tire

Abstract

To provide a tire 2 that can attain reduction of rolling resistance while securing required wet performance.SOLUTION: A shape index of a reference grounding surface of a tire 2 is 1.20 or more and 1.50 or less. A tread 4 of a tire 2 comprises a cap layer 38, an intermediate layer 40 whose loss tangent at 30°C is lower than a loss tangent at 30°C of the cap layer 38, and a base layer 42 whose loss tangent at 30°C is lower than the loss tangent at 30°C of the intermediate layer 40. In a radial direction, the intermediate layer 40 is positioned outside the base layer 42, and the cap layer 38 is positioned outside the intermediate 40. In a shoulder land part 28s of the tread 4, a thickness Tm of the cap layer 38 at a center Pm of a width in an axial direction is smaller than a thickness Tg of the cap layer 38 at a shoulder side circumferential groove 26s side.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a tire. [Background technology]

[0002] When low heat generation rubber is used in the tread, a tire with low rolling resistance is obtained. The grip of low heat generation rubber is inferior to that of high heat generation rubber that can exert a high grip. For this reason, when low heat generation rubber is used in the tread, for example, braking performance on wet road surfaces (hereinafter also referred to as wet performance) decreases. It is difficult to achieve a good balance between rolling resistance and wet performance. Various studies have been conducted with the aim of reducing rolling resistance and improving wet performance (for example, Patent Document 1 below). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2018-2008 A Summary of the Invention [Problem to be solved by the invention]

[0004] A cap layer made of heat-generating rubber is formed on the outer surface of the tread in order to provide high grip. The tread has circumferential grooves to define a plurality of land portions. As disclosed in the above-mentioned Patent Document 1, the cap layer is usually formed so that each land portion has a substantially uniform thickness.

[0005] However, depending on the shape of the tire's ground contact, the land portion may not wear uniformly as a whole, but may wear only partially. Specifically, in the shoulder land portion formed on the axially outer side of the tread, the sidewall side portion of the tread edge may hardly wear. In such a case, a part of the cap layer remains unworn in the shoulder land portion of the tire that has been worn down by running.

[0006] Considering the impact on the environment, there is a demand for further reduction in tire rolling resistance. Because the cap layer is made of heat-generating rubber, it is expected that further reduction in rolling resistance can be achieved by replacing the remaining cap layer with a rubber that is less likely to heat up than the cap layer.

[0007] The present invention has been made in view of the above circumstances, and has an object to provide a tire that can achieve reduced rolling resistance while ensuring the required wet performance. [Means for solving the problem]

[0008] A tire according to one aspect of the present invention includes a tread that comes into contact with a road surface. The tire is mounted on a regular rim, the internal pressure of the tire is adjusted to 230 kPa, and a load of 70% of the regular load is applied to the tire as a vertical load, and the tire is brought into contact with a flat road surface to obtain a contact surface, which is a reference contact surface. In the reference contact surface, a shape index expressed as a ratio of an equatorial contact length measured along the equator of the tire to a reference contact length at a position corresponding to a width of 80% of the maximum contact width is 1.20 or more and 1.50 or less. At least four land portions arranged in parallel in the axial direction are formed by carving at least three circumferential grooves in the tread. Of the at least three circumferential grooves, the circumferential groove located on the outer side in the axial direction is a shoulder circumferential groove. In the axial direction, the land portion located on the outer side of the shoulder circumferential groove is a shoulder land portion, and the land portion located on the inner side of the shoulder circumferential groove is a middle land portion. The tread includes a cap layer, an intermediate layer having a loss tangent at 30° C. lower than that of the cap layer at 30° C., and a base layer having a loss tangent at 30° C. lower than that of the intermediate layer at 30° C. In the radial direction, the intermediate layer is located outside the base layer, and the cap layer is located outside the intermediate layer. In the shoulder land portion, the thickness of the cap layer at the center of the axial width is thinner than the thickness of the cap layer on the shoulder circumferential groove side.

[0009] Preferably, in the tire, when the shape index is F and the ratio of the thickness of the cap layer at the center of the axial width of the shoulder land portion to the effective groove depth of the shoulder circumferential groove is Rm (unit: %), the ratio Rm satisfies the following formulas (1) and (2). Rm≦33.333×F-18 (1) Rm≧33.333×F-22 (2)

[0010] Preferably, in the tire, a ratio of a thickness of the cap layer on the shoulder circumferential groove side in the shoulder land portion to an effective groove depth of the shoulder circumferential groove is equal to or greater than 40% and equal to or less than 60%.

[0011] Preferably, in the tire, in the middle land portion, a ratio of a thickness of the cap layer to an effective groove depth of the shoulder circumferential groove is equal to or greater than 40% and is equal to or less than 60%.

[0012] Preferably, in the tire, the axial width of the cap layer is equal to or smaller than the axial width of the intermediate layer.

[0013] Preferably, in the tire, the difference between the axial width of the intermediate layer and the axial width of the cap layer is equal to or greater than 10 mm and equal to or less than 30 mm.

[0014] Preferably, in the tire, a difference between an axial width of the cap layer and a width of the tread is equal to or greater than -10 mm and equal to or less than 10 mm.

[0015] Preferably, in the tire, a ratio of a loss tangent at 30° C. of the cap layer to a loss tangent at 30° C. of the intermediate layer is 110% or greater and 250% or less. Effect of the Invention

[0016] According to the present invention, a tire can be obtained that can achieve reduced rolling resistance while ensuring the required wet performance. [Brief description of the drawings]

[0017] [Figure 1] FIG. 1 is a cross-sectional view showing a part of a tire according to one embodiment of the present invention. [Diagram 2] FIG. 2 is an enlarged cross-sectional view showing the contour of a shoulder portion of the tire of FIG. [Diagram 3] FIG. 3 is an enlarged cross-sectional view showing a portion of the tire of FIG. [Figure 4] FIG. 4 is an image diagram for explaining a method for calculating the shape index of the reference contact surface. [Diagram 5] FIG. 5 is a graph plotting the relationship between shape index and cap layer thickness ratio. [Figure 6] FIG. 6 is an enlarged cross-sectional view showing a part of the tire of Comparative Example 1. As shown in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Hereinafter, the present invention will be described in detail based on preferred embodiments with appropriate reference to the drawings.

[0019] In this disclosure, a state in which a tire is mounted on a standard rim, the internal pressure of the tire is adjusted to the standard internal pressure, and no load is applied to the tire is referred to as a standard state. A state in which a tire is mounted on a standard rim, the internal pressure of the tire is adjusted to 230 kPa, and no load is applied to the tire is referred to as a standard state.

