JP7767103B2 - pneumatic tires - Google Patents

Description

The present invention relates to a pneumatic tire.

Patent Document 1 discloses a pneumatic tire in which the heel of the bead portion has a heel cut portion that is chamfered to form a linear cross section, while the base surface of the bead portion has a two-stage taper consisting of a heel side portion and a toe side portion that are at different angles relative to the tire axial direction, with the toe side portion having a larger inclination angle relative to the tire axial direction than the heel side portion (similar to Patent Document 2).

Japanese Patent Application Laid-Open No. 2015-209032 International Publication No. 2014/126098

In the pneumatic tires of Patent Documents 1 and 2, the bead base surface is configured with a two-stage taper, with the toe side having a larger inclination angle than the heel side. This, combined with the bead heel, which is the axially outer and inner diameter side of the bead, being chamfered, means that the bead portion tends to collapse axially outward when mounted on the rim. As a result, the fitting pressure on the bead base surface decreases while the fitting pressure on the bead back surface tends to increase, leaving room for improvement in rim fit.

The objective of the present invention is to provide a pneumatic tire that can improve rim fitting while also improving handling stability.

The present invention provides
The tire has a tread, a sidewall extending radially inward from an axial end of the tread, and a bead portion continuing to the radially inward end of the sidewall,
In a meridian cross section, the bead portion has an outer surface, a bead base surface extending along a first straight line inclined radially outward toward the tire axially outer side at an inner end of the bead portion in the tire radial direction;
a bead back surface extending radially outward from an axially outer end of the bead portion and having an inner end in the tire radial direction extending along a second straight line along the tire radial direction;
a bead heel surface extending along a third straight line that is inclined further outward in the tire radial direction than the first straight line toward the tire axially outward between an outer end of the bead base surface in the tire axial direction and an inner end of the bead back surface in the tire radial direction ,
The pneumatic tire provides a tire in which the third straight line is geometrically defined as a straight line connecting both end points of an imaginary arc that tangently and continuously connects the first straight line and the second straight line to each other with a radius of curvature greater than 3 mm and less than 8 mm in a meridian cross section .

According to the present invention, the bead base surface is configured with a single inclined surface (sometimes referred to as a single-step taper), which prevents the bead portion from tipping axially outward when assembled to the rim, compared to when the bead base surface has an additional inclined surface that increases in the radially inward direction toward the axially inward direction of the tire (sometimes referred to as a two-step taper). As a result, when assembled to the rim, the mating pressure at the back surface of the bead is reduced, while the mating pressure at the bead base surface is increased, compared to when the bead base surface is two-step tapered.

Furthermore, the bead heel surface is configured as a chamfer that linearly connects the bead base surface and bead back surface, preventing the bead heel surface from interfering with the rim heel of the original rim when the rim is assembled. As a result, the bead portion can be fitted to the rim seat of the original rim at the correct axial position, preventing the bead portion from collapsing axially outward due to insufficient fit and preventing an increase in fitting pressure at the bead back surface.

Therefore, when a load is applied, the bead base surface with increased fitting pressure makes it easier to maintain fitting with the rim seat of the regular rim, improving rim fitting, while the bead back surface with reduced fitting pressure makes it easier for the sidewall to flex and deform, improving handling stability.
Furthermore, the third straight line is geometrically defined as a straight line connecting the end points of an imaginary arc that tangentially connects the first and second straight lines with a radius of curvature greater than 3 mm and less than 8 mm in a meridian cross section. As a result, the bead heel surface is formed with an appropriate size, so that when the pneumatic tire is mounted on a standard rim and inflated to the specified internal pressure, the bead heel surface can easily abut against the rim heel of the standard rim, ensuring rim fit. If the curvature radius of the imaginary arc is 3 mm or less, the bead heel surface will interfere with the rim heel during rim mounting, making it difficult to fit the tire to the appropriate axial position. On the other hand, if the curvature radius of the imaginary arc is 8 mm or more, it will be difficult to abut the bead heel surface against the rim heel even after inflation during rim mounting, making it difficult to fit the tire to the appropriate axial position.

The inclination angle of the first straight line relative to the tire axial direction may be greater than or equal to 5° and less than or equal to 15°.

With this configuration, the bead base surface is moderately inclined, which increases the mating pressure at the bead base surface while suppressing an increase in mating pressure at the bead back surface. If the inclination angle of the first straight line is less than 5°, the sidewall will deform excessively due to insufficient mating pressure at the bead back surface, which is likely to lead to a decrease in handling stability. On the other hand, if the inclination angle of the first straight line is greater than 15°, excessive mating pressure at the bead back surface will hinder the sidewall's flexural deformation, which is likely to lead to a decrease in the sidewall's load support performance.

In the meridian section,
an intersection point between the first straight line and the second straight line is defined as a first intersection point;
an intersection of the first straight line and the third straight line is a second intersection;
When the intersection of the second line and the third line is defined as a third intersection,
The bead heel surface may have a radial dimension, which is the dimension in the tire radial direction between the first intersection and the third intersection, that is equal to or greater than an axial dimension, which is the dimension in the tire axial direction between the first intersection and the second intersection, and less than twice the axial dimension.

With this configuration, the bead heel surface is configured to be long in the tire radial direction, making it easier to ensure a bead base surface and prevent the bead portion from tipping axially outward. If the radial dimension of the bead heel surface is less than the axial dimension, it is likely to be insufficient to prevent the bead heel from interfering with the rim seat of a standard rim when assembled to the rim. On the other hand, if the radial dimension of the bead heel surface is more than twice the axial dimension, the bead back surface becomes excessively small, causing fitting pressure to be concentrated locally on the bead back surface, hindering flexural deformation of the sidewall.

A bead core is embedded in the bead portion,
In the meridian cross section, the bead heel surface is located axially outward of a fourth straight line extending in the tire radial direction at a position axially inwardly spaced a predetermined length from an axially outer end of the bead core,
The predetermined length may be 2 mm, or may be 30% of the axial dimension of the bead core of the tire.

This configuration prevents the bead heel surface from intruding excessively axially inward in the radially inner region of the bead core. As a result, it is easier to ensure a bead base surface on the radially inner side of the bead portion, and prevents the bead portion from collapsing axially outward.

The bead portion is provided in a pair,
the pair of bead portions are positioned at a distance from each other so that the width between the outer surfaces in the axial direction of the tire in an unassembled state is wider than the rim width of a corresponding regular rim,
Each of the pair of bead portions has a recess formed on the back surface of the bead in an unrim-assembled state, the recess being recessed axially inward of an axially outer end of the bead heel surface,
When assembled to the rim, the bead back surface may be in close contact with substantially the entire surface of a radial portion of the rim flange of the regular rim that extends parallel to the tire radial direction.

With this configuration, a recess is formed in the bead back surface, so that when the tire is not mounted to the rim, the inner diameter side portion of the bead back surface, located on the inner diameter side from the deepest part of the recess, slopes radially inward and axially outward in the tire's direction. When the pair of bead portions are mounted to the rim with the width of a standard rim close to that of a standard rim, this inner diameter side portion tends to bend axially inward from the periphery of the deepest part of the recess and generally follow the tire's radial direction. In other words, when mounted to the rim, the bead back surface can easily be brought into close contact with almost the entire radial portion of the rim flange while eliminating the recess, thereby increasing the contact area.

