Pneumatic tire and manufacturing method therefor

Abstract

The problem invention provides a pneumatic tire, and a manufacturing method therefor, that enables the occurrence of groove cracks to be suppressed while improving tire appearance and rim assemblability. The present invention pertains to a pneumatic tire comprising a tread portion 1, a pair of sidewall portions 2, and a pair of bead portions 3, a plurality of main grooves extending in the circumferential direction of the tire being formed in the tread portion 1, and the tire cross-sectional height SH being set to 120 mm or more, wherein: the bead base width TBW is set to greater than or equal to 107% of the tread deployment width TDW and less than or equal to 92% of the nominal tire width; a contour line PL forming the tread surface of the tread portion 1 is constituted of two or more arcs C1, C2 having different curvature radii from one another from the tire center line CL to tread edges E1, E2 of the tread portion 1; and the curvature radius TR1 of the arc C1 positioned near the tire center line CL and the curvature radius TR2 of the arc C2 positioned on the outermost side in the tire width direction satisfy the relationship 0.2≤TR2 / TR1≤0.4.

Description

Pneumatic tire and manufacturing method thereof

[0001] The present invention relates to a pneumatic tire and a manufacturing method thereof, and more particularly to a pneumatic tire and a manufacturing method thereof that can suppress the occurrence of groove cracks while improving the appearance and rim assembly properties of the tire.

[0002] In pneumatic tires, it has been proposed to improve the rim assembly performance by modifying the mold shape (see, for example, Patent Documents 1 and 2). Furthermore, to prevent poor rim assembly performance and poor appearance due to side profile collapse, the bead base width of the bead portion is set wider than the developed tread width. However, if the bead base width is set too wide, the shoulder main grooves open in the width direction during rim assembly, generating tensile stress at the groove bottom and compressive stress directly below the contact patch, resulting in a large change in groove width during rolling. This increases the strain generated at the groove bottom of the main groove, raising the risk of groove cracking. This problem is particularly pronounced in pneumatic tires with a large cross-sectional height (pneumatic tires with a high section height).

[0003] Japanese Unexamined Patent Publication No. 2008-094147 Japanese Unexamined Patent Publication No. 8-132448

[0004] An object of the present invention is to provide a pneumatic tire that can suppress the occurrence of groove cracks while improving the appearance and rim assembly properties of the tire, and a method for manufacturing the same.

[0005] In order to achieve the above object, the pneumatic tire of the present invention comprises a tread portion extending circumferentially to form an annular shape, a pair of sidewall portions disposed on either side of the tread portion, and a pair of bead portions disposed radially inward of the sidewall portions, the tread portion having a plurality of main grooves extending circumferentially, and the tire cross-sectional height SH is set to 120 mm or greater, the bead base width TBW of the bead portions being set to 107% or greater of the tread developed width TDW of the tread portion and 92% or less of the nominal tire width, the contour line forming the ground contact surface of the tread portion being composed of two or more arcs having mutually different radii of curvature from the tire centerline to an end of the tread developed width TDW of the tread portion, and the radius of curvature TR1 of the arc located closer to the tire centerline and the radius of curvature TR2 of the arc located outermost in the tire width direction satisfy the relationship 0.2≦TR2 / TR1≦0.4.

[0006] Further, in order to achieve the above object, a method of manufacturing a pneumatic tire of the present invention is provided, which comprises a tread portion extending in a circumferential direction of the tire to form an annular shape, a pair of sidewall portions disposed on both sides of the tread portion, and a pair of bead portions disposed radially inward of the sidewall portions, wherein a plurality of main grooves extending in the circumferential direction of the tire are formed in the tread portion, and the tire cross-sectional height SH is set to 120 mm or more, and wherein a bead base width TBW of the bead portion is 1 / 2 of a developed tread width TDW of the tread portion. The tire is characterized in that the tire width is at least 0.07% and at most 92% of the nominal tire width, and the contour line forming the tread surface of the tread portion is composed of two or more arcs having different radii of curvature from the tire center line to the end of the tread developed width TDW of the tread portion, and a mold is used that is formed so that the radius of curvature TR1 of the arc located closer to the tire center line and the radius of curvature TR2 of the arc located outermost in the tire width direction satisfy the relationship 0.2≦TR2 / TR1≦0.4, and the pneumatic tire is vulcanized in this mold.

