Pneumatic tire

A pneumatic tire includes a tread portion 16, a sidewall portion 14, a buttress portion 18, and a hollow portion 50. A cross-sectional shape of a bottom surface of the hollow portion 50 is a curved shape in which a plurality of arcs having different radii of curvature are disposed in a tire radial direction and adjacent arcs are connected at a contact point C where the adjacent arcs have a common tangent line, or is a shape in which a plurality of arcs having different radii of curvature are connected by a straight line connecting contact points where the adjacent arcs have a common tangent line. An outside end 32a, in the tire radial direction, of a sidewall rubber 32 provided on the sidewall portion 14 is located on an outside Ro in the tire radial direction from the groove bottom 36B1 of the shoulder main groove 36B.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a pneumatic tire.

Background Art

In a pneumatic tire, a ground contact pressure is usually high in the vicinity of a ground contact end of a tread portion during traveling, so that uneven wear, which means that an amount of wear in the vicinity of the ground contact end is larger than those in other portions, may become a problem. In order to suppress such uneven wear, it is known that an annular recess groove extending in a tire circumferential direction is provided in a buttress portion provided between the tread portion and a sidewall portion (for example, Japanese Unexamined Patent Publication No. 2017-136936). In a pneumatic tire provided with an annular recess portion in the buttress portion, the rigidity in the vicinity of the ground contact end of the tread portion can be decreased to reduce the ground contact pressure and the occurrence of uneven wear can be suppressed.

SUMMARY OF THE INVENTION

However, in a pneumatic tire having an annular recess portion in the buttress portion, if the annular recess portion is provided at a position away from the ground contact end of the tread portion inwardly in a radial direction, the rigidity in the vicinity of the ground contact end cannot be sufficiently reduced and uneven wear cannot be suppressed.

In order to suppress the uneven wear, it is conceivable to provide an annular recess portion close to the ground contact end of the tread portion. However, in that case, cracks are likely to occur in rubber starting from the outside end portion, in the tire radial direction, of the sidewall rubber provided on the sidewall portion.

That is, if the annular recess portion is provided close to the ground contact end of the tread portion, the annular recess portion is close to the outside end portion, in the tire radial direction, of the sidewall rubber provided on the sidewall portion. In a pneumatic tire having an annular recess portion in the buttress portion, a strain generated by a load received in the vicinity of the ground contact end of the tread portion tends to concentrate in the annular recess groove. As a result, cracks are likely to occur at the end portion of the sidewall rubber close to the annular recess portion under the influence of the strain stress concentrated in the annular recess groove.

In view of the above points, an object of an embodiment of the present invention is to provide a pneumatic tire in which cracks can be suppressed in the sidewall rubber provided in the sidewall portion while reducing the rigidity in the vicinity of the ground contact end of the tread portion to suppress the occurrence of uneven wear.

According to an embodiment of the present invention, there is provided a pneumatic tire including: a tread portion; a sidewall portion; a buttress portion provided between the tread portion and the sidewall portion; a hollow portion provided on the buttress portion in a tire circumferential direction; and a shoulder main groove provided in the tire circumferential direction on an outside of the tread portion in a tire width direction. A cross-sectional shape of a bottom surface of the hollow portion is a curved shape in which a plurality of arcs having different radii of curvature are disposed side by side in a tire radial direction and adjacent arcs are connected at a contact point where the adjacent arcs have a common tangent line, or a shape in which a plurality of arcs having different radii of curvature are connected by a straight line connecting contact points where the adjacent arcs have a common tangent line. The contact point is located on an inside in the tire radial direction from a groove bottom of the shoulder main groove. An outside end, in the tire radial direction, of a sidewall rubber provided on the sidewall portion is located on the outside in the tire radial direction from the groove bottom of the shoulder main groove.

DETAILED DESCRIPTION OF THE INVENTION

Unless otherwise specified, each dimension described above in the present specification is provided in a regular state with no load in which a pneumatic tire is mounted on a regular rim and filled with a regular internal pressure. Further, in the present specification, a ground contact end E is an end portion, in a tire width direction, of a tread surface that is in contact with a ground of a road surface, in a regular load state where a pneumatic tire is rim-assembled on a regular rim and is vertically placed on a flat road surface in a state of being filled with a regular internal pressure, and a regular load is applied.

