Heavy-duty tires

Abstract

To provide a heavy-duty tire 2 that ensures rutting performance, snow performance, wear resistance, and crack resistance. [Solution] This tire 2 has a tread 4 made of cross-linked rubber. The tread 4 has a shoulder circumferential groove that extends continuously in the circumferential direction of the tire, a shoulder land portion located axially outward of the shoulder circumferential groove, and a crown land portion located axially inward of the shoulder circumferential groove. The cross-linked rubber constituting the tread 4 includes a first cross-linked rubber and a second cross-linked rubber that is harder than the first cross-linked rubber. The radially outward portion of the shoulder land portion is made of the first cross-linked rubber, and at least the radially outward portion of the crown land portion is made of the second cross-linked rubber.

Description

Technical Field

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[0001] The present invention relates to a heavy-duty tire.

Background Art

[0002] An all-season tire is required to have driving performance not only on dry and wet road surfaces but also on icy and snowy road surfaces. In order to increase the frictional force on an icy and snowy road surface, for example, in Patent Documents 1 and 2, it has been proposed to form blocks in which sipes are arranged in the tread portion.

[0003] Also, for example, in Patent Document 3, in order to achieve both ice performance and wear resistance, it has been proposed to configure the tread portion such that three different rubber layers are laminated in the radial direction.

Prior Art Documents

Patent Documents

[0007] According to the present invention, a heavy-duty tire is provided that ensures rutting performance (wandering performance), snow performance, wear resistance, and crack resistance. [Brief explanation of the drawing]

[0008] [Figure 1] Figure 1 is a cross-sectional view showing a part of a heavy-duty tire according to one embodiment of the present invention. [Figure 2] Figure 2 is a cross-sectional view showing a portion of the tire in Figure 1. [Figure 3] Figure 3 is an unfolded view showing a portion of the tire tread surface shown in Figure 1. [Modes for carrying out the invention]

[0009] The tire of this invention is mounted on a rim. Air is filled inside the tire, and the internal pressure of the tire is regulated. A tire mounted on a rim is also called a tire-rim assembly. A tire-rim assembly comprises a rim and a tire mounted on this rim.

[0010] In this invention, the 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 the standard state.

[0011] In this invention, unless otherwise specified, the dimensions and angles of each part of the tire are measured under normal conditions. The dimensions and angles of each part of the tire in the meridional cross-section, which cannot be measured when the tire is mounted on a standard rim, are measured at the tire's cross-section, obtained by cutting the tire along a plane containing the axis of rotation. In this measurement, the tire is set so that the distance between the left and right beads matches the distance between the beads in a tire mounted on a standard rim. The tire's structure, which cannot be confirmed when the tire is mounted on a standard rim, is confirmed at the aforementioned cross-section.

[0012] A genuine rim refers to a rim defined 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 all considered genuine rims.

[0013] Regular tire pressure refers to the internal pressure specified in the tire's standard. 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 all considered regular tire pressures.

[0014] The normal load means the load defined in the standard on which the tire relies. The "maximum load capacity" in the JATMA standard, the "maximum value" published in "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES" in the TRA standard, and the "LOAD CAPACITY" in the ETRTO standard are normal loads.

[0015] In the present invention, the crosslinked rubber is a molded body obtained by pressurizing and heating a rubber composition. The crosslinked rubber is a crosslinked product of the rubber composition. This rubber composition is a material obtained by mixing a base rubber and chemicals in a kneader such as a Banbury mixer. The base rubber of the rubber composition is not crosslinked. The base rubber of the crosslinked rubber is crosslinked. The crosslinked rubber is also referred to as vulcanized rubber, and the rubber composition is also referred to as unvulcanized rubber.

[0016] 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, anti-aging agents, processing aids, sulfur, and vulcanization accelerators. The selection of the base rubber and chemicals, the content of the selected chemicals, etc. are appropriately determined according to the specifications of each element such as the tread and sidewall to which the rubber composition is applied.

[0017] In the present invention, among the elements constituting the tire, the loss tangent (tanδ) and complex elastic modulus of the element made of crosslinked rubber are measured using a viscoelastic spectrometer in accordance with the provisions of JIS K6394. The measurement conditions are as follows. Initial strain = 10% Dynamic strain = ±1% Frequency = 10 Hz Mode = elongation mode Temperature = 70 °C In this measurement, the test specimen (40 mm long x 4 mm wide x 1 mm thick) is sampled from the tire. The length of the test specimen is aligned with the circumferential direction of the tire. If it is not possible to sample a test specimen from the tire, a test specimen is sampled from a sheet of cross-linked rubber (hereinafter also referred to as a rubber sheet) obtained by pressurizing and heating the rubber composition used to form the element to be measured at a temperature of 170°C for 12 minutes. In this invention, the loss tangent and complex modulus are expressed as the loss tangent and complex modulus at 70°C.