In this way, when the tire is mounted to the rim and in an unloaded state, surface pressure acts across almost the entire surface of the bead back surface, which is in close contact with the radial portion of the rim flange. This means that the load borne by the entire bead back surface is distributed more evenly than when surface pressure acts only on a portion of the bead back surface. In other words, compared to when high surface pressure acts locally on the bead back surface, there is more room for compression to allow the bead back surface to elastically deform.

Therefore, when a load or lateral force is applied, the back surface of the bead can be further compressed by the amount of the available compression. In this case, the sidewall can also deform in the area close to the bead due to further compression of the back surface of the bead, so the sidewall can bend and deform overall from the bead side to the tread side. This improves the efficiency of load support at the sidewall, thereby improving handling stability.

The recess may have an axial depth of less than 1.0 mm.

With this configuration, the inner diameter side portion is moderately inclined axially outward when the tire is not mounted to the rim, so when it is bent axially inward when mounted to the rim, the axially outward inclination is easily eliminated and it easily fits perfectly into the radial portion of the rim flange. If the recess depth is 1.0 mm or more, the inner diameter side portion is likely to be inclined excessively axially outward when the tire is not mounted to the rim, making it difficult to eliminate the axially outward inclination even when it is bent axially inward when mounted to the rim. In this case, the recess is unlikely to disappear when mounted to the rim, making it difficult to abut the back surface of the bead over substantially the entire radial portion of the rim flange.

The bead heel surface may be formed, at least in part, from rim strip rubber and may have vent hole traces.

This configuration improves the flow of rubber around the bead heel surface during vulcanization molding, so while a linear bead heel surface is formed on the outer surface of the bead portion, problems such as bare spots caused by poor rubber flow during vulcanization molding are suppressed.

The present invention makes it possible to improve rim fitting while also improving handling and stability.

1 is a meridian cross-sectional view of a pneumatic tire according to one embodiment of the present invention. A meridian cross section of the bead area of a pneumatic tire before it is mounted on a rim. FIG. 3 is an enlarged view of the bead heel surface and its surroundings in FIG. 2 . FIG. 4 is a front view of a bead heel surface as viewed from an arrow A in FIG. 3 . 1 is a meridian cross-sectional view of the vicinity of a bead portion of a pneumatic tire assembled to a rim. 6 is a graph showing fitting pressure on the outer surface of the bead portion during inflation. 1 is a meridian cross-sectional view of the periphery of a bead portion of a pneumatic tire according to Comparative Example 1 in a rim-assembled state; FIG. 10 is a meridian cross-sectional view of the periphery of a bead portion of a pneumatic tire according to Comparative Example 2 in a rim-assembled state.

Embodiments of the present invention will now be described with reference to the accompanying drawings. Note that the following description is merely exemplary in nature and is not intended to limit the present invention, its applications, or its uses. The drawings are schematic, and the proportions of the various dimensions may differ from those of the actual product.

Figure 1 is a meridian cross-sectional view of a pneumatic tire 1 according to one embodiment of the present invention, showing only one side relative to the tire equator line CL. The pneumatic tire 1 comprises a tread 10, a pair of sidewalls 20 extending radially inward from both axial ends of the tread 10, and a pair of bead portions 30 each continuing radially inward from the pair of sidewalls 20.

A bead core 31 and a bead filler 32 connected to the outer side in the tire radial direction are embedded in the bead portion 30. The bead core 31 is constructed by covering an annular bundle formed by multiple windings of a bead wire made of steel wire with rubber. The cross-sectional shape of the bead core 31 is polygonal, with a bead core radial outer end surface 31a extending in the tire axial direction at the outer end in the tire radial direction. In this embodiment, the height Hb of the bead core radial outer end surface 31a in the tire radial direction based on the nominal rim diameter (specified in JIS 4102) NR (also known as the reference rim diameter) is 6.7 mm.

The bead filler 32 is made of hard rubber and extends annularly along the radially outer end surface 31a of the bead core. Its cross-sectional shape in the meridian direction is triangular, narrowing axially toward the tire radially outward.

A carcass ply 2 is laid across the tread 10 and sidewall 20 between a pair of bead cores 31. The carcass ply 2 is folded back around the bead cores 31 from the inner side of the tire to the outer side of the tire. An inner liner 3 is provided on the inner side of the carcass ply 2 to maintain air pressure.

In the tread 10, a belt layer 11 and a belt reinforcing layer 12 are laminated in this order on the radially outer side of the carcass ply 2. In this embodiment, the belt layer 11 is composed of two layers. A tread rubber 13 is laminated on the radially outer side of the belt reinforcing layer 12. The tread rubber 13 forms the radially outer surface of the pneumatic tire 1.

A tire side rubber 21 is arranged axially outward of the carcass ply 2, spanning the sidewall 20 and bead portion 30. The tire side rubber 21 includes a sidewall rubber 21a extending radially inward from the axial end of the tread rubber 13, and a rim strip rubber 21b connected to the inner diameter end and extending further radially inward. The tire side rubber 21 forms the outer surface of the pneumatic tire 1 in the axial direction.

The sidewall rubber 21a constitutes the majority of the sidewall 20. The rim strip rubber 21b is provided to correspond to at least the portion of the tire side rubber 21 that abuts against the rim flange 53 when the tire is assembled to a corresponding standard rim 50 (see Figure 2) (referred to as the rim-assembled state). The rim strip rubber 21b is made of rubber with superior abrasion resistance compared to the sidewall rubber 21a.

A rim protector 4 that protrudes axially outward is formed on the tire side rubber 21. The rim protector 4 is located radially inward of the tire's maximum width position Z. The maximum width position Z is the position where the profile line of the outer surface of the sidewall 20 is farthest axially from the tire equator line CL. In other words, the tire side rubber 21 gradually increases in thickness from the maximum width position Z toward the rim protector 4, and gradually decreases in thickness from the rim protector 4 toward the tire's radially inward direction.

The rim protector 4 bends axially inward from the end that extends radially inward from the maximum width position Z, and has a vertex 4a where it is thickest. The vertex 4a is located at a height H3 radially outward from the nominal rim diameter NR. The height H3 of the vertex 4a is located radially outward from the rim flange 53 of the corresponding genuine rim 50 when assembled to the rim.

In this specification, the portion located radially outward from the tip 32a of the bead filler 32 in the tire radial direction is referred to as the sidewall 20, and the portion located radially inward is referred to as the bead portion 30. The rim protector 4 is located in the bead portion 30. In addition, in this specification, the thickness of the tire side rubber 21 is defined as the direction perpendicular to the outer surface of the carcass ply 2.

Figure 2 shows an enlarged view of the bead portion 30 and its surroundings when not assembled to a genuine rim 50 (referred to as an unassembled state), along with the surroundings of the corresponding rim flange 53 of the genuine rim 50. The genuine rim 50 has a rim seat 51 extending axially outward, a rim heel 52 curved in an arc from the axially outer end of the rim to the radially outer side of the tire, and a rim flange 53 extending radially outward from the radially outer end of the rim.

The rim seat 51 is inclined radially outward toward the tire axially outward, with an inclination angle A0 relative to a line parallel to the tire axis. The rim flange 53 has a flange radial portion 53a extending radially outward from the rim heel 52 parallel to the tire radial direction, and a flange curved portion 53b continuing from the radially outer end of the flange radial portion 53a and curved axially outward in an arc shape.

