before vulcanization, to a region including a predetermined position, i.e., the position where the circumferential main groove 40 is formed. The coating agent mainly consists of the above-mentioned diene-based rubber material and non-diene-based rubber material, and also contains carbon, a vulcanizing agent, and a vulcanization accelerator. Subsequently, by going through the vulcanization process, a tire 10 can be obtained in which a stress-relieving layer 52 is formed at least at the groove bottom 44 of the circumferential main groove 40. In the vulcanization process, a vulcanization mold is used, which has convex and concave portions corresponding to a predetermined tread pattern formed on its inner wall, and is further provided with vent holes connected to an exhaust passage.

[0051] (Mechanism of Action and Effects) Generally, during the extrusion process in tire manufacturing, residual stress tends to occur at the groove bottom 44 of the circumferential main groove 40 when the tread portion 18 is extruded. This residual stress can cause groove cracks to occur depending on the usage conditions.

[0052] The tire 10 according to this embodiment can alleviate residual stress and suppress the occurrence of groove cracks by providing a stress-relieving layer 52 at the bottom of the groove 44. In particular, the tire 10 according to this embodiment can uniformly form the stress-relieving layer 52 during the vulcanization process by satisfying the relationship 50 ≤ W / Ga ≤ 500 between the thickness Ga of the stress-relieving layer 52 and the distance W in the tire width direction between the center of the vent mark 43 in the tire width direction and the edge 48.

[0053] In the case of the lower limit of the above range, the stress relaxation layer 52 is thicker, and the vent marks 43 are formed closer to the edge 48. A thicker stress relaxation layer 52 is more effective in suppressing groove cracks, but it becomes more difficult to obtain the original properties of the tread rubber. When the vent marks 43 are formed closer to the edge 48, it is easier to suppress chipping of the tread surface 37 by releasing gas from the vent holes during the vulcanization process. On the other hand, when the vent holes are located near the outer edge of the stress relaxation layer 52 in the tire width direction, the stress relaxation layer 52 is more susceptible to the effects of rubber flow due to gas release during the vulcanization process.

[0054] By being above the lower limit of the above range, the stress relaxation layer 52 can obtain the inherent properties of the tread rubber while suppressing the occurrence of groove cracks. Furthermore, by being above the lower limit of the above range, the stress relaxation layer 52 is positioned in a location that is less affected by the rubber flow during the vulcanization process, so the stress relaxation layer 52 can obtain excellent durability. Therefore, the tire 10 can improve the durability of the stress relaxation layer 52. Moreover, the smaller the distance W, the more likely local deformation may occur in the circumferential main groove due to the vent marks 43 during contact. By being above the lower limit of the above range, the position of the vent marks 43 can be set to a position that is less likely to affect the stress relaxation layer 52 formed in the circumferential main groove 40, so the durability of the stress relaxation layer 52 can be further improved.

[0055] In the case of the upper limit of the above range, the stress relaxation layer 52 is thinner, and the vent marks 43 are formed further away from the edge 48. When the stress relaxation layer 52 is thinner, the effect of suppressing groove cracks is less obtained due to the thinner thickness of the stress relaxation layer 52, and the stress relaxation layer 52 is more susceptible to the influence of rubber flowing into the vent holes along with the gas discharged from the vent holes during the vulcanization process. Also, when the vent marks 43 are formed further away from the edge 48, it does not affect the formation of the stress relaxation layer 52, but the effect of suppressing chipping of the tread surface 37 during the vulcanization process is less obtained.

[0056] By being below the upper limit of the above range, the stress relaxation layer 52 has sufficient thickness, which suppresses the formation of extremely thin areas during the vulcanization process, thus providing excellent durability. Furthermore, by being below the upper limit of the above range, vent holes can be provided in a position that effectively suppresses chipping of the tread surface 37 while avoiding affecting the stress relaxation layer 52.

[0057] By satisfying the relationship -400 ≤ (Dm - 5W) / Ga ≤ 400 between the depth Dm of the circumferential main groove 40, where the stress relaxation layer 52 is provided, and the distance W in the tire width direction between the vent mark 43 and the edge 48, it is possible to form a circumferential main groove 40 with an appropriate groove depth and a stress relaxation layer 52 with excellent durability during the vulcanization process. Furthermore, the deeper the groove depth Dm of the circumferential main groove 40, the lower the block rigidity.