[0018] In this invention, the rubber hardness of the cross-linked rubber element among the elements constituting the tire is the durometer A hardness measured using a type A durometer in accordance with the provisions of JIS K6253-3 under a temperature atmosphere of 23°C. In this measurement, a test piece of a predetermined size is sampled from the tire, and the measurement is performed using this test piece. If it is not possible to sample test specimens from the tires, test specimens are prepared from the aforementioned rubber sheets.

[0019] [Practices that formed the basis of this invention] When sipes are placed in blocks on the tire tread, the friction force on icy and snowy roads is improved. While the technique of placing sipes on blocks is effective in improving performance on ice and snow, the rigidity of the blocks with sipes is reduced, which may decrease dry performance and handling stability. Therefore, it is not easy to achieve a balance between performance on icy and snowy roads and performance such as handling stability and wear resistance on dry roads.

[0020] Furthermore, as proposed in Patent Document 3, if the tread section is constructed as a three-layer structure made of different rubbers, the tread surface that contacts the road surface, both the shoulder and crown sections, is made of the same rubber. Therefore, if a rubber suitable for ensuring ice performance is selected as the rubber that makes up the tread surface, the tread surface of the shoulder section is also made of this rubber, which can worsen rutting performance (wandering performance) and reduce handling stability. In particular, in heavy-duty tires, if a highly rigid rubber is used as the rubber that makes up the tread surface in order to suppress the deformation of the blocks formed in the tread section and improve ice and snow performance, then the tread surface of the shoulder section will also be made of highly rigid rubber, which can worsen rutting performance (wandering performance) and reduce handling stability.

[0021] Therefore, in order to simultaneously ensure rutting performance, snow performance, wear resistance, and crack resistance, the inventors considered using different rubbers for the axial outer portion (shoulder portion) and the axial central portion (crown portion) of the tire tread, and have completed the invention described below.

[0022] [Details of the Embodiments of the Invention] The present invention will now be described in detail, with reference to drawings as appropriate, based on preferred embodiments.

[0023] In this invention, the tread portion of a tire is the part of the tire that makes contact with the road surface. The bead portion is the part of the tire that is fitted onto the rim. The side portion is the part of the tire that spans the space between the tread portion and the bead portion. The tire comprises the tread portion, a pair of bead portions, and a pair of side portions.

[0024] Figure 1 shows a part of a heavy-duty tire 2 (hereinafter also simply referred to as "tire 2") according to one embodiment of the present invention. Figure 2 shows a part of the cross-section of tire 2 shown in Figure 1. This tire 2 is mounted on vehicles such as trucks and buses. Preferably, it is mounted on a 19.5-inch drive shaft. This tire 2 is an all-season tire.

[0025] This tire 2 is suitable for use on vehicles where light loads (approximately 60% of the standard load) are common. Tires used in light load driving tend to have a rounded contact patch, which makes the axial center portion (crown portion) of the tire prone to wear. Therefore, this tire 2 employs a configuration suitable for suppressing crown portion wear when mounted on vehicles such as trucks and buses. As will be described in detail later, a relatively high-elasticity cross-linked rubber is used in the crown portion of tire 2.

[0026] Figure 1 shows a portion of the cross-section of tire 2 (hereinafter referred to as the meridian cross-section) along the plane containing the rotation axis of tire 2. In Figure 1, the left-right direction is the axial direction of tire 2, and the up-down direction is the radial direction of tire 2. The direction perpendicular to the plane of paper in Figure 1 is the circumferential direction of tire 2. The dashed line CL represents the equatorial plane of tire 2.

[0027] Tire 2 is mounted on rim R. Rim R is the standard rim. Air is filled inside tire 2, and the internal pressure of tire 2 is adjusted. Tire 2 mounted on rim R is also called a tire-rim assembly. A tire-rim assembly consists of rim R and tire 2 mounted on this rim R.

[0028] The tire 2 comprises a tread 4, a pair of sidewalls 6, a pair of chafers 8, a pair of beads 10, a carcass 12, a pair of steel reinforcement layers 16, an inner liner 18, a pair of cushioning layers 20, and a belt 34.

[0029] The tread 4 constitutes the tread portion. The tread 4 makes contact with the road surface at its tread surface 22. In other words, the tread 4 has a tread surface 22 that makes contact with the road surface. In Figure 1, the symbol PC is the intersection point of the tread surface 22 and the equatorial plane. The intersection point PC is the equator of the tire 2. The equatorial PC is the radial outer end of the tire 2.

[0030] In Figure 1, the symbol PE represents the edge of the tread surface 22. The length indicated by the double arrow WT is the width of the tread surface 22. The width WT of the tread surface 22 is the axial distance from one end PE to the other end PE of the tread surface 22.

[0031] Tread 4 is made of cross-linked rubber. Tread 4 is composed of two types of cross-linked rubber with different hardness levels. This will be explained in more detail later.

