JP7583243B2 - Pneumatic tires - Google Patents

Description

The present invention relates to a pneumatic tire, and more specifically, to a pneumatic tire that, by defining the relationship between the thermal expansion coefficient and cross-sectional area of the rubber that constitutes the tread rubber layer in the center region and shoulder region, makes it possible to suppress groove cracks by mitigating the initial tensile strain caused by thermal contraction of the rubber after vulcanization and the tensile strain caused by the opening of the grooves after assembly with the rim and inflation, while maintaining cornering power and rolling resistance.

When cracks occur in the main grooves formed in the tread of a pneumatic tire, the tire may need to be replaced before it reaches the end of its wear life. Such groove cracks expand from two sources: the initial tensile strain caused by thermal contraction of the rubber after vulcanization, and the tensile strain caused by the opening of the grooves after the tire is fitted to the rim and inflated.

In order to suppress group cracks in the tread portion, the compounding of the rubber that constitutes the tread portion has been devised (for example, see Patent Document 1). However, there has been no attempt to specify the relationship between the thermal expansion coefficient and the cross-sectional area of the rubber that constitutes the tread portion in a specific region of the tread portion.

Patent No. 6025494

The object of the present invention is to provide a pneumatic tire that, by defining the relationship between the thermal expansion coefficient and cross-sectional area of the rubber that constitutes the tread rubber layer in the center region and shoulder region, can maintain cornering power and rolling resistance while mitigating the initial tensile strain caused by thermal contraction of the rubber after vulcanization and the tensile strain caused by the opening of the grooves after assembly to the rim and inflation, thereby suppressing groove cracks.

In order to achieve the above object, a pneumatic tire of the present invention includes at least one carcass layer mounted between a pair of bead portions, two or more belt layers disposed on the outer peripheral side of the carcass layer in a tread portion, and a tread rubber layer disposed on the outer peripheral side of the belt layer and made of two or more types of rubber, wherein at least two main grooves extending in the tire circumferential direction are formed in the tread portion, and the pneumatic tire has a plurality of land portions separated by these main grooves, wherein a length CW obtained by dividing a length in the tire width direction measured along the belt layer in a belt crossing region where belt cords constituting the belt layer cross between layers, and a length GW in the tire width direction measured along the belt layer from the tire centerline to the groove center of the outermost main groove in the tire width direction satisfy a relationship of 0.35≦GW/CW≦0.55, and a tire meridian cross section of the main grooves satisfies the relationship of 0.35≦GW/CW≦0.55, The present invention is characterized in that, when a groove bottom reference line that defines a groove bottom position and an outer reference line that passes through the outer end of the belt crossing region and is perpendicular to the belt layer are defined, and a shoulder region defined by the groove bottom reference line and the outer reference line in a land portion adjacent to the outer side in the tire width direction of the outermost main groove in the tire width direction, and a center region defined by the groove bottom reference line in a land portion adjacent to the inner side in the tire width direction of the outermost main groove in the tire width direction, a sum CE calculated from the following formula (1) which is a relational expression between the thermal expansion coefficient EX_CEn and the cross-sectional area CS_CEn of each rubber constituting the tread rubber layer in the center region, and a sum SH calculated from the following formula (2) which is a relational expression between the thermal expansion coefficient EX_SHn and the cross-sectional area CS_SHn of each rubber constituting the tread rubber layer in the shoulder region satisfy the relationship CE<SH. Here, "n" is the number of types of rubber constituting the tread rubber layer.
CE=EX_CE1 ² ×CS_CE1+EX_CE2 ² ×CS_CE2+...+EX_CEn ² ×CS_CEn (1)
SH=EX_SH1 ² ×CS_SH1+EX_SH2 ² ×CS_SH2+...+EX_SHn ² ×CS_SHn (2)

The inventors have conducted extensive research into pneumatic tires that have at least one carcass layer mounted between a pair of bead portions, two or more belt layers arranged on the outer periphery of the carcass layer in the tread portion, and a tread rubber layer made of two or more types of rubber arranged on the outer periphery of the belt layer, in which at least two main grooves extending in the tire circumferential direction are formed in the tread portion, and which have a plurality of land portions separated by these main grooves. As a result, the inventors have discovered that, in order to suppress groove cracks as described above, the position at which the tensile strain is maximum after the tire is assembled on the rim and inflated varies depending on the position of the main groove, and that the maximum value of the tensile strain can be reduced by controlling the thermal contraction of the rubber after vulcanization in the land portions (areas) located on both sides of the main groove where groove cracks are likely to occur, and have arrived at the present invention.