[0007] In the present invention, in a pneumatic tire, by setting the tire cross-sectional height SH to 120 mm or more and the bead base width TBW of the bead portion to 107% or more of the tread developed width TDW of the tread portion, it is possible to effectively suppress deterioration of rim assembly performance and crushing of the sidewall portion due to an increase in the tread developed width TDW. Furthermore, by setting the bead base width TBW of the bead portion to 92% or less of the nominal tire width, it is possible to reduce the amount of change in the groove width of the shoulder main grooves of the tire before, during, and after rim assembly, thereby suppressing distortion at the groove bottom. Furthermore, by having the curvature radius TR1 of the arc located on the tire centerline side and the curvature radius TR2 of the arc located on the outermost side in the tire width direction, which constitute the tread surface of the tread portion, satisfy the relationship 0.2≦TR2 / TR1≦0.4, it is possible to reduce the amount of change in the groove width of the shoulder main grooves during rolling, thereby further suppressing distortion at the groove bottom. In this way, it is possible to suppress the occurrence of groove cracks while improving the tire's appearance and rim assembly performance.

[0008] In the present invention, the bead base width TBW, the developed tread width TDW, and the radii of curvature TR1 and TR2 of the arcs constituting the ground contact surface of the tread portion are all dimensions in a mold. Therefore, when manufacturing a pneumatic tire with a tire section height SH set to 120 mm or more as described above, a mold is used that is formed so that the bead base width TBW of the bead portion is 107% or more of the developed tread width TDW of the tread portion and 92% or less of the nominal tire width, and so that the radius of curvature TR1 of the arcs constituting the ground contact surface of the tread portion located on the tire centerline side and the radius of curvature TR2 of the arcs located on the outermost sides in the tire width direction satisfy the relationship 0.2≦TR2 / TR1≦0.4, and the pneumatic tire is vulcanized in this mold.

[0009] In the present invention, it is preferable that the arc length L1 of the arc located on the tire centerline side is 60% or less of half the tread developed width TDW, and that the widthwise center position A of the shoulder main groove located on the outermost side in the tire width direction among the plurality of main grooves is located more inward in the tire width direction than the end of the arc located on the tire centerline side. This makes the shoulder main groove less susceptible to the effects of groove widening during rim assembly and groove contraction during contact with the tire, thereby effectively suppressing distortion at the groove bottom. When manufacturing such a pneumatic tire, it is sufficient that the mold is formed so that the arc length L1 of the arc located on the tire centerline side is 60% or less of half the tread developed width TDW, and so that the widthwise center position A of the shoulder main groove located on the outermost side in the tire width direction among the plurality of main grooves is more inward in the tire width direction than the end of the arc located on the tire centerline side.

[0010] The arc length Lg of the contour line between the widthwise center position A of the shoulder main groove located at the outermost side in the tire width direction and the tire centerline is preferably 56% or less of half the tread developed width TDW. This makes the shoulder main groove less susceptible to the effects of groove widening during assembly to the rim and groove contraction during contact with the tire, thereby effectively suppressing distortion at the groove bottom. When manufacturing such a pneumatic tire, it is sufficient that the mold is formed so that the arc length Lg of the contour line between the widthwise center position A of the main groove located at the outermost side in the tire width direction and the tire centerline is 56% or less of half the tread developed width TDW.

[0011] The bead base width TBW is preferably in the range of 120% or more and less than 130% of the rim width RW of the standard rim. This effectively improves rim assembly. When manufacturing such a pneumatic tire, it is sufficient that the mold is formed so that the bead base width TBW is in the range of 120% or more and less than 130% of the rim width RW of the standard rim.