[0032] Multiple circumferential grooves 24 are carved into the tread 4. This tread 4 is composed of multiple land areas 26, each separated by multiple circumferential grooves 24. In this tire 2, four circumferential grooves 24 are carved into the surface, forming five land areas 26.

[0033] The four circumferential grooves 24 are arranged in parallel in the axial direction and extend continuously in the circumferential direction. Of these circumferential grooves 24, the circumferential groove 24 located on the outer side in the axial direction is the shoulder circumferential groove 24s. The circumferential groove 24 located on the inner side in the axial direction of the shoulder circumferential groove 24s is the middle circumferential groove 24m. The four circumferential grooves 24 of this tire 2 are composed of a pair of middle circumferential grooves 24m and a pair of shoulder circumferential grooves 24s. In this invention, the circumferential groove 24 located axially inward of the shoulder circumferential groove 24s is also called the crown circumferential groove. Therefore, both of the pair of middle circumferential grooves 24m are crown circumferential grooves.

[0034] The five land sections 26 are arranged in parallel in the axial direction and extend continuously in the circumferential direction. Of these land sections 26, the land section 26 located on the outer side in the axial direction is the shoulder land section 26s. The shoulder land section 26s is the land section 26 located on the axially outer side of the shoulder circumferential groove 24s. The shoulder land section 26s includes the edge PE of the tread surface 22. The land section 26 located axially inward of the shoulder land section 26s is the middle land section 26m. The land section 26 located axially inward of the middle land section 26m is the center land section 26c. The five land sections 26 of this tire 2 consist of a center land section 26c, a pair of middle land sections 26m, and a pair of shoulder land sections 26s. In this invention, the land area 26 located axially inward of the shoulder land area 26s is also called the crown land area. Therefore, each of the pair of middle land areas 26m and the center land area 26c is a crown land area.

[0035] In this tire 2, the cross-linked rubber constituting the tread 4 is composed of two types of cross-linked rubber with different hardnesses (first cross-linked rubber and second cross-linked rubber). Here, the second cross-linked rubber is harder than the first cross-linked rubber. In other words, the first cross-linked rubber is softer than the second cross-linked rubber. In this specification, the first cross-linked rubber is also referred to as low-elasticity cross-linked rubber, and the second cross-linked rubber is also referred to as high-elasticity cross-linked rubber. The tread 4 contains low-elasticity cross-linked rubber LR and high-elasticity cross-linked rubber HR, which is harder than the low-elasticity cross-linked rubber LR. The hardness of the cross-linked rubber is compared using the complex modulus of elasticity described above. Therefore, the low-elasticity cross-linked rubber LR has a lower complex modulus of elasticity than the high-elasticity cross-linked rubber HR. As mentioned above, the hardness (complex modulus) of the low-elasticity crosslinked rubber LR and the high-elasticity crosslinked rubber HR can be adjusted by selecting the base rubber and chemicals, and determining the content of the selected chemicals.

[0036] The tread 4 of this tire 2 is made of high-elasticity cross-linked rubber HR in the radially outer portions of the middle land area 26m and the center land area 26c. The remaining portions of the tread 4 are made of low-elasticity cross-linked rubber LR. The radially inner portion of the tread 4 and the shoulder land area 26s of this tire 2 are made of low-elasticity cross-linked rubber LR.

[0037] Therefore, the tread surface 22 of this tire 2 is composed of a relatively soft, low-elasticity cross-linked rubber (first cross-linked rubber) in the shoulder area and a relatively hard, high-elasticity cross-linked rubber (second cross-linked rubber) in the crown area. In this case, the cross-linked rubber compound that makes up the tread 4 can be formulated to be advantageous for snow performance while reducing the load on the shoulder during cornering and the reaction force generated on the shoulder when crossing ruts. Therefore, it is possible to ensure both snow performance and rut performance.

[0038] Furthermore, as mentioned above, tires used on buses, trucks, etc., which are used for light load driving tend to have wear on the crown portion. However, since the tread surface 22 side of the crown portion of tire 2 is made of high-elasticity cross-linked rubber HR, the wear energy during driving can be reduced, resulting in good wear resistance. In this invention, the portion of the tire tread from the shoulder circumferential groove to the shoulder land portion is also referred to as the shoulder portion, and the axially inner portion sandwiched between the shoulder portions is also referred to as the crown portion.