In the present invention, the length CW obtained by dividing the tire width direction length measured along the belt layer in the belt crossing region where the belt cords constituting the belt layer cross between the layers satisfies the relationship of 0.35≦GW/CW≦0.55, and the sum CE calculated from the above formula (1) which is the relation between the thermal expansion coefficient EX_CEn and the cross-sectional area CS_CEn of each rubber constituting the tread rubber layer in the center region, and the sum SH calculated from the above formula (2) which is the relation between the thermal expansion coefficient EX_SHn and the cross-sectional area CS_SHn of each rubber constituting the tread rubber layer in the shoulder region, satisfy the relationship of CE<SH. That is, focusing on the main groove (outermost main groove) on the outermost side in the tire width direction where groove cracks are likely to occur, the position of the outermost main groove is specified, and the relation between the thermal expansion coefficient and the cross-sectional area of each rubber constituting the tread rubber layer in the center region and the shoulder region located on both sides of the outermost main groove is specified. As described above, by satisfying the relationship of 0.35≦GW/CW≦0.55 between the length CW and the length GW, the outermost main groove is positioned closer to the inner side in the tire width direction of the tread portion, which acts in the direction of alleviating the tensile strain in the groove bottom region of the outermost main groove after the tire is assembled to the rim and inflated. In addition, by satisfying the relationship of CE<SH between the sum CE of the center region calculated from the above formula (1) and the sum SH of the shoulder region calculated from the above formula (2), the tensile strain in the groove bottom region of the outermost main groove after the tire is assembled to the rim and inflated can be alleviated, and the thermal contraction of the rubber after vulcanization can be controlled, so that the maximum value of the tensile strain can be reduced. This makes it possible to suppress groove cracks in the outermost main groove, which is prone to groove cracks. Furthermore, by arranging the outermost main groove closer to the inner side in the tire width direction of the tread portion, the cross-sectional area of the center region becomes relatively smaller than the cross-sectional area of the shoulder region, so that cornering power and rolling resistance can be maintained without deterioration.

In the pneumatic tire of the present invention, it is preferable that the sum CE and the sum SH satisfy the relationship 0.50≦CE/SH≦0.85. This makes it possible to sufficiently reduce the tensile strain at the bottom of the outermost main groove, where groove cracks are likely to occur, and effectively suppress groove cracks.

Of the two or more types of rubber constituting the tread rubber layer, it is preferable that the thermal expansion coefficient of the cap rubber located at the outermost position in the tire radial direction is smaller than the thermal expansion coefficient of the rubber other than the cap rubber, and the rubber thickness of the rubber other than the cap rubber of the tread rubber layer in the center region is thinner than the rubber thickness of the rubber other than the cap rubber of the tread rubber layer in the shoulder region. This makes it possible to adjust the position at which the tensile strain of the groove bottom of the outermost main groove is maximized while suppressing the thermal contraction of the cap rubber, thereby effectively suppressing groove cracks.

The bottom of the outermost main groove in the tire width direction is composed of two or more arcs, and it is preferable that the radius of curvature Rsh of the arc on the shoulder region side at the bottom of the outermost main groove in the tire width direction is larger than the radius of curvature Rce of the arc on the center region side. This makes it possible to effectively suppress groove cracks after the tire is assembled to the rim and inflated.

It is preferable that the tread portion has three or more of the above-mentioned main grooves, the groove width of the outermost main groove in the tire width direction is larger than the groove width of the main groove located on the inner side of the outermost main groove in the tire width direction, and the average value of the radius of curvature R of the arc constituting the groove bottom of the outermost main groove in the tire width direction is larger than the average value of the radius of curvature R of the arc constituting the groove bottom of the main groove located on the inner side of the outermost main groove in the tire width direction. This makes it possible to effectively suppress groove cracks after assembly to the rim and inflation.

The groove bottom of the outermost main groove in the tire width direction is composed of two or more arcs, and it is preferable that the radius of curvature Rce of the arc on the center region side at the groove bottom of the outermost main groove in the tire width direction and the radius of curvature Rsh of the arc on the shoulder region side at the groove bottom of the outermost main groove in the tire width direction satisfy the relationship 0.25≦Rce/Rsh≦0.50. This makes it possible to sufficiently reduce the tensile strain at the groove bottom of the outermost main groove, and effectively suppress groove cracks.