[0058] In the case of the lower limit of the above range, the thickness Ga of the stress relaxation layer 52 is greater, and the value of the distance W relative to the groove depth Dm of the circumferential main groove 40 is greater. Because it is above the lower limit of the above range, the groove depth Dm of the circumferential main groove 40 is shallow, but the value of the distance W is greater, so the vent holes are positioned sufficiently far from the circumferential main groove 40, and the stress relaxation layer 52, which is provided around the groove bottom 44 during the vulcanization process, can be made less susceptible to the effects of rubber flow. Therefore, the tire 10 has excellent dimensional stability of the stress relaxation layer 52. In addition, even if the value of the distance W is large, the effect of suppressing chipping of the tread surface 37 is obtained because the groove depth Dm is shallow, and the decrease in drainage performance can be suppressed because the circumferential main groove 40 has a groove depth Dm of a certain level or more. Furthermore, although a shallower groove depth Dm is more susceptible to the effects of ultraviolet rays, a stress relaxation layer 52 with sufficient thickness Ga can be obtained.

[0059] When the upper limit of the above range is reached, the thickness Ga of the stress relaxation layer 52 is thinner, and the value of the distance W relative to the groove depth Dm of the circumferential main groove 40 is smaller. Because the distance W is smaller but the groove depth Dm of the circumferential main groove 40 is deeper, the stress relaxation layer 52, which is provided mainly around the groove bottom 44 during the vulcanization process, can be made less susceptible to the effects of rubber flow. Furthermore, even if the groove depth Dm is deep, the smaller value of the distance W suppresses the occurrence of chipping of the tread surface 37, thereby suppressing a decrease in the rigidity of the tread portion 18. Moreover, a stress relaxation layer 52 with a thickness Ga suitable for the groove depth Dm can be obtained.

[0060] By ensuring that the spacing La between adjacent vent marks in the circumferential direction of the tire and the groove width GW of the circumferential main groove 40 in which the vent marks 43 are provided satisfy the relationship 0.5 ≤ La / GW ≤ 6, it is possible to form a stress-relieving layer 52 with excellent durability while suppressing the occurrence of chipping of the tread surface 37.

[0061] When the above range is lower, the groove width GW of the circumferential main groove 40 is larger, and the spacing La between the vent marks 43 is smaller. When the groove width GW of the circumferential main groove 40 is large, the rigidity of the tread portion 18 decreases, and the land portion 42 adjacent to the circumferential main groove 40 becomes more prone to deformation. By being above the lower limit of the above range, the deformation of the stress relaxation layer 52 provided in the circumferential main groove 40 is suppressed, and excellent durability of the stress relaxation layer 52 is obtained.

[0062] At the upper limit of the above range, the groove width GW of the circumferential main groove 40 is smaller, and the spacing La between the vent marks 43 is larger. When the spacing La between the vent marks 43 is large, it becomes more difficult to obtain the effect of suppressing the occurrence of chipping of the tread surface 37. By keeping it below the upper limit of the above range, the effect of suppressing the occurrence of chipping of the tread surface 37 can be obtained more reliably.

[0063] By ensuring that the hardness Ha of the stress relaxation layer 52 and the hardness Hc of the tread rubber 38 satisfy the relationship |Ha-Hc|≦5, the ability of the stress relaxation layer 52 to follow the deformation of the tread portion 18 can be improved. In other words, the tire 10 can achieve excellent durability of the stress relaxation layer 52 by reducing the internal stress generated between the stress relaxation layer 52 and the tread portion 18 when in contact with the ground.

[0064] By ensuring that the thickness Ga of the stress relaxation layer 52 and the length Wa of the surface portion 58 in the tire width direction satisfy the relationship 10 ≤ Wa / Ga ≤ 300, the decrease in wet performance in the initial stages of wear can be suppressed, and the excellent durability of the stress relaxation layer 52 can be obtained.

[0065] When the value is at the lower limit of the above range, the thickness Ga of the stress relaxation layer 52 is greater, and the length Wa of the surface portion 58 is smaller. If the thickness Ga of the stress relaxation layer 52 is greater, the period during which the area of ​​the tread rubber 38 exposed on the tread surface 37 is small is longer, and as a result, the wet performance in the initial stages of wear tends to decrease. By being above the lower limit of the above range, the surface portion 58 disappears in the initial stages of wear, thereby exposing the tread rubber 38, and thus the decrease in wet performance in the initial stages of wear can be suppressed.