The regular rim is an application rim defined for each tire by a standard in a market standard where the tire is used. For example, if it is JATMA, the regular rim is an application rim, if it is TRA, the regular rim is “Approved Rim Contours”, and if it is ETRTO, the regular rim is “Approved Rim”. The regular internal pressure is an air pressure defined for each tire by each standard in the market where the tire is used. If it is JATMA, the regular internal pressure is an air pressure corresponding to a maximum load capacity of a target tire, and if it is TRA, the regular internal pressure is an air pressure corresponding to a maximum load capacity of a target tire described in a table of “TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES”, and if it is ETRTO, the regular internal pressure corresponds to “INFLATION PRESSURE”. For example, in a case where a tire having a tire size of 295/75R22.5 (LR=G), it may be set to 760 kPa. In addition, the regular load is a permissible load defined for each tire by each standard in the market where the tire is used. If it is JATMA, the regular load is a single wheel maximum load capacity value of the target tire, if it is TRA, the regular load is a single wheel maximum load capacity value of the target tire described in the table described above, and if it is ETRTO, the regular load is a single wheel “LOAD CAPACITY PER AXLE” value of the target tire.

First Embodiment

A pneumatic tire10of a first embodiment of the present invention will be described with reference to the drawings.FIG.1is a sectional view of a tire meridian illustrating an example of the pneumatic tire10according to the present embodiment. Since the pneumatic tire10is left-right symmetrical, only a right half is illustrated in the drawing.

The pneumatic tire10ofFIG.1includes a pair of left and right bead portions12, a pair of left and right sidewall portions14extending outside in a radial direction from the bead portions12, a tread portion16forming a tread surface, and a pair of left and right buttress portions18disposed on an inside Ri of the tread portion16in the tire radial direction. Here, the buttress portion18is a boundary region between the tread portion16and the sidewall portion14, and is provided so as to connect between the tread portion16and the sidewall portion14.

The pneumatic tire10includes a carcass ply20provided so as to be bridged between the pair of bead portions12in a toroidal shape. A ring-shaped bead core22is embedded in each of the pair of bead portions12. A bead filler34made of a hard rubber material that tapers outward in the tire radial direction is disposed on an outer peripheral side of the bead core22.

The carcass ply20extend from the tread port ion16through the buttress portion18and the sidewall portion14and is locked by the bead core22at the bead portion12. The carcass ply20reinforces the bead portion12, the sidewall portion14, the tread portion16, and the buttress portion18. In this example, the carcass ply20is locked at both ends by each being folded around the bead core22from an inside Wi to an outside Wo in a tire width direction. An inner liner24for holding an air pressure is disposed inside the carcass ply20.

The carcass ply20is formed of at least one ply coated with topping rubber in which a metal cord such as a steel cord and an organic fiber cord such as polyester fiber, rayon fiber, aramid fiber, and nylon fiber are disposed at a predetermined angle (for example, 70° to 90°) with respect to the tire circumferential direction. The carcass ply20in this example is configured of one ply. As the cord constituting the carcass ply20, for example, the metal cord such as the steel cord is preferably used.

A belt26is disposed on an outer peripheral side of the carcass ply20in the tread portion16. That is, the belt26is provided between the carcass ply20and a tread rubber28in the treadportion16. The belt26is configured of a plurality of crossed belt plies in which belt cords are disposed at a predetermined angle (for example, 10° to 75°) with respect to the tire circumferential direction. As the belt cord, a steel cord or an organic fiber cord having high tension is used.

In this example, the belt26has a four-layer structure of a first belt26A located closest to the inside Ri in the tire radial direction, a second belt26B and a third belt26C stacked in order on the outer peripheral side thereof, and a fourth belt26D located closest to an outside Ro in the tire radial direction. The second belt26B is a maximum width belt having a widest width among the four-layers of the belts26A,26B,26C, and26D.

A plurality of main grooves36extending in a tire circumferential direction S are provided on a surface of the tread portion16. Specifically, the main groove36is configured of a pair of shoulder main grooves36B provided at positions close to a ground contact end E of the tread portion16and a pair of center main grooves36A provided between the pair of shoulder main grooves36B. The outside Wo in the tire width direction refers to a side away from a tire equatorial plane CL in the tire width direction W.

Due to the four main grooves36described above, in the tread portion16, a central land portion38is formed between the two center main grooves36A, an intermediate land portion40is formed between the center main groove36A and the shoulder main groove36B, and a shoulder land portion42is formed on the outside Wo of the two shoulder main grooves36B in the tire width direction.