[0039] The tread 4 of this tire 2 is entirely made of low-elasticity cross-linked rubber LR on its radially inner portion. Therefore, the belt 34 is covered with low-elasticity cross-linked rubber LR on its radially outer portion. The groove bottoms 24sb of the shoulder circumferential grooves 24s and 24mb of the middle circumferential grooves 24m, which are provided in the tread 4, are made of low-elasticity cross-linked rubber LR. Therefore, in the tread 4 of tire 2, the low-elasticity cross-linked rubber LR, which is relatively softer than the high-elasticity cross-linked rubber HR, is exposed at the bottom of the circumferential grooves 24. As a result, stress concentration near the bottom of the circumferential grooves 24 can be avoided, and the occurrence of groove bottom cracks can be suppressed. In this respect, tire 2 has crack resistance. In particular, since the area near the groove bottom 24sb of the shoulder circumferential groove 24s is prone to deformation during driving, it is preferable from the viewpoint of suppressing the occurrence of groove bottom cracks to expose the low-elasticity crosslinked rubber LR at the groove bottom 24sb of the shoulder circumferential groove 24s. Furthermore, in the heavy-duty tire according to the present invention, the statement that the groove bottom of the circumferential groove is made of low-elasticity crosslinked rubber (first crosslinked rubber) means that at least the deepest portion of the groove bottom of the circumferential groove is made of low-elasticity crosslinked rubber.

[0040] In the tread 4 of this tire 2, the ratio [TA / TB] of the thickness TA of the high-elasticity cross-linked rubber portion to the thickness TB of the low-elasticity cross-linked rubber portion LR in the crown area is between 0.83 and 2.0. If this ratio [TA / TB] is less than 0.83, the thickness of the portion made of high-elasticity cross-linked rubber HR is thin, making it difficult to ensure snow performance and wear resistance. Furthermore, if this ratio [TA / TB] exceeds 2.0, the thickness of the portion made of low-elasticity crosslinked rubber LR becomes thin, making it difficult to expose the low-elasticity crosslinked rubber LR at the bottom of the circumferential grooves 24. When the bottom of the groove becomes the boundary between the high-elasticity crosslinked rubber HR and the low-elasticity crosslinked rubber LR, strain concentrates at that boundary, making it easier for cracks to occur at the bottom of the groove.

[0041] In the tire of the present invention, the thickness TA of the portion made of high-elasticity crosslinked rubber HR and the thickness TB of the portion made of low-elasticity crosslinked rubber LR in the crown land area are radial distances measured at the equatorial plane of the tire. However, if a crown circumferential groove is provided at a position overlapping with the equatorial plane of the tire, the radial distance is measured at the portion of the crown land area adjacent to the crown circumferential groove and close to the equatorial plane of the tire.

[0042] In the tread 4 of this tire 2, the ratio [TA / DS] of the thickness TA of the high-elasticity cross-linked rubber portion HR in the crown area to the depth DS of the shoulder circumferential groove 24s is between 0.65 and 0.95. If this ratio [TA / DS] is less than 0.65, the proportion of the high-elasticity cross-linked rubber portion becomes small, and the snow performance and wear resistance cannot be fully demonstrated. Furthermore, if this ratio [TA / DS] exceeds 0.95, the groove bottom 24sb of the shoulder circumferential groove 24s is composed of low-elasticity cross-linked rubber LR, and even if the low-elasticity cross-linked rubber LR is exposed at the groove bottom 24sb, its exposed area is small, and the effect of suppressing the occurrence of groove bottom cracks becomes poor.

[0043] In the tread 4 of this tire 2, the ratio [TA / DM] of the thickness TA of the high-elasticity cross-linked rubber portion HR in the crown area to the depth DM of the middle circumferential groove 24m is between 0.65 and 0.95. If this ratio [TA / DM] is less than 0.65, the proportion of high-elasticity cross-linked rubber becomes small, and snow performance and wear resistance cannot be fully achieved. If this ratio [TA / DM] exceeds 0.95, the groove bottom 24mb of the middle circumferential groove 24m is composed of low-elasticity cross-linked rubber LR, and even if the low-elasticity cross-linked rubber LR is exposed at the groove bottom 24mb, its exposed area is small, and the effect of suppressing the occurrence of groove bottom cracks becomes poor.

[0044] In this invention, the groove depth of the circumferential groove is expressed by the distance from the line segment connecting the opening edges of the circumferential groove to the deepest part of the groove bottom of the circumferential groove.

[0045] In the tread 4 of this tire 2, the preferred complex modulus of elasticity E*(2) of the high-elasticity crosslinked rubber HR at 70°C is between 6.6 MPa and 14.0 MPa. When the complex modulus of elasticity E*(2) is within this range, sufficient rigidity is ensured in the radially outer portion of the crown land area, which is suitable for ensuring wear resistance. In particular, as will be described later, if blocks are provided on the crown of tire 2 (middle land area 26m and center land area 26c), the increased rigidity of the blocks can suppress uneven wear on the blocks. Furthermore, by increasing the rigidity of the blocks, block collapse is suppressed, and the contact area can be increased, thereby improving ice performance. In addition, higher block rigidity increases the shearing force of snow columns, which can also improve snow performance. From a similar viewpoint, a more preferable complex modulus E*(2) of high-elasticity crosslinked rubber HR at 70°C is 10.0 MPa or more and 12.0 MPa or less.