[0066] At the upper limit of the above range, the thickness Ga of the stress relaxation layer 52 is thinner, and the length Wa of the surface portion 58 is greater. If the length Wa of the surface portion 58 is smaller than the thickness Ga of the stress relaxation layer 52, the stress relaxation layer 52 is more susceptible to the deformation of the tread portion 18, and is more likely to peel off from the tread portion 18 at the edges in the tire width direction. Durability tends to decrease. By keeping it below the upper limit of the above range, peeling off from the tread portion 18 at the edges, for example, the surface portion 58, can be suppressed.

[0067] The edges 48F on the other side in the tire width direction of each circumferential main groove 40 do not have a center lug groove 41C and are continuous in the circumferential direction of the tire, thereby suppressing a decrease in the rigidity of the land portion 42 adjacent to the circumferential main groove 40. Therefore, the stress relaxation layer 52 provided in the circumferential main groove 40 is deformed during contact with the ground, resulting in excellent durability.

[0068] On at least one side of the circumferential main groove 40 in the tire width direction, the diameter d of the vent mark 43 and the distance Lb in the tire circumferential direction between the center lug groove 41C connected to the circumferential main groove 40 on which the stress relaxation layer 52 is provided satisfy the relationship 3.5 ≤ Lb / d ≤ 45. This allows for the formation of a stress relaxation layer 52 with excellent durability while suppressing the occurrence of chipping of the tread surface 37.

[0069] At the lower limit of the above range, the diameter d of the vent marks 43 is larger, and the spacing Lb of the center lug grooves 41C in the tire circumferential direction is smaller. When the spacing Lb is small, the rigidity of the tread portion decreases, and the land portion 42 adjacent to the circumferential main groove 40 becomes more prone to deformation. By being above the lower limit of the above range, the deformation of the stress relaxation layer 52 provided in the circumferential main groove 40 is suppressed, and excellent durability of the stress relaxation layer 52 is obtained.

[0070] At the upper limit of the above range, the diameter d of the vent marks 43 is smaller and the spacing Lb is larger. When the diameter d of the vent marks 43 is small, it becomes more difficult to obtain the effect of suppressing the occurrence of chipping of the tread surface 37. By keeping it below the upper limit of the above range, the effect of suppressing the occurrence of chipping of the tread surface 37 can be obtained more reliably.

[0071] By ensuring that the surface area 58 is 12% or less of the contact area of ​​the tread portion 18, the amount of tread rubber 38 exposed on the tread surface 37 is maintained at a certain level or higher, thereby suppressing a decrease in wet performance in the initial stages of wear.

[0072] (modified version) Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment, and various modifications are possible without departing from the spirit of the invention.

[0073] In the above embodiment, the vent mark 43 is described as being cylindrical and protruding radially outward from the tread surface 37, but the present invention is not limited to this. For example, the vent mark 43 may have an annular recess 60 provided on the tread surface 37, as shown in Figure 4. The vent mark 43B is formed by molding with a mold having a vent plug loaded into a vent hole, although this is not shown. The vent mark 43B has an annular recess 60 and a central portion 62 surrounded by the recess 60. As shown in Figure 5A, the recess 60 is recessed radially inward from the tread surface 37. The central portion 62 is at the same height as the tread surface 37.

[0074] The central portion 62 is not limited to being at the same height as the tread surface 37; as shown in Figure 5B, it may be recessed inward in the tire radial direction from the tread surface 37. In this case, the height of the central portion 62 in the tire radial direction is between the tread surface 37 and the recess 60. Furthermore, as shown in Figure 5C, the central portion 62 may protrude outward in the tire radial direction from the tread surface 37.

[0075] In the above embodiment, the stress relaxation layer 52 was described as having a bottom portion 54, a wall portion 56, and a surface portion 58, but the present invention is not limited thereto. For example, as shown in Figure 6A, the stress relaxation layer 52 does not have to have a surface portion 58. That is, the stress relaxation layer 52 shown in Figure 6A has a bottom portion 54 and a pair of wall portions 56. One end 56U of the pair of wall portions 56, which is on the inner side in the tire radial direction, is connected to the bottom portion 54, and the other end 56T, which is on the outer side in the tire radial direction, is positioned on the edge 48.