[0020] In this disclosure, unless otherwise specified, the dimensions and angles of each part of the tire are measured in a normal state. The dimensions and angles of each part in a meridian section of the tire that cannot be measured when the tire is mounted on a normal rim are measured by matching the distance between the left and right beads in the tire section obtained by cutting the tire along a plane including the rotation axis to the distance between the beads in the tire when mounted on a normal rim.

[0021] A genuine rim is a rim that is specified in the standard on which the tire is based. The "standard rim" in the JATMA standard, the "Design Rim" in the TRA standard, and the "Measuring Rim" in the ETRTO standard are genuine rims.

[0022] Normal tire pressure means the internal pressure specified in the standard on which the tire is based. The "maximum air pressure" in the JATMA standard, the "maximum value" listed in "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES" in the TRA standard, and the "INFLATION PRESSURE" in the ETRTO standard are normal tire pressures.

[0023] Normal load refers to the load specified in the standard on which the tire is based. The "maximum load capacity" in the JATMA standard, the "maximum value" listed in "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES" in the TRA standard, and the "LOAD CAPACITY" in the ETRTO standard are normal loads.

[0024] In the present disclosure, the crosslinked rubber is a molded product of the rubber composition obtained by pressurizing and heating the rubber composition. The rubber composition is an uncrosslinked rubber obtained by mixing a base rubber and chemicals in a kneader such as a Banbury mixer. The crosslinked rubber is also called vulcanized rubber, and the rubber composition is also called unvulcanized rubber.

[0025] Examples of the base rubber include natural rubber (NR), butadiene rubber (BR), styrene butadiene rubber (SBR), isoprene rubber (IR), ethylene propylene rubber (EPDM), chloroprene rubber (CR), acrylonitrile butadiene rubber (NBR), and butyl rubber (IIR). Examples of the chemicals include reinforcing agents such as carbon black and silica, plasticizers such as aromatic oil, fillers such as zinc oxide, lubricants such as stearic acid, antioxidants, processing aids, sulfur, and vulcanization accelerators. The selection of the base rubber and chemicals, the content of the selected chemicals, etc. are appropriately determined depending on the specifications of each element such as the tread and sidewall to which the rubber composition is applied.

[0026] In the present disclosure, the loss tangent (also referred to as tan δ) at a temperature of 30°C of an element made of crosslinked rubber among the elements constituting a tire is measured in accordance with the provisions of JIS K6394 using a viscoelasticity spectrometer ("VES" manufactured by Iwamoto Seisakusho Co., Ltd.) under the following conditions. Initial distortion = 10% Dynamic strain = 2% Frequency = 10Hz Deformation mode = tension In this measurement, the test specimen is sampled from a tire. If it is not possible to sample the test specimen from a tire, the test specimen is sampled from a sheet-like crosslinked rubber (hereinafter also referred to as a rubber sheet) obtained by pressing and heating the rubber composition used to form the element to be measured at a temperature of 170°C for 12 minutes.

[0027] Fig. 1 shows a portion of a tire 2 according to one embodiment of the present invention. This tire 2 is a tire for passenger cars. Fig. 1 shows a portion of a cross section (hereinafter also referred to as a meridian cross section) of the tire 2 along a plane including the rotation axis of the tire 2. In Fig. 1, the left-right direction is the axial direction of the tire 2, and the up-down direction is the radial direction of the tire 2. The direction perpendicular to the paper surface of Fig. 1 is the circumferential direction of the tire 2.

[0028] 1, the dashed dotted line EL is the equatorial plane of the tire 2. The tire 2 is symmetrical with respect to the equatorial plane, except for the tread pattern and decorations such as designs and letters engraved on the outer surface.

[0029] In FIG. 1, a tire 2 is mounted on a rim R. The rim R is a regular rim. Air is filled inside the tire 2, and the internal pressure of the tire 2 is adjusted. The tire 2 mounted on the rim R is also called a tire-rim assembly. The tire-rim assembly includes the rim R and the tire 2 mounted on the rim R.

[0030] 1, the position indicated by the symbol PW is the axially outer end of the tire 2. If there is a decoration such as a pattern or lettering on the outer surface, the outer end PW is identified based on a virtual outer surface obtained by assuming that there is no decoration.

[0031] In Fig. 1, the length indicated by the symbol WA is the maximum width of the tire 2, i.e., the section width (see JATMA, etc.). The section width WA of the tire 2 is the axial distance from one outer end PW to the other outer end PW. The outer end PW is the position where the tire 2 shows the maximum width (hereinafter, the maximum width position). The section width WA is measured with the tire 2 in a standard state.

[0032] The tire 2 includes a tread 4 , a pair of sidewalls 6 , a pair of clinches 8 , a pair of beads 10 , a carcass 12 , a belt 14 , a band 16 , a pair of cushions 18 , a pair of chafers 20 , and an inner liner 22 .

[0033] The outer surface of the tread 4 comes into contact with the road surface. Grooves 24 are cut into the tread 4. This forms a tread pattern.

[0034] In this tire 2, at least three circumferential grooves 26 extending continuously in the circumferential direction are cut into the tread 4. As a result, at least four land portions 28 arranged in parallel in the axial direction are formed in the tread 4. In the tire 2 shown in Fig. 1, four land portions 28 are formed by cutting the three circumferential grooves 26 into the tread 4. The circumferential grooves 26 form part of the grooves 24 that form the tread pattern.

[0035] In Fig. 1, the position indicated by the symbol PE is the equator of the tire 2. The equator PE is the intersection point between the outer surface of the tread 4 and the equatorial plane. As shown in Fig. 1, when a groove 24 is present on the equatorial plane, the equator PE is determined based on a virtual outer surface of the tread 4 obtained by assuming that the groove 24 does not exist.

[0036] Each sidewall 6 is continuous with an end of the tread 4. The sidewall 6 is located radially inward of the tread 4. The sidewall 6 extends from the end of the tread 4 toward the clinch 8 along the carcass 12. The sidewall 6 is made of crosslinked rubber in consideration of cut resistance.

[0037] Each clinch 8 is located radially inside the sidewall 6. The clinch 8 contacts the rim R. The clinch 8 is made of crosslinked rubber in consideration of wear resistance.

[0038] Each bead 10 is located axially inside the clinch 8. The bead 10 includes a core 30 and an apex 32. Although not shown, the core 30 includes a steel wire.

[0039] The apex 32 is located radially outward of the core 30. The apex 32 tapers outward. The apex 32 is made of crosslinked rubber having high rigidity.

[0040] The carcass 12 is located inside the tread 4, the pair of sidewalls 6, and the pair of clinches 8. The carcass 12 bridges between one bead 10 and the other bead 10. This carcass 12 has a radial structure.