[0012] The bead base width TBW is preferably 115% or less of the tread developed width TDW. This reduces the amount of change in the groove width of the main grooves from before to after inflation of the tire. When manufacturing such a pneumatic tire, it is sufficient that the mold is formed so that the bead base width TBW is 115% or less of the tread developed width TDW.

[0013] It is preferable that a narrow groove extending in the tire circumferential direction is formed in at least one of a pair of shoulder land portions located outward in the tire width direction from a pair of shoulder main grooves located at the outermost positions in the tire width direction among the plurality of main grooves, thereby making it possible to prevent the shoulder main grooves from widening in the tire width direction when assembled to a rim.

[0014] Preferably, the number of main grooves is at least three, the groove bottoms of the main grooves are configured to include two arcs, the radii of curvature r of these arcs are in the range of 2 mm to 4 mm, and the radii of curvature ra of the inner arc in the tire width direction and the radii of curvature rb of the outer arc in the tire width direction that constitute the groove bottoms of the main grooves satisfy the relationship ra≦rb. This suppresses distortion at the groove bottoms of the main grooves, thereby contributing to the prevention of groove cracks.

[0015] Fig. 1 is a meridian cross-sectional view showing an example of a pneumatic tire according to an embodiment of the present invention. Fig. 2 is a plan view showing the tread pattern of the pneumatic tire of Fig. 1. Fig. 3 is a meridian cross-sectional view showing a standard rim on which a pneumatic tire according to an embodiment of the present invention is mounted. Fig. 4 is a cross-sectional view showing an enlarged view of one of the main grooves of the tread portion. Fig. 5 is a meridian cross-sectional view showing a mold used during vulcanization of a pneumatic tire according to an embodiment of the present invention.

[0016] The configuration of the present invention will be described in detail below with reference to the accompanying drawings, in which Fig. 1 shows a pneumatic tire according to an embodiment of the present invention, Fig. 2 shows the tread pattern of the pneumatic tire, and Fig. 3 shows a standard rim on which the pneumatic tire is mounted.

[0017] As shown in FIG. 1 , the pneumatic tire of this embodiment includes a tread portion 1 extending circumferentially in a ring shape, a pair of sidewall portions 2 disposed on both sides of the tread portion 1, and a pair of bead portions 3 disposed radially inward of the sidewall portions 2.

[0018] At least one carcass layer 4 (one layer in FIG. 1 ) made of a plurality of carcass cords arranged in the radial direction is mounted between a pair of bead portions 3. The carcass layer 4 is folded back from the inside to the outside of the tire around a bead core 5 arranged in each bead portion 3. Organic fiber cords such as nylon or polyester are preferably used as the carcass cords that make up the carcass layer 4. A bead filler 6 made of a rubber composition and having a triangular cross section is arranged on the outer periphery of the bead core 5. An inner liner layer 9 is arranged on the inner surface of the tire along the carcass layer 4.

[0019] Meanwhile, multiple belt layers 7 (two layers in FIG. 1 ) are embedded on the tire outer circumferential side of the carcass layer 4 in the tread portion 1. The belt layer 7 includes multiple reinforcing cords that are inclined with respect to the tire circumferential direction, and are arranged so that the reinforcing cords cross each other between the layers. The two belt layers 7 include an inner belt layer 7A located on the inner side in the tire radial direction and an outer belt layer 7B located on the outer side of the inner belt layer 7A, and the width of the inner belt layer 7A is wider than the width of the outer belt layer 7B. In the belt layers 7, the inclination angle of the reinforcing cords with respect to the tire circumferential direction is set in the range of 10° to 40°, for example. Steel cords are preferably used as the reinforcing cords of the belt layers 7.