[0046] In the tread 4 of this tire 2, the preferred complex modulus of elasticity E*(1) of the low-elasticity crosslinked rubber LR at 70°C is between 4.0 MPa and 5.3 MPa. When the complex modulus of elasticity E*(1) is within this range, the shoulder area 26s has low elasticity, resulting in a smaller reaction force when crossing ruts, thus improving rut performance. Furthermore, TLC (tread loose crack) is less likely to occur. Moreover, it is suitable for suppressing the occurrence of groove bottom cracks near the bottom of the circumferential grooves. From a similar viewpoint, a more preferable complex modulus E*(1) of low-elasticity crosslinked rubber LR at 70°C is 4.5 MPa or more and 5.0 MPa or less.

[0047] In the tread 4 of this tire 2, the ratio [E*(2) / E*(1)] of the complex modulus of elasticity of the high-elasticity crosslinked rubber HR at 70°C to the complex modulus of elasticity of the preferred low-elasticity crosslinked rubber LR at 70°C E*(1) is between 1.25 and 3.50. In this case, it is particularly suitable for ensuring the rutting performance, snow performance, wear resistance, and crack resistance of tire 2.

[0048] In the tread 4 of this tire 2, the loss tangent (tanδ) of the preferred low-elasticity crosslinked rubber LR at 70°C is 0.02 or more and 0.06 or less. In the tread 4 of this tire 2, the loss tangent (tanδ) of the preferred high-elasticity crosslinked rubber HR at 70°C is 0.07 or more and 0.11 or less. In this case, heat generation in the radially inner portion of the tread 4, which is made of low-elasticity cross-linked rubber LR with a relatively low tanδ, is suppressed, making groove bottom cracks less likely to occur. In particular, the shoulder portion is prone to deformation and heat generation during driving, so lowering the tanδ of the low-elasticity cross-linked rubber LR is effective in suppressing the occurrence of groove bottom cracks. Furthermore, when tire 2 is used for light load driving, the contact area of ​​the crown tends to increase. Therefore, by making the high-elasticity crosslinked rubber HR that constitutes the tread surface 22 side of the crown of tire 2 a crosslinked rubber with a relatively high tanδ, tire 2 can exhibit high grip on snowy or wet roads.

[0049] In this tire 2, the width WM of the middle track section 26m is greater than or equal to the width WC of the center track section 26c (WC ≤ WM). In this tire 2, the width WS of the shoulder track section 26s is wider than the width WM of the middle track section 26m (WM <WS)。 In this tire 2, the sum of the widths WS of the 26s shoulder area is wider than the sum of the widths WC of the 26c center area and WM of the 26m middle area, from the perspective of ensuring contact patch during cornering (2WS > WC + 2WM). In this invention, the width of the land portion 26 is represented by the axial width of the top surface of the land portion 26 which forms part of the tread surface 22.

[0050] In this tire 2, the width WC of the center 26c is between 14% and 16% of the width WT of the tread surface 22, from the perspective of ensuring traction performance. The width WM of the middle 26m is between 14% and 16% of the width WT of the tread surface 22, from the perspective of ensuring traction performance. The width WS of the shoulder 26s is between 28% and 30% of the width WT of the tread surface 22, from the perspective of ensuring contact area during cornering.

[0051] In this tire 2, of the land portions 26 that make up the tread 4, the land portion 26 located in the axial direction, i.e., the center land portion 26c, is located on the equatorial plane. This tire 2 has a tread 4 configured such that the land portion 26 is located on the equatorial plane. The tread 4 may also be configured such that the circumferential grooves 24 are located on the equatorial plane.

[0052] In the tire according to an embodiment of the present invention, the circumferential grooves formed in the tread 4 may be located on the equatorial plane. In this case, the circumferential groove located on the equatorial plane is also called the center circumferential groove. This center circumferential groove is the crown circumferential groove. In a tire equipped with a center circumferential groove, the land area located between the center circumferential groove and the shoulder circumferential groove in the axial direction becomes the crown land area.

[0053] Each sidewall 6 is connected to the edge of the tread 4. The sidewall 6 extends radially inward from the edge of the tread 4. The sidewall 6 is located radially inward of the tread 4. The sidewall 6 is made of cross-linked rubber with cut resistance in mind.

[0054] Each chafer 8 is located radially inward of the sidewall 6. The chafer 8 is in contact with the rim R. The chafer 8 is made of cross-linked rubber with wear resistance in mind.