[0041] The carcass 12 includes at least one carcass ply 34. From the viewpoint of weight reduction, the carcass 12 of the tire 2 is configured with one carcass ply 34.

[0042] Although not shown, the carcass ply 34 includes a number of carcass cords arranged in parallel. These carcass cords are covered with a topping rubber. Each carcass cord crosses the equator plane. The carcass cord is a cord made of an organic fiber. Examples of the organic fiber include nylon fiber, rayon fiber, polyester fiber, and aramid fiber.

[0043] The belt 14 is located on the inside of the tread 4 in the radial direction. The belt 14 is layered on the carcass 12 from the outside in the radial direction. In FIG. 1, the length indicated by the symbol WR is the axial width of the belt 14. The axial width WR is the axial distance from one end of the belt 14 to the other end. In this tire 2, the axial width WR of the belt 14 is 65% or more and 85% or less of the cross-sectional width WA.

[0044] The belt 14 is composed of at least two layers 36 laminated in the radial direction. The belt 14 of the tire 2 is composed of two layers 36 laminated in the radial direction. Of the two layers 36, the layer 36 located on the inner side is the inner layer 36a, and the layer 36 located on the outer side is the outer layer 36b. As shown in FIG. 1, the inner layer 36a is wider than the outer layer 36b. The length from an end of the outer layer 36b to an end of the inner layer 36a is 3 mm or more and 10 mm or less.

[0045] Although not shown, each of the inner layer 36a and the outer layer 36b includes a number of belt cords arranged in parallel. These belt cords are covered with a topping rubber. Each belt cord is inclined with respect to the equator plane. The material of the belt cord is steel.

[0046] The band 16 is located between the tread 4 and the belt 14 in the radial direction. The band 16 is laminated on the belt 14 on the inside of the tread 4.

[0047] Although not shown, the band 16 includes a band cord wound in a spiral shape. The band cord extends substantially in the circumferential direction. Specifically, the angle that the band cord forms with the circumferential direction is 5° or less. The band 16 has a jointless structure. In the tire 2, a cord made of an organic fiber is used as the band cord. Examples of the organic fiber include nylon fiber, rayon fiber, polyester fiber, and aramid fiber.

[0048] The band 16 of the tire 2 is composed of a full band whose two ends face each other across the equator PE. The band 16 is wider than the belt 14. The length from the end of the belt 14 to the end of the band 16 is 3 mm or more and 10 mm or less. The band 16 covers the entire belt 14. The band 16 may include a pair of edge bands that are spaced apart in the axial direction and cover the ends of the full band and the ends of the belt 14. The band 16 may be composed of only a pair of edge bands.

[0049] The cushions 18 are disposed at intervals in the axial direction. The cushions 18 are located between the ends of the belt 14 and the band 16 and the ply body 34a of the carcass 12. The cushions 18 are made of crosslinked rubber having low rigidity. In the tire 2, the cushions 18 may not be provided.

[0050] Each chafer 20 is located radially inside the bead 10. The chafer 20 contacts the rim R. The chafer 20 of the tire 2 is made of a cloth and rubber impregnated into the cloth.

[0051] The inner liner 22 is positioned inside the carcass 12. The inner liner 22 constitutes the inner surface of the tire 2. The inner liner 22 is made of a crosslinked rubber having a low gas permeability coefficient. The inner liner 22 maintains the internal pressure of the tire 2.

[0052] 1, the position indicated by the symbol PH is a position on the outer surface of the tread 4. The position PH corresponds to the axially outer end of the contact surface of the tire 2 with the road surface.

[0053] The contact patch for identifying the position PH is obtained, for example, by using a contact patch shape measuring device (not shown). This contact patch is obtained by applying a load of 70% of the normal load as a vertical load to the tire 2 with the camber angle of the tire 2 in a standard state set to 0°, and bringing the tire 2 into contact with a flat road surface. In the tire 2, the contact patch obtained in this manner is the reference contact patch, and the position on the outer surface of the tread 4 that corresponds to the axial outer end of the reference contact patch is the above-mentioned position PH. In the tire 2, this position PH is the reference contact end.

[0054] Fig. 2 shows a part of the tire 2 shown in Fig. 1. In Fig. 2, the left-right direction is the axial direction of the tire 2, and the up-down direction is the radial direction of the tire 2. The direction perpendicular to the paper surface of Fig. 2 is the circumferential direction of the tire 2.

[0055] Fig. 2 shows the contour of a shoulder portion of a tire 2 in a meridian cross section. The contour shown in Fig. 2 is obtained by measuring the outer surface shape of the tire 2 in a standard state with a displacement sensor.

[0056] In a meridian cross section, the contour of the outer surface of the tire 2 (hereinafter, tire outer surface TS) is formed by connecting multiple contour lines consisting of straight lines or circular arcs. In this disclosure, a contour line consisting of a straight line or a circular arc is simply referred to as a contour line. A contour line consisting of a straight line is referred to as a straight contour line, and a contour line consisting of a circular arc is referred to as a curved contour line.

[0057] The tire outer surface TS includes a tread surface T and a pair of side surfaces S connected to the ends of the tread surface T. In a meridian cross section, the contour of the tread surface T includes a plurality of curved contour lines having different radii. In this tire 2, the curved contour line having the smallest radius among the plurality of curved contour lines included in the contour of the tread surface T is located at the end portion of the tread surface T and connected to the side surface S. In a meridian cross section, the contour of the tire outer surface TS includes, at the end portion of the tread surface T, a curved portion that is a curved contour line consisting of an arc having the smallest radius among the plurality of curved contour lines included in the contour of the tread surface T and connected to the side surface S. In FIG. 2, this curved portion is indicated by the symbol RS.

[0058] In the contour of the tire outer surface TS, the curved portion RS is tangent to a contour line adjacent to the axially inner side (hereinafter, referred to as an inner adjacent contour line NT) at a tangent point CT. This curved portion RS is tangent to a contour line constituting the contour of the side surface S adjacent to the axially outer side (hereinafter, referred to as an outer adjacent contour line NS) at a tangent point CS. This contour of the tire outer surface TS includes an inner adjacent contour line NT located axially inside the curved portion RS and tangent to the curved portion RS, and an outer adjacent contour line NS located axially outside the curved portion RS and tangent to the curved portion RS.

[0059] In Fig. 2, a solid line LT is a tangent to the curved portion RS at a contact point CT between the inner adjacent contour line NT and the curved portion RS. A solid line LS is a tangent to the curved portion RS at a contact point CS between the outer adjacent contour line NS and the curved portion RS. The position indicated by the symbol PT is the intersection point between the tangent line LT and the tangent line LS. In this tire 2, this intersection point PT is the imaginary tread edge.