The outer surface of the rim heel 52, located on the side where the pneumatic tire 1 is fitted, extends in an arc shape with a radius of curvature R11, centered on a center of curvature O11 located closer to the pneumatic tire 1 than the outer surface. A pair of flange radial portions 53a are arranged in the tire axial direction, separated by the rim width W0. The flange radial portions 53a extend radially outward in the tire direction from the nominal rim diameter NR to a height H11. The outer surface of the flange curved portion 53b, located on the side where the pneumatic tire 1 is fitted, extends in an arc shape with a radius of curvature R12, centered on a center of curvature O12 located on the outer surface of the tire.

Note that a genuine rim 50 is a rim defined for each tire by a standard system that includes the standard on which the tire is based. For example, it is called a "standard rim" in the case of JATMA, and a "measuring rim" in the cases of TRA and ETRTO.

The regular rim 50 according to this embodiment conforms to the flange symbol J for a 5° deep rim specified by JATMA, with the inclination angle A0 of the rim seat 51 being 5°, the radius of curvature R11 of the rim heel 52 being 6.5 mm, the height H11 of the flange radial portion 53a being 8 mm, and the radius of curvature R12 of the flange curved portion 53b being 9.5 mm. Furthermore, the angle between the rim seat 51 and the flange radial portion 53a of the regular rim 50 according to this embodiment is 95°.

The bead portion 30 has, on its outer surface in the meridian direction, a bead base surface 34 extending in the axial direction at its radially inner end, a bead back surface 35 extending radially outward at its axially outer end and continuing to the rim protector 4, and a bead heel surface 36 that chamfers the connection between the axially outer end of the bead base surface 34 and the radially inner end of the bead back surface 35.

When not mounted on a rim, the bead base surface 34 extends along a first straight line L1 that is inclined radially outward toward the tire axially outward. The inclination angle of the first straight line L1 relative to a line parallel to the tire axis is A1. That is, the bead base surface 34 is composed of a single straight line. Specifically, the inclination angle A1 is greater than the inclination angle A0 of the rim seat 51. Preferably, the inclination angle A1 is greater than or equal to 5° and less than or equal to 15°. More preferably, the difference between the inclination angle A1 and the inclination angle A0 is less than or equal to 8°. In this embodiment, the inclination angle A1 is 12°, which is 7° greater than the inclination angle A0.

The bead back surface 35 has at least a straight portion 61 extending along a second straight line L2 along the tire radial direction at its radially inner end, a first curved portion 62 (recess) that continues from the straight portion 61 and extends radially outward in the tire direction and constitutes at least a part of a recess 70 (described below), and a second curved portion 63 that is located radially outward in the tire direction and reaches the apex 4a of the rim protector 4.

The first curved portion 62 extends from a first point P1, which is the radially outer end point of the straight portion 61, toward the radially outer side of the tire, in a direction tilting axially inward, and then curves axially outward to a second point P2. The second point P2 is located axially outward of the first point P1. The first curved portion 62 is composed of an arc-shaped portion with a curvature radius R1 centered at a curvature center O1 located closer to the tire outer surface than the outer surface of the bead portion 30.

The height H1 of the center of curvature O1 in the tire radial direction, based on the nominal rim diameter NR, is equal to or greater than the height H11 of the flange radial portion 53a of the corresponding regular rim 50. Preferably, the height H1 of the center of curvature O1 is equal to or less than 1.5 times the height H11 of the flange radial portion 53a. In this embodiment, the height H11 of the flange radial portion 53a is 8 mm, so the height H1 of the center of curvature O1 is set to 8 mm or greater and 12 mm or less. Preferably, the height H1 of the center of curvature O1 is equal to or greater than 0.2 and equal to or less than 0.6 times the height H3 of the apex 4a of the rim protector 4.

The center of curvature O1 is located in a radial range W between a radial position V1 that is 2 mm radially inward in the tire direction and a radial position V2 that is 9 mm radially outward in the tire direction, based on the bead core radial outer end surface 31a.

Furthermore, the height H1 of the center of curvature O1 is set to less than 0.25 times the tire cross-sectional height H0 (see Figure 1). The tire cross-sectional height H0 is calculated by subtracting the nominal rim diameter from the outer diameter of the pneumatic tire 1 and dividing the result by 2.

The radius of curvature R1 is larger than the radius of curvature R12 of the corresponding flange curved portion 53b of the regular rim 50. Preferably, the radius of curvature R1 is 1.4 times or more the radius of curvature R12, and more preferably 1.6 to 2.4 times the radius of curvature R12. In this embodiment, since the radius of curvature R12 of the flange curved portion 53b is 9.5 mm, the radius of curvature R1 is set to 14 mm or more, and more preferably 16 mm to 22 mm.

The second curved portion 63 extends from a third point P3 located at the apex 4a of the rim protector 4, curving radially inward toward the tire axial direction and reaching a fourth point P4. The second curved portion 63 is composed of an arc-shaped portion having a radius of curvature R2 centered at a center of curvature O2 located closer to the tire outer surface than the outer surface of the bead portion 30. The radius of curvature R2 is set to be equal to or greater than the radius of curvature R1 of the first curved portion 62. Preferably, the radius of curvature R2 is equal to or greater than 1.2 times the radius of curvature R1.

As shown in the enlarged view of Figure 2, a virtual curve 62a extending the first curved portion 62 radially outward from the tire and a virtual curve 63a extending the second curved portion 63 axially inward from the tire intersect at a virtual intersection point P5 so as to be convex toward the tire's inner surface.

The height H5 of the virtual intersection point P5 in the tire radial direction, based on the nominal rim diameter NR, is greater than 1.5 and less than 3.0 times the height H1 of the center of curvature O1 of the first curved portion 62. More preferably, the height H5 of the virtual intersection point P5 is greater than 2 and less than 2.5 times the height H1 of the center of curvature O1. Furthermore, the height H5 of the virtual intersection point P5 is greater than 0.7 times the height H3 of the apex 4a of the rim protector 4. The height H5 of the virtual intersection point P5 is, for example, 15 mm or greater and 25 mm or less, and preferably 20 mm or greater and 24 mm or less.

Furthermore, the height H5 of the virtual intersection point P5 is smaller than the height H9 of the tip 32a of the bead filler 32, which is based on the nominal rim diameter NR. Specifically, the height H9 of the tip 32a of the bead filler 32 is preferably at least 1.1 times, and more preferably at least 1.3 times, the height H5 of the virtual intersection point P5.

The intersection angle A3 between the imaginary curves 62a and 63a, i.e., the angle between the tangent line 62b to the first curved portion 62 (imaginary curve 62a) extending from the imaginary intersection point P5 and the tangent line 63b to the second curved portion 63 (imaginary curve 63a) extending from the imaginary intersection point P5, is greater than 0° and less than or equal to 45°. If the intersection angle A3 exceeds 45°, strain is likely to concentrate between the first curved portion 62 and the second curved portion 63, which may result in a deterioration in bead durability. Preferably, the intersection angle A3 is less than or equal to 30°. The intersection angle A3 is defined as the angle between the tangent lines 62b and 63b extending axially outward from the imaginary intersection point P5.

The bead back surface 35 also has a third curved portion 64 that connects the second point P2 and the fourth point P4 in an arc shape.