[0055] Each bead 10 is located axially inward of the chafer 8. The bead 10 is located radially inward of the sidewall 6. The bead 10 comprises a core 28 and an apex 30. The core 28 extends in the circumferential direction. The core 28 contains wound steel wire. The core 28 has a roughly hexagonal cross-sectional shape. Apex 30 is located radially outward from core 28. Apex 30 comprises an inner apex 30u and an outer apex 30s. The inner apex 30u extends radially outward from core 28. The outer apex 30s is located radially outward from the inner apex 30u. The inner apex 30u is made of rigid cross-linked rubber. The outer apex 30s is made of softer cross-linked rubber than the inner apex 30u.

[0056] The carcass 12 is located inside the tread 4, a pair of sidewalls 6, and a pair of chafers 8. The carcass 12 spans between one bead 10 of a pair of beads 10 and the other bead 10.

[0057] The carcass 12 comprises at least one carcass ply 32. The carcass 12 of this tire 2 consists of one carcass ply 32. The carcass ply 32 is folded back axially from the inside to the outside around each core 28. The carcass ply 32 has a ply body 32a that spans between one core 28 of a pair of cores and the other core 28, and a pair of folded portions 32b that are connected to the ply body 32a and are folded back axially from the inside to the outside around each core 28.

[0058] Although not shown, the carcass ply 32 contains numerous parallel carcass cords. These carcass cords are covered with topping rubber. Each carcass cord intersects the equatorial plane. In this tire 2, the angle that the carcass cords make with respect to the equatorial plane is between 70° and 90°. This carcass 12 has a radial structure. The carcass cords of this tire 2 are steel cords.

[0059] Each steel reinforcement layer 16 is located in the bead area. The steel reinforcement layer 16 is folded axially from the inside to the outside around the core 28 along the carcass ply 32. Although not shown, the steel reinforcement layer 16 contains numerous parallel filler cords. These filler cords are covered with topping rubber. Steel cords are used as filler cords.

[0060] The belt 34 is located radially inward of the tread 4. The belt 34 also constitutes the tread portion. The belt 34 is located radially inward of the low-elasticity crosslinked rubber LR. In the radial direction, the belt 34 is located between the tread 4 and the carcass 12.

[0061] The belt 34 comprises three belt plies 38. These belt plies 38 are arranged radially. Each belt ply 38 is positioned so that its ends 38e face each other across the equatorial plane. Each belt ply 38 intersects the equatorial plane. The ends 38e of each belt ply 38 are located on either side of the equatorial plane. The three belt plies 38 are the first belt ply 38A, the second belt ply 38B, and the third belt ply 38C. Of the three belt plies 38, the first belt ply 38A is the one located radially inward. The second belt ply 38B is located radially outward from the first belt ply 38A. The third belt ply 38C is located radially outward from the second belt ply 38B. The third belt ply 38C is the belt ply 38 that is radially outward from the three belt plies 38. In the tire 2 of the embodiment of the present invention, the number of belt plies 38 is not particularly limited. The number of belt plies 38 may be one, two, or four or more.

[0062] As shown in Figures 1 and 2, the ends 38Ae of the first belt ply 38A, 38Be of the second belt ply 38B, and 38Ce of the third belt ply 38C are all located axially outward from the shoulder circumferential groove 24s.

[0063] In this tire 2, the second belt ply 38B has the widest width, and the first belt ply 38A has the narrowest width. The end 38Ce of the third belt ply 38C is located axially between the end 38Ae of the first belt ply 38A and the end 38Be of the second belt ply 38B. The width of each belt ply 38 is the axial distance from the first end 38e to the second end 38e of the belt ply 38.

[0064] Each belt ply 38 constituting the belt 34 contains numerous belt cords arranged in parallel. The belt cords are covered with topping rubber. The belt cord on this tire 2 is a metal cord. Specifically, the belt cord is a steel cord.

[0065] Each cushion layer 20 is located between the belt 34 and the carcass 12 at the end of the belt 34. The cushion layer 20 is made of cross-linked rubber.

[0066] The inner liner 18 is located inside the carcass 12. The inner liner 18 is bonded to the inner surface of the carcass 12 via an insulation (not shown) made of cross-linked rubber. The inner liner 18 constitutes the inner surface of the tire 2. The inner liner 18 is made of cross-linked rubber with excellent air-shielding properties. The inner liner 18 maintains the internal pressure of the tire 2.

[0067] Figure 3 shows a portion of the tread surface of tire 2 shown in Figure 1. This Figure 3 is an unfolded view of a portion of the tread surface. Figure 3 shows portions of the middle and center sections. Both the middle and center sections are part of the crown section.

[0068] As shown in Figures 1 to 3, the tread 4 of tire 2 has shoulder circumferential grooves 24s and middle circumferential grooves 24m formed as circumferential grooves 24. The tread 4 is divided into a shoulder land area 26s and a crown land area (middle land area 26m and center land area 26c) by these circumferential grooves 24. The shoulder circumferential grooves 24s and the middle circumferential grooves 24m are both zigzag circumferential grooves (see Figure 3).