[0020] At least one belt cover layer 8 (two layers in Fig. 1 ) is disposed on the tire outer circumferential side of the belt layer 7, with the aim of improving high-speed durability. The belt cover layer 8 has reinforcing cords arranged at an angle of, for example, 5° or less with respect to the tire circumferential direction. In Fig. 1 , the belt cover layer 8 located on the inner side in the tire radial direction constitutes a full cover that covers the entire width of the belt layer 7, while the belt cover layer 8 located on the outer side in the tire radial direction constitutes an edge cover layer that covers only the ends of the belt layer 7. As the reinforcing cords of the belt cover layer 8, organic fiber cords such as nylon and aramid are preferably used.

[0021] Furthermore, a tread rubber layer 10A is disposed on the outside of the belt cover layer 8 in the tread portion 1, a sidewall rubber layer 10B is disposed on the outside of the carcass layer 4 in the sidewall portion 2, and a rim cushion rubber layer 10C is disposed on the outside of the carcass layer 4 in the bead portion 3. The above-described tire internal structure shows a typical example of a pneumatic tire, but is not limited to this.

[0022] As shown in Figure 2, the tread portion 1 has multiple main grooves formed extending in the tire circumferential direction. These main grooves include a pair of center main grooves 11, 12 located on both sides of the tire centerline CL and a pair of shoulder main grooves 13, 14 located on the outermost sides in the tire width direction. These four main grooves define multiple rows of land portions in the tread portion 1. These land portions include a center land portion 21 located on the tire centerline CL, a pair of intermediate land portions 22, 23 located outboard of the center land portion 21 in the tire width direction, and a pair of shoulder land portions 24, 25 located outboard of each intermediate land portion 22, 23 in the tire width direction.

[0023] More specifically, multiple sipes 41 inclined in the same direction relative to the tire width direction are arranged at intervals in the tire circumferential direction in the central land portion 21. One end of each sipe 41 opens into the central main groove 12, while the other end terminates within the central land portion 21 without reaching the tire center line CL.

[0024] In the intermediate land portion 22, a plurality of sipes 42 inclined in the same direction relative to the tire width direction are arranged at intervals in the tire circumferential direction. One end of each sipe 42 opens into the shoulder main groove 13, while the other end terminates within the intermediate land portion 22. In addition, a plurality of sipes 43 inclined in the same direction relative to the tire width direction are arranged at intervals in the tire circumferential direction. One end of each sipe 43 opens into the center main groove 11, while the other end terminates within the intermediate land portion 22. Such sipes 42, 43 are arranged alternately in the tire circumferential direction, and the sipes 42, 43 are arranged in a staggered pattern in the tire circumferential direction throughout the intermediate land portion 22 as a whole.

[0025] A single narrow groove 15 extending in the tire circumferential direction is formed in the intermediate land portion 23. A plurality of sipes 44 inclined in the same direction relative to the tire width direction are arranged at intervals in the tire circumferential direction in the intermediate land portion 23. One end of each of these sipes 44 opens into the narrow groove 15, while the other end opens into the shoulder main groove 14.

[0026] A plurality of lug grooves 31 inclined in the same direction relative to the tire width direction are arranged at intervals in the tire circumferential direction in the shoulder land portion 24. These lug grooves 31 are not connected to the shoulder main grooves 13.

[0027] A plurality of lug grooves 32 inclined in the same direction relative to the tire width direction are arranged at intervals in the tire circumferential direction in the shoulder land portion 25. These lug grooves 32 are not connected to the shoulder main grooves 14. A single narrow groove 16 extending in the tire circumferential direction is formed so as to intersect with these lug grooves 32.

[0028] In the pneumatic tire, the tire section height SH is set to 120 mm or more. The tire section height SH is preferably set to 130 mm or more, with the upper limit being 160 mm or less. Furthermore, the aspect ratio is preferably set in the range of 55% or more and less than 75%.