The thermal expansion coefficient of each rubber constituting the tread rubber layer is preferably in the range of 0.8×10 ⁻⁴ /° C. to ^{3.0×10 −4} /° C. This makes it possible to sufficiently obtain the effect of reducing the tensile strain at the groove bottom of the outermost main groove, and effectively suppress groove cracks.

1 is a meridian cross-sectional view showing an example of a pneumatic tire according to an embodiment of the present invention. 2A and 2B are enlarged cross-sectional views showing a main portion of the pneumatic tire of FIG. 1, where (a) shows various dimensions and (b) shows the division between the center region and the shoulder region. 2 is a cross-sectional view showing dimensions of a groove bottom of a main groove formed in the pneumatic tire of FIG. 1 . FIG. FIG. 4 is a cross-sectional view showing a modified example of an outermost main groove formed in a pneumatic tire according to an embodiment of the present invention.

The configuration of the present invention will be described in detail below with reference to the attached drawings. Figures 1 to 3 show a pneumatic tire according to an embodiment of the present invention.

As shown in FIG. 1, a pneumatic tire according to an embodiment of the present invention includes a tread portion 1 that extends in the circumferential direction of the tire to form an annular shape, a pair of sidewall portions 2 disposed on both sides of the tread portion 1, and a pair of bead portions 3 disposed on the radially inner side of the sidewall portions 2. The pneumatic tire shown in FIG. 1 has a symmetrical structure on both sides of the tire center line CL.

At least one carcass layer 4 (one layer in FIG. 1) is fitted between a pair of bead portions 3. This carcass layer 4 includes multiple reinforcing cords extending in the tire radial direction, and is folded from the inside to the outside of the tire around the bead cores 5 arranged in each bead portion 3. As the reinforcing cords of the carcass layer 4, organic fiber cords such as polyester are preferably used, but steel cords may also be used. A bead filler 6 made of a rubber composition with a triangular cross section is arranged on the outer periphery of the bead cores 5.

On the other hand, multiple belt layers 7 are embedded on the outer peripheral side of the carcass layer 4 in the tread portion 1. In FIG. 1, the belt layer 7 includes an inner belt 7A adjacent to the outer side of the carcass layer 4 in the tire radial direction, and an outer belt 7B located on the outer side of the inner belt 7A in the tire radial direction. These belt layers 7 include multiple reinforcing cords that are inclined with respect to the tire circumferential direction, and are arranged so that the reinforcing cords cross each other between the layers. In the belt layer 7, the inclination angle of the reinforcing cords with respect to the tire circumferential direction is set to, for example, a range of 10° to 40°. Steel cords are preferably used as the reinforcing cords of the belt layer 7. At least one belt cover layer 8 is arranged on the outer peripheral side of the belt layer 7, in which reinforcing cords are arranged at an angle of, for example, 5° or less with respect to the tire circumferential direction, in order to improve high-speed durability. In FIG. 1, the belt cover layer 8 constitutes a pair of edge cover layers that locally cover both ends of the belt layer 7. As the reinforcing cords of the belt cover layer 8, organic fiber cords such as nylon and aramid are preferably used.

At least two main grooves 9 extending in the tire circumferential direction are formed in the tread portion 1. In FIG. 1, the main grooves 9 include a main groove 9 (inner main groove 9A) located on the tire center line CL and a pair of main grooves 9 (outermost main grooves 9B) located on the tire width direction outer side. These main grooves 9 divide the tread portion 1 into multiple rows of land portions 10 extending in the tire circumferential direction. The land portions 10 include a land portion 10 (center land portion 10A) adjacent to the outer side in the tire width direction of the outermost main groove 9B, and a land portion 10 (shoulder land portion 10B) adjacent to the outer side in the tire width direction of the outermost main groove 9B.

In the above pneumatic tire, the region where the belt cords constituting the belt layer 7 cross each other is called the belt crossing region. The length in the tire width direction measured along the belt layer 7 of this belt crossing region is called the length BW (see FIG. 1). The length BW corresponds to the total width of the outer belt 7B, which has a narrower belt width, out of the inner belt 7A and the outer belt 7B. The length obtained by dividing the length BW in half is called the length CW, and the length in the tire width direction measured along the belt layer 7 from the tire center line CL to the groove center of the outermost main groove 9B is called the length GW (see FIG. 2(a)). In this case, the length CW and the length GW satisfy the relationship 0.35≦GW/CW≦0.55.