[0060] The portion of the tread 4 from one imaginary tread edge PT to the other imaginary tread edge PT is an area (hereinafter also referred to as a normal contact area) that is expected to come into contact with the road surface under normal driving conditions of the tire 2. From the viewpoint of effective reinforcement of the portion of the tread 4 (hereinafter also referred to as a tread portion), the belt 14 and band 16 described above are disposed in this normal contact area.

[0061] In Fig. 1, the length indicated by the double-headed arrow WT is the width of the tread 4. This width of the tread 4 is the axial distance from one imaginary tread edge PT to the other imaginary tread edge PT. The length indicated by the double-headed arrow WH is the axial width of the reference ground contact surface. The axial width WH is the axial distance from one reference ground contact edge PH to the other reference ground contact edge PH.

[0062] In this tire 2, the ratio (WT / WA) of the width WT of the tread 4 to the cross-sectional width WA is 70% or more and 90% or less. The virtual tread edge PT is located axially outboard of the reference ground contact edge PH. In other words, the axial width WH of the reference ground contact surface is narrower than the width WT of the tread 4. Specifically, the ratio (WH / WT) of the axial width WH to the width WT of the tread 4 is 70% or more and 90% or less.

[0063] As described above, three circumferential grooves 26 are formed in the tread 4 of the tire 2. There are no particular limitations on the arrangement, groove depth, and groove width of the three circumferential grooves 26 in the tire 2. The arrangement, groove depth, and groove width of the circumferential grooves of a tire are generally applied to the tread 4.

[0064] In the tire 2, the circumferential groove 26 located on the outer side in the axial direction among the three circumferential grooves 26 is a shoulder circumferential groove 26s. The circumferential groove 26 located on the inner side of the shoulder circumferential groove 26s is a middle circumferential groove 26m.

[0065] As described above, the tread 4 of the tire 2 is formed with four land portions 28. Of the four land portions 28, the land portion 28 located on the equatorial plane side is the middle land portion 28m, and the land portion 28 located on the outer side of the middle land portion 28m is the shoulder land portion 28s.

[0066] The shoulder land portion 28s is a land portion 28 located on the outer side of the shoulder circumferential groove 26s in the axial direction. The shoulder land portion 28s includes a reference ground contact edge PH. The middle land portion 28m is a land portion 28 located on the inner side of the shoulder circumferential groove 26s in the axial direction. The middle circumferential groove 26m is between the left and right middle land portions 28m. In the tire 2, the middle circumferential groove 26m is located on the equatorial plane. The middle circumferential groove 26m is also referred to as a center circumferential groove.

[0067] Fig. 3 shows a part of the tire 2 shown in Fig. 1. Fig. 3 shows a tread 4 of the tire 2. In Fig. 3, the left-right direction is the axial direction of the tire 2, and the up-down direction is the radial direction of the tire 2. The direction perpendicular to the paper surface of Fig. 3 is the circumferential direction of the tire 2.

[0068] The tread 4 of the tire 2 includes a cap layer 38, an intermediate layer 40, and a base layer 42. In the radial direction, the intermediate layer 40 is located outside the base layer 42, and the cap layer 38 is located outside the intermediate layer 40. As shown in FIG. 3 , the cap layer 38 is laminated on the intermediate layer 40, and the intermediate layer 40 is laminated on the base layer 42.

[0069] In FIG. 3, the position indicated by the symbol PC is the outer end of the cap layer 38. The length indicated by the symbol WC is the axial width of the cap layer 38. The axial width WC is the axial distance from one outer end PC to the other outer end PC. The position indicated by the symbol PM is the outer end of the intermediate layer 40. The length indicated by the symbol WM is the axial width of the intermediate layer 40. The axial width WM is the axial distance from one outer end PM to the other outer end PM. The position indicated by the symbol PB is the outer end of the base layer 42. The length indicated by the symbol WB is the axial width of the base layer 42. The axial width WB is the axial distance from one outer end PB to the other outer end PB.

[0070] An outer end PC of the cap layer 38 is located axially inside an outer end PM of the intermediate layer 40. The outer end PC of the cap layer 38 is located axially at approximately the same position as an outer end PB of the base layer 42. In the tire 2, the position of the outer end PC of the cap layer 38 is appropriately adjusted between the reference ground edge PH and the outer end PM of the intermediate layer 40, taking into consideration wet performance and rolling resistance.

[0071] An outer end PB of the base layer 42 is located axially inside an outer end PM of the intermediate layer 40. In the tire 2, a difference (WB-WT) between an axial width WB of the base layer 42 and a width WT of the tread 4 is -10 mm or more and 10 mm or less. In other words, the axial width WB of the base layer 42 is approximately equal to the width WT of the tread 4.

[0072] In Fig. 3, the position indicated by the symbol Pg is the edge of the shoulder land portion 28s on the shoulder circumferential groove 26s side. The axial distance from this edge Pg to the virtual tread edge PT is the axial width of the shoulder land portion 28s. The position indicated by the symbol Pm is a position on the outer surface of the tread 4, and is the center of the axial width of the shoulder land portion 28s. This center Pm is located inside the reference ground contact edge PH in the axial direction.

[0073] The length indicated by the symbol Tg is the thickness of the cap layer on the shoulder circumferential groove 26s side of the shoulder land portion 28s. This thickness Tg is measured along a straight line extending in the radial direction through the edge Pg of the shoulder land portion 28s on the shoulder circumferential groove 26s side. When the shape of the boundary between the cap layer 38 and the intermediate layer 40 changes according to the shape of the shoulder circumferential groove 26s, the thickness measured along a straight line extending in the radial direction through a position on the outer surface of the tread 4 1 mm away from the edge Pg toward the virtual tread edge PT is used as the thickness Tg.

[0074] The length indicated by the symbol Tm is the thickness of the cap layer 38 at the center Pm of the axial width of the shoulder land portion 28s. This thickness Tm is measured along a straight line that passes through the center Pm of the axial width of the shoulder land portion 28s and extends in the radial direction.

[0075] The position indicated by the symbol Pe is the intersection point between a straight line passing through the imaginary tread edge PT and extending in the radial direction and the outer surface of the tread 4. This intersection point Pe is also referred to as the reference edge of the tread 4. When the outer end PC of the cap layer is located axially outboard of the imaginary tread edge PT, the thickness of the cap layer 38 measured along a straight line passing through the reference end Pe and extending in the radial direction is represented as the thickness Te of the cap layer 38 at the reference end Pe of the tread 4.