[0069] As shown in Figure 3, multiple transverse grooves 62 are formed in both the middle land area 26m and the center land area 26c. More specifically, multiple middle transverse grooves 62m are provided at equal intervals in the middle land area 26m. The middle transverse grooves 62m are provided to traverse the middle land area 26m. Multiple center transverse grooves 62c are provided at equal intervals in the center land area 26c. The center transverse grooves 62c are provided to traverse the center land area 26c.

[0070] Multiple blocks 64 are provided in both the middle track area 26m and the center track area 26c by horizontal grooves 62. The blocks 64 formed in the middle track area 26m are middle blocks 64m, and the blocks 64 formed in the center track area 26c are center blocks 64c.

[0071] The depth of the transverse groove 62 is shallower than the depth of each circumferential groove 24 (shoulder circumferential groove 24s and middle circumferential groove 24m). The depth of the middle lateral groove 62m is preferably 40% to 95% of the depth of the shoulder circumferential groove 24s. More preferably, it is 40% to 75%. The depth of the center lateral groove 62c is preferably 40% to 95% of the depth of the middle circumferential groove 24m. More preferably, it is 40% to 75%.

[0072] The middle lateral groove 62m and the center lateral groove 62c are formed to be inclined with respect to the axial direction of the tire 2. The inclination angle θ of these lateral grooves with respect to the axial direction is independently, for example, between 3° and 15°.

[0073] In the crown section, the positions of the transverse grooves 62 provided on each land area, for example, the formation position of the center transverse groove 62c and the formation position of the middle transverse groove 62m, are offset. Here, the misalignment of the lateral groove 62 formation positions means that, in the tire axial direction, one lateral groove 62 and the other lateral groove 62 do not overlap at all.

[0074] Each of the middle blocks 64m and center blocks 64c provided on the tread 4 has two narrow lateral grooves 66. Each narrow lateral groove 66 is provided almost parallel to the lateral groove 62. The narrow lateral grooves 66 are shallower in depth and narrower in width than the lateral groove 62. The depth of the fine horizontal groove 66 is, for example, 12% to 30% of the depth of the horizontal groove 62.

[0075] Furthermore, a zigzag-shaped sipe 68 is provided at the bottom of each narrow horizontal groove 66. The sipe 68 is a cut that is even narrower than the narrow horizontal groove 66. The depth of sipe 68 is, for example, between 30% and 95% of the depth of transverse groove 62. Also, the combined depth of the depth of the fine transverse groove 66 and the depth of sipe 68 is, for example, between 40% and 120% of the depth of transverse groove 62. The tire 2 has crown blocks on its tread 4 (blocks 64), and each crown block is provided with narrow lateral grooves 66 and sipes 68. This further improves performance on ice and snow.

[0076] In this tire 2, the lateral grooves that separate block 64 have a groove width of at least 3.0 mm. On the other hand, in tire 2, the sipes are lateral grooves with a groove width of 1.5 mm or less. Furthermore, in tire 2, lateral grooves with a groove width exceeding 1.5 mm and less than 3.0 mm are referred to as narrow lateral grooves.

[0077] Although not shown in the diagram, a transverse groove (shoulder transverse groove) is also formed on the shoulder land portion 26s, and a block equipped with sipes is provided.

[0078] In the embodiment of the present invention, the tire does not necessarily need to have blocks separated by lateral grooves on the tread, but it is preferable to have blocks from the viewpoint of ensuring performance on ice and snow. Furthermore, it is preferable that sipes are arranged on the provided blocks.

[0079] As is clear from the above description, the present invention provides a heavy-duty tire that ensures rutting performance (wandering performance), snow performance, wear resistance, and crack resistance. [Industrial applicability]

[0080] The technology described above, which provides heavy-duty tires with ensured rutting performance, snow performance, wear resistance, and crack resistance, is suitably applied to all-season tires.

[0081] [Note] The present invention includes the following embodiments.

[0082] [1] A tire having a tread surface that contacts the road surface and having a tread made of cross-linked rubber, The tread includes at least three circumferential grooves extending continuously in the circumferential direction of the tire, and land portions divided by the circumferential grooves. Of the multiple circumferential grooves, the two circumferential grooves located on the outer side in the axial direction are shoulder circumferential grooves. Of the multiple circumferential grooves, the circumferential groove located axially inward from the shoulder circumferential groove is the crown circumferential groove. Of the multiple land portions, the land portion located axially outward of the shoulder circumferential groove is the shoulder land portion. Of the multiple land portions, the land portion located axially inward of the shoulder circumferential groove is the crown land portion. The cross-linked rubber constituting the tread includes a first cross-linked rubber and a second cross-linked rubber. The second crosslinked rubber is harder than the first crosslinked rubber. At least the radially outer portion of the shoulder land area is made of the first cross-linked rubber, A heavy-duty tire in which at least the radially outer portion of the crown's land area is made of a second cross-linked rubber.