In this example, the central land portion38, the intermediate land portion40, and the shoulder land portion42are configured of ribs continuous in the tire circumferential direction. The central land portion38, the intermediate land portion40, and the shoulder land portion42may be block columns divided in the tire circumferential direction by lateral grooves.

An outside end of the tread surface of the shoulder land portion42in the tire width direction forms the ground contact end E. The buttress portion18extending to the inside Ri in the tire radial direction and forming an upper portion of the side surface of the tire is connected to the ground contact end E.

Then, as illustrated inFIGS.1and2, an outer surface of the buttress portion18is formed of an inclined portion48extending from the ground contact end E toward the inside Ri in the tire radial direction and a hollow portion50provided on the inside Ri in the tire radial direction from the inclined portion48.

The inclined portion48is inclined so as to expand in the tire width direction W from the ground contact end E toward the inside Ri in the tire radial direction (that is, to reduce a diameter toward the outside Wo in the tire width direction). The inclined portion48weakens the rigidity of the shoulder land portion42on a ground contact end E side and improves a wandering performance when climbing over a step on the road surface such as a rut.

In the present embodiment, as illustrated inFIG.2, a bent portion49bent to the inside Ri in the tire radial direction is provided on a tip side (outside in the tire width direction) of the inclined portion48, and an outside end50aof the hollow portion50in the tire width direction is connected to the tip of the bent portion49.

The hollow portion50is an annular recess groove extending in the tire circumferential direction, which sinks from the outer surface of the buttress portion18to the inside Wi in the tire width direction. A cross-sectional shape of a bottom surface of the hollow portion50is a curved shape in which two arcs having different radii of curvature are disposed side by side in the tire radial direction, and adjacent arcs are connected at a contact point C where the adjacent arcs have a common tangent line.

Specifically, the bottom surface of the hollow portion50is formed of an upper arc portion51aprovided on the outside Ro in the tire radial direction and a lower arc portion51bprovided on the inside Ri of the upper arc portion51ain the tire radial direction. The upper arc portion51ais a curved surface having a cross section formed of an arc having a radius of curvature ra, and the lower arc portion51bis a curved surface having a cross section formed of an arc having a radius of curvature rb which is smaller than the radius of curvature ra. A length L1of the upper arc portion51ain the tire radial direction R is longer than a length L2of the lower arc portion51bin the tire radial direction R. As an example, the radius of curvature ra of the upper arc portion51amay be set to 10 to 40 mm, the radius of curvature rb of the lower arc portion51bmay be set to 3 to 15 mm, the length L1of the upper arc portion51ain the tire radial direction R may be set to 5 to 14 mm, and the length L2of the lower arc portion51bin the tire radial direction R may be set to 2 to 9 mm.

As described above, the upper arc portion51aand the lower arc portion51bforming the bottom surface of the hollow portion50are connected so as to have the common tangent line at the contact point C, and are smoothly connected at the contact point C without any ridge protruding from the bottom surface of the hollow portion50to the outside Wo in the tire width direction.

The hollow portion50is disposed in the buttress portion18so that the contact point C between the upper arc portion51aand the lower arc portion51bis located on the inside Ri in the tire radial direction from a groove bottom36B1of the shoulder main groove36B.

A protrusion portion60extending in the tire circumferential direction is provided at the outside end50aof the hollow portion50in the tire radial direction. The protrusion portion60is provided so as to straddle the hollow portion50and the bent portion49. The protrusion portion60is a protrusion formed by a narrow groove called a saw cut provided in a tire mold. As an example, a length (width of the protrusion portion60) of the protrusion portion60in the tire radial direction R may be set to 0.50 to 1.50 mm, and a height of the protrusion portion60may be set to 0.50 to 1.50 mm. The protrusion portion60may be an annular protrusion continuously extending in the tire circumferential direction, or may be a protrusion intermittently extending in the tire circumferential direction. In a case where the protrusion portions60intermittently extends in the tire circumferential direction, a distance between the protrusion portions60adjacent to each other in the tire circumferential direction may be set to 8 to 50 mm.

It is desirable that the hollow portion50is provided such that a distance in the tire radial direction R from the ground contact end E of the tread portion16to the contact point C is 30 mm or less. Further, it is desirable that a distance D from an end26B1of the widest second belt26B to the contact point C in the tire width direction is 12 mm or more.