[0029] The bead base width TBW of the bead portion 3 is set to 107% or more of the tread developed width TDW of the tread portion 1, but preferably set to 115% or less as the upper limit. The bead base width TBW is the width of the bead base between a pair of bead portions 3 measured based on the surface of the bead portion 3 that abuts against the rim flange. On the other hand, the tread developed width TDW is the distance from the tread edge E1 to the tread edge E2 measured along the outline of the tread surface of the tread portion 1, when the tread edges E1 and E2 are defined as the intersections of the extensions of the arcs that define the outlines of the shoulder regions on both sides of the tread portion 1 and the extensions of the arcs that define the outlines of the buttress regions on both sides outside the contact area of ​​the tread portion 1, respectively. Furthermore, the bead base width TBW of the bead portion 3 is set to 92% or less of the nominal tire width.

[0030] In such a pneumatic tire, the contour line PL forming the contact surface of the tread portion 1 is composed of two or more (two in FIG. 1 ) arcs C1, C2 having different radii of curvature from the tire centerline CL to the ends (tread edges E1, E2) of the tread developed width TDW of the tread portion 1. Of these arcs C1, C2, the radius of curvature TR1 of the arc C1 located closer to the tire centerline CL and the radius of curvature TR2 of the arc C2 located outermost in the tire width direction are configured to satisfy the relationship 0.2≦TR2 / TR1≦0.4. The arc C1 is an arc extending from the tire centerline CL to an inflection point where the radius of curvature changes, and the arc C2 is an arc extending from the inflection point to the ends of the tread developed width TDW, both of which have centers on the inner side in the tire radial direction. The arc C2 does not mean the actual contour line constituting the ground contact surface near the tread ends E1 and E2, but means an extension of the arc forming the contour of the shoulder region.

[0031] In the pneumatic tire described above, by setting the bead base width TBW to 107% or more of the tread developed width TDW while also setting the high section height, the spacing between the bead portions 3 is optimized before the tire is inflated, effectively suppressing deterioration of rim assembly and crushing of the sidewall portion 2 even when the tread developed width TDW is increased. Furthermore, by setting the bead base width TBW to 92% or less of the nominal tire width, the amount of change in the groove width of the shoulder main grooves 13, 14 before, during, and upon contact with the tire's rim can be reduced, thereby suppressing distortion at the groove bottoms. Furthermore, by having the curvature radius TR1 of the arc C1 and the curvature radius TR2 of the arc C2 that form the tread surface of the tread portion 1 satisfy the relationship 0.2≦TR2 / TR1≦0.4, the amount of change in the groove width of the shoulder main grooves 13, 14 during rolling can be reduced, further contributing to suppression of distortion at the groove bottoms. In this way, the occurrence of groove cracks can be suppressed while improving the appearance and rim mounting properties of the tire.

[0032] If the ratio of the bead base width TBW to the expanded tread width TDW (TBW / TDW x 100%) is less than 107%, the tire's appearance and rim assembly performance cannot be maintained in a good condition. If the ratio of the bead base width TBW to the tire nominal width (TBW / nominal width x 100%) is more than 92%, the groove crack suppression effect cannot be sufficiently obtained.

[0033] In the pneumatic tire described above, the arc length L1 of the arc C1 located on the tire centerline CL side is preferably 60% or less of the half width (TDW / 2) of the tread developed width TDW, with a lower limit of 40% or more being more preferable. In this case, the widthwise center position A of the shoulder main grooves 13, 14 is preferably located on the inner side of the tire widthwise outer end of the arc C1. This means that the arc length L1 of the arc C1 is longer than the arc length Lg described below. This configuration makes the shoulder main grooves 13, 14 less susceptible to the effects of groove widening during assembly to the rim and groove contraction during ground contact, thereby effectively suppressing distortion at the groove bottom.

[0034] The arc length Lg of the contour line PL between the widthwise center position A of the shoulder main groove 13, 14 and the tire centerline CL is preferably 56% or less of the half width of the tread developed width TDW. In this case, for the purpose of suppressing external noise, the arc length Lg is preferably set to 45% or more of the half width of the tread developed width TDW. This configuration makes the shoulder main grooves 13, 14 less susceptible to the effects of groove widening during rim assembly and groove contraction during contact with the tire, thereby effectively suppressing distortion at the groove bottom.