[0083] [2] The radially inner portion of the tread is made of the first crosslinked rubber, The heavy-duty tire described in [1] above, wherein the groove bottom of the circumferential groove is made of the first cross-linked rubber.

[0084] [3] The heavy-duty tire according to [1] or [2] above, wherein the ratio [TA / DS] of the thickness TA of the portion made of the second cross-linked rubber in the crown land area to the depth DS of the shoulder circumferential groove is 0.65 or more and 0.95 or less.

[0085] [4] The heavy-duty tire described in [2] above, wherein the ratio [TA / TB] of the thickness of the portion made of the second cross-linked rubber to the thickness TB of the portion made of the first cross-linked rubber in the crown land area is 0.83 or more and 2.0 or less.

[0086] [5] The heavy-duty tire described in any of the above [1] to [4], wherein the complex modulus E*(2) of the second crosslinked rubber at 70°C is 6.6 MPa or more and 14.0 MPa or less.

[0087] [6] The heavy-duty tire described in any of [1] to [5] above, wherein the complex modulus E*(1) of the first crosslinked rubber at 70°C is 4.0 MPa or more and 5.3 MPa or less.

[0088] [7] The heavy-duty tire described in [1] to [6] above, wherein the ratio [E*(2) / E*(1)] of the complex modulus of elasticity E*(2) of the second crosslinked rubber at 70°C to the complex modulus of elasticity E*(1) of the first crosslinked rubber at 70°C is 1.25 or more and 3.50 or less.

[0089] [8] The heavy-duty tire according to [1] to [7] above, wherein the crown land portion includes a plurality of crown blocks separated by a plurality of crown lateral grooves, and each block is provided with sipes. [Explanation of Symbols]

[0090] 2... Tires 4. Tread 6. Sidewall 8. Chafer 10...bead 12...Carcass 16. Steel reinforcement layer 18. Inner Liner 20... cushion layer 22...Tread surface 24, 24m, 24s...Circumferential groove 26, 26c, 26m, 26s...Land 28 cores 30...Apex 34... belt 38, 38A, 38B, 38C... Belt ply 62, 62c, 62m... Yokomizo 64, 64c, 64m...block 66...Narrow horizontal groove 68... Sipes

Claims

1. A tire having a tread surface that contacts the road surface and having a tread made of cross-linked rubber, The tread includes at least three circumferential grooves extending continuously in the circumferential direction of the tire, and land portions divided by the circumferential grooves. Of the multiple circumferential grooves, the two circumferential grooves located on the outer side in the axial direction are shoulder circumferential grooves. Of the multiple circumferential grooves, the circumferential groove located axially inward from the shoulder circumferential groove is the crown circumferential groove. Of the multiple land portions, the land portion located axially outward of the shoulder circumferential groove is the shoulder land portion. Of the multiple land portions, the land portion located axially inward of the shoulder circumferential groove is the crown land portion. The cross-linked rubber constituting the tread includes a first cross-linked rubber and a second cross-linked rubber. The second crosslinked rubber is harder than the first crosslinked rubber. At least the radially outer portion of the shoulder land area is made of the first cross-linked rubber, A heavy-duty tire in which at least the radially outer portion of the crown's land area is made of a second cross-linked rubber.

2. The radially inner portion of the tread is made of the first cross-linked rubber, The heavy-duty tire according to claim 1, wherein the groove bottom of the circumferential groove is made of the first cross-linked rubber.

3. The heavy-duty tire according to claim 1 or 2, wherein the ratio [TA / DS] of the thickness TA of the portion made of the second cross-linked rubber in the crown land area to the depth DS of the shoulder circumferential groove is 0.65 or more and 0.95 or less.

4. The heavy-duty tire according to claim 2, wherein the ratio [TA / TB] of the thickness TA of the portion made of the second cross-linked rubber to the thickness TB of the portion made of the first cross-linked rubber in the crown land portion is 0.83 or more and 2.0 or less.

5. The heavy-duty tire according to claim 1 or 2, wherein the complex modulus E*(2) of the second crosslinked rubber at 70°C is 6.6 MPa or more and 14.0 MPa or less.

6. The heavy-duty tire according to claim 1 or 2, wherein the complex modulus E*(1) of the first crosslinked rubber at 70°C is 4.0 MPa or more and 5.3 MPa or less.

7. The heavy-duty tire according to claim 1 or 2, wherein the ratio [E*(2) / E*(1)] of the complex modulus of elasticity E*(2) of the second crosslinked rubber at 70°C to the complex modulus of elasticity E*(1) of the first crosslinked rubber at 70°C is 1.25 or more and 3.50 or less.

8. The heavy-duty tire according to claim 1 or 2, wherein the crown land portion includes a plurality of crown blocks separated by a plurality of crown lateral grooves, and each block is provided with sipes.

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