The straight portion 61 is tangentially continuous with the first curved portion 62. In other words, the first point P1 constitutes the point of contact between the first curved portion 62 and the straight portion 61. The straight portion 61 extends parallel to a second straight line L2 that is inclined axially inward toward the tire radially outward, and has an inclination angle A2 with respect to the line parallel to the tire radial direction. The inclination angle A2 is set to be smaller than the inclination angle A1 of the bead base surface 34. In this embodiment, the inclination angle A2 is 10° or less. Furthermore, the inclination angle A2 is set so that the angle A4 between the straight portion 61 and the bead base surface 34 is 95° or greater and 105° or less. The inclination angle A2 is set to be smaller than the value obtained by subtracting the inclination angle A0 of the rim seat 51 from the inclination angle A1 of the bead base surface 34 (A2 < A1 - A0).

The third curved portion 64 tangentially connects the first curved portion 62 and the second curved portion 63, and is formed by an arc-shaped portion with a curvature radius of R3 centered at a center of curvature O3 located on the tire outer surface side of the bead portion 30. In other words, the second point P2 forms the point of contact between the first curved portion 62 and the third curved portion 64, and the fourth point P4 forms the point of contact between the second curved portion 63 and the third curved portion 64.

The radius of curvature R3 of the third curved portion 64 is smaller than the radii of curvature R1 and R2 of the first curved portion 62 and the second curved portion 63.

Figure 3 is an enlarged view of the bead heel surface 36 and its surroundings in Figure 2. As shown in Figure 3, the bead heel surface 36 is located radially inward and axially outward relative to the bead core 31. The bead heel surface 36 extends axially outward along a third straight line L3 that is inclined radially outward from the first straight line L1 between the axially outer end of the bead base surface 34 and the radially inner end of the bead back surface 35.

Specifically, when the intersection of the first line L1 and the second line L2 is defined as the first intersection X1, the intersection of the first line L1 and the third line L3 is defined as the second intersection X2, and the intersection of the second line L2 and the third line L3 is defined as X3, the radial dimension K of the bead heel surface 36, which is the dimension in the tire radial direction between the first intersection X1 and the third intersection X3, is equal to or greater than the axial dimension G, which is the dimension in the tire axial direction between the first intersection X1 and the second intersection X2, but is less than twice the axial dimension G.

The bead heel surface 36 is located axially inward of a fourth straight line L4 that extends radially from the bead core axially outer end surface 31b, which is the axially outer end surface of the bead core 31, a predetermined distance J inward. In other words, the value obtained by adding the predetermined length J to the axial dimension Q from the third intersection point X3 to the bead core axially outer end surface 31b is longer than the axial dimension T between the third intersection point X3 and the second intersection point X2. For example, the predetermined length J is 2 mm. The predetermined length J may also be 30% of the axial dimension M of the bead core 31.

The second intersection point X2 and the third intersection point X3 are geometrically set as the endpoints of an imaginary circular arc 38 that tangentially connects the first line L1 and the second line L2 with a radius of curvature Ra greater than 3 mm and less than 8 mm in the meridian cross section. In other words, the third line L3 is defined as a straight line that linearly connects the endpoints X2 and X3 of the imaginary circular arc 38 that tangentially connects the first line L1 and the second line L2 with a radius of curvature Ra greater than 3 mm and less than 8 mm.

The bead heel surface 36 has a bead heel straight portion 36a that extends linearly along the third straight line L3, a bead heel first R-chamfered portion 36b that tangentially connects the bead heel straight portion 36a to the bead base surface 34, and a bead heel second R-chamfered portion 36c that tangentially connects the bead heel straight portion 36a to the bead back surface 35. The radii of curvature of the bead heel first R-chamfered portion 36b and the bead heel second R-chamfered portion 36c are 2 mm or greater and equal to or less than the radius of curvature Ra of the imaginary arc 38.

The bead heel surface 36 is at least partially formed by the rim strip rubber 21b. Figure 4 is a front view of the bead heel surface 36 as viewed from the arrow A in Figure 3. As shown in Figure 4, the bead heel surface 36 is formed by the rim strip rubber 21b, and has a vent hole mark 36z formed thereon, which is a trace of a gas release vent provided in a tire mold (not shown) that vulcanizes and molds the pneumatic tire 1. The vent hole mark 36z may be a vent spew formed by a vent hole, or a burr or unevenness formed by a spring vent.

Returning to Figure 2, in the unrim-mounted state, a recess 70 is formed on the radially inner side of the bead portion 30, spanning the bead heel surface 36 and the bead back surface 35, and recessed axially inward. The recess 70 refers to the portion of the bead heel surface 36 and the bead back surface 35 that is located axially inward with respect to the fifth straight line L5, which is a tangent to the bead heel second R chamfer portion 36c and extends parallel to the tire radial direction. In other words, the recess 70 is composed of the portion of the bead heel second R chamfer portion 36c that is located radially outward, the straight portion 61, and the portion of the first curved portion 62 that is located radially inward.

The recess 70 has a deepest portion 71 that is deepest on the axially inner side of the tire. The deepest portion 71 is located on the first curved portion 62. The deepest portion 71 is located radially outward from the straight portion 61. In other words, the straight portion 61 is located radially inward from the deepest portion 71. The depth D of the deepest portion 71 is set to less than 1.0 mm, preferably 0.8 mm or less, and more preferably 0.3 mm or more and 0.5 mm or less, based on the fifth straight line L5.

The height H10 of the deepest part 71, based on the nominal rim diameter NR, is equal to or greater than the height H11 of the flange radial portion 53a of the corresponding regular rim 50. Preferably, the height H10 of the deepest part 71 is equal to or less than 1.5 times the height H11 of the flange radial portion 53a. In this embodiment, the height H11 of the flange radial portion 53a is 8 mm, so the height H11 of the deepest part 71 is set to be equal to or greater than 8 mm and equal to or less than 12 mm.

In this embodiment, the deepest part 71 is located on the sixth straight line L6 that extends parallel to the tire axial direction from the center of curvature O1 of the arc-shaped first curved part 62, so the height H10 of the deepest part 71 is equal to the height H1 of the center of curvature O1 of the first curved part 62.

Here, in the pneumatic tire 1, when not mounted on a rim, the pair of bead portions 30 are spaced apart at a distance wider than the rim width W0. Specifically, the outer width W1 (i.e., the distance between the fifth straight lines) in the tire axial direction between the pair of bead heel surfaces 36 (specifically, the bead heel second R chamfered portions 36c) is greater than the rim width W0. For example, the difference between the outer width W1 and the rim width W0 is 1.5 inches or less, and preferably 1 inch or less.

Figure 5 shows the area around the bead portion 30 before the pneumatic tire 1 is mounted on a corresponding regular rim 50 and inflated to the specified internal pressure. The imaginary line shows the pneumatic tire 1 before mounting on the rim, and the dashed line shows the pneumatic tire 1 when a load is applied after inflation. The pneumatic tire 1 has an outer width W1 between a pair of bead portions 30 that is wider than the rim width W0 of the corresponding regular rim 50. Therefore, when mounting the pneumatic tire 1 on a rim, the pair of bead portions 30 must be brought closer to each other axially inward. At this time, the pneumatic tire 1 is deformed so that it tilts axially inward toward the tire's radial inner side across the sidewall 20 and the bead portion 30 (arrow Y1 in the figure).