[0035] In the pneumatic tire, it is also preferable that a narrow groove 16 extending in the tire circumferential direction is formed in at least one of the pair of shoulder land portions 24, 25. The narrow groove 16 preferably has a groove width in the range of 1.5 mm to 3.0 mm. By providing the narrow groove 16 in at least one of the shoulder land portions 24, 25 in this manner, it is possible to prevent the shoulder main grooves 13, 14 from widening in the width direction when the tire is assembled to a rim.

[0036] Furthermore, as shown in FIG. 4 , the groove bottom of the main groove is configured to include two arcs c1 and c2. The radius of curvature r of the arcs c1 and c2 is preferably in the range of 2 mm to 4 mm. The radius of curvature ra of the arc c1 on the inner side in the tire width direction and the radius of curvature rb of the arc c2 on the outer side in the tire width direction preferably satisfy the relationship ra≦rb. This suppresses distortion at the groove bottom of the main groove, contributing to the prevention of groove cracks. Here, when the radius of curvature r of the arcs c1 and c2 at the groove bottom is 2 mm or more, distortion concentration at the groove bottom and cracks can be effectively suppressed. When the radius of curvature r of the arcs c1 and c2 at the groove bottom is 4 mm or less, the effect of suppressing distortion at the groove bottom can be fully enjoyed. Note that the radii of curvature of the arcs constituting the groove bottoms of the main grooves 11 to 14 do not necessarily have to be set to the same dimension.

[0037] In the above pneumatic tire, the bead base width TBW is preferably in the range of 120% or more and less than 130% of the rim width RW of the standard rim R (see FIG. 3). By appropriately setting the ratio of the bead base width TBW to the rim width RW (TBW / RW x 100%) in this way, rim assembly properties can be effectively improved. Here, a ratio TBW / RW of 120% or more tends to improve rim assembly properties.

[0038] Figure 5 shows a mold used when vulcanizing a pneumatic tire according to an embodiment of the present invention. In Figure 5, CL is the tire centerline, and T is the tire. As shown in Figure 5, the mold 50 includes a sector mold 51 for molding the tread portion 1 of the tire T, a side plate 52 for molding the sidewall portion 2 of the tire T, and a bead ring 53 for molding the bead portion 3 of the tire T. During vulcanization, a rubber bladder is inserted inside the tire T, and the tire T is pressed against the inner surface of the mold 50 by the expansion of the bladder.

[0039] The mold 50 configured in this manner is machined so that the bead base width TBW of the tire T is 107% or more of the tread developed width TDW and 92% or less of the nominal tire width, and the radius of curvature TR1 of the arc C1 and the radius of curvature TR2 of the arc C2 that form the tread surface of the tread portion 1 satisfy the relationship 0.2≦TR2 / TR1≦0.4.

[0040] Then, a mold 50 is used that is formed so that the bead base width TBW has a predetermined ratio to the tread developed width TDW, the bead base width TBW has a predetermined ratio to the nominal tire width, and the radii of curvature TR1, TR2 of the arcs C1, C2 that make up the tread surface of the tread portion 1 satisfy a predetermined relationship, and the tire T is vulcanized in the mold 50, thereby making it possible to manufacture a pneumatic tire such as that shown in FIG.

[0041] In a pneumatic tire having a tire size of 235 / 60R18 (i.e., an aspect ratio of 60%) and four main grooves formed in the tread portion, the tire cross-sectional height SH, the tread developed width TDW, the bead base width TBW, the ratio of the bead base width TBW to the tire nominal width (TBW / nominal width x 100%), the ratio of the bead base width TBW to the tread developed width TDW (TBW / TDW x 100%), the ratio of the curvature radius TR2 to the curvature radius TR1 (TR2 / TR1), the tread developed width T Tires of the conventional example, comparative example and examples 1 to 12 were manufactured with the ratio of arc length L1 to half width of DW (L1 / (TDW / 2) × 100%), center position A of the shoulder main groove, ratio of arc length Lg to half width of tread developed width TDW (Lg / (TDW / 2) × 100%), rim width RW of the standard rim, ratio of bead base width TBW to rim width RW of the standard rim (TBW / RW × 100%), presence or absence of fine grooves, radius of curvature of the arc at the groove bottom ra, and radius of curvature of the arc at the groove bottom rb set as shown in Table 1.

