tire

Abstract

To provide a tire having both low rolling resistance performance and wet performance.SOLUTION: In a tire 1, the ratio of the grounding width Wce of a center land part 32 to a tire grounding width TW is in a range of 0.55≤Wce / TW≤0.70. The center land part 32 comprises a plurality of the first center lug grooves 323A which open on a first circumferential main groove 2A and extend to a tire width direction to cross with a tire equator face CL, and the plurality of the second center lug grooves 323B which open on a second circumferential main groove 2B in one end part and extend in the tire width direction to cross with the tire equator face CL. Besides, the first and second center lug grooves 323A and 323B have the S-shaped forms in the area from the tire equator face CL to one end part. Further, the S-shaped form becomes convex toward a tire rotation direction on the tire equator face and becomes convex toward a tire reversal direction on one end part.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a tire, and more particularly to a tire that can achieve both low rolling resistance performance and wet performance. [Background technology]

[0002] In recent years, heavy-duty tires fitted to vehicles for long-distance transport such as trucks and buses have adopted a configuration in which only a pair of shoulder main grooves are provided on the tread surface, with no other wide circumferential grooves in the center region of the tread, in order to reduce the rolling resistance of the tire while ensuring the wet performance of the tire.

[0003] As a conventional heavy duty tire employing such a configuration, the technology described in Patent Document 1 is known. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2019-081490 A Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide a tire that can achieve both low rolling resistance performance and wet performance. [Means for solving the problem]

[0006] In order to achieve the above object, a tire according to the present invention is a tire including first and second circumferential main grooves extending in a tire circumferential direction, and a pair of shoulder land portions and a single center land portion partitioned by the first and second circumferential main grooves, wherein a ground contact width Wce of the center land portion is in a range of 0.55≦Wce / TW≦0.70 relative to a tire ground contact width TW, and the center land portion includes a plurality of first center lug grooves that open into the first circumferential main groove at one end, extend in the tire width direction and intersect with the tire equatorial plane, and a plurality of second center lug grooves that open into the second circumferential main groove at one end, extend in the tire width direction and intersect with the tire equatorial plane, and the first and second center lug grooves have an S-shape in a region from the tire equatorial plane to the one end. death, The S-shape is convex in the tire rotation direction at the tire equatorial plane and is convex in the tire turning direction at the one end. and the first and second center lug grooves adjacent to each other do not overlap each other in the tire circumferential direction. It is characterized by: Effect of the Invention

[0007] In the tire according to the present invention, (1) the S-shape of the center lug groove is convex in the tire rotation direction at the tire equatorial plane, so that the edge of the center land portion defined by the first and second center lug grooves is convex in the tire rotation direction at the leading side during tire rotation and at the center of the center land portion in the tire width direction. This has the advantage of increasing the rigidity of the center land portion in the tire width direction and improving the rolling resistance of the tire. In addition, (2) the S-shape of the center lug groove is convex in the tire inversion direction at the one end (i.e., the opening end with respect to the circumferential main groove), so that the inclination angle of the first and second center lug grooves with respect to the tire width direction gradually decreases toward the opening with respect to the circumferential main groove. This has the advantage of improving the drainage at the end of the center land portion in the tire width direction and improving the wet traction of the tire. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a cross-sectional view in the tire meridian direction showing a tire according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a plan view showing a tread surface of the tire shown in FIG. [Diagram 3] FIG. 3 is a plan view showing the center land portion of the tire 1 shown in FIG. [Figure 4] FIG. 4 is an enlarged view showing the first circumferential narrow groove depicted in FIG. [Diagram 5] FIG. 5 is a cross-sectional view taken along line A of the first circumferential narrow groove shown in FIG. [Figure 6] FIG. 6 is a cross-sectional view taken along line B of the second circumferential narrow groove shown in FIG. [Figure 7] FIG. 7 is a cross-sectional view taken along line C of the center lug groove shown in FIG. [Figure 8] FIG. 8 is a cross-sectional view taken along line D of the center auxiliary groove shown in FIG. [Figure 9] FIG. 9 is a table showing the results of performance tests of the tire according to the embodiment of the present invention. [Figure 10] FIG. 10 is a table showing the results of performance tests of the tire according to the embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] The present invention will be described in detail below with reference to the drawings. Note that the present invention is not limited to the embodiments. The components of the embodiments include those that can be substituted and are obvious substitutes while maintaining the identity of the invention. The multiple modified examples described in the embodiments can be arbitrarily combined within the scope of what is obvious to those skilled in the art.

[0010] [tire] Fig. 1 is a cross-sectional view in the tire meridian direction showing a tire 1 according to an embodiment of the present invention. The figure shows a cross-sectional view of one side region in the tire radial direction. The figure also shows a heavy-duty tire for trucks and buses as an example of a tire.

[0011] In the figure, the cross section in the tire meridian direction refers to a cross section of the tire cut by a plane including the tire rotation axis (not shown). The symbol CL denotes the tire equatorial plane, which is a plane that passes through the center point of the tire in the tire rotation axis direction and is perpendicular to the tire rotation axis. The tire width direction refers to the direction parallel to the tire rotation axis, and the tire radial direction refers to the direction perpendicular to the tire rotation axis.

[0012] The tire 1 has an annular structure centered on the tire rotation axis, and includes a pair of bead cores 11, 11, a pair of bead fillers 12, 12, a carcass layer 13, a belt layer 14, a tread rubber 15, a pair of sidewall rubbers 16, 16, and a pair of rim cushion rubbers 17, 17 (see Figure 1).

[0013] The pair of bead cores 11, 11 are formed by winding one or more steel bead wires in an annular manner in multiple layers, and are embedded in the bead portions to form the cores of the left and right bead portions. The pair of bead fillers 12, 12 are made up of a lower filler 121 and an upper filler 122, and are disposed on the outer periphery of the pair of bead cores 11, 11 in the tire radial direction, respectively, to reinforce the bead portions.

[0014] The carcass layer 13 has a single-layer structure consisting of one carcass ply or a multi-layer structure consisting of multiple carcass plies stacked together, and is toroidally stretched between the left and right bead cores 11, 11 to form the tire framework. Both ends of the carcass layer 13 are wrapped around and secured to the outside in the tire width direction so as to envelop the bead cores 11 and the bead fillers 12. The carcass ply of the carcass layer 13 is formed by covering multiple carcass cords made of steel or organic fiber material (e.g., aramid, nylon, polyester, rayon, etc.) with coating rubber and rolling them, and has a carcass angle (defined as the inclination angle of the carcass cords in the longitudinal direction with respect to the tire circumferential direction) of 80 degrees or more and 90 degrees or less in absolute value.