[0076] Furthermore, as shown in Figure 6B, the other end 56T of the pair of wall portions 56 of the stress relaxation layer 52, which is on the radially outer side of the tire, may be positioned within the groove wall 46.

[0077] The depth Dm of the circumferential main groove 40, which is provided with the stress relaxation layer 52, and the distance W in the tire width direction between the vent mark 43 and the edge 48 are not limited to satisfying the relationship -400 ≤ (Dm - 5W) / Ga ≤ 400, but (Dm - 5W) / Ga may be less than -400 or greater than 400.

[0078] The distance La between adjacent vent marks 43 in the circumferential direction of the tire and the groove width GW of the circumferential main groove 40 in which the vent marks 43 are provided do not necessarily satisfy the relationship 0.5 ≤ La / GW ≤ 6; La / GW may be less than 0.5 or greater than 6.

[0079] The relationship between the hardness Ha of the stress relaxation layer 52 and the hardness Hc of the tread rubber 38 is not limited to the case where |Ha-Hc|≦5 is satisfied; |Ha-Hc| may also be greater than 5.

[0080] The thickness Ga of the stress relaxation layer 52 and the length Wa of the surface portion 58 in the tire width direction are not limited to satisfying the relationship 10 ≤ Wa / Ga ≤ 300; Wa / Ga may be less than 10 or greater than 300.

[0081] The relationship between the diameter d of the vent mark 43 and the distance Lb in the tire circumferential direction of the center lug groove 41C connected to the circumferential main groove 40 provided with the stress relaxation layer 52 is not limited to cases where Lb / d is less than 3.5 or greater than 45.

[0082] The area of ​​the surface portion 58 is not limited to 12% or less of the contact area of ​​the tread portion 18, but may be greater than 12%.

[0083] In the above embodiment, the case described is one in which the edge 48F on the other side in the tire width direction of each circumferential main groove 40 does not have a center lug groove 41C and is continuous in the tire circumferential direction, but the present invention is not limited to this. That is, in at least a portion of each circumferential main groove 40, the edge 48F on the other side in the tire width direction does not have a center lug groove 41C and is continuous in the tire circumferential direction. [Examples]

[0084] This paper describes the results of manufacturing a tire corresponding to the invention defined in the claims of this application and evaluating the durability of the stress relaxation layer.

[0085] (sample) A test tire with tire size 225 / 65R17 was mounted on a rim with rim size 17×6.5J, and this test tire was subjected to an internal pressure of 230 [kPa] and a normal load of 6.0 [kN]. The tire conditions for Examples 1 to 16 and Reference Examples 1 to 2 are as shown in Tables 1-1 and 1-2 below.

[0086] (Evaluation of crack resistance performance) After leaving tires in a room maintained at an ozone concentration of 100±5 pphm, a temperature of 25±2°C, and an internal pressure of 230±2 kPa for 24 hours, the number of cracks formed in the circumferential main grooves of the tires was measured. Based on these measurement results, an index was expressed with a reference example set as the baseline (100). In this evaluation, a higher numerical value is preferable.

[0087] (Evaluation of the durability performance of the stress relaxation layer) In evaluating the durability performance of the stress relaxation layer, an indoor drum testing machine with a drum diameter of 1707 mm is used, and the distance traveled until delamination of the stress relaxation layer occurs is measured under conditions of a travel speed of 120 km / h. Based on these measurement results, an index evaluation is performed with Example 1 as the baseline (100). A higher numerical value in this evaluation is preferable.

[0088] [Table 1-1]

[0089] [Table 1-2]