In addition, in the above pneumatic tire, a tread rubber layer 20 made of two or more types of rubber is disposed on the outer peripheral side of the belt layer 7 and the belt cover layer 8. In Fig. 2(a) and (b), the tread rubber layer 20 is composed of two types of rubber: a cap rubber 21 that forms the tread surface of the tread portion 1, and a base rubber 22 that is adjacent to the cap rubber 21 on the radially inner side of the tire.

2(a), in the tire meridian cross section, a groove bottom reference line GL (dashed line in the figure) that defines the groove bottom position of the main groove 9 and an outer reference line OL that passes through the outer end of the belt crossing region (the outer end of the outer belt 7B) and is perpendicular to the belt layer 7 are defined. Here, the groove bottom reference line GL is a line that connects a plurality of points that are a reference distance from the belt layer 7 or belt cover layer 8 to the tire radial direction outer side at 3 mm intervals from the tire center line CL toward the tire width direction outer side, and the lowest point of the main groove 9. This reference distance is, for example, the average value of the rubber thickness a of the tread rubber layer 20 measured along a line L1 that passes through the lowest point of the inner main groove 9A and is perpendicular to the belt layer 7, and the rubber thickness b of the tread rubber layer 20 measured along a line L2 that passes through the lowest point of the outermost main groove 9B and is perpendicular to the belt layer 7. In addition, in FIGS. 2(a) and 2(b), the tire center line CL and the line L1 coincide with each other.

Furthermore, a center region Ace is defined by the groove bottom reference line GL in the land portion 10 (center land portion 10A) adjacent to the inner side of the outermost main groove 9B in the tire width direction, and a shoulder region Ash is defined by the groove bottom reference line GL and the outer reference line OL in the land portion 10 (shoulder land portion 10B) adjacent to the outer side of the outermost main groove 9B in the tire width direction. That is, as shown in FIG. 2(b), the center region Ace is the entire shaded area in the center land portion 10A, and the shoulder region Ash is the entire shaded area in the shoulder land portion 10B.

For the center region Ace and the shoulder region Ash, a relational expression (the product of the square of the thermal expansion coefficient and the cross-sectional area) between the thermal expansion coefficient and the cross-sectional area of the rubber constituting the tread rubber layer 20 is specified. Specifically, the sum CE is calculated from the following formula (1), which is a relational expression between the thermal expansion coefficients EX_CE1, EX_CE2 of the cap rubber 21 and the base rubber 22 in the center region Ace and the cross-sectional areas CS_CE1, CS_CE2, and the sum SH is calculated from the following formula (2), which is a relational expression between the thermal expansion coefficients EX_SH1, EX_SH2 of the cap rubber 21 and the base rubber 22 in the shoulder region Ash and the cross-sectional areas CS_SH1, CS_SH2. The calculated sums CE and SH satisfy the relationship CE<SH.
CE=EX_CE1 ² ×CS_CE1+EX_CE2 ² ×CS_CE2 (1)
SH=EX_SH1 ² ×CS_SH1+EX_SH2 ² ×CS_SH2 (2)

In FIG. 2B, the cross-sectional area CS_CE1 of the cap rubber 21 in the center region Ace corresponds to the area of the region Ace1 (diagonal lines drawn upwards to the right), and the cross-sectional area CS_CE2 of the base rubber 22 in the center region Ace corresponds to the area of the region Ace2 (diagonal lines drawn downwards to the right). The cross-sectional area CS_SH1 of the cap rubber 21 in the shoulder region Ash corresponds to the area of the region Ash1 (diagonal lines drawn upwards to the right), and the cross-sectional area CS_SH2 of the base rubber 22 in the shoulder region Ash corresponds to the area of the region Ash2 (diagonal lines drawn downwards to the right).

The thermal expansion coefficient [10 ^-4 /°C] indicates the rate at which the length or volume of an object expands due to an increase in temperature. In the present invention, it is an index showing the degree of thermal contraction of rubber after vulcanization, and the larger the thermal expansion coefficient, the easier it is to contract. This thermal expansion coefficient can be changed by changing the rubber polymer or by compounding a large amount of silica in the rubber composition. For example, when a large amount of silica is compounded, the value of the thermal expansion coefficient of rubber becomes large.

In general, the area of the tread portion that is radially inward of the main grooves is structurally constrained, so the rubber is less likely to thermally shrink after vulcanization. Similarly, the area that is radially outward of the belt layer is structurally constrained, so there is little effect on groove cracks in the outermost main grooves. Therefore, in order to suppress groove cracks, it is effective to specify the relationship between the thermal expansion coefficient and cross-sectional area of the rubber that constitutes the tread rubber layer in the area adjacent to the outermost main groove, and to reduce the tensile strain in the groove bottom area of the main groove after assembly and inflation to the rim, and the tensile strain due to thermal shrinkage of the rubber after vulcanization.