[0015] The belt layer 14 is formed by laminating a high-angle belt 141, a pair of cross belts 142 and 143, and a belt cover 144, and is disposed around the outer periphery of the carcass layer 13. The high-angle belt 141 is formed by covering a plurality of belt cords made of steel or organic fiber material with a coating rubber and rolling it, and has a belt angle (defined as the inclination angle of the longitudinal direction of the belt cord with respect to the tire circumferential direction) of 45 degrees or more and 70 degrees or less in absolute value. The pair of cross belts 142 and 143 are formed by covering a plurality of belt cords made of steel or organic fiber material with a coating rubber and rolling it, and have a belt angle of 10 degrees or more and 55 degrees or less in absolute value. In addition, the pair of cross belts 142 and 143 have belt angles of opposite signs to each other, and are laminated with the longitudinal directions of the belt cords crossing each other (so-called cross-ply structure). The belt cover 144 is formed by covering a plurality of belt cover cords made of steel or organic fiber material with coating rubber and rolling it, and has a belt angle of 10 degrees or more and 55 degrees or less in absolute value.

[0016] The tread rubber 15 is disposed on the outer periphery of the carcass layer 13 and the belt layer 14 in the tire radial direction to form the tread portion of the tire. A pair of sidewall rubbers 16, 16 are disposed on the outer sides of the carcass layer 13 in the tire width direction to form left and right sidewall portions. A pair of rim cushion rubbers 17, 17 extend from the inner side in the tire radial direction of the left and right bead cores 11, 11 and the turned-up portions of the carcass layer 13 to the outer side in the tire width direction to form the rim fitting surface of the bead portion.

[0017] [Tread pattern] Fig. 2 is a plan view showing the tread surface of the tire 1 shown in Fig. 1. In the drawing, the tire circumferential direction refers to the direction around the tire rotation axis. Also, the symbol T indicates the tread edge, and the dimension symbol TW indicates the tire contact width. In the configuration of Fig. 2, the tire 1 has a square-shaped shoulder portion, and the tread edge T coincides with the tire contact edge.

[0018] The tire rotation direction is defined as the rotation direction that is frequently used when the tire is in use, more specifically, the rotation direction when the vehicle is moving forward. The tire rotation direction also defines the leading side (the so-called leading side or toe side) and trailing side (the so-called trailing side or heel side) of the block when the tire touches the ground. The tire is also equipped with a rotation direction indicator (not shown) that indicates the tire rotation direction. The rotation direction indicator is, for example, configured by a mark or a concave or convex portion provided on the sidewall of the tire.

[0019] As shown in FIG. 2, the tire 1 has, on its tread surface, first and second circumferential main grooves 2A, 2B extending in the tire circumferential direction, and a pair of shoulder land portions 31, 31 and a single center land portion 32 partitioned by these circumferential main grooves 2A, 2B.

[0020] The main groove is a groove that is required to display a wear indicator as stipulated by JATMA, and in particular in heavy-duty tires, has a groove width Wm (see FIG. 2) of 6.0 mm or more and a groove depth Hm (not shown) of 10.0 mm or more.

[0021] The groove width is measured as the distance between the left and right groove walls at the groove opening when the tire is mounted on a specified rim, inflated to a specified internal pressure, and in an unloaded state. In a configuration in which the land portion has a notch or chamfered portion at the edge, the groove width is measured at the intersection of the tread surface and the extension of the groove wall in a cross-sectional view normal to the groove length direction.

[0022] The 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 specified rim, inflated to a specified internal pressure, and in a no-load state. If the groove has partial unevenness or sipes at the bottom of the groove, the groove depth is measured excluding these.

[0023] The specified rim refers to the "standard rim" specified by JATMA, the "design rim" specified by TRA, or the "measuring rim" specified by ETRTO. The specified internal pressure refers to the "maximum air pressure" specified by JATMA, the maximum value of the "tire load limits at various cold inflation pressures" specified by TRA, or the "inflation pressures" specified by ETRTO. The specified load refers to the "maximum load capacity" specified by JATMA, the maximum value of the "tire load limits at various cold inflation pressures" specified by TRA, or the "load capacity" specified by ETRTO. However, in JATMA, for passenger car tires, the specified internal pressure is 180 kPa, and the specified load is 88% of the maximum load capacity at the specified internal pressure.

[0024] [Center Land] Fig. 3 is a plan view showing the center land portion 32 of the tire 1 shown in Fig. 2. Fig. 4 and Fig. 5 are an enlarged view (Fig. 4) and a cross-sectional view (Fig. 5) of the first circumferential narrow groove 321 shown in Fig. 3. Fig. 6 is a cross-sectional view taken from B of the second circumferential narrow groove 322 shown in Fig. 3. Fig. 7 is a cross-sectional view taken from C of the center lug groove 323A (323B) shown in Fig. 3. Fig. 8 is a cross-sectional view taken from D of the center auxiliary groove 324A (324B) shown in Fig. 3.

[0025] As shown in Fig. 2, in this tire 1, the ground contact width Wce of the center land portion 32 is in the range of 0.55 < Wce / TW < 0.70, preferably 0.55 < Wce / TW < 0.60, relative to the tire ground contact width TW. The wide structure of the center land portion 32 increases the ground contact area of ​​the tread center region and increases its rigidity. This reduces the rolling resistance of the tire.

[0026] The contact width Wce of the center land portion 32 and the tire contact width TW are measured as the maximum linear distance in the axial direction of the tire at the contact surface between the tire and a flat plate when the tire is mounted on a specified rim, pressurized to a specified internal pressure, and placed perpendicular to a flat plate in a stationary state and subjected to a load corresponding to a specified load.

[0027] The central land portion 32 includes first and second circumferential narrow grooves 321, 322, first and second central lug grooves 323A, 323B, first and second central auxiliary grooves 324A, 324B, and first and second sipes 325A, 325B.

[0028] The circumferential narrow grooves 321, 322 are longitudinal grooves that extend in the tire circumferential direction over the entire circumference of the tire, and function as grooves by opening when the tire is in contact with the ground. The multiple circumferential narrow grooves 321, 322 are arranged at a predetermined interval in the tire width direction. The circumferential narrow groove 321 arranged at a position closest to the tire equatorial plane CL is defined as a first circumferential narrow groove, and the circumferential narrow grooves 322, 322 arranged between the first circumferential narrow groove 321 and the circumferential main grooves 2A, 2B are defined as second circumferential narrow grooves.

[0029] For example, in the configuration of Fig. 2, a single first circumferential narrow groove 321 is disposed on the tire equatorial plane CL. However, this is not limiting, and the first circumferential narrow groove 321 may be disposed at a position deviated from the tire equatorial plane CL (not shown). In this case, it is preferable that the distance from the tire equatorial plane CL to the groove center line of the first circumferential narrow groove 321 (dimension symbols omitted in the drawing) is within a range of 10[%] or less with respect to the tire ground contact width TW.

[0030] 2, one second circumferential narrow groove 322 is disposed between the tire equatorial plane CL and each of the pair of circumferential main grooves 2A, 2B. The distance in the tire width direction from the tire equatorial plane CL to the second circumferential narrow groove 322 (dimension symbols are omitted in the drawing) is in the range of 0.18% to 0.32% of the contact width Wce of the center land portion 32, and preferably in the range of 0.22% to 0.28%.