[0076] 3, the position indicated by the symbol PS is the outer end of the tread 4 on the tire outer surface TS. In this tire 2, the outer end PS of the tread 4 is located axially outboard of the outer end PC of the cap layer 38. Of the tire outer surface TS, a portion from the outer end PC to the outer end PS is formed by the intermediate layer 40. In this tire 2, a portion of the intermediate layer 40 is exposed to the tire outer surface TS.

[0077] In Fig. 3, the length indicated by the symbol DS is the groove depth of the shoulder circumferential groove 26s. This groove depth DS is represented by the maximum groove depth of this shoulder circumferential groove. Although not shown, a protrusion such as a wear indicator is provided at the groove bottom of the shoulder circumferential groove 26s. In this tire 2, a value obtained by subtracting 1.6 mm from the groove depth DS (unit: mm) is used as the effective groove depth DSe of the shoulder circumferential groove.

[0078] In this tire 2, the cap layer 38, the intermediate layer 40, and the base layer 42 are each made of a crosslinked rubber having a different heat buildup property. In this tire 2, the cap layer 38 is most likely to generate heat, and the base layer 42 is least likely to generate heat. The intermediate layer 40 has a heat buildup property between that of the cap layer 38 and that of the base layer 42. In this tire 2, the loss tangent LTm of the intermediate layer 40 at 30°C is lower than the loss tangent LTc of the cap layer 38 at 30°C. The loss tangent LTb of the base layer 42 at 30°C is lower than the loss tangent LTm of the intermediate layer 40 at 30°C.

[0079] The loss tangent LTb of the base layer 42 at 30°C is preferably 0.11 or less. This is because the base layer 42 effectively contributes to reducing rolling resistance. From this viewpoint, the loss tangent LTb is more preferably 0.10 or less, and further preferably 0.09 or less. The smaller the loss tangent LTb of the base layer 42, the more preferable it is, and therefore no preferable lower limit is set.

[0080] The loss tangent Ltm of the intermediate layer 40 at 30°C is preferably 0.15 or less. This is because the intermediate layer 40 effectively contributes to reducing rolling resistance. From this viewpoint, the loss tangent Ltm is more preferably 0.14 or less, and even more preferably 0.13 or less. The loss tangent Ltm of the intermediate layer 40 at 30°C is preferably 0.11 or more. This is because the intermediate layer 40 can ensure the necessary rigidity and effectively contribute to improving wet performance. From this viewpoint, the loss tangent LTm is more preferably 0.12 or more.

[0081] The loss tangent LTc of the cap layer 38 at 30°C is preferably 0.15 or more. This is because the cap layer 38 can contribute to improving wet performance. From this viewpoint, the loss tangent LTc is more preferably 0.16 or more, and even more preferably 0.17 or more. The cap layer 38 contacts the road surface. From the viewpoint of improving wet performance, the higher the loss tangent LTc, the more preferable. However, a high loss tangent LTc leads to heat generation. There is a concern that the heated cap layer 38 will increase the temperature of the intermediate layer 40 more than expected. From the viewpoint of keeping the temperature state of the entire tread 4 stable and maintaining low rolling resistance, the loss tangent LTc of the cap layer 38 at 30°C is preferably 0.30 or less, more preferably 0.28 or less, and even more preferably 0.27 or less.

[0082] A model of the contour shape of the reference contact patch is shown in Figure 4. In Figure 4, the up-down direction corresponds to the circumferential direction of the tire 2, and the left-right direction corresponds to the axial direction of the tire 2. The direction perpendicular to the paper surface of Figure 4 corresponds to the radial direction of the tire 2.

[0083] In Fig. 4, the dashed dotted line LP is a straight line on the reference contact surface that corresponds to the equator PE of the tire 2. When it is difficult to identify the equator PE on the reference contact surface, the axial center line of the reference contact surface is used as the straight line that corresponds to the equator PE. The double-headed arrow P100 is the length of the intersection line between a plane including the line LP and the reference contact surface. In this tire 2, the length P100 of the intersection line is the equatorial contact length measured along the equator PE on the reference contact surface.

[0084] In FIG. 4, the solid line LM is a straight line that passes through the reference ground edge PH of the reference ground surface and is parallel to the straight line LP. The solid line L80 is a straight line that is located between the straight lines LM and LP and is parallel to the straight lines LM and LP. The double-headed arrow A100 represents the axial distance from the straight line LP to the straight line LM. This distance A100 corresponds to half of the maximum ground contact width of the reference ground surface. The double-headed arrow A80 represents the axial distance from the straight line LP to the straight line L80. In this tire 2, the ratio of the distance A80 to the distance A100 is set to 80%. That is, the straight line L80 represents a position that corresponds to 80% of the maximum ground contact width of the reference ground surface. The double-headed arrow P80 is the length of the intersection line between the plane including the straight line L80 and the reference ground surface. In this tire 2, the length P80 of this intersection line is the reference ground contact length at a position on the reference ground surface that corresponds to 80% of the maximum ground contact width.

[0085] In the tire 2, the ratio (P100 / P80) of the equatorial contact length P100 to the reference contact length P80, which is expressed using the equatorial contact length P100 and the reference contact length P80 specified in the reference contact surface, is used as a shape index of the contour shape of the reference contact surface. A larger value of this shape index indicates that the reference contact surface has a rounder contour.

[0086] As described above, the profile of the tread surface T is formed by connecting a plurality of contour lines consisting of straight lines or circular arcs. Although not described in detail, the shape index F of the reference contact surface is controlled by adjusting the specifications of the contour lines constituting the profile of the tread surface T (for example, the length of the straight contour lines and the radius and length of the curved contour lines).

[0087] In this tire 2, the shape index of the reference contact patch is 1.20 or more and 1.50 or less. In this tire 2, the shoulder land portion 28s does not wear uniformly as a whole, but tends to wear partially. In this tire 2, among the end portions of the tread 4, particularly the portion on the sidewall 6 side hardly wears at all, so there is a concern that the cap layer 38 will remain in the axially outer portion of the shoulder land portion 28s even if the tire 2 wears with use.

[0088] In the tire 2, the cap layer 38 in the middle land portion 28m has a generally uniform thickness overall. In contrast, the cap layer 38 in the shoulder land portion 28s has a thickness equal to that of the cap layer 38 in the middle land portion 28m on the shoulder circumferential groove 26s side, but the thickness of the cap layer 38 gradually decreases from the shoulder circumferential groove 26s side toward the axially outer side. In the shoulder land portion 28s, the thickness Tm of the cap layer 38 at the center Pm of the axial width is thinner than the thickness Tgs of the cap layer 38 on the shoulder circumferential groove 26s side.