Furthermore, because the inclination angle A1 of the bead base surface 34 is greater than the inclination angle A0 of the rim seat 51, when the bead base surface 34 is fitted radially to the rim seat 51, the bead base surface 34 rotates clockwise in Figure 3 (arrow Y2 in the figure) by an angle calculated by subtracting the amount by which the bead base surface 34 is compressed from the difference between the inclination angles A0 and A1. The bead base surface 34 rotates so that the inclination angle A1 becomes smaller, and is fitted radially to the rim seat 51.

Here, the bead portion 30 is formed with a linear bead heel surface 36, so the bead heel surface 36 does not ride up onto or interfere with the rim heel 52 of the genuine rim 50, and can be moved from the axially inner side to the axially outer side relative to the rim seat 51 and rim heel 52 to assemble it in the correct axial position.

As a result, in the bead portion 30, the portion located radially inward of the deepest portion 71 of the recess 70 is tilted clockwise in FIG. 3 compared to the non-rim-mounted state (arrow Y3 in the figure). As the bead base surface 34 rotates, the bead back surface 35 rotates axially inward from the periphery of the deepest portion 71 so that the inclination angle A2 becomes zero, i.e., so that the straight portion 61 extends along the tire radial direction. In this embodiment, the inclination angle A1 is 7° greater than the inclination angle A0, and therefore the portion located radially inward of the deepest portion 71 rotates by an angle of 7° or less.

As a result, when the pneumatic tire 1 is mounted on a rim, the recess 70 disappears and the portion of the bead back surface 35 located radially inward of the deepest part 71 deforms so as to extend generally along the tire radial direction. Therefore, the bead base surface 34 and the portion of the bead back surface 35 located radially inward of the deepest part 71 of the bead portion 30 are in close contact with the rim seat 51 and the radial portion 53a of the rim flange 53 over substantially their entire surfaces.

On the other hand, when the tire is mounted on the rim, the bead heel surface 36 is spaced apart from the rim heel 52. Next, when the pneumatic tire 1 is inflated by filling it with the specified internal pressure while it is mounted on the rim, the bead heel surface 36 of the bead portion 30 deforms and comes into substantial contact with the rim heel 52.

As the bead core 31 moves from the non-rim-assembled state to the inflated state, it rotates clockwise in Figure 5 (arrow Y2 in the figure), just like the bead base surface 34. The rotation angle A6 of the bead core 31 is approximately equal to the inclination angle A2 with respect to a line parallel to the tire radial direction. In this embodiment, the rotation angle A6 of the bead core 31 is defined by the angle between an extension line L8 of the bead core's radially outer end surface 31a in the non-rim-assembled state and an extension line L9 of the bead core's radially outer end surface 31a in the inflated state.

In this rim-assembled state, the bead back surface 35 is formed so as to be sufficiently spaced apart in the tire radial direction from the flange curved portion 53b. Specifically, the bead back surface 35 is formed so that the distance in the tire radial direction between the apex P10 of the flange curved portion 53b located at the outermost position in the tire radial direction and the intersection point P11 between the bead back surface 35 and a radial line L7 passing through apex P10 and extending in the tire radial direction is 4 mm or more.

As shown by the dashed line in Figure 5, the bead back surface 35 is formed so that it is sufficiently spaced apart in the tire radial direction from the flange curved portion 53b even when a load corresponding to the load index set for the pneumatic tire 1 is applied. Specifically, the bead back surface 35 is formed so that the distance in the tire radial direction between the apex P10 and the intersection P12 between the radial line L7 and the bead back surface 35 in the load-applied state is 3 mm or more.

Figure 6 is a graph showing the fitting pressure at the fitting portion between the bead portion 30 and the regular rim 50 when the pneumatic tire 1 is mounted on the regular rim 50 and inflated. The fitting pressure was measured using a sheet-type pressure sensor sandwiched between the bead portion 30 and the regular rim 50. In this graph, the horizontal axis represents each position along the outer surface of the bead portion 30, from the axially inner end of the bead base surface 34 to the bead back surface 35, and the vertical axis represents the fitting pressure.

Figure 6 also shows the fitting pressure of the bead portions of pneumatic tires according to Comparative Examples 1 and 2. As shown in Figure 7, pneumatic tire 100 according to Comparative Example 1 differs from pneumatic tire 1 in that the bead heel surface 136 is formed on an arc that generally matches the rim heel 52 and does not have a recess. As shown in Figure 8, pneumatic tire 200 according to Comparative Example 2 differs from pneumatic tire 1 in that the bead heel surface 236 is formed on an arc that generally matches the rim heel 52, and differs from pneumatic tire 100 according to Comparative Example 1 in that it has a recess 270. In Figure 6, the fitting pressure for pneumatic tire 1 is shown by a thick solid line, the fitting pressure for pneumatic tire 100 is shown by a dashed line, and the fitting pressure for pneumatic tire 200 is shown by a thin solid line.

As shown in Figures 6 and 7, in the pneumatic tire of Comparative Example 1, fitting pressure is generated locally at two points: the bead base surface 134 and the contact portion 135a of the bead back surface 135, which is located on the radially outer side of the tire. The peak at the bead base surface 134 is approximately fitting pressure A, while the peak at the contact portion 135a is lower than fitting pressure A and slightly exceeds fitting pressure B, which is lower than fitting pressure A. In other words, no fitting pressure is generated at the non-contact portion 135b, which is located between the bead base surface 134 and the bead back surface 135 and does not contact the regular rim 50.

Therefore, the pneumatic tire 100 is tightly fitted locally at two locations: the bead base surface 134 and the abutment portion 135a, and the bead portion 130 is strongly compressed at these two locations.

In the pneumatic tire 200 according to Comparative Example 2, fitting pressure is generated between the tire and the regular rim 50 from the bead base surface 234 to the bead heel surface 236 and the bead back surface 235. In other words, unlike the pneumatic tire 100, the bead heel surface 236 and the inner diameter portion 235b of the bead back surface 235 also abut against the regular rim 50.

Specifically, the fitting pressure of the pneumatic tire 200 is lower than that of the pneumatic tire 100 in the areas corresponding to the locally tightly fitted portions of the pneumatic tire 100. That is, the peak at the bead base surface 234 is approximately fitting pressure B, and the peak at the bead back surface 235 is slightly lower than fitting pressure C, which is lower than fitting pressure B. On the other hand, the pneumatic tire 200 generates fitting pressure in the areas corresponding to the unfitted portions of the pneumatic tire 100. That is, the pneumatic tire 200 adheres to the standard rim 50 with fitting pressure from the bead base surface 234 to the bead back surface 235, and local compression is suppressed.

On the other hand, in the pneumatic tire 1 according to this embodiment, fitting pressure is generated between the bead base surface 34, the bead back surface 35, and the bead heel surface 36 in the areas on the bead base surface 34 side and the bead back surface 35 side and between the bead base surface 34 and the bead back surface 35 side and the bead heel surface 36 and the regular rim 50.

Specifically, the peak fitting pressure at the bead base surface 34 of the pneumatic tire 1 is higher than that of either of the pneumatic tires 100 or 200, while the peak fitting pressure at the bead back surface 35 is lower than that of either of the pneumatic tires 100 or 200. This is thought to be because, unlike the pneumatic tires 100 and 200, the bead heel surface 36 of the pneumatic tire 1 is formed with a linear chamfer, making it easier to fit the tire at the correct axial position without it getting caught in the rim heel 52 when assembled to the rim.