In contrast, in the pneumatic tire of the present invention, the length CW obtained by dividing the tire width direction length measured along the belt layer 7 in the belt intersection region in half and the length GW in the tire width direction measured along the belt layer 7 from the tire centerline CL to the groove center of the outermost main groove 9B satisfy the relationship 0.35≦GW/CW≦0.55, and the sum CE calculated from the relationship between the thermal expansion coefficients EX_CE1, EX_CE2 of the cap rubber 21 and the base rubber 22 in the center region Ace and the cross-sectional areas CS_CE1, CS_CE2 and the sum SH calculated from the relationship between the thermal expansion coefficients EX_SH1, EX_SH2 of the cap rubber 21 and the base rubber 22 in the shoulder region Ash and the cross-sectional areas CS_SH1, CS_SH2 satisfy the relationship CE<SH. That is, focusing on the outermost main groove 9B where groove cracks are likely to occur, the position of the outermost main groove 9B is specified, and the relational expression between the thermal expansion coefficient and the cross-sectional area of the rubber constituting the tread rubber layer 20 in the center region Ace and shoulder region Ash located on both sides of the outermost main groove 9B is specified. As described above, by making the length CW and the length GW satisfy the relationship of 0.35≦GW/CW≦0.55, the outermost main groove 9B is positioned closer to the inner side in the tire width direction of the tread portion 1, which acts in the direction of alleviating the tensile strain in the groove bottom region of the outermost main groove 9B after the tire is assembled on the rim and inflated. In addition, by satisfying the relationship CE<SH between the sum CE of the center region Ace calculated from the above formula (1) and the sum SH of the shoulder region Ash calculated from the above formula (2), the tensile strain in the groove bottom region of the outermost main groove 9B after assembly to the rim and inflation can be alleviated, and the thermal contraction of the rubber after vulcanization can be controlled, so that the maximum value of the tensile strain can be reduced. This makes it possible to suppress groove cracks in the outermost main groove 9B, where groove cracks are likely to occur. Furthermore, since the outermost main groove 9B is positioned closer to the inner side in the tire width direction of the tread portion 1, the cross-sectional area of the center region Ace becomes relatively smaller than the cross-sectional area of the shoulder region Ash, so that cornering power and rolling resistance can be maintained without deterioration.

Here, if the ratio GW/CW is less than 0.35, the outermost main groove 9B is positioned too far inward in the tire width direction, which tends to deteriorate cornering power and rolling resistance. Conversely, if the ratio GW/CW is greater than 0.55, it becomes easier to reduce rolling resistance and increase cornering power, but it becomes more difficult to suppress groove cracks.

In the above pneumatic tire, it is preferable that the sum CE and the sum SH satisfy the relationship 0.50≦CE/SH≦0.85. By appropriately setting the ratio CE/SH of the sum CE and the sum SH in this way, it is possible to sufficiently reduce the tensile strain at the bottom of the outermost main groove 9B, where groove cracks are likely to occur, and groove cracks can be effectively suppressed.

If the ratio CE/SH of the sum CE to the sum SH is less than 0.50, the difference in thermal contraction between the center region Ace and the shoulder region Ash becomes too great, and the tensile strain at the bottom of the outermost main groove 9B becomes large. Conversely, if the ratio CE/SH is greater than 0.85, the difference in thermal contraction between the center region Ace and the shoulder region Ash does not become large, and the effect of reducing the tensile strain at the bottom of the outermost main groove 9B cannot be fully obtained.

Of the two or more types of rubber constituting the tread rubber layer 20, the thermal expansion coefficient of the cap rubber 21 is preferably smaller than the thermal expansion coefficient of the rubber other than the cap rubber 21. This relationship may be satisfied in at least one of the center region Ace and the shoulder region Ash. Furthermore, the rubber thickness g (see FIG. 2(b)) of the rubber other than the cap rubber 21 in the center region Ace is preferably thinner than the rubber thickness g of the rubber other than the cap rubber 21 in the shoulder region Ash. Here, the rubber thickness g of the rubber other than the cap rubber 21 is the average value of the rubber thicknesses measured every 1 mm in the tire width direction along the direction perpendicular to the belt layer 7 for the rubber other than the cap rubber 21 in each of the center region Ace and the shoulder region Ash. In this embodiment, the rubber constituting the tread rubber layer 20 is the cap rubber 21 and the base rubber 22, so the rubber other than the cap rubber 21 is only the base rubber 22.