[0031] In FIG. 2, the groove widths W1 and W2 of the first and second circumferential narrow grooves 321 and 322 are in the range of 10% to 40% of the groove width Wm of the circumferential main groove 2A (2B), and preferably in the range of 13% to 32%. The groove widths W1 and W2 of the first and second circumferential narrow grooves 321 and 322 are in the range of 1.0 mm to 5.0 mm, and preferably in the range of 1.6 mm to 4.0 mm. The lower limit ensures the function of the circumferential narrow grooves 321 and 322 as grooves when the tire is in contact with the ground, and ensures the drainage of the tread center region. The upper limit suppresses the reduction in rigidity of the tread center region caused by the groove widths W1 and W2 of the circumferential narrow grooves 321 and 322 being excessively large, and ensures the reduction in the rolling resistance of the tire.

[0032] The first and second circumferential narrow grooves 321, 322 may have a straight shape, a zigzag shape, or a wavy shape. For example, in the configuration of Fig. 2, as shown in Fig. 3, the first circumferential narrow groove 321 has a zigzag shape with amplitude in the tire width direction, and the second circumferential narrow groove 322 has a straight shape.

[0033] 4, the amplitude As of the zigzag shape of the first circumferential narrow groove 321 is in the range of 0.01≦As / Wce≦0.08 with respect to the contact width Wce (see FIG. 3) of the center land portion 32, and preferably in the range of 0.01≦As / Wce≦0.04. The amplitude As of the first circumferential narrow groove 321 is in the range of 0.80≦As / W1≦3.70 with respect to the groove width W1 of the first circumferential narrow groove 321, and preferably in the range of 0.90≦As / W1≦2.70. The lower limit ensures the amplitude As of the first circumferential narrow groove 321, and the drainage improvement effect of the zigzag shape of the first circumferential narrow groove 321 is ensured. The upper limit suppresses the deterioration of rolling resistance caused by the amplitude As of the zigzag shape becoming excessive. 4, the wavelength λs of the zigzag shape of the first circumferential narrow groove 321 is in the range of 0.40≦λs / P3≦2.20, preferably 0.80≦λs / P3≦1.20, relative to the pitch length P3 (see FIG. 3) of the first center lug groove 323A. This improves the drainage performance of the first circumferential narrow groove 321.

[0034] The amplitude As and wavelength λs of the zigzag shape are measured as the amplitude and wavelength of the groove center line of the first circumferential narrow groove 321 in a plan view of the tread when the tire is mounted on a specified rim, pressurized to a specified internal pressure, and in an unloaded state.

[0035] 5 and 6, the groove depths H1, H2 of the first and second circumferential narrow grooves 321, 322 are in the range of 80% to 110% of the groove depth Hm (not shown) of the circumferential main groove 2A.

[0036] As shown in FIG. 5, the first circumferential narrow groove 321 has a widened portion 3211 and a chamfered portion 3212 .

[0037] The widened portion 3211 is formed at the bottom of the first circumferential narrow groove 321 to expand the cross-sectional area of ​​the first circumferential narrow groove 321. The widened portion 3211 is formed around the entire circumference of the first circumferential narrow groove 321. This enhances the drainage effect of the first circumferential narrow groove 321. In addition, in FIG. 5, the maximum width W1' of the widened portion 3211 is in the range of 1.10≦W1' / W1≦4.00 with respect to the groove width W1 of the first circumferential narrow groove 321, and preferably in the range of 1.50≦W1' / W1≦3.00. In addition, the distance D1 from the tread surface to the widening start point of the widened portion 3211 is in the range of 0.30≦D1 / H1≦0.60 with respect to the groove depth H1 of the first circumferential narrow groove 321.

[0038] The maximum width W1' of the widened portion 3211 is measured in a cross section perpendicular to the groove length direction of the first circumferential narrow groove 321 when the tire is mounted on a specified rim, a specified internal pressure is applied, and no load is applied.

[0039] As shown in Fig. 4 and Fig. 5, the chamfered portion 3212 is formed at the groove opening of the zigzag-shaped first circumferential narrow groove 321, and is formed over the entire circumference of the first circumferential narrow groove 321. As shown in Fig. 4, the chamfered portion 3212 has a straight edge portion on the tread surface and surrounds the entire zigzag-shaped first circumferential narrow groove 321. Specifically, the width Wc of the chamfered portion 3212 is larger than the amplitude As of the zigzag shape of the first circumferential narrow groove 321.

[0040] In this configuration, since the main body of the first circumferential narrow groove 321 has a zigzag shape, the groove length of the first circumferential narrow groove 321 is ensured and the drainage effect of the first circumferential narrow groove 321 in the tread center region is enhanced, compared to a configuration in which the chamfered portion has a zigzag edge portion along the circumferential narrow groove (not shown). Also, since the chamfered portion 3212 has a straight edge portion on the tread surface, the drainage effect of the first circumferential narrow groove 321 is further enhanced and the rigidity of the edge portion is ensured, thereby maintaining the low rolling resistance performance of the tire in the early stage of wear.

[0041] Also, the width Wc (see FIG. 4) of the chamfered portion 3212 is in the range of 0.025≦Wc / Wce≦0.060 with respect to the contact width Wce (see FIG. 3) of the central land portion 32, and preferably in the range of 0.030≦Wc / Wce≦0.050. Also, in FIG. 5, the depth Hc of the chamfered portion 3212 is in the range of 0.14≦Hc / H1≦0.25 with respect to the groove depth H1 of the first circumferential narrow groove 321, and preferably in the range of 0.15≦Hc / H1≦0.20. Also, in the configurations of FIG. 4 and FIG. 5, the depth Hc of the chamfered portion 3212 is constant, and the inclination angle of the wall surface of the chamfered portion 3212 with respect to the tread surface changes in the tire circumferential direction, so that the chamfered portion 3212 is connected to the opening of the first circumferential narrow groove 321 at a constant depth Hc.

[0042] The width Wc of the chamfered portion 3212 is measured as the maximum width of the ridge line at the connection between the tread surface and the chamfered portion when the tire is mounted on a specified rim, a specified internal pressure is applied, and the tire is in an unloaded state.

[0043] The depth Hc of the chamfered portion 3212 is measured in a cross section perpendicular to the groove length direction of the first circumferential narrow groove 321 when the tire is mounted on a specified rim, a specified internal pressure is applied, and no load is applied.

[0044] As shown in FIG. 6, the second circumferential narrow groove 322 has a widened portion 3221 .

[0045] The widened portion 3221 is formed at the bottom of the second circumferential narrow groove 322 to expand the cross-sectional area of ​​the second circumferential narrow groove 322. The widened portion 3221 is formed over the entire circumference of the second circumferential narrow groove 322. This enhances the drainage effect of the second circumferential narrow groove 322. In addition, in FIG. 6, the maximum width W2' of the widened portion 3221 is in the range of 1.10≦W2' / W2≦4.00 with respect to the groove width W2 of the second circumferential narrow groove 322, and preferably in the range of 1.50≦W2' / W2≦3.00. In addition, the distance D2 from the tread surface to the widening start point of the widened portion 3221 is in the range of 0.30≦D2 / H2≦0.60 with respect to the groove depth H2 of the second circumferential narrow groove 322.