For example, as shown by the dashed line in Figure 7, if the radius of curvature of the bead heel surface 136 in the pneumatic tire 100 is excessively smaller than the radius of curvature of the rim heel 52, and the bead heel surface 136 is axially inwardly mated with the rim heel 52 relative to the appropriate axial position, the gap between the bead back surface 135 and the rim flange 53 will cause the bead portion 130 to easily collapse axially outward during inflation, causing the bead back surface 135 to locally come into contact with the rim flange 53.

In other words, because the bead heel surfaces 136, 236 of the pneumatic tires 100, 200 are not linearly chamfered, depending on the magnitude of the curvature of the bead heel surfaces 136, 236, they are more likely to bend against the rim heel 52 than the pneumatic tire 1, and the bead portions 130, 230 are more likely to collapse axially outward in the tire. As a result, in the pneumatic tires 100, 200, the mating pressure between the bead base surfaces 134, 234 and the rim seat 51 is reduced, while the mating pressure between the bead back surfaces 135, 235 and the rim flange 53 is increased.

The pneumatic tire 1 according to this embodiment provides the following advantages:

(1) Because the bead base surface 34 is configured with a single inclined surface (single-step taper), the bead portion 30 is less likely to collapse axially outward when assembled to the rim, compared to when the bead base surface 34 has an additional inclined surface that increases in the radially inward direction toward the axially inward direction of the tire (two-step taper). As a result, when assembled to the rim, the mating pressure at the bead back surface 35 is reduced, while the mating pressure at the bead base surface 34 is increased, compared to when the bead base surface 34 is two-step tapered.

Furthermore, because the bead heel surface 36 is configured as a chamfer that connects the bead base surface 34 and the bead back surface 35 in a straight line, interference between the bead heel surface 36 and the rim heel 52 of the genuine rim 50 is reduced during rim assembly. As a result, the bead portion 30 can be fitted to the genuine rim 50 at the correct axial position, preventing the bead portion 30 from collapsing axially outward due to an insufficient fitting position that is too far to the axially inner side of the tire, and also reducing an increase in fitting pressure at the bead back surface 35.

As a result, under load, the bead base surface 34 with increased fitting pressure makes it easier to maintain fitting with the rim seat 51 of the standard rim 50, improving rim fitting, while the bead back surface 35 with reduced fitting pressure makes it easier for the sidewall 20 to flex and deform, improving handling stability.

(2) The inclination angle A1 of the first straight line L1 relative to the tire axial direction is 5° or greater and 15° or less. In other words, the bead base surface 34 is moderately inclined, which increases the mating pressure at the bead base surface 34 while suppressing an increase in mating pressure at the bead back surface 35. If the inclination angle A1 of the first straight line L1 is less than 5°, the sidewall 20 will deform excessively due to insufficient mating pressure at the bead back surface 35, which is likely to lead to a decrease in handling stability. On the other hand, if the inclination angle A1 of the first straight line L1 is greater than 15°, the excessive mating pressure at the bead back surface 35 will hinder the flexural deformation of the sidewall 20, which is likely to lead to a decrease in the load support performance of the sidewall 20.

(3) The radial dimension K of the bead heel surface 36 is equal to or greater than the axial dimension G but less than twice the axial dimension G. As a result, the bead heel surface 36 is configured to be long in the tire radial direction, making it easier to ensure the bead base surface 34 and to prevent the bead portion 30 from tipping axially outward. If the radial dimension K of the bead heel surface 36 is less than the axial dimension G, it is likely that interference of the bead heel surface 36 with the standard rim 50 during rim assembly will be insufficient. On the other hand, if the radial dimension K of the bead heel surface 36 is equal to or greater than twice the axial dimension G, the bead back surface 35 will be excessively small, causing fitting pressure to be locally concentrated on the bead back surface 35, hindering flexural deformation of the sidewall 20.

(4) The bead heel surface 36 is located axially outward of the fourth straight line L4. As a result, the amount of axial inward intrusion of the bead heel surface 36 into the radially inner region of the bead core 31 is prevented from becoming excessive. This makes it easier to ensure the bead base surface 34 on the radially inner side of the bead portion 30, and prevents the bead portion 30 from tipping axially outward.

(5) Because the bead back surface 35 has a recess 70, in the non-rim-mounted state, the inner diameter side portion of the bead back surface 35, located radially inward from the deepest point 71 of the recess 70, is inclined axially outward toward the inner diameter side of the tire. In the rim-mounted state where the pair of bead portions 30 are brought close to the rim width W0 of the standard rim 50, this inner diameter side portion tends to bend axially inward from the periphery of the deepest point 71 of the recess 70 and generally follow the tire radial direction. In other words, in the rim-mounted state, the bead back surface 35 is easily brought into close contact with substantially the entire radial portion 53a of the rim flange 53 while eliminating the recess 70, thereby increasing the contact area.

In this way, when the tire is mounted to the rim and no load is applied, surface pressure acts over almost the entire surface of the bead back surface 35, which is in close contact with the radial portion 53a of the rim flange 53. This distributes the load across the entire bead back surface 35, compared to when surface pressure acts only on a portion of the bead back surface 35. In other words, compared to when high surface pressure acts locally on the bead back surface 35, there is more room for compression to allow the bead back surface 35 to elastically deform.

Therefore, when a load or lateral force is applied, the bead back surface 35 can be further compressed by the amount of the available compression. In this case, the sidewall 20 can also deform in the area adjacent to the bead portion 30 due to further compression of the bead back surface 35, so the sidewall 20 can be deformed by bending overall from the bead portion 30 side to the tread 10 side. This improves the efficiency of load support in the sidewall 20, thereby improving handling stability.

(6) If the depth D of the recess 70 is less than 1.0 mm, the portion located radially inward of the deepest part 71 is moderately inclined axially outward in the unrim-mounted state, and therefore, when bent axially inward in the rim-mounted state, the inclination to the axially outward side of the tire is easily eliminated, and the recess 70 is likely to fit exactly along the radial part 53a of the rim flange 53. If the depth of the recess 70 is 1.0 mm or more, the portion located radially inward of the deepest part 71 is likely to be excessively inclined axially outward in the unrim-mounted state, and therefore, even when bent axially inward in the rim-mounted state, the inclination to the axially outward side of the tire is difficult to eliminate. In this case, the recess 70 is unlikely to disappear in the rim-mounted state, and it is difficult to abut the bead back surface 35 over substantially the entire radial part 53a of the rim flange 53. Furthermore, when the depth D of the recess 70 is 0.8 mm or less, the portion located radially inward of the deepest part 71 is moderately inclined axially outward in the unrim-mounted state, and is more likely to fit closely to the radial part 53a of the rim flange 53 when mounted on the rim. Furthermore, when the depth D of the recess 70 is 0.3 mm or more and 0.5 mm or less, the portion located radially inward of the deepest part 71 is even more moderately inclined axially outward in the unrim-mounted state, and is more likely to fit closely to the radial part 53a of the rim flange 53 when mounted on the rim.

(7) The third straight line L3 is geometrically defined as a straight line connecting the end points of an imaginary arc 38 that tangently connects the first straight line L1 and the second straight line L2 with a curvature radius Ra greater than 3 mm and less than 8 mm. As a result, the bead heel surface 36 is formed with an appropriate size, and when the pneumatic tire 1 is mounted on a standard rim 50 and inflated to the specified internal pressure, the bead heel surface 36 can easily come into contact with the rim heel 52, ensuring rim fitting.