[0090] Examples 1-16 showed improved durability of the stress relaxation layer compared to Reference Examples 1 and 2 when W / Ga was between 50 and 500. In contrast, Reference Example 1 had a W / Ga of 40, and Reference Example 2 had a W / Ga of 520, so no improvement in the durability of the stress relaxation layer was observed. Examples 3 and 4 showed improved durability and crack resistance of the stress relaxation layer compared to Examples 1 and 2 by satisfying the relationship -400 ≤ (Dm-5W) / Ga ≤ 400. Examples 5 and 6 showed improved durability and crack resistance of the stress relaxation layer compared to Examples 3 and 4 by satisfying the relationship 0.5 ≤ La / GW ≤ 6. Examples 7 and 8 showed improved durability and crack resistance of the stress relaxation layer compared to Examples 5 and 6 by satisfying the relationship 0.8 ≤ La / GW ≤ 3. Example 9 showed improved durability and crack resistance of the stress relaxation layer compared to Examples 7 and 8 by satisfying the relationship |Ha-Hc| ≤ 5. Examples 10 and 11 showed improved durability and crack resistance of the stress relaxation layer compared to Example 9 by satisfying the relationship 10 ≤ Wa / Ga ≤ 300. Example 12 showed improved crack resistance compared to Example 11 by satisfying the relationship 12 ≤ Wa / Ga ≤ 250. Examples 13 to 15 showed improved durability and crack resistance of the stress relaxation layer compared to Example 12 by satisfying the relationship 3.5 ≤ Lb / d ≤ 45. In particular, Example 15 showed improved durability and crack resistance of the stress relaxation layer compared to Examples 13 and 14 because Lb / d was 15. [Explanation of symbols]

[0091] 10 tires 12 Bead section 14 Sidewall section 16 Shoulder section 18 Tread section 20 Carcass Layers 22 Reinforcement layer 23 Inner liner layer 24 Bead Core 26 Bead Filler 28 Belt Layer 30 belts 32 Belt cover layer 34a Belt cover 34b Edge Cover 36 Sidewall rubber 37 Tread surface 38 Tread Rubber 40 Circumferential main groove 40S outermost main groove 41 Sub-groove 41S Shoulder lug groove 41C Center lug groove 41G one end 41L other end 42 Land 42S Shoulder Track and Field Club 42C Center Track and Field Club 43, 43B Vent marks 44 Groove bottom 46 Ditch wall 48 Edge 48N One side edge 48F Other side edge 50 Tire inner surface 52 Stress relaxation layer 54 Bottom 56 Wall 56U one end 56T other end 58 Surface part 58G one end 58L other end 60 recesses 62 Central part CP Tire Equatorial Plane E Ground end

Claims

1. It has a tread surface and a tread portion provided with tread rubber, The tread surface has a plurality of circumferential main grooves, a plurality of land areas formed by the plurality of circumferential main grooves, and a plurality of vent marks provided along the edges of the plurality of circumferential main grooves. At least one of the plurality of circumferential main grooves has a stress-relieving layer on the surface of the groove bottom, The stress-relieving layer mainly consists of diene-based rubber material and non-diene-based rubber material, and also contains carbon and a vulcanizing agent. A tire in which the thickness Ga of the stress relaxation layer and the distance W in the tire width direction from the edge to the plurality of vent marks satisfy the relationship 50 ≤ W / Ga ≤ 500.

2. The tire according to claim 1, wherein the thickness Ga, the maximum depth Dm of the circumferential main groove having the stress relaxation layer, and the distance W in the tire width direction from the edge to the plurality of vent marks satisfy the relationship -400 ≤ (Dm - 5W) / Ga ≤ 400.

3. The tire according to claim 1, wherein the average value La of the distance between two adjacent vent marks in the tire circumferential direction among the plurality of vent marks provided along the edge, and the groove width GW of the circumferential main groove in which the plurality of vent marks are provided satisfy the relationship 0.5 ≤ La / GW ≤ 6.

4. The tire according to claim 1, wherein the hardness Ha of the stress relaxation layer and the hardness Hc of the tread rubber satisfy the relationship |Ha - Hc| ≤ 5.

5. The tire according to claim 1, wherein the stress-relieving layer integrally comprises a bottom portion provided at the bottom of the circumferential main groove, a wall portion provided on at least one of the pair of groove walls of the circumferential main groove, and a surface portion provided on the tread surface along the edge, wherein the length Wa in the tire width direction of the surface portion and the thickness Ga satisfy the relationship 10 ≤ Wa / Ga ≤ 300.

6. The tire according to claim 1, wherein at least one of the pair of edges of the circumferential main groove having the stress-relieving layer is continuous in the circumferential direction of the tire.

7. The tire according to claim 1, wherein the diameter d of the plurality of vent marks in a plan view and the average value Lb of the distance between the sub-grooves penetrating the circumferential main groove having the stress relaxation layer in the tire circumferential direction satisfy the relationship 3.5 ≤ Lb / d ≤ 45.

8. The tire according to claim 5, wherein the area of ​​the surface portion is 12% or less of the contact area of ​​the tread portion.

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