[0046] As shown in FIG. 3, the center lug grooves 323A and 323B are lateral grooves extending in the tire width direction, and function as grooves by opening when the tire is in contact with the ground. A plurality of center lug grooves 323A and 323B are arranged at a predetermined interval in the tire circumferential direction. The center lug grooves 323A and 323B open to the circumferential main grooves 2A and 2B at one end, extend in the tire width direction, and intersect with the tire equatorial plane CL. Here, the center lug groove 323A opening to the first circumferential main groove 2A is defined as a first center lug groove, and the center lug groove 323B opening to the second circumferential main groove 2B is defined as a second center lug groove. In this configuration, the center lug grooves 323A and 323B improve the drainage of the center land portion 32, and improve the wet performance of the tire.

[0047] In the configuration of FIG. 3, the first and second center lug grooves 323A, 323B have a semi-closed structure in which the other end ends are terminated within the center land portion 32. The first and second center lug grooves 323A, 323B cross the tire equatorial plane CL and terminate between the first circumferential narrow groove 321 and the second circumferential narrow groove 322. The extension length D3 (D3A; D3B) of the first and second center lug grooves 323A; 323B in the tire width direction is in the range of 0.55≦D3 / Wce≦0.75 with respect to the contact width Wce of the center land portion 32, and preferably in the range of 0.58≦D3 / Wce≦0.65. The lower limit ensures the drainage action of the center lug grooves 323A, 323B, and the upper limit ensures the rigidity of the center land portion 32, thereby reducing the rolling resistance of the tire.

[0048] The extension length D3 (D3A; D3B) of the center lug grooves 323A; 323B is measured as the maximum linear distance in the axial direction of the tire at the contact surface between the tire and a flat plate when the tire is mounted on a specified rim, pressurized to a specified internal pressure, and placed perpendicular to a flat plate in a stationary state and subjected to a load corresponding to a specified load.

[0049] 3, adjacent center lug grooves 323A, 323B are offset from each other so as not to overlap each other in the tire circumferential direction. The offset amount Dp between the first center lug groove 323A and the second center lug groove 323B in the tire circumferential direction is in the range of 0≦Dp / P3≦0.06 with respect to the pitch length P3 of the first center lug groove 323A, and preferably in the range of 0.02≦Dp / P3≦0.04. The lower limit ensures the rigidity of the center land portion 32 and reduces the rolling resistance of the tire, and the upper limit ensures the drainage function of the center lug grooves 323A, 323B.

[0050] The pitch length P3 and the offset amount Dp are measured with the tire mounted on a specified rim, with a specified internal pressure applied, and under no load.

[0051] 3, the first and second center lug grooves 323A, 323B are alternately arranged in the tire circumferential direction, and therefore the pitch numbers of both are equal. However, this is not limited thereto, and the first and second center lug grooves 323A, 323B may be irregularly arranged in the tire circumferential direction (not shown). In this case, it is preferable that the ratio of the pitch numbers of the first and second center lug grooves 323A, 323B is in the range of 0.40 to 0.60.

[0052] As shown in FIG. 3, the first and second center lug grooves 323A; 323B have an S-shape in the region from the tire equatorial plane CL to the one end, i.e., the opening to the circumferential main grooves 2A; 2B. The S-shape is convex in the tire rotation direction at the tire equatorial plane CL, and is convex in the tire inversion direction at the opening to the circumferential main grooves 2A; 2B. In other words, the first and second center lug grooves 323A; 323B are formed by connecting a first groove portion (reference numeral omitted in the figure) that is arranged on the tire equatorial plane CL and convex in the tire rotation direction, and a second groove portion (reference numeral omitted in the figure) that is connected to the circumferential main grooves 2A; 2B and convex in the tire inversion direction. The first and second center lug grooves 323A; 323B have a single inflection point in the region from the tire equatorial plane CL to the opening to the circumferential main grooves 2A; 2B.

[0053] In addition, as shown in FIG. 3, in a configuration in which the center lug grooves 323A, 323B intersect and are divided by the circumferential narrow grooves 321, 322, the openings of the center lug grooves 323A, 323B relative to the circumferential narrow grooves 321, 322 must be at approximately the same positions in the tire circumferential direction. Specifically, as shown in FIG. 4, the tire circumferential distance Ga of the opening of the center lug groove 323A relative to the first circumferential narrow groove 321 must be in the range of 0≦Ga / P3≦0.06, preferably 0≦Ga / P3≦0.03, relative to the pitch length P3 (see FIG. 3) of the first center lug groove 323A. This ensures the continuity of the center lug grooves 323A, 323B. Similarly, the tire circumferential distance Ga' (not shown) of the opening of the center lug groove 323A relative to the second circumferential narrow groove 322 must be within the above range.

[0054] The distance Ga is measured as the offset amount of the left and right groove center lines of the center lug groove 323A from the groove center line of the circumferential narrow groove 321 when the tire is mounted on a specified rim, pressurized to a specified internal pressure, and in an unloaded state.

[0055] In the above configuration, (1) the S-shape of the center lug grooves 323A and 323B is convex in the tire rotation direction at the tire equatorial plane CL, so that the edge of the center land portion 32 defined by the first and second center lug grooves 323A and 323B is convex in the tire rotation direction at the leading side during tire rotation and at the center of the center land portion 32 in the tire width direction. This increases the rigidity of the center land portion 32 in the tire width direction and improves the rolling resistance of the tire. In addition, (2) the S-shape of the center lug grooves 323A and 323B is convex in the tire inversion direction at the one end (i.e., the opening end relative to the circumferential main grooves 2A and 2B), so that the inclination angle of the first and second center lug grooves 323A and 323B relative to the tire width direction (dimension symbols omitted in the figure) gradually decreases toward the opening relative to the circumferential main grooves 2A and 2B. This improves drainage at the end of the center land portion 32 in the tire width direction, improving the wet traction of the tire. Furthermore, (3) since the first and second center lug grooves 323A, 323B have an S-shape with a single inflection point, compared to a lug groove (not shown) having a wavy shape with multiple inflection points, the rigidity of the edges of the center lug grooves 323A, 323B is ensured, reducing the rolling resistance of the tire, and the drainage resistance of the center lug grooves 323A, 323B is reduced, improving the wet traction of the tire.

[0056] In the configuration of Fig. 3, the maximum convex positions (reference numbers omitted in the figure) of the first and second center lug grooves 323A, 323B in the tire rotation direction are on the tire equatorial plane CL. However, this is not limited thereto, and the maximum convex positions of the first and second center lug grooves 323A, 323B in the tire rotation direction may be located at positions that are off the tire equatorial plane CL. In addition, it is preferable that the distance (not shown) from the maximum convex positions of the first and second center lug grooves 323A, 323B in the tire rotation direction to the tire equatorial plane CL is in the range of 0% to 10% of the tire contact width TW.