By setting the thermal expansion coefficient and rubber thickness of the rubber that constitutes the tread rubber layer 20 in this way, it is possible to adjust the position at which the tensile strain at the bottom of the outermost main groove 9B is maximized while suppressing the thermal contraction of the cap rubber 21, thereby effectively suppressing groove cracks.

The thermal expansion coefficient of the rubber constituting the tread rubber layer 20 is preferably in the range of 0.8×10 ⁻⁴ /° C. to 3.0 ^{×10 −4} /° C. In this way, by appropriately setting the thermal expansion coefficient of the rubber constituting the tread rubber layer 20, it is possible to sufficiently obtain the effect of reducing the tensile strain at the groove bottom of the outermost main groove 9B, and to effectively suppress groove cracks.

Figure 4 shows a modified example of the outermost main groove formed in a pneumatic tire according to an embodiment of the present invention. As shown in Figure 4, the groove bottom of the outermost main groove 9B is composed of two or more arcs r, and it is preferable that the radius of curvature Rsh of the arc r2 on the shoulder region Ash side of the outermost main groove 9B is larger than the radius of curvature Rce of the arc r1 on the center region Ace side. In the present invention, as shown in Figure 3, the radius of curvature R of the arc r constituting the groove bottom of the main groove 9 (e.g., the outermost main groove 9B) is calculated based on three arbitrary points P on the arc r within a range of 1.6 mm radially outward from the lowest point of the main groove 9, by dividing the groove bottom with a line L (e.g., L2) that passes through the lowest point of the main groove 9 and is perpendicular to the belt layer 7.

In the present invention, the relationship between CW and the length GW is 0.35≦GW/CW≦0.55, so the outermost main groove 9B is positioned closer to the inner side in the tire width direction of the tread portion 1, but after mounting to the rim and inflation, the tensile strain in the groove bottom region on the outer side in the tire width direction of the outermost main groove 9B tends to increase. Therefore, by relatively increasing the radius of curvature Rsh of the arc r2 on the shoulder region Ash side (outer side in the tire width direction) in the groove bottom region of the outermost main groove 9B, the tensile strain after mounting to the rim and inflation can be reduced, and groove cracks can be effectively suppressed.

In particular, it is preferable that the radius of curvature Rce of the arc r1 on the center region Ace side at the groove bottom of the outermost main groove 9B and the radius of curvature Rsh of the arc r2 on the shoulder region Ash side satisfy the relationship 0.25≦Rce/Rsh≦0.50. In this case, it is preferable that the radius of curvature Rce of the arc r1 on the center region Ace side is 1.4 mm or more. In this way, by appropriately setting the ratio Rce/Rsh of the radius of curvature Rce and the radius of curvature Rsh, it is possible to sufficiently reduce the tensile strain at the groove bottom of the outermost main groove 9B, and effectively suppress groove cracks.

If the ratio Rce/Rsh is less than 0.25, the tensile strain at the bottom of the outermost main groove 9B on the center region Ace side becomes excessively large, and if the ratio Rce/Rsh is greater than 0.50, the effect of reducing the tensile strain at the bottom of the outermost main groove 9B cannot be sufficiently achieved.

In the above pneumatic tire, three or more main grooves 9 are formed in the tread portion 1, and the groove width Wb (see FIG. 2(a)) of the outermost main groove 9 in the tire width direction is preferably larger than the groove width Wa (see FIG. 2(a)) of the main groove 9 located on the inner side of the main groove 9 in the tire width direction. This allows the arc r constituting the groove bottom of the outermost main groove 9 in the tire width direction to be larger. Furthermore, it is preferable that the average value of the curvature radius R of the arc r constituting the groove bottom of the outermost main groove 9 in the tire width direction is larger than the average value of the curvature radius R of the arc r constituting the groove bottom of the main groove 9 located on the inner side of the main groove 9 in the tire width direction. By configuring in this way, groove cracks can be effectively suppressed after assembly to the rim and inflation.

In the above explanation, an example was shown in which the tread rubber layer 20 is made of two types of rubber, the cap rubber 21 and the base rubber 22, but this is not limited to the above, and the tread rubber layer 20 can be made by laminating three or more types of rubber. In this case, the sums CE and SH are calculated according to the number of types of rubber (n number) that make up the tread rubber layer 20.