[0089] In this tire 2, the cap layer 38 is prevented from remaining unworn in the axially outer portion of the shoulder land portion 28s. In other words, the cap layer 38 of a required thickness is provided near the reference ground contact edge PH of the shoulder land portion 28s. In this tire 2, a part of the portion where the cap layer 38 was provided in the conventional tire is replaced with an intermediate layer 40 that is less likely to generate heat than the cap layer 38. This tire 2 can achieve reduced rolling resistance while ensuring required wet performance.

[0090] As described above, the shape index of the reference contact surface is 1.20 or more and 1.50 or less in the tire 2. For example, if the equatorial contact length P100 is the same, the reference contact length P80 of a reference contact surface having a large shape index is shorter than the reference contact length P80 of a reference contact surface having a small shape index.

[0091] The larger the shape index, the shorter the reference contact length P80, so the larger the shape index of the tire 2, the higher the contact pressure near the reference contact end PH of the reference contact surface. Since high contact pressure promotes wear, when the shape index of the reference contact surface is large, the tire 2 can ensure good wet performance by increasing the thickness Tm of the cap layer 38 at the center Pm of the axial width of the shoulder land portion 28s. Conversely, when the shape index of the reference contact surface is small, wear does not progress easily in the shoulder land portion 28s, so by reducing the thickness Tm of the cap layer 38, the tire 2 can further reduce the rolling resistance.

[0092] In this tire 2, from the viewpoints of ensuring wet performance and reducing rolling resistance, the ratio (Tm / DSe) of the thickness Tm of the cap layer 38 at the center Pm of the axial width of the shoulder land portion 28s to the effective groove depth DSe of the shoulder circumferential groove 26s is preferably 15% or more and 35% or less. In particular, in this tire 2, from the viewpoint of effectively configuring the cap layer 38 of the required thickness at the required position in the shoulder land portion 28s according to the shape index of the reference contact patch, it is more preferable that when the shape index is F and the ratio (Tm / DSe) is Rm (unit: %), this ratio Rm satisfies the following formulas (1) and (2). Rm≦33.333×F-18 (1) Rm≧33.333×F-22 (2)

[0093] In the tire 2, from the viewpoints of ensuring wet performance and reducing rolling resistance, it is more preferable that the ratio Rm satisfy the following formula (3). Rm=33.333×F-20 (3)

[0094] 5 plots the relationship between the shape index F and the ratio Rm (hereinafter, the thickness ratio Rm of the cap layer 38) of the thickness Tm of the cap layer 38 at the center Pm of the axial width of the shoulder land portion 28s to the effective groove depth of the shoulder circumferential groove 26s. In this Fig. 2, the horizontal axis represents the shape index F, and the vertical axis represents the thickness ratio Rm of the cap layer.

[0095] In Fig. 5, the region indicated by the symbol Y is a region formed by the tire 2 that satisfies the above-mentioned formulas (1) and (2). In the tire 2 included in this region Y, when the shape index F of the reference contact surface is large, the thickness Tm of the cap layer 38 at the center Pm of the axial width of the shoulder land portion 28 is thick, and conversely, when the shape index F of the reference contact surface is small, the thickness Tm of the cap layer 38 is thin. In this tire 2, the cap layer 38 of the required thickness can be effectively formed at the required position in the shoulder land portion 28s according to the shape index F of the reference contact surface. This tire 2 can effectively ensure good wet performance and can further reduce rolling resistance.

[0096] In the tire 2, in the shoulder land portion 28s, the ratio (Tg / DSe) of the thickness Tg of the cap layer 38 on the shoulder circumferential groove 26s side to the effective groove depth DSe of the shoulder circumferential groove 26s is preferably 40% or more and 60% or less.

[0097] Setting the ratio (Tg / DSe) to be equal to or greater than 40% can contribute to ensuring good wet performance of the cap layer 38. From this viewpoint, the ratio (Tg / DSe) is more preferably equal to or greater than 45%.

[0098] Setting the ratio (Tg / DSe) to be equal to or less than 60% suppresses the effect of the cap layer 38 on the rolling resistance. From this viewpoint, the ratio (Tg / DSe) is more preferably equal to or less than 55%.

[0099] As described above, the cap layer 38 in the middle land portion 28m has a generally uniform thickness as a whole. When the thickness of the cap layer at the widthwise center of the middle land portion 28m is defined as the thickness TM of the middle land portion 28m, in the tire 2, from the viewpoints of ensuring good wet performance and reducing rolling resistance, the ratio (TM / DSe) of the thickness TM of the cap layer 38 in the middle land portion 28m to the effective groove depth DSe of the shoulder circumferential groove 26s is preferably 40% or more and 60% or less.

[0100] Setting the ratio (TM / DSe) to be equal to or greater than 40% allows the cap layer 38 to contribute to ensuring good wet performance. From this viewpoint, the ratio (Tg / DSe) is more preferably equal to or greater than 45%.

[0101] Setting the ratio (TM / DSe) to be equal to or less than 60% suppresses the effect of the cap layer 38 on the rolling resistance. From this viewpoint, the ratio (Tg / DSe) is more preferably equal to or less than 55%.

[0102] In the tire 2, from the viewpoint of allowing the cap layer 38 to effectively contribute to wet performance, the ratio (WC / WA) of the axial width WC of the cap layer 38 to the cross-sectional width WA of the tire 2 is preferably 70% or more, and more preferably 75% or more. From the viewpoint of effectively suppressing the effect of the cap layer 38 on rolling resistance, the ratio (WC / WA) is preferably 90% or less, and more preferably 85% or less.

[0103] During braking, a large load acts on the tire 2. This tends to increase the contact patch width of the tire 2. In this tire 2, the outer end PC of the cap layer 38 is located outside the reference contact edge PH in the axial direction. Even during braking, the cap layer 38 can make sufficient contact with the road surface. With this tire 2, good wet performance is obtained. From this viewpoint, it is preferable that the outer end PC of the cap layer 38 is located outside the reference contact edge PH in the axial direction.

[0104] In the tire 2, from the viewpoint of suppressing the effect of the cap layer 38 on the rolling resistance, the axial width WC of the cap layer 38 is preferably equal to or smaller than the axial width WM of the intermediate layer 40. From the viewpoint of further reducing the rolling resistance, it is more preferable that the cap layer 38 is narrower than the intermediate layer 40.