[0042] In Table 1, with respect to the "center position A of the shoulder main groove," "outside" means that the center position A of the shoulder main groove in the width direction is located outside the tire width direction of the end of the arc located on the tire centerline side, and "inside" means that the center position A of the shoulder main groove in the width direction is located inside the tire width direction of the end of the arc located on the tire centerline side.

[0043] These test tires were evaluated for appearance, rim mounting properties, and groove crack resistance by the following test methods. The results are shown in Table 1.

[0044] Tire appearance: After each test tire was vulcanized and left to stand for one hour, the spacing between the bead portions was measured at four points on the tire circumference, and the average value was calculated. The evaluation results were expressed as an index, with the conventional example being set at 100. The larger the index value, the wider the spacing between the bead portions and the better the tire appearance.

[0045] Rim assembly performance: The work time required to assemble each test tire onto a standard rim was measured. The evaluation results were expressed as an index using the reciprocal of the measured value, with the conventional example being set at 100. The higher the index value, the better the rim assembly performance.

[0046] Groove crack resistance: Each test tire was mounted on a wheel with a rim size of 18 x 7.5J, mounted on a test vehicle with an engine displacement of 2000cc, and the air pressure was adjusted to 200 kPa. After driving 10,000 km on a rough road, the number of groove cracks that occurred at the bottom of the main groove was counted. The evaluation results were expressed as an index using the reciprocal of the measured value, with the conventional example being set at 100. The higher the index value, the better the groove crack resistance.

[0047]

[0048] As can be seen from Table 1, the pneumatic tires of Examples 1 to 12 were able to improve the tire appearance, rim assembly properties, and groove crack resistance compared to the conventional tire.

[0049] In the comparative example pneumatic tire, the ratio of the radius of curvature TR2 to the radius of curvature TR1 (TR2 / TR1) was set higher than the range specified in the present invention, and therefore the groove crack resistance was deteriorated.

[0050] DESCRIPTION OF SYMBOLS 1 tread portion 2 sidewall portion 3 bead portion 4 carcass layer 5 bead core 6 bead filler 7 belt layer 8 belt cover layer 9 inner liner layer 50 mold C1, C2 arc CL tire center line E1, E2 tread edge PL contour line T tire

Claims

1. A pneumatic tire comprising a circumferentially extending annular tread portion, a pair of sidewall portions disposed on either side of the tread portion, and a pair of bead portions disposed radially inward of the sidewall portions, wherein the tread portion has a plurality of circumferentially extending main grooves, and the tire cross-sectional height SH is set to 120 mm or greater, wherein the bead base width TBW of the bead portions is set to 107% or greater of the tread developed width TDW of the tread portion and 92% or less of the nominal tire width, and wherein the contour line forming the ground contact surface of the tread portion is made up of two or more arcs with different radii of curvature from the tire centerline to the ends of the tread developed width TDW of the tread portion, and wherein the radius of curvature TR1 of the arc located closer to the tire centerline and the radius of curvature TR2 of the arc located outermost in the tire width direction satisfy the relationship 0.2≦TR2 / TR1≦0.

4.

2. The pneumatic tire described in claim 1, characterized in that the arc length L1 of the arc located on the tire centerline side is 60% or less of half the tread developed width TDW, and the widthwise center position A of the shoulder main groove located on the outermost side in the tire width direction among the plurality of main grooves is located more inward in the tire width direction than the end of the arc located on the tire centerline side.

3. A pneumatic tire as described in claim 1 or 2, characterized in that the arc length Lg of the contour line between the widthwise center position A of the shoulder main groove located at the outermost side of the plurality of main grooves and the tire center line is 56% or less of half the tread developed width TDW.