Although an example in which three main grooves are formed in the tread portion 1 has been shown, this is not limiting. For example, if two main grooves 9 are formed in the tread portion 1, the tread portion 1 is divided into three land portions 10, and the center region Ace is an area defined by the groove bottom reference line GL and the tire center line CL in the land portion 10 adjacent to the inner side of the main groove 9 in the tire width direction. In this case, the reference distance defining the groove bottom reference line GL is the rubber thickness of the tread rubber layer 20 measured along a line that passes through the lowest point of the main groove 9 on either side of the tire center line CL and is perpendicular to the belt layer 7. In addition, when four or more main grooves 9 are formed in the tread portion 1, the tread portion 1 is divided into five or more land portions 10, and the center region Ace is the region defined by the groove bottom reference line GL in the land portion 10 adjacent to the inner side in the tire width direction of the outermost main groove 9 in the tire width direction, and the shoulder region Ash is the region defined by the groove bottom reference line GL and the outer reference line OL in the land portion 10 adjacent to the outer side in the tire width direction of the outermost main groove 9 in the tire width direction.

A pneumatic tire having a tire size of 155/65R14, at least one carcass layer mounted between a pair of bead portions, two or more belt layers arranged on the outer periphery of the carcass layer in the tread portion, and a tread rubber layer made of two types of rubber arranged on the outer periphery of the belt layer, in which three main grooves extending in the tire circumferential direction are formed in the tread portion and which has a plurality of land portions separated by these main grooves, the length CW, length GW, ratio GW/CW, cross-sectional area of the cap rubber in the center region, cross-sectional area of the base rubber in the center region, cross-sectional area of the cap rubber in the shoulder region, and cross-sectional area of the base rubber in the shoulder region are The tires of the conventional example, comparative examples 1 and 2, and examples 1 to 7 were manufactured with the following settings as shown in Table 1 for the area, thermal expansion coefficient of the cap rubber in the center region, thermal expansion coefficient of the base rubber in the center region, thermal expansion coefficient of the cap rubber in the shoulder region, thermal expansion coefficient of the base rubber in the shoulder region, magnitude relationship between the sum CE and the sum SH, ratio CE/SH, thickness of the base rubber in the center region, thickness of the base rubber in the shoulder region, groove width of the inner main groove, groove width of the outermost main groove, radius of curvature R of the groove bottom of the inner main groove, radius of curvature R of the groove bottom of the outermost main groove, radius of curvature Rce of the groove bottom of the outermost main groove, radius of curvature Rsh of the groove bottom of the outermost main groove, and ratio Rce/Rsh.

In Table 1, the "radius of curvature R of the bottom of the outermost main groove" is the average value of the radius of curvature Rce and the radius of curvature Rsh ((Rce+Rsh)/2).

These test tires were evaluated for groove crack resistance, rolling resistance, and cornering power using the following test methods, and the results are shown in Table 1.

Groove crack resistance:
Each test tire was mounted on a wheel with a rim size of 14 x 4.5J, and left for 8 hours for 3 days in an environment with an air pressure of 50 kPa, a temperature of 50°C, and an ozone concentration of 100 pphm, after which the number of cracks that occurred in the main grooves formed in the tread portion was counted. The evaluation results were expressed as an index using the reciprocal of the measured value, with the conventional example being set at 100. The higher the index value, the better the groove crack resistance.

Rolling resistance:
Each test tire was mounted on a wheel with a rim size of 14 x 4.5J and attached to a rolling resistance tester, and the rolling resistance was measured according to ISO conditions at an air pressure of 210 kPa and a load of 3.04 kN. The evaluation results were expressed as an index using the reciprocal of the measured value, with the conventional example being set at 100. The higher the index value, the better the rolling resistance.

Cornering Power:
Each test tire was mounted on a wheel with a rim size of 14 x 4.5J and attached to a flat belt testing machine, and run under conditions of an air pressure of 240 kPa and a load of 2.7 kN. The cornering power was measured when the slip angle was set to ±1°, and the absolute average was calculated. The evaluation results were shown as an index with the conventional example being 100. The higher the index value, the better the cornering power.

As can be seen from Table 1, Examples 1 to 7 showed significantly improved groove crack resistance compared to the conventional example while maintaining rolling resistance and cornering power.

On the other hand, in Comparative Example 1, the relationship between the sum CE and the sum SH was the opposite of that specified in the present invention, so although rolling resistance could be maintained, cornering power deteriorated and no improvement in groove crack resistance was obtained. In Comparative Example 2, the length GW was set small, falling outside the range of the ratio GW/CW specified in the present invention, so rolling resistance and cornering power deteriorated.