[0105] As described above, in the tire 2, the outer end PC of the cap layer 38 is located at approximately the same position as the outer end PB of the base layer 42 in the axial direction. The outer end PM of the intermediate layer 40 is located outside the outer end PC of the cap layer 38 in the axial direction. From the viewpoint that the intermediate layer 40 can effectively contribute to preventing exposure of the base layer 42 during critical running, the difference (WM-WC) between the axial width WM of the intermediate layer 40 and the axial width WC of the cap layer 38 is preferably 10 mm or more. From the viewpoint of reducing rolling resistance, the difference (WM-WC) is preferably 30 mm or less, more preferably 20 mm or less, and even more preferably 10 mm or less. In the tire 2, from the viewpoint of effectively reducing rolling resistance while preventing exposure of the base layer 42, it is particularly preferable that the difference (WM-WC) is 10 mm.

[0106] In this tire 2, the difference (WC-WT) between the axial width WC of the cap layer 38 and the width WT of the tread 4 is preferably -10 mm or more and 10 mm or less. In other words, it is preferable that the axial width WC of the cap layer 38 is substantially equal to the width WT of the tread 4. This allows the cap layer 38 to make sufficient contact with the road surface not only during straight running but also during braking under the action of a large load. In this tire 2, good wet performance is obtained. From this viewpoint, the difference (WC-WT) is preferably -5 mm or more and 5 mm or less.

[0107] In the tire 2, the portion of the tread 4 consisting of the cap layer 38 and the intermediate layer 40 corresponds to the cap layer of the tread of a conventional tire consisting of the cap layer and the base layer. From the viewpoint that the tread 4 can effectively contribute to improving the wet performance and reducing the rolling resistance, the ratio (Ltc / Ltm) of the loss tangent LTc of the cap layer 38 at 30°C to the loss tangent LTm of the intermediate layer 40 at 30°C is preferably 110% or more and 250% or less. This ratio (Ltc / Ltm) is more preferably 130% or more, and even more preferably 150% or more. This ratio (Ltc / Ltm) is more preferably 240% or less, and even more preferably 230% or less.

[0108] As described above, according to the present invention, a tire can be obtained that can achieve reduced rolling resistance while ensuring the required wet performance. EXAMPLES

[0109] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0110] [Experiment 1] [Example 1] A pneumatic tire for passenger cars (tire size=205 / 55R16) having the basic structure shown in FIG. 1 and the specifications shown in Table 1 below was obtained.

[0111] A tread having a configuration shown in Figure 3 was used, including a cap layer, an intermediate layer, and a base layer. The loss tangent LTc of the cap layer at 30°C was 0.27. The loss tangent LTm of the intermediate layer at 30°C was 0.12. The loss tangent LTb of the base layer at 30°C was 0.10.

[0112] The difference between the axial width WC of the cap layer and the width WT of the tread (WC-WT) was 0 mm. The difference between the axial width WB of the base layer and the width WT of the tread (WB-WT) was 0 mm. The difference between the axial width WM of the intermediate layer and the axial width WB of the base layer (WM-WB) was 10 mm.

[0113] In the shoulder land portion, the ratio Rg of the cap layer thickness Tg on the shoulder circumferential groove side to the effective groove depth DSe of the shoulder circumferential groove was 50%. The ratio Rm of the cap layer thickness Tm at the center of the axial width to the effective groove depth DSe of the shoulder circumferential groove was 30%. The axial width WC of the cap layer was equal to the tread width WT, and the ratio Re of the cap layer thickness Te at the reference end Pe of the tread to the effective groove depth DSe of the shoulder circumferential groove was 0%.

[0114] [Comparative Example 1] A tire of Comparative Example 1 was obtained in the same manner as in Example 1, except that the ratio Rm, the ratio Re, and the difference (WC-WT) were set as shown in the following Table 1. The configuration of the tread of Comparative Example 1 is shown in FIG.

[0115] [Example 2] A tire of Example 2 was obtained in the same manner as in Example 1, except that the ratio Re and the difference (WC-WT) were set as shown in Table 1 below.

[0116] [Example 3] A tire of Example 3 was obtained in the same manner as in Example 1, except that the ratio Rm was set as shown in Table 1 below.

[0117] [Rolling resistance coefficient (RRC)] Using a rolling resistance tester, the rolling resistance coefficient (RRC) was measured when the prototype tire was running on a drum at a speed of 80 km / h under the following conditions. The results are shown in Table 1 below as an index, with Comparative Example 1 being set at 100. The smaller the value, the lower the rolling resistance of the tire. Rim: 16×6.5J Internal pressure: 210kPa Vertical load: 4.82kN

[0118] [Wet performance of new tires (WET)] A new prototype tire was mounted on a rim (size = 16 x 6.5J), and air was filled to adjust the tire's internal pressure to 230 kPa. The tire was mounted on a test vehicle (passenger car). The test vehicle was driven on a test course with a wet road surface (water film thickness = 1.4 mm). The test vehicle was braked while traveling at a speed of 100 km / h, and the running distance (braking distance) from braking to stopping was measured. The results are shown in the "NEW" column of Table 1 below as an index, with Comparative Example 1 set at 100. The higher the value, the shorter the braking distance and the better the tire's wet performance. In this evaluation, an index of 95 or more is acceptable as it ensures good wet performance.

[0119] [Wet performance of worn tires (WET)] A new prototype tire was mounted on a rim (size = 16 x 6.5J) and air was filled to adjust the internal pressure of the tire to 230 kPa. The tire was mounted on a test vehicle (passenger car). The test vehicle was run on a test course with a dry asphalt road surface to wear the tire. When the effective groove depth reached 50% of the effective groove depth of the new tire, the wet performance of the worn tire was evaluated in the same manner as the wet performance of the new tire described above. The results are shown in the "OLD" column in Table 1 below as an index, with Comparative Example 1 set to 100. The higher the value, the shorter the braking distance and the better the wet performance of the tire. In this evaluation, if the index is 95 or higher, the tire is acceptable as maintaining good wet performance.

[0120] [comprehensive evaluation] An overall evaluation was made based on the index obtained in each evaluation. In this overall evaluation, the smaller the value for the rolling resistance coefficient, the more preferable it is, so the evaluation value was obtained by subtracting the index from 100. In the case of the wet performance of new tires and worn tires, the higher the value, the more preferable it is, so the evaluation value was obtained by subtracting 100 from the index. The sum of each evaluation value was calculated, and an overall evaluation was made based on this sum. The results are shown in the "Overall Evaluation" column in Table 1 below. The higher this value, the more preferable it is.

[0121] [Table 1]

[0122] [Experiment 2] [Examples 4-5 and Comparative Example 2] The specifications of the contour line constituting the contour of the tread surface were adjusted to set the shape index F as shown in Table 2 below, and the ratio Tg, ratio Rm, ratio Re, and difference (WC-WT) were set as shown in Table 2. Except for this, tires of Examples 4-5 and Comparative Example 2 were obtained in the same manner as Example 1.