[0057] In addition, in FIG. 3, the intersection angle (dimension symbols omitted in the drawing) between the groove center lines of the first and second center lug grooves 323A and 323B and the tire equatorial plane CL is in the range of 80 [deg] to 100 [deg], preferably in the range of 85 [deg] to 95 [deg]. Therefore, it is preferable that the groove center lines of the first and second center lug grooves 323A and 323B are substantially perpendicular to the tire equatorial plane CL. This ensures the block rigidity of the tread center region, reducing the rolling resistance of the tire. In addition, the intersection angle (dimension symbols omitted in the drawing) between the groove center lines of the first and second center lug grooves 323A and 323B and the groove center lines of the first and second circumferential main grooves 2A and 2B is in the range of 75 [deg] to 100 [deg], preferably in the range of 80 [deg] to 95 [deg]. Therefore, it is preferable that the groove center lines of the first and second center lug grooves 323A and 323B are substantially perpendicular to the first and second circumferential main grooves 2A and 2B, which improves the drainage of the center land portion 32 and the wet traction of the tire.

[0058] The intersection angle of the groove center lines is measured with the tire mounted on a specified rim, with a specified internal pressure applied, and in an unloaded state.

[0059] 3, the radius of curvature Rs2 (dimension symbol omitted in the figure) of the S-shape at the opening position of the one end of the center lug groove 323A; 323B relative to the radius of curvature Rs1 (dimension symbol omitted in the figure) of the S-shape of the center lug groove 323A; 323B at the tire equatorial plane CL satisfies the relationship of 1.00≦Rs2 / Rs1≦3.50, and preferably satisfies the relationship of 1.50≦Rs2 / Rs1≦2.50. In this configuration, the radius of curvature Rs2 of the S-shape at the opening position relative to the circumferential main groove 2A; 2B is set large, which has the advantage of improving the drainage effect of the center lug groove 323A; 323B.

[0060] The curvature radius Rs1 of the S-shape at the tire equatorial plane CL is defined as the curvature radius of an arc passing through three points: the intersection of the groove center line of the center lug groove 323A; 323B with the tire equatorial plane CL, the inflection point of the S-shape, and the midpoint of these intersections and inflection points, when the tire is mounted on a specified rim, a specified internal pressure is applied, and no load is applied. Similarly, the curvature radius Rs2 of the S-shape at the opening position of the circumferential main groove 2A; 2B is defined as the curvature radius of an arc passing through three points: the intersection of the groove center line of the center lug groove 323A; 323B with the groove center line of the circumferential main groove 2A; 2B, the inflection point of the S-shape, and the midpoint of these intersections and inflection points, when the tire is mounted on a specified rim, a specified internal pressure is applied, and no load is applied.

[0061] For example, in the configuration of FIG. 3, the inflection point (reference numerals omitted in the drawing) of the S-shaped center lug groove 323A; 323B is located closer to the tire equatorial plane CL than the circumferential main groove 2A; 2B. Specifically, the inflection point of the S-shaped is located in the region between the tire equatorial plane CL and the second circumferential narrow groove 322. As a result, the distance from the inflection point of the S-shaped to the circumferential main groove 2A; 2B is set large, and the extension length of the groove portion having a large radius of curvature Rs2 is set long. However, without being limited thereto, the inflection point of the S-shaped may be located on the second circumferential narrow groove 322, or may be located in the region between the second circumferential narrow groove 322 and the circumferential main groove 2A; 2B (not shown).

[0062] 3, the extension length L3 of the first and second center lug grooves 323A, 323B in the tire circumferential direction is in the range of 0.10≦L3 / Wce≦0.30, more preferably 0.15≦L3 / Wce≦0.25, relative to the contact width Wce of the center land portion 32. The lower limit ensures the inclination angle of the center lug grooves 323A, 323B, and ensures the drainage effect of the center lug grooves 323A, 323B. The upper limit ensures the rigidity of the center land portion 32, and reduces the rolling resistance of the tire.

[0063] In addition, the groove width W3 of the center lug grooves 323A and 323B (see FIG. 3 and FIG. 7) is in the range of 0.10≦W3 / Wm≦0.55, preferably 0.15≦W3 / Wm≦0.30, relative to the groove width Wm of the circumferential main grooves 2A and 2B. In addition, the groove width W3 of the center lug grooves 323A and 323B is in the range of 1.5 [mm]≦W3≦6.0 [mm], preferably 1.5 [mm]≦W3≦3.0 [mm]. The above lower limit ensures the function of the center lug grooves 323A and 323B as a groove when the tire is in contact with the ground, and ensures the drainage of the tread center region. The above upper limit suppresses the reduction in rigidity of the tread center region caused by the groove width W3 of the center lug grooves 323A and 323B being excessively large, and ensures the reduction in the rolling resistance of the tire.

[0064] 7, the groove depth H3 of the center lug grooves 323A and 323B is in the range of 0.10≦H3 / Hm≦0.30, preferably 0.15≦H3 / Hm≦0.20, relative to the groove depth Hm (not shown) of the circumferential main grooves 2A and 2B. Therefore, the center lug grooves 323A and 323B have a sufficiently shallow groove depth relative to the circumferential main grooves 2A and 2B. The lower limit ensures the drainage effect of the center lug grooves 323A and 323B. The upper limit ensures the rigidity of the center land portion 32, improving the rolling resistance of the tire.

[0065] As shown in FIG. 3, the center auxiliary grooves 324A, 324B are lateral grooves extending in the tire width direction, and function as grooves by opening when the tire is in contact with the ground. A plurality of center auxiliary grooves 324A, 324B are arranged at a predetermined interval in the tire circumferential direction. The center auxiliary grooves 324A, 324B are arranged on the extension line of the center lug grooves 323A, 323B, and have a semi-closed structure in which one end opens into the circumferential main grooves 2B, 2A and the other end terminates within the center land portion 32. Here, the center auxiliary groove 324A arranged on the extension line of the first center lug groove 323A is defined as the first center auxiliary groove, and the center auxiliary groove 324B arranged on the extension line of the second center lug groove 323B is defined as the second center auxiliary groove. In this configuration, the center auxiliary grooves 324A, 324B improve the drainage of the center land portion 32, and the wet performance of the tire is improved.

[0066] 3, the opening position of the center auxiliary groove 324A; 324B relative to the circumferential main groove 2B; 2A is substantially the same in the tire circumferential direction as the opening position of the center lug groove 323A; 323B relative to the circumferential main groove 2A; 2B. Specifically, the circumferential distance between them (not shown) is in the range of 0% to 10% of the pitch length P3 of the center lug groove 323A; 323B, preferably in the range of 0% to 5%. This makes the stress acting on the blocks uniform in the tire width direction when the tire rolls, reducing the rolling resistance of the tire.