If the curvature radius Ra of the imaginary arc 38 is 3 mm or less, the bead heel surface 36 will interfere with the rim heel 52 when assembled to the rim, making it difficult to fit the tire to the appropriate axial position. On the other hand, if the curvature radius Ra of the imaginary arc 38 is 8 mm or more, it will be difficult to bring the bead heel surface 36 into contact with the rim heel 52, even after inflation when assembled to the rim, which can lead to poor rim fitting.

(8) At least a portion of the bead heel surface 36 is formed from the rim strip rubber 21b and has vent hole marks 36z. As a result, the rubber flow around the bead heel surface 36 during vulcanization is improved, so while a linear bead heel surface 36 is formed on the outer surface of the bead portion 30, problems such as bare spots caused by poor rubber flow during vulcanization are suppressed.

(9) Because the deepest portion 71 is located at a radial position equal to or higher than the outer diameter end of the radial portion 53a of the rim flange 53, when assembled to the rim, the portion of the recess 70 located inward of the deepest portion 71 can easily abut against substantially the entire surface of the radial portion 53a of the rim flange 53, starting from the radially inner side. If the deepest portion 71 of the recess 70 is located inward of the outer diameter end of the radial portion 53a of the rim flange 53 of a genuine rim 50, the deepest portion 71 will face the inner side of the radial portion 53a of the rim flange 53 relative to the outer diameter end. As a result, the portion of the recess 70 located outward of the deepest portion 71 extends axially outward toward the radially outer side of the tire, making it easy for it to locally abut strongly against the radial portion 53a of the rim flange 53.

(10) The deepest part 71 is located radially in the tire at a distance less than 1.5 times the height H11 to the outer diameter end of the radial portion 53a of the rim flange 53, based on the nominal rim diameter NR. This makes it easier for the inner diameter portion of the recess 70 to bend axially inward from the periphery of the deepest part 71 when assembled to the rim. In other words, if the deepest part 71 of the recess 70 is located radially outward more than 1.5 times the length to the outer diameter end of the radial portion 53a, based on the nominal rim diameter NR, the distance from the bead base surface 34 to the deepest part 71 of the recess 70 is likely to be excessive, and the deformation of the bead portion 30 when assembled to the rim is likely to start on the inner diameter side of the deepest part 71 of the recess 70. In this case, it is difficult to bend the entire inner diameter portion of the recess 70 from the deepest part 71, making it difficult for the bead back surface 35 to abut substantially the entire radial portion 53a of the rim flange 53.

(11) The first curved portion 62 constitutes the deepest part 71 of the recess 70, and its radius of curvature R2 is larger than the radius of curvature R12 of the opposing curved portion 53b of the rim flange 53. As a result, when assembled to the rim, the curved portion 53b of the rim flange 53 is easily enclosed within the recess 70, and the recess 70 is less likely to be obscured by the curved portion 53b, making it easier for the portion located on the inner side of the deepest part 71 to abut against substantially the entire surface of the radial portion 53a of the rim flange 53, starting from the radially inner side. If the radius of curvature R2 of the first curved portion 62 were smaller than the radius of curvature of the curved portion of the rim flange, when assembled to the rim, the curved portion 53b would be more likely to be obscured by the inside of the recess 70, and it would be more difficult for the portion located on the inner side of the deepest part 71 of the recess 70 to abut against substantially the entire surface of the radial portion 53a of the rim flange 53, starting from the radially inner side.

(12) In the pneumatic tire 1, the height H1 of the center of curvature O1 of the first curved portion 62 is located in the tire radial direction within a range of 8 mm to 12 mm, and the radius of curvature R1 is 12 mm or greater. Therefore, the effects of the above invention are effectively achieved when the tire is mounted on a regular rim 50 in which the height H11 of the outer diameter end of the radial portion 53a of the rim flange 53 is 8 mm or less and the radius of curvature R12 of the curved portion 53b of the rim flange 53 is 12 mm or less, such as a regular rim with flange symbol J for a 5° deep rim as specified by JATMA.

Note that the present invention is not limited to the configuration described in the above embodiment, and various modifications are possible.

In the above embodiment, the recess 70 is formed by a portion of the bead heel second R chamfered portion 36c, the straight portion 61, and at least a portion of the first curved portion 62, but this is not limited to this. It may also be configured to directly connect the bead heel second R chamfered portion 36c and the first curved portion 62 without using the straight portion 61. In this case, the bead heel second R chamfered portion 36c and the first curved portion 62 may be connected tangent to each other.

In addition, while in this embodiment the recess 70 is configured as an arc-shaped portion, this is not limited to this. In other words, the recess 70 may be formed in a trapezoidal or triangular shape, and various other configurations can be adopted. Furthermore, by configuring the recess 70 as a tangent-continuous arc-shaped portion, as in this embodiment, it becomes easier to smoothly deform the bead back surface 35 so that it extends parallel to the tire radial direction when assembled to the rim, resulting in excellent rim fitting.

(Reference example)
Evaluation tests were conducted on the average fitting pressure at the back surface of the bead between the pneumatic tire and the regular rim, bead durability, and steering stability for the pneumatic tires of Reference Comparative Examples 1 and 2 and Reference Examples 1 to 4. In Reference Comparative Examples 1 and 2 and Reference Examples 1 to 4, the bead heel surface is not formed as a linear chamfer, but is formed as a curved portion along the rim heel 52.

In Reference Comparative Example 1, no recesses are provided. In Reference Comparative Example 2, the depth D of the recess 70 is 2 mm, which is 1 mm or more and exceeds the upper limit of the present invention, which is less than 1.0 mm. In Reference Examples 1 to 4, the depth D of the recess 70 is within the above numerical range. In Reference Example 3, the depth D of the recess 70 is 0.9 mm, which is near the upper limit of the above numerical range. In Reference Example 4, the depth D of the recess 70 is 0.8 mm, which is the upper limit of the above numerical range, which is a more preferable range of 0.8 mm or less. In Reference Examples 1 and 2, the depth D of the recess 70 is 0.4 mm, which is the median of the above numerical range, which is 0.3 mm or more and 0.5 mm or less, which is an even more preferable range. The height H10 of the deepest part 71 of the recess 70 is 10 mm in Reference Comparative Example 2 and Reference Examples 1 and 2. The radius of curvature R1 of the first curved portion 62 is 18 mm for Reference Comparative Example 1, 12 mm for Reference Comparative Example 2, 18 mm for Reference Examples 1 and 4, and 22 mm for Reference Examples 2 and 3. The regular rim used for the evaluation conforms to the flange symbol J for a 5° deep rim specified by JATMA.

The average mating pressure at the bead back surface 35 was measured by measuring the average mating pressure in the contact area of the bead back surface 35 against the radial portion 53a of the rim flange 53 during inflation, and the average mating pressure in Reference Comparative Example 2 and Reference Examples 1-4 is expressed as an index, with Reference Comparative Example 1 set to 100. A larger value indicates a stronger mating over a narrower contact area, and a lower value indicates that the mating pressure is distributed over a wider contact area.

To evaluate bead durability, a drum durability test was conducted to induce bead failure, and the distance traveled until failure was measured. The distance traveled for Reference Comparative Example 2 and Reference Examples 1-4 was expressed as an index, with Reference Comparative Example 1 set at 100. The higher the value, the better the bead durability.