4. A pneumatic tire according to any one of claims 1 to 3, characterized in that the bead base width TBW is in the range of 120% or more and less than 130% of the rim width RW of a standard rim.

5. A pneumatic tire according to any one of claims 1 to 4, characterized in that the bead base width TBW is 115% or less of the tread developed width TDW.

6. A pneumatic tire according to any one of claims 1 to 5, characterized in that a narrow groove extending in the tire circumferential direction is formed in at least one of a pair of shoulder land portions located outside in the tire width direction of a pair of shoulder main grooves located at the outermost positions in the tire width direction among the plurality of main grooves.

7. A pneumatic tire according to any one of claims 1 to 6, characterized in that the number of main grooves is at least three, the groove bottoms of the main grooves are configured to include two arcs, the radii of curvature r of these arcs are in the range of 2 mm to 4 mm, and the radius of curvature ra of the arc on the inner side in the tire width direction at the groove bottoms of the main grooves and the radius of curvature rb of the arc on the outer side in the tire width direction satisfy the relationship ra≦rb.

8. A method for manufacturing a pneumatic tire comprising a circumferentially extending annular tread portion, a pair of sidewall portions disposed on either side of the tread portion, and a pair of bead portions disposed radially inward of the sidewall portions, wherein the tread portion has a plurality of circumferentially extending main grooves, and the tire cross-sectional height SH is set to 120 mm or greater, wherein the bead base width TBW of the bead portions is 107% or greater of the tread developed width TDW of the tread portion and 92% or less of the nominal tire width, and the contour line forming the ground contact surface of the tread portion is composed of two or more arcs having mutually different radii of curvature from the tire centerline to the ends of the tread developed width TDW of the tread portion, and wherein a mold is used in which the radius of curvature TR1 of the arc located closer to the tire centerline and the radius of curvature TR2 of the arc located outermost in the tire width direction satisfy the relationship 0.2≦TR2 / TR1≦0.4, and the pneumatic tire is vulcanized in this mold.

9. A method for manufacturing a pneumatic tire as described in claim 8, characterized in that the mold is formed so that the arc length L1 of the arc located on the tire center line side is 60% or less of half the tread developed width TDW, and the widthwise center position A of the shoulder main groove located on the outermost side in the tire width direction among the plurality of main grooves is located more inward in the tire width direction than the end of the arc located on the tire center line side.

10. A method for manufacturing a pneumatic tire as described in claim 8 or 9, characterized in that the mold is formed so that the arc length Lg of the contour line between the widthwise center position A of the shoulder main groove located at the outermost side of the plurality of main grooves in the tire width direction and the tire center line is 56% or less of half the tread developed width TDW.

11. A method for manufacturing a pneumatic tire as set forth in any one of claims 8 to 10, characterized in that the mold is formed so that the bead base width TBW is in the range of 120% or more and less than 130% of the rim width RW of a standard rim.

12. The method for manufacturing a pneumatic tire according to any one of claims 8 to 11, wherein the mold is formed so that the bead base width TBW is 115% or less of the tread developed width TDW.

13. A method for manufacturing a pneumatic tire according to any one of claims 8 to 12, characterized in that a narrow groove extending in the tire circumferential direction is formed in at least one of a pair of shoulder land portions located outside in the tire width direction of a pair of shoulder main grooves located at the outermost positions in the tire width direction among the plurality of main grooves.

14. A method for manufacturing a pneumatic tire as set forth in any one of claims 8 to 13, characterized in that the number of main grooves is at least three, the groove bottoms of the main grooves are configured to include two arcs, the radii of curvature r of these arcs are in the range of 2 mm to 4 mm, and the radius of curvature ra of the arc on the inner side in the tire width direction that constitutes the groove bottoms of the main grooves and the radius of curvature rb of the arc on the outer side in the tire width direction satisfy the relationship ra≦rb.

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