[0067] As shown in FIG. 3, the center auxiliary grooves 324A and 324B are spaced apart from the center lug grooves 323A and 323B. Specifically, the distance D4 (D4A; D4B) between the end of the center auxiliary groove 324A and 324B in the center land portion 32 and the center lug grooves 323A and 323B in the tire width direction is in the range of 0.05≦D4 / Wce≦0.25, preferably 0.10≦D4 / Wce≦0.20, relative to the ground contact width Wce of the center land portion 32. The lower limit ensures the rigidity of the center land portion 32 and ensures the reduction of the rolling resistance of the tire. The upper limit ensures the improvement of the drainage of the center land portion 32 by the center lug grooves 323A and 323B and the center auxiliary grooves 324A and 324B.

[0068] 3, the center auxiliary grooves 324A; 324B have an arc shape that is convex in the tire inversion direction. Therefore, the center auxiliary grooves 324A; 324B have a shape that is approximately symmetrical with respect to the S-shaped portion of the center lug grooves 323A; 323B on the circumferential main grooves 2A; 2B side. In addition, the inclination angle of the center auxiliary grooves 324A; 324B with respect to the tire width direction (dimension symbols omitted in the figure) gradually decreases toward the openings with respect to the circumferential main grooves 2A; 2B. This improves drainage at the ends of the center land portion 32 in the tire width direction, improving the wet traction of the tire.

[0069] In addition, the groove width W4 of the center auxiliary grooves 324A and 324B (see FIG. 3 and FIG. 8) is in the range of 0.10≦W4 / Wm≦0.55, preferably 0.15≦W4 / Wm≦0.30, relative to the groove width Wm of the circumferential main grooves 2A and 2B. The groove width W4 of the center auxiliary grooves 324A and 324B is in the range of 1.5 [mm]≦W4≦6.0 [mm], preferably 1.5 [mm]≦W4≦3.0 [mm]. The above lower limit ensures the function of the center auxiliary grooves 324A and 324B as a groove when the tire is in contact with the ground, and ensures the drainage of the tread center region. The above upper limit suppresses the reduction in rigidity of the tread center region caused by the groove width W4 of the center auxiliary grooves 324A and 324B being excessively large, and ensures the reduction in the rolling resistance of the tire.

[0070] 8, the groove depth H4 of the center auxiliary grooves 324A and 324B is in the range of 0.10≦H4 / Hm≦0.30, preferably 0.15≦H4 / Hm≦0.20, relative to the groove depth Hm (not shown) of the circumferential main grooves 2B and 2A. Therefore, the center auxiliary grooves 324A and 324B have a sufficiently shallow groove depth relative to the circumferential main grooves 2B and 2A. The lower limit ensures the drainage function of the center auxiliary grooves 324A and 324B. The upper limit ensures the rigidity of the center land portion 32, improving the rolling resistance of the tire.

[0071] 3, as described above, the other end of the center lug grooves 323A; 323B terminates within the center land portion 32, and the center auxiliary grooves 324A; 324B are disposed on the extensions of the center lug grooves 323A; 323B while being spaced apart from the center lug grooves 323A; 323B. However, this is not limiting, and the center lug grooves 323A; 323B and the center auxiliary grooves 324A; 324B may be connected to each other so that one lug groove 323A, 324A; 323B, 324B penetrates the center land portion 32 in the tire width direction and opens to the left and right circumferential main grooves 2A, 2B (not shown).

[0072] The sipes 325A and 325B are cuts formed at the bottom of the center lug grooves 323A and 323B, and are closed when the tire touches the ground. These sipes 325A and 325B improve the drainage of the center region of the tread. The width W5 (see FIG. 7) of the sipes 325A and 325B is in the range of 0.5 mm to 1.5 mm, which is narrower than the groove width W3 of the center lug grooves 323A and 323B. The depth H5 (see FIG. 7) of the sipes 325A and 325B is in the range of 0.50≦H5 / Hm≦0.95 with respect to the groove depth Hm (not shown) of the circumferential main groove 2A.

[0073] In the configuration of Fig. 3, the sipes 325A, 325B extend along the groove bottoms of the center lug grooves 323A, 323B and the center auxiliary grooves 324A, 324B, and connect the center lug grooves 323A, 323B and the center auxiliary grooves 324A, 324B. In addition, the sipes 325A, 325B each penetrate the center land portion 32 in the tire width direction and open into the left and right circumferential main grooves 2A, 2B. This improves the drainage of the center land portion 32.

[0074] In the configuration of FIG. 3, the central land portion 32 includes the sipes 325A, 325B as described above, but the present invention is not limited to this, and these sipes 325A, 325B may be omitted (not shown).

[0075] [Shoulder land area] 2, each of the pair of shoulder land portions 31, 31 includes a plurality of shoulder lug grooves 311. The shoulder lug grooves 311 penetrate the shoulder land portion 31 in the tire width direction and open to the circumferential main grooves 2A, 2B and the tire ground contact edge T. The shoulder lug grooves 311 of one shoulder land portion 31 are disposed on an extension line of the first center lug groove 323A, and the shoulder lug grooves 311 of the other shoulder land portion 31 are disposed on an extension line of the second center lug groove 323B. The number of pitches of the shoulder lug grooves 311 of one shoulder land portion 31 is equal to the number of pitches of the first center lug groove 323A or the second center lug groove 323B.

[0076] The groove width (dimension symbols omitted in the figure) of the shoulder lug groove 311 is in the range of 60% to 110% of the groove width Wm of the circumferential main grooves 2A and 2B. The groove depth (not shown) of the shoulder lug groove 311 is in the range of 65% to 90% of the groove depth Hm (not shown) of the circumferential main grooves 2A and 2B.

[0077] As shown in FIG. 2, the shoulder land portion 31 has multiple chamfered portions (reference symbols omitted in the figure) at the edge portions on the circumferential main grooves 2A and 2B side. The chamfered portions are disposed between adjacent shoulder lug grooves, and open into the shoulder lug grooves while gradually increasing the chamfer width from the leading side to the trailing side in the tire rotation direction. This improves the drainage of the shoulder region of the tread portion. The maximum width of the chamfered portions (dimension symbols omitted in the figure) is in the range of 5% to 20% of the ground contact width of the shoulder land portion 31 (dimension symbols omitted in the figure). The maximum depth of the chamfered portions (dimension symbols omitted in the figure) is in the range of 14% to 25% of the groove depth Hm (not shown) of the circumferential main grooves 2A and 2B.

[0078] [effect] As described above, the tire 1 includes the first and second circumferential main grooves 2A, 2B extending in the tire circumferential direction, and a pair of shoulder land portions 31, 31 and a single center land portion 32 partitioned by the first and second circumferential main grooves 2A, 2B (see FIG. 2). The ground contact width Wce of the center land portion 32 is in the range of 0.55≦Wce / TW≦0.70 with respect to the tire ground contact width TW. The center land portion 32 includes a plurality of first center lug grooves 323A that open into the first circumferential main groove 2A at one end, extend in the tire width direction, and intersect with the tire equatorial plane CL, and a plurality of second center lug grooves 323B that open into the second circumferential main groove 2B at one end, extend in the tire width direction, and intersect with the tire equatorial plane CL (see FIG. 3). The first and second center lug grooves 323A, 323B have an S-shape in the region from the tire equatorial plane CL to one end. In addition, the S-shape is convex in the tire rotation direction at the tire equatorial plane CL and is also convex in the tire turning direction at one end.