1 Tread portion 2 Sidewall portion 3 Bead portion 4 Carcass layer 7 Belt layer 9 Main groove 9A Inner main groove 9B Outermost main groove 10 Land portion Ace Center region Ash Shoulder region CL Tire center line CW Length obtained by dividing the length in the tire width direction measured along the belt layer of the belt intersection region in half GW Length in the tire width direction measured along the belt layer from the tire center line to the groove center of the outermost main groove

Claims (7)

A pneumatic tire comprising at least one carcass layer mounted between a pair of bead portions, two or more belt layers disposed on the outer peripheral side of the carcass layer in a tread portion, and a tread rubber layer disposed on the outer peripheral side of the belt layer and made of two or more types of rubber, wherein at least two main grooves extending in the tire circumferential direction are formed in the tread portion, and the tire has a plurality of land portions separated by the main grooves,
a length CW obtained by dividing a length in the tire width direction measured along the belt layer in a belt crossing region where belt cords constituting the belt layer cross between layers, and a length GW in the tire width direction measured along the belt layer from the tire centerline to the groove center of the outermost main groove in the tire width direction satisfy a relationship of 0.35≦GW/CW≦0.55,
a groove bottom reference line that defines a groove bottom position of the main groove, and an outer reference line that passes through an outer end of the belt crossing region and is perpendicular to the belt layer, in a tire meridian cross section; a shoulder region that is defined by the groove bottom reference line and the outer reference line in a land portion adjacent to the outer side in the tire width direction of the outermost main groove in the tire width direction, and a center region that is defined by the groove bottom reference line in a land portion adjacent to the inner side in the tire width direction of the outermost main groove in the tire width direction, wherein a sum CE calculated from the following formula (1) which is a relational expression between a thermal expansion coefficient EX_CEn and a cross-sectional area CS_CEn of each rubber that constitutes the tread rubber layer in the center region, and a sum SH calculated from the following formula (2) which is a relational expression between a thermal expansion coefficient EX_SHn and a cross-sectional area CS_SHn of each rubber that constitutes the tread rubber layer in the shoulder region satisfy a relationship of CE<SH.
CE=EX_CE1 ² ×CS_CE1+EX_CE2 ² ×CS_CE2+...+EX_CEn ² ×CS_CEn (1)
SH=EX_SH1 ² ×CS_SH1+EX_SH2 ² ×CS_SH2+...+EX_SHn ² ×CS_SHn (2) The pneumatic tire according to claim 1, characterized in that the sum CE and the sum SH satisfy the relationship 0.50≦CE/SH≦0.85. The pneumatic tire according to claim 1 or 2, characterized in that the thermal expansion coefficient of the cap rubber located at the outermost position in the tire radial direction among the two or more types of rubber constituting the tread rubber layer is smaller than the thermal expansion coefficient of the rubber other than the cap rubber, and the rubber thickness of the rubber other than the cap rubber of the tread rubber layer in the center region is thinner than the rubber thickness of the rubber other than the cap rubber of the tread rubber layer in the shoulder region. A pneumatic tire according to any one of claims 1 to 3, characterized in that the groove bottom of the outermost main groove in the tire width direction is composed of two or more arcs, and the radius of curvature Rsh of the arc on the shoulder region side at the groove bottom of the outermost main groove in the tire width direction is greater than the radius of curvature Rce of the arc on the center region side. The pneumatic tire according to any one of claims 1 to 4, characterized in that the tread portion has three or more of the main grooves, the groove width of the outermost main groove in the tire width direction is larger than the groove width of the main groove located inward in the tire width direction from the outermost main groove, and the average value of the radius of curvature R of the arc constituting the groove bottom of the outermost main groove in the tire width direction is larger than the average value of the radius of curvature R of the arc constituting the groove bottom of the main groove located inward in the tire width direction from the outermost main groove. A pneumatic tire according to any one of claims 1 to 5, characterized in that the groove bottom of the outermost main groove in the tire width direction is composed of two or more arcs, and the radius of curvature Rce of the arc on the center region side and the radius of curvature Rsh of the arc on the shoulder region side at the groove bottom of the outermost main groove in the tire width direction satisfy the relationship 0.25≦Rce/Rsh≦0.50. 7. The pneumatic tire according to claim 1, wherein the thermal expansion coefficient of each rubber constituting the tread rubber layer is in the range of 0.8×10 ^-4 /°C to 3.0 ^{×10 -4} /°C.

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