[0123] [Rolling resistance coefficient (RRC)] The rolling resistance coefficient (RRC) was measured in the same manner as in Experiment 1. The results are shown in Table 2 below as an index, with Comparative Example 2 being set at 100. The smaller the value, the lower the rolling resistance of the tire.

[0124] [Wet performance of new tires (WET)] The wet performance of new tires was evaluated in the same manner as in Experiment 1. The results are shown in the "NEW" column of Table 2 below as an index, with Comparative Example 2 set at 100. The larger the index, the shorter the braking distance and the better the wet performance of the tire. In this evaluation, an index of 95 or more is acceptable as it ensures good wet performance.

[0125] [Wet performance of worn tires (WET)] The wet performance of the worn tires was evaluated in the same manner as in Experiment 1. The results are shown in the "OLD" column of Table 2 below as an index, with Comparative Example 2 being 100. The larger the index, the shorter the braking distance and the better the wet performance of the tire. In this evaluation, an index of 95 or more is acceptable as the tire maintains good wet performance.

[0126] [comprehensive evaluation] Similar to experiment 1, an overall evaluation was made based on the indices obtained in each evaluation. The results are shown in the "Overall Evaluation" column in Table 2 below. The higher the value, the more preferable it is.

[0127] [Table 2]

[0128] [Experiment 3] [Examples 6-7 and Comparative Example 3] Tires of Examples 6-7 and Comparative Example 3 were obtained in the same manner as in Example 1, except that the specifications of the contour line constituting the contour of the tread surface were adjusted to make the shape index F as shown in Table 3 below, and the ratio Tg, ratio Rm, ratio Re and difference (WC-WT) were set as shown in Table 2. In Example 7, the outer end of the cap layer is located inside the virtual tread edge PT in the axial direction, so the difference (WC-WT) is expressed as a negative number.

[0129] [Rolling resistance coefficient (RRC)] The rolling resistance coefficient (RRC) was measured in the same manner as in Experiment 1. The results are shown in Table 3 below as an index, with Comparative Example 3 being set at 100. The smaller the value, the lower the rolling resistance of the tire.

[0130] [Wet performance of new tires (WET)] The wet performance of new tires was evaluated in the same manner as in Experiment 1. The results are shown in the "NEW" column of Table 3 below as an index, with Comparative Example 3 set at 100. The larger the index, the shorter the braking distance and the better the wet performance of the tire. In this evaluation, an index of 95 or more is acceptable as it ensures good wet performance.

[0131] [Wet performance of worn tires (WET)] The wet performance of the worn tires was evaluated in the same manner as in Experiment 1. The results are shown in the "OLD" column of Table 3 below as an index, with Comparative Example 3 being 100. The larger the index, the shorter the braking distance and the better the wet performance of the tire. In this evaluation, an index of 95 or more is acceptable as the tire maintains good wet performance.

[0132] [comprehensive evaluation] Similar to Experiment 1, an overall evaluation was made based on the indices obtained in each evaluation. The results are shown in the "Overall Evaluation" column in Table 3 below. The higher the value, the more preferable it is.

[0133] [Table 3]

[0134] As shown in Table 1-3, it has been confirmed that the Examples can achieve a reduction in rolling resistance while ensuring the necessary wet performance. From these evaluation results, the superiority of the present invention is clear. [Industrial Applicability]

[0135] The above-described technology capable of achieving reduced rolling resistance while ensuring necessary wet performance can be applied to various tires. [Explanation of symbols]

[0136] 2. Tires 4. Tread 6. Sidewall 12. Carcass 14. Belt 16...Band 26, 26s, 26m...Circumferential groove 28, 28s, 28m...Land 34, 34a, 34b... Carcass ply 36, 36a, 36b...layer 38 Cap layer 40. Middle class 42 Base layer

Claims

1. A tire having a tread that comes into contact with a road surface, The tire is mounted on a standard rim, the internal pressure of the tire is adjusted to 230 kPa, and a load of 70% of the standard load is applied to the tire as a vertical load. The tire is brought into contact with a flat road surface, and the contact surface obtained is the reference contact surface. a shape index expressed as a ratio of an equatorial contact length measured along the equator of the tire to a reference contact length at a position corresponding to a width that is 80% of a maximum contact width in the reference contact patch is 1.20 or more and 1.50 or less, At least three circumferential grooves are formed in the tread to form at least four land portions arranged in parallel in the axial direction; Among the at least three circumferential grooves, a circumferential groove located on an outer side in the axial direction is a shoulder circumferential groove, In the axial direction, a land portion located outside the shoulder circumferential groove is a shoulder land portion, and a land portion located inside the shoulder circumferential groove is a middle land portion, The tread comprises a cap layer, an intermediate layer having a loss tangent at 30°C lower than the loss tangent at 30°C of the cap layer, and a base layer having a loss tangent at 30°C lower than the loss tangent at 30°C of the intermediate layer, In a radial direction, the intermediate layer is located outside the base layer, and the cap layer is located outside the intermediate layer; In the shoulder land portion, a thickness of the cap layer at a center of an axial width is thinner than a thickness of the cap layer on a side of the shoulder circumferential groove, When the shape index is F and the ratio of the thickness of the cap layer at the center of the axial width of the shoulder land portion to the effective groove depth of the shoulder circumferential groove is Rm (unit: %), the ratio Rm satisfies the following formulas (1) and (2): Rm≦33.333×F−18 (1) Rm≧33.333×F−22 (2) tire.

2. In the shoulder land portion, a ratio of a thickness of the cap layer on the shoulder circumferential groove side to an effective groove depth of the shoulder circumferential groove is 40% or more and 60% or less.

2. The tire of claim 1.

3. In the middle land portion, a ratio of a thickness of the cap layer to an effective groove depth of the shoulder circumferential groove is 40% or more and 60% or less.

3. A tire according to claim 1 or 2.

4. The axial width of the cap layer is equal to or less than the axial width of the intermediate layer. A tire according to any one of claims 1 to 3.

5. The difference between the axial width of the intermediate layer and the axial width of the cap layer is 10 mm or more and 30 mm or less.

5. The tire of claim 4.

6. The difference between the axial width of the cap layer and the width of the tread is -10 mm or more and 10 mm or less. A tire according to any one of claims 1 to 5.

7. A ratio of a loss tangent at 30° C. of the cap layer to a loss tangent at 30° C. of the intermediate layer is 110% or more and 250% or less. A tire according to any one of claims 1 to 6.

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