[0079] In this configuration, (1) the S-shape of the center lug grooves 323A and 323B is convex in the tire rotation direction at the tire equatorial plane CL, so that the edge of the center land portion 32 defined by the first and second center lug grooves 323A and 323B is convex in the tire rotation direction at the leading side during tire rotation and at the center of the center land portion 32 in the tire width direction. This has the advantage of increasing the rigidity of the center land portion 32 in the tire width direction and improving the rolling resistance of the tire. In addition, (2) the S-shape of the center lug grooves 323A and 323B is convex in the tire inversion direction at the one end (i.e., the opening end relative to the circumferential main grooves 2A and 2B), so that the inclination angle (dimension symbols omitted in the figure) of the first and second center lug grooves 323A and 323B relative to the tire width direction gradually decreases toward the opening relative to the circumferential main grooves 2A and 2B. This improves drainage at the ends of the central land portion 32 in the tire width direction, which has the advantage of improving the wet traction of the tire.

[0080] In addition, in the tire 1, the adjacent first and second center lug grooves 323A, 323B do not overlap each other in the tire circumferential direction (see FIG. 3). This has the advantage of ensuring the rigidity of the center land portion 32 and reducing the rolling resistance of the tire.

[0081] In addition, in the tire 1, the radius of curvature Rs2 of the S-shape at one end has a relationship of 1.00≦Rs2 / Rs1≦3.50 with respect to the radius of curvature Rs1 of the S-shape at the tire equatorial plane CL (see FIG. 3. Dimensional symbols in the figure are omitted.) In this configuration, the radius of curvature Rs2 of the S-shape at the opening position relative to the circumferential main grooves 2A; 2B is set large, which has the advantage of improving the drainage effect of the center lug grooves 323A; 323B.

[0082] In addition, in this tire 1, the groove width W3 (see FIG. 3) of the first and second center lug grooves 323A, 323B is in the range of 0.10≦W3 / Wm≦0.55 with respect to the groove width Wm (not shown) of the circumferential main grooves 2A, 2B. This has the advantage of optimizing the groove width W3 of the center lug grooves 323A, 323B. Specifically, the above-mentioned lower limit ensures the groove function of the center lug grooves 323A, 323B when the tire is in contact with the ground, and ensures the drainage of the tread center region. The above-mentioned upper limit suppresses the reduction in rigidity of the tread center region caused by the groove width W3 of the center lug grooves 323A, 323B being excessively large, and ensures the reduction in the rolling resistance of the tire.

[0083] In addition, in the tire 1, the groove depth H3 (see FIG. 7) of the first and second center lug grooves 323A, 323B is in the range of 0.10≦H3 / Hm≦0.30 with respect to the groove depth Hm (not shown) of the circumferential main grooves 2A, 2B. This has the advantage of optimizing the groove depth H3 of the center lug grooves 323A, 323B. Specifically, the lower limit ensures the drainage effect of the center lug grooves 323A, 323B. The upper limit ensures the rigidity of the center land portion 32, improving the rolling resistance of the tire.

[0084] In addition, in this tire 1, the first and second center lug grooves 323A, 323B terminate at the other end within the center land portion 32 (see FIG. 3). The extension length D3 (D3A, D3B) of the first and second center lug grooves 323A, 323B in the tire width direction is in the range of 0.55≦D3 / Wce≦0.75 with respect to the contact width Wce of the center land portion 32. This has the advantage of optimizing the extension length D3 of the center lug grooves 323A, 323B. Specifically, the lower limit ensures the drainage function of the center lug grooves 323A, 323B, and the upper limit ensures the rigidity of the center land portion 32, reducing the rolling resistance of the tire.

[0085] In addition, in the tire 1, the extension length L3 of the first and second center lug grooves 323A, 323B in the tire circumferential direction is in the range of 0.10≦L3 / Wce≦0.30 with respect to the contact width Wce of the center land portion 32 (see FIG. 3). This has the advantage of optimizing the inclination angle of the center lug grooves 323A, 323B. Specifically, the lower limit ensures the inclination angle of the center lug grooves 323A, 323B, and ensures the drainage function of the center lug grooves 323A, 323B. The upper limit ensures the rigidity of the center land portion 32, and reduces the rolling resistance of the tire.

[0086] In addition, in this tire 1, the center land portion 32 includes a plurality of circumferential narrow grooves 321, 322 extending in the tire circumferential direction (see FIG. 2). The groove widths W1, W2 of the circumferential narrow grooves 321, 322 are in the range of 10% to 40% of the groove width Wm of the circumferential main grooves 2A, 2B. This has the advantage of optimizing the groove widths W1, W2 of the circumferential narrow grooves 321, 322. Specifically, the lower limit ensures the function of the circumferential narrow grooves 321, 322 as grooves when the tire is in contact with the ground, and ensures the drainage of the tread center region. The upper limit suppresses the reduction in rigidity of the tread center region caused by the groove widths W1, W2 of the circumferential narrow grooves 321, 322 being excessively large, and ensures the reduction in the rolling resistance of the tire.

[0087] In addition, in the tire 1, the multiple circumferential narrow grooves 321, 322 include a first circumferential narrow groove 321 having a zigzag or wavy shape with amplitude in the tire width direction and disposed at a position closest to the tire equatorial plane CL, and a second circumferential narrow groove 322 having a straight shape and disposed between the first circumferential narrow groove 321 and the circumferential main groove. This has the advantage of improving drainage in the center region of the tread.

[0088] In addition, in this tire 1, the first circumferential narrow groove 321 has a straight-shaped chamfered portion 3212 (see FIG. 4 and FIG. 5). In this configuration, the main body of the first circumferential narrow groove 321 has a zigzag shape, so compared to a straight-shaped circumferential narrow groove (not shown), the groove length of the first circumferential narrow groove 321 is ensured, and there is an advantage that the drainage effect of the first circumferential narrow groove 321 in the tread center region is enhanced. In addition, since the chamfered portion 3212 has a straight edge portion on the tread surface, compared to a configuration (not shown) in which the chamfered portion has a zigzag edge portion along the circumferential narrow groove, there is an advantage that the drainage effect of the first circumferential narrow groove 321 is further enhanced, and the rigidity of the edge portion is ensured, so that the low rolling resistance performance of the tire at the initial stage of wear is maintained.

[0089] In addition, in the tire 1, the center land portion 32 is provided with a plurality of first center auxiliary grooves 324A arranged on an extension line of the first center lug groove 323A, one end of which opens into the second circumferential main groove 2B and the other end of which terminates within the center land portion 32, and a plurality of second center auxiliary grooves 324B arranged on an extension line of the second center lug groove 323B, one end of which opens into the first circumferential main groove 2A and the other end of which terminates within the center land portion 32 (see FIG. 3). In this configuration, the center auxiliary grooves 324A, 324B improve the drainage of the center land portion 32, which has the advantage of improving the wet performance of the tire.