A sidewall rubber32is provided on the outside (that is, on the tire outer surface side) of the carcass ply20in the sidewall portion14. The sidewall rubber32is a rubber member that constitutes the surface (outer surface) of the sidewall portion14, and is formed of a rubber composition having a high cut resistance and weather resistance for sidewall formulation.

The sidewall rubber32extends from the outside Wo of the bead filler34of the bead portion12in the tire width direction to the outside Ro in the tire radial direction along the outer surface of the tire. An outside end32aof the sidewall rubber32in the tire radial direction is located on the outside Ro in the tire radial direction from the groove bottom36B1of the shoulder main groove36B. That is, the outside end32aof the sidewall rubber32in the tire radial direction is located on the outside Ro in the tire radial direction from the contact point C of the hollow portion50.

It is desirable that the outside end32aof the sidewall rubber32in the tire radial direction is located at a position of 50% or less of a groove depth H from the groove bottom36B1of the shoulder main groove36B. That is, it is desirable that the outside end32aof the sidewall rubber32in the tire radial direction is at the same position in the tire radial direction R as a position P of 50% of the groove depth H from the groove bottom36B1of the shoulder main groove36B, or located on the inside Ri in the tire radial direction from the position P. Further, it is desirable that the outside end32aof the sidewall rubber32in the tire radial direction is separated from the contact point C between the upper arc portion51aand the lower arc portion51bby 4 mm or more on the outside Ro in the tire radial direction.

In the pneumatic tire10of the present embodiment as described above, the contact point C between the upper arc portion51aand the lower arc portion51bis disposed on the inside Ri in the tire radial direction from the groove bottom36B1of the shoulder main groove36B, and the outside end32aof the sidewall rubber32in the tire radial direction is disposed on the outside Ro in the tire radial direction from the groove bottom36B1of the shoulder main groove36B.

Therefore, the outside end32aof the sidewall rubber32in the tire radial direction can be disposed in the vicinity of the ground contact end E of the tread portion16to improve the cut resistance and weather resistance in the buttress portion18, and improve a rough road running performance and durability.

Moreover, the outside end32aof the sidewall rubber32in the tire radial direction is disposed so as to avoid the contact point C of the hollow portion50where strain tends to concentrate when the tire is in contact with the ground. Therefore, it is possible to suppress cracks in the sidewall rubber32starting from the outside end32aof the sidewall rubber32in the tire radial direction, while suppressing uneven wear in the vicinity of the ground contact end of the tread portion by the hollow portion50.

Further, in the present embodiment, if the outside end32aof the sidewall rubber32in the tire radial direction is located at a position of 50% or less of the groove depth from the groove bottom of the shoulder main groove, since the sidewall rubber32is not exposed on the outer surface of the tread portion16until an end stage of wear, different rubber is not disposed on the outer surface of the tread portion16from an early stage to a middle stage of wear, and a desired tire performance can be maintained.

Further, in the present embodiment, if the distance D from the end26B1of the widest second belt26B in the tire width direction to the contact point C is 12 mm or more, the rubber thickness is sufficiently ensured from the end portion of the belt26to the bottom surface of the hollow portion50, and separation that the tread rubber peels off from the end portion of the belt26is less likely to occur.

Further, in the present embodiment, if the narrow protrusion portion60is provided at the outside end50aof the hollow portion50in the tire radial direction, the narrow groove is provided at an outside end of a protruded hollow forming portion in the tire radial direction, which forms the hollow portion50in a tire mold for vulcanization molding of the pneumatic tire10, and a rubber flow in the vicinity of the hollow forming portion is improved, and a vulcanization molding defect such as bare (recess on the tire surface) can be suppressed.

Further, in the present embodiment, in a case where the radius of curvature R1of the upper arc portion51aprovided on the outside Ro of the hollow portion50in the tire radial direction is larger than the radius of curvature R2of the lower arc portion51bprovided on the inside Ri of the hollow portion50in the tire radial direction, and the length L1of the upper arc portion51ain the tire radial direction R is longer than the length L2of the lower arc portion51bin the tire radial direction R, the following effects are obtained.