The handling stability was evaluated by a sensory relative assessment by the driver when the tires were fitted to an actual vehicle and driven on the road. The maximum score was 10, with 6.0 being the median value, and higher scores indicating better handling stability.

As is clear from Table 1, the pneumatic tires of Reference Examples 1 to 4, in which the depth D of the deepest part 71 of the recess 70 is less than 1.0 mm, have a lower average fitting pressure at the bead back surface 35 and the fitting pressure is distributed over a wider contact area compared to Reference Comparative Examples 1 and 2. This is thought to be because in Reference Examples 1 to 4, the depth D of the deepest part 71 of the recess 70 is appropriate, so the recess 70 disappears when assembled to the rim, making it easier for the bead back surface 35 to conform to the radial portion 53b of the rim flange 53.

In Reference Comparative Example 2, the depth D of the deepest part 71 of the recess 70 is 1.0 mm or more, so even when assembled to the rim and inflated, the recess 70 does not disappear and the bead back surface 35 comes into localized contact, resulting in a localized contact area and therefore a higher average fitting pressure than in Reference Examples 1 and 2. Furthermore, in Reference Comparative Example 2, the depth D of the deepest part 71 is excessively large at 2.0 mm, resulting in poor adhesion to the rim over the entire area and insufficient effectiveness, resulting in bead durability and handling stability similar to those of Reference Comparative Example 1.

With regard to Reference Examples 1 to 4, the average engagement pressure at the bead back surface 35 decreases in the order of Reference Example 3, which is near the upper limit of the above numerical range (less than 1.0 mm), Reference Example 4, which is at the upper limit of a more preferable range (0.8 mm or less), and Reference Examples 1 and 2, which are at the median of an even more preferable range (0.3 mm to 0.5 mm). This results in improved bead durability and handling stability in this order. Specifically, in Reference Example 1, the depth D of the deepest part of the recess 70 is an appropriate 0.4 mm. Therefore, when assembled to the rim and inflated, the recess 70 disappears, and the bead portion 30, from the bead base surface 34 to the bead back surface 35, abuts almost the entire radial portion 53a of the rim flange 53. This results in a lower average engagement pressure at the bead back surface 35 than in Reference Comparative Example 1, resulting in better bead durability and handling stability than in Reference Comparative Example 1.

Like Reference Example 1, Reference Example 2 also has a moderate depth D of 0.4 mm at the deepest point of the recess 70. Therefore, when the tire is assembled to the rim and inflated, the recess 70 disappears, and the bead portion 30 is in almost complete contact with the radial portion 53a of the rim flange 53 from the bead base surface 34 to the bead back surface 35. Note that Reference Example 2 exhibits smaller improvements in both bead durability and handling stability than Reference Example 1. This is presumably due to the fact that Reference Example 2 has a larger radius of curvature R1 of the first curved portion 62 than Reference Example 1, which tends to position the virtual intersection point P5 axially inward. This results in a larger intersection angle A3 at the virtual intersection point P5, which increases the concentration of strain near the virtual intersection point P5. Reference Examples 3 and 4 have a higher average fitting pressure at the bead back surface 35 than Reference Examples 1 and 2, resulting in inferior bead durability and handling stability compared to Reference Examples 1 and 2. In Reference Example 4, the depth D of the recess 70 is in a more preferable range than in Reference Example 3, and therefore the average fitting pressure at the bead back surface 35 is lower than in Reference Example 3, resulting in improved bead durability and handling stability.

DESCRIPTION OF SYMBOLS 1 Pneumatic tire 4 Rim protector 10 Tread 20 Sidewall 30 Bead portion 31 Bead core 31a Bead core radial outer end surface 31b Bead core axial outer end surface 32 Bead filler 32a Bead filler tip 34 Bead base surface 35 Bead back surface 36 Bead heel surface 36a Bead heel straight portion 36b Bead heel first R chamfered portion 36c Bead heel second R chamfered portion 36z Vent hole mark 38 Virtual arc 50 Regular rim 51 Rim seat 52 Rim heel 53 Rim flange 53a Radial portion 53b Curved portion 61 Straight portion 62 First curved portion 63 Second curved portion 64 Third curved portion 70 Recess 71 Deepest portion L1 First straight line L2 Second line L3 Third line L4 Fourth line L5 Fifth line X1 First intersection X2 Second intersection X3 Third intersection K Radial dimension of rim heel surface G Axial dimension of rim heel surface J Specified length M Axial dimension of bead core

Claims (8)

The tire has a tread, a sidewall extending radially inward from an axial end of the tread, and a bead portion continuing to the radially inward end of the sidewall,
In a meridian cross section, the bead portion has an outer surface, a bead base surface extending along a first straight line inclined outward in the tire radial direction at an inner end of the bead portion in the tire radial direction, and
a bead back surface extending radially outward from an axially outer end of the bead portion and having an inner end in the tire radial direction extending along a second straight line along the tire radial direction;
a bead heel surface extending along a third straight line that is inclined further outward in the tire radial direction than the first straight line toward the tire axially outward between an outer end of the bead base surface in the tire axial direction and an inner end of the bead back surface in the tire radial direction ,
the third straight line is geometrically defined as a straight line connecting both end points of an imaginary arc that tangently connects the first straight line and the second straight line to each other with a curvature radius of more than 3 mm and less than 8 mm in a meridian cross section . an inclination angle of the first straight line with respect to the tire axial direction is equal to or greater than 5° and equal to or less than 15°;
The pneumatic tire according to claim 1 . In the meridian section,
an intersection point between the first straight line and the second straight line is defined as a first intersection point;
an intersection of the first straight line and the third straight line is a second intersection;
When the intersection of the second line and the third line is defined as a third intersection,
a radial dimension of the bead heel surface, which is a dimension in the tire radial direction between the first intersection point and the third intersection point, that is equal to or greater than an axial dimension, which is a dimension in the tire axial direction between the first intersection point and the second intersection point, and that is less than twice the axial dimension;
The pneumatic tire according to claim 1 or 2. A bead core is embedded in the bead portion,
In the meridian cross section, the bead heel surface is located axially outward of a fourth straight line extending in the tire radial direction at a position axially inwardly spaced a predetermined length from an axially outer end of the bead core,
The predetermined length is 2 mm.
The pneumatic tire according to any one of claims 1 to 3. A bead core is embedded in the bead portion,
In the meridian cross section, the bead heel surface is located axially outward of a fourth straight line extending in the tire radial direction at a position axially inwardly spaced a predetermined length from an axially outer end of the bead core,
The predetermined length is 30% of the axial dimension of the bead core.
The pneumatic tire according to any one of claims 1 to 3. The bead portion is provided in a pair,
the pair of bead portions are positioned at a distance from each other so that the width between the outer surfaces in the axial direction of the tire in an unassembled state is wider than the rim width of a corresponding regular rim,
Each of the pair of bead portions has a recess formed on the back surface of the bead in an unrim-assembled state, the recess being recessed axially inward of an axially outer end of the bead heel surface,
When assembled to a rim, the back surface of the bead is in close contact with substantially the entire surface of a radial portion of the rim flange of the regular rim that extends parallel to the tire radial direction.
The pneumatic tire according to any one of claims 1 to 5. The recess has a depth of less than 1.0 mm in the tire axial direction.
The pneumatic tire according to claim 6. At least a portion of the bead heel surface is formed by rim strip rubber and has vent hole traces.
The pneumatic tire according to any one of claims 1 to 7 .