[0090] In addition, in the tire 1, the center land portion 32 is provided with sipes 325A; 325B formed at the groove bottoms of the first and second center lug grooves 323A; 323B and connecting the first and second center lug grooves 323A; 323B to the first and second center auxiliary grooves 324A; 324B (see FIG. 3). This has the advantage of improving drainage in the center region of the tread.

[0091] Applies to The tire 1 is a heavy-duty tire that is mounted on a drive axle of a vehicle. By applying the present invention to such tires, it is advantageous in that the tire's low rolling resistance and wet performance can be effectively improved.

[0092] In addition, in this embodiment, as described above, a pneumatic tire has been described as an example of a tire. However, the present invention is not limited to this, and the configuration described in this embodiment can be arbitrarily applied to other tires within the scope of what is obvious to a person skilled in the art. Examples of other tires include airless tires and solid tires. EXAMPLES

[0093] 9 and 10 are tables showing the results of performance tests of the tire according to the embodiment of the present invention.

[0094] In this performance test, several types of test tires were evaluated for (1) low rolling resistance performance and (2) wet performance. Test tires with a tire size of 315 / 70R22.5 were also manufactured.

[0095] (1) In the evaluation of low rolling resistance performance, a drum testing machine with a drum diameter of 1707 mm is used, and the reciprocal of the rolling resistance coefficient of the test tire is calculated and evaluated under the conditions of a load of 31.26 kN, an air pressure of 900 kPa, and a speed of 80 km / h in accordance with ISO 28580. This evaluation is performed using an index evaluation with the conventional example as the standard (100), and the higher the value, the better.

[0096] (2) In the evaluation of wet performance, the test tire is mounted on an ETRTO-specified rim, and the test tire is subjected to the JATMA-specified internal pressure and load. The test tire is also mounted on a test vehicle for the prescribed μ-S evaluation. The test vehicle then runs on a test course on a wet road surface at a speed of 60 km / h, and the friction coefficient during braking is measured and evaluated. This evaluation is performed using an index evaluation with the conventional example as the standard (100), and the higher the value, the better.

[0097] The test tire of the embodiment has the configuration of Figures 1 and 2, and a single center land portion 32 partitioned into a pair of circumferential main grooves 2A and 2B has three circumferential narrow grooves 321 and 322 and a plurality of center lug grooves 323A and 323B. The groove width Wm of the circumferential main grooves 2A and 2B is 11.0 [mm], and the groove depth Hm is 13.5 [mm]. The tire ground contact width TW is 282 [mm], and the ratio Wce / TW is 0.60. The groove widths W1 and W2 of the circumferential narrow grooves 321 and 322 are 2.2 [mm], and the groove depths H1 and H2 are 13.5 [mm].

[0098] The test tire of the comparative example is the test tire of Example 1, in which the center lug grooves 323A, 323B have a wavy shape having two inflection points in the region from the tire equatorial plane CL to the edge portion of the center land portion 32.

[0099] As the test results show, the test tires of the examples achieve both low rolling resistance performance and wet performance. [Explanation of symbols]

[0100] 1 tire; 2A, 2B circumferential main groove; 11 bead core; 12 bead filler; 121 lower filler; 122 upper filler; 13 carcass layer; 14 belt layer; 141 high angle belt; 142, 143 cross belt; 144 belt cover; 15 tread rubber; 16 sidewall rubber; 17 rim cushion rubber; 31 shoulder land portion; 311 shoulder lug groove; 32 center land portion; 321, 322 circumferential narrow groove; 3211 widening portion; 3212 chamfered portion; 3221 widening portion; 323A, 323B center lug groove; 324A, 324B center auxiliary groove; 325A, 325B sipe

Claims

1. A tire including first and second circumferential main grooves extending in a tire circumferential direction, and a pair of shoulder land portions and a single center land portion defined by the first and second circumferential main grooves, a ground contact width Wce of the center land portion is in a range of 0.55≦Wce / TW≦0.70 with respect to a tire ground contact width TW, the central land portion includes a plurality of first center lug grooves that open into the first circumferential main groove at one end, extend in the tire width direction and intersect with the tire equatorial plane, and a plurality of second center lug grooves that open into the second circumferential main groove at one end, extend in the tire width direction and intersect with the tire equatorial plane, the first and second center lug grooves have an S-shape in a region from the tire equatorial plane to the one end, The S-shape is convex in the tire rotation direction at the tire equatorial plane and is convex in the tire turning direction at the one end, and The tire is characterized in that adjacent first and second center lug grooves do not overlap each other in the tire circumferential direction.

2. The tire according to claim 1 , wherein a radius of curvature Rs2 of the S-shape at the one end has a relationship with a radius of curvature Rs1 of the S-shape at the tire equatorial plane that satisfies 1.00≦Rs2 / Rs1≦3.

50.

3. 3. The tire according to claim 1, wherein a groove width W3 of the first and second center lug grooves is in a range of 0.10≦W3 / Wm≦0.55 relative to a groove width Wm of the circumferential main groove.

4. The tire according to any one of claims 1 to 3, wherein a groove depth H3 of the first and second center lug grooves is in a range of 0.10≦H3 / Hm≦0.30 relative to a groove depth Hm of the circumferential main groove.

5. The tire according to any one of claims 1 to 4, wherein the first and second center lug grooves terminate at the other end within the center land portion, and the extension length D3 of the first and second center lug grooves in the tire width direction is in the range of 0.55≦D3 / Wce≦0.75 with respect to the contact width Wce of the center land portion.

6. The tire according to any one of claims 1 to 5, wherein an extension length L3 of the first and second center lug grooves in the tire circumferential direction is in a range of 0.10≦L3 / Wce≦0.30 with respect to a contact width Wce of the center land portion.

7. The tire according to any one of claims 1 to 6, wherein the center land portion has a plurality of circumferential narrow grooves extending in the tire circumferential direction, and the groove width of the circumferential narrow grooves is in the range of 10% to 40% of the groove width of the circumferential main groove.

8. 8. The tire according to claim 7, wherein the plurality of circumferential narrow grooves include a first circumferential narrow groove having a zigzag or wavy shape having amplitude in the tire width direction and disposed at a position closest to the tire equatorial plane, and a second circumferential narrow groove having a straight shape and disposed between the first circumferential narrow groove and the circumferential main groove.

9. The tire according to claim 8 , wherein the first circumferential narrow groove has a chamfered portion having a straight shape.

10. 10. The tire according to claim 1, wherein the center land portion comprises: a plurality of first center auxiliary grooves arranged on an extension line of the first center lug groove, one end of which opens into the second circumferential main groove and the other end of which terminates within the center land portion; and a plurality of second center auxiliary grooves arranged on an extension line of the second center lug groove, one end of which opens into the first circumferential main groove and the other end of which terminates within the center land portion.

11. 11. The tire according to claim 10, wherein the center land portion includes sipes formed at bottoms of the first and second center lug grooves and connecting the first and second center lug grooves and the first and second center auxiliary grooves.

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