That is, in order to reduce the ground contact pressure in the vicinity of the ground contact end of the tread portion, even if the hollow portion50is disposed close to the ground contact end E of the tread portion16, the contact point C of the hollow portion50can be disposed on the inside Ri in the tire radial direction. Therefore, the outside end32aof the sidewall rubber32in the tire radial direction can be disposed at a position away from the contact point C of the hollow portion50where strain is likely to be concentrated, and uneven wear and the separation of the sidewall rubber32can be compatible at a high level.

Second Embodiment

Next, a pneumatic tire100of a second embodiment of the present invention will be described focusing on portions different from those of the first embodiment based onFIG.3. The same components as those of the first embodiment are designated by the same reference numerals, and detailed description thereof will be omitted.

In the first embodiment described above, a case where the cross-sectional shape of the bottom surface of the hollow portion50has the curved shape in which two arcs having different radii of curvature are connected at the contact point C where the adjacent arcs have the common tangent line is described.

In the present embodiment, as illustrated inFIG.3, a cross-sectional shape of a bottom surface of a hollow portion150has a shape in which three arcs having different radii of curvature are disposed side by side in the tire radial direction and the adjacent arcs are connected by contact points C1and C2where the adjacent arcs have a common tangent line.

Specifically, the bottom surface of the hollow portion150includes an upper arc portion151aprovided closest to the outside Ro in the tire radial direction, a lower arc portion151bprovided closest to the inside Ri in the tire radial direction, and an intermediate arc portion151cprovided between the upper arc portion151aand the lower arc portion151b. The upper arc portion151ais a curved surface having a cross section formed of an arc having a radius of curvature ra, and the lower arc portion151bis a curved surface having a cross section formed of an arc having a radius of curvature rb which is smaller than the radius of curvature ra. The intermediate arc portion151cis a curved surface having a cross section formed an arc having a radius of curvature rc which is larger than the radius of curvature ra and the radius of curvature rb. The length L1of the upper arc portion151ain the tire radial direction R is longer than those of the lower arc portion151band the intermediate arc portion151c.

The upper arc port ion151aand the intermediate arc portion151care connected so as to have a common tangent line at the contact point C1. The intermediate arc portion151cand the lower arc portion151bare connected so as to have a common tangent line at the contact point C2. Therefore, the upper arc portion151a, the intermediate arc portion151c, and the lower arc portion151bare smoothly connected to each other at the contact points C1and C2without causing ridges.

As an example, the radius of curvature ra of the upper arc portion151amay be set to 10 to 40 mm, the radius of curvature rb of the lower arc portion151bmay be set to 3 to 15 mm, the radius of curvature rc of the intermediate arc portion151cmay be set to 100 to 250 mm, the length L1of the upper arc portion151ain the tire radial direction R may be set to 5 to 14 mm, the length L2of the lower arc portion151bin the tire radial direction R may be set to 2 to 9 mm, and the length L3of the intermediate arc portion151cin the tire radial direction R may be set to 1 to 4 mm.

In the hollow portion150, the contact point C1between the upper arc portion151aand the intermediate arc portion151c, and the contact point C2between the intermediate arc portion151cand the lower arc portion151bare disposed in the buttress portion18so as to be located on the inside Ri in the tire radial direction from the groove bottom36B1of the shoulder main groove36B. In other words, the hollow portion150is disposed in the buttress portion18such that both the contact points C1and C2are located on the inside Ri in the tire radial direction from the groove bottom36B1of the shoulder main groove36B.

It is desirable that the hollow portion150is provided such that a distance in the tire radial direction R from the ground contact end E of the tread portion16to the contact point C1of the upper arc portion151ais 30 mm or less. Further, it is desirable that a distance D1from the end26B1of the widest second belt26B in the tire width direction to the contact point C1and a distance D2to the contact point C2are 12 mm or more.

Similar to the first embodiment, it is desirable that the outside end32aof the sidewall rubber32in the tire radial direction is located at a position of 50% or less of the groove depth H from the groove bottom36B1of the shoulder main groove36B. Further, it is desirable that the outside end32aof the sidewall rubber32in the tire radial direction is separated from the contact point C2of the lower arc portion151bby 4 mm or more on the outside Ro in the tire radial direction.

Even with the pneumatic tire100of the present embodiment as described above, the same effects as those of the first embodiment described above can be achieved.

Third Embodiment

Next, a pneumatic tire200of a third embodiment of the present invention will be described focusing on portions different from those of the first embodiment based onFIG.4. The same components as those of the first embodiment are designated by the same reference numerals, and detailed description thereof will be omitted.

In the first embodiment described above, a case where the cross-sectional shape of the bottom surface of the hollow portion50has a curved shape in which two arcs having different radii of curvature are connected at the contact point C where the adjacent arcs have the common tangent line is described. In the present embodiment, as illustrated inFIG.4, a cross-sectional shape of a bottom surface of a hollow portion250has a shape in which two arcs having different radii of curvature are connected by a straight line connecting contact points C3and C4where the adjacent arcs have a common tangent line.

Specifically, the bottom surface of the hollow portion250includes an upper arc portion251aprovided on the outside Ro in the tire radial direction, a lower arc portion251bprovided on the inside Ri of the upper arc portion251ain the tire radial direction, and a straight line portion251cthat connects the upper arc portion251aand the lower arc portion251b. The upper arc portion251ais a curved surface having a cross section formed of an arc having a radius of curvature ra, and the lower arc portion251bis a curved surface having a cross section formed of an arc having a radius of curvature rb which is smaller than the radius of curvature ra.

Since the tangent line of the upper arc portion251aat the contact point C3is common to, that is, coincides with the tangent line of the lower arc portion251bat the contact point C4, the straight line portion251cconnecting the contact point C3and the contact point C4coincides with the tangent line of the upper arc portion251aat the contact point C3, and also coincides with the tangent line of the lower arc portion251bat the contact point C4. Therefore, the upper arc portion251aand the lower arc portion251bare smoothly connected to each other via the straight line portion251cwithout causing ridges at the contact points C3and C4.

As an example, the radius of curvature ra of the upper arc portion251amay be set to 10 to 40 mm, the radius of curvature rb of the lower arc portion251bmay be set to 3 to 15 mm, the length L1of the upper arc portion251ain the tire radial direction R may be set to 5 to 14 mm, the length L2of the lower arc portion251bin the tire radial direction R may be set to 2 to 9 mm, and the length L3of the straight line portion251cin the tire radial direction R may be set to 1 to 4 mm.

The hollow portion250is disposed in the buttress portion18such that the contact point C3of the upper arc portion251aand the contact point C4of the lower arc portion251bare located on the inside Ri in the tire radial direction from the groove bottom36B1of the shoulder main groove36B. In other words, the hollow portion250is disposed in the buttress portion18such that the straight line portion251cis located on the inside Ri in the tire radial direction from the groove bottom36B1of the shoulder main groove36B.

It is desirable that the hollow portion250is provided such that a distance in the tire radial direction R from the ground contact end E of the tread portion16to the contact point C3of the upper arc portion251ais 30 mm or less. Further, it is desirable that a distance D3from the end26B1of the widest second belt26B in the tire width direction to the contact point C3and a distance D4to the contact point C4are 12 mm or more.

Similar to the first embodiment, it is desirable that the outside end32aof the sidewall rubber32in the tire radial direction is located at a position of 50% or less of the groove depth H from the groove bottom36B1of the shoulder main groove36B. Further, it is desirable that the outside end32aof the sidewall rubber32in the tire radial direction is separated from the contact point C4of the lower arc portion251bby 4 mm or more on the outside Ro in the tire radial direction.

Even with the pneumatic tire200of the present embodiment as described above, the same effects as those of the first embodiment described above can be achieved.

MODIFIED EXAMPLES

The embodiments described above are presented as examples and are not intended to limit the scope of the invention. This novel embodiments can be implemented in various other embodiments, and various omissions, replacements, and changes can be made without departing from the gist of the invention.

For example, in the embodiments described above, the case where the hollow port ion50forms a completely continuous annular shape in the tire circumferential direction is described, but it may be intermittent at some points in the circumferential direction.

Further, in the first and second embodiments described above, cases where the cross-sectional shapes of the bottom surface of the hollow portion are respectively configured of two and three arcs having different radii of curvature are described, but a curved shape may be employed in which four or more arcs may be disposed side by side in the tire radial direction, and adjacent arcs are connected at a contact point where the adjacent arcs have a common tangent line. Further, in the third embodiment described above, the case where the cross-sectional shape of the bottom surface of the hollow portion has the shape in which two arcs having different radii of curvature are connected by the straight line connecting the contact points where the adjacent arcs have the common tangent line is described. However, it may have a shape in which three or more arcs are provided and adjacent arcs are connected by a straight line connecting the contact points where the adjacent arcs have the common tangent line.