Patent Description:
In order to improve tire ground contact characteristics, recent pneumatic tires have a two-layer structure including a cap tread including a tread rubber exposed at a tread surface to form an outer surface of a tread portion and an undertread disposed below the cap tread. The technology described in Patent Document <NUM> is known as a conventional pneumatic tire that is configured in this manner.

Patent Document <NUM> describes a pneumatic tire having an annular structure centered around the tire rotational axis and including a pair of bead cores, a pair of bead fillers, a carcass layer, a belt layer, a tread rubber, a pair of side wall rubbers and a pair of rim cushion rubbers. The pair of bead fillers is disposed on a periphery of each of the pair of bead cores in the tire radial direction so as to reinforce the bead portions. The tread rubber includes an undertread. An earthing tread is exposed to a road contact surface of a tread rubber and penetrates the undertread to contact the belt layer in an electrically conductive manner. Patent Document <NUM> describes a pneumatic tire including a tread portion on which a tread pattern is formed, side portions provided to both sides in the tire lateral direction of the tread portion and bead portions connected to the side portions. The tire further includes a carcass, a belt layer disposed outward from the carcass in the tire radial direction and a bead core. The carcass, the belt layer, and the bead core function as a reinforcing member of the tire. Patent Literature <NUM> describes a pneumatic tire comprising a tread portion, a pair of sidewall portions, a pair of bead portions, a carcass extending between the bead portions, a breaker disposed radially outside the carcass, and a band disposed radially outside the breaker.

An object of the invention is to provide a pneumatic tire that can reduce tire rolling resistance while maintaining tire durability performance.

To achieve the object described above, an embodiment of the present invention provides a pneumatic tire including a carcass layer, a pair of cross belts disposed on an outer side of the carcass layer in a radial direction, a tread rubber including a cap tread and an undertread that are layered, the tread rubber being disposed on the outer side of the cross belts in the radial direction, a pair of shoulder main grooves and at least one center main groove that are formed in the tread surface, and a pair of shoulder land portions and two or more center land portions that are defined by the shoulder main grooves and the center main grooves,.

The maximum gauge UT_sh of the undertread in the shoulder land portion has a relationship <NUM> ≤ UT_sh/Ga_sh with respect to a total gauge Ga_sh of the tread rubber at a measurement point of the maximum gauge UT_sh.

In the pneumatic tire according to an embodiment of the invention, (<NUM>) the undertread in the center land portion has a low loss tangent tanδ_ut and a thick maximum gauge UT_ce, and thus heat build-up in the tread portion center region during rolling of the tire is suppressed, improving tire rolling resistance. Additionally, (<NUM>) the undertread of the shoulder land portion has a thin maximum gauge UT_sh, thus relatively ensuring the gauge of the cap tread corresponding to high breakdown resistance. This suppresses cracks and/or fractures in the shoulder land portions caused by the significant effect of repeated strain during rolling of the tire, improving the durability of the tire. This is advantageous in that both tire rolling resistance performance and durability performance are provided in a compatible manner.

<FIG> is a cross-sectional view in a tire meridian direction illustrating a pneumatic tire according to an embodiment of the invention. The same drawing illustrates a cross-sectional view of a half region in a tire radial direction. <FIG> also illustrates a radial tire for a passenger vehicle as an example of a pneumatic tire.

In the same drawing, the cross-section in the tire meridian direction is defined as a cross-section of the tire taken along a plane that includes a tire rotation axis (not illustrated). Further, a tire equatorial plane CL is defined as a plane perpendicular to the tire rotation axis through a midpoint between measurement points in a tire cross-sectional width defined by JATMA. Furthermore, the tire width direction is defined as a direction parallel with the tire rotation axis, and the tire radial direction is defined as a direction perpendicular to the tire rotation axis. In addition, point T denotes a tire ground contact edge.

Furthermore, an inner side in a vehicle width direction and an outer side in the vehicle width direction are defined with respect to the vehicle width direction in a case where the tire is mounted on a vehicle. Additionally, left and right regions demarcated by the tire equatorial plane are defined as an outer region in the vehicle width direction and an inner region in the vehicle width direction. Further, the pneumatic tire includes a mounting direction indicator portion (not illustrated) that indicates the tire mounting direction with respect to a vehicle. The mounting direction indicator portion, for example, is composed of a mark or recesses/protrusions on a sidewall portion of the tire. For example, Economic Commission for Europe Regulation <NUM> (ECE R30) stipulates that the vehicle mounting direction indicator portion be provided on the sidewall portion on the outer side in the vehicle width direction in a case where the tire is mounted on a vehicle.

A pneumatic tire <NUM> has an annular structure with the tire rotation axis as its center and includes: a pair of bead cores <NUM>, <NUM>, a pair of bead fillers <NUM>, <NUM>, a carcass layer <NUM>, a belt layer <NUM>, a tread rubber <NUM>, a pair of sidewall rubbers <NUM>, <NUM>, and a pair of rim cushion rubbers <NUM>, <NUM> (see <FIG>).

The pair of bead cores <NUM>, <NUM> include one or a plurality of bead wires made of steel and wound annularly multiple times and are embedded in bead portions to configure cores of the left and right bead portions. The pair of bead fillers <NUM>, <NUM> are respectively disposed on an outer circumference of the pair of bead cores <NUM>, <NUM> in the tire radial direction and reinforce the bead portions.

The carcass layer <NUM> has a single layer structure made of one carcass ply or a multilayer structure made of a plurality of carcass plies being layered and extends between the left and right bead cores <NUM>, <NUM> in a toroidal shape, forming the framework of the tire. Additionally, both end portions of the carcass layer <NUM> are wound and turned back toward an outer side in the tire width direction so as to wrap the bead cores <NUM> and the bead fillers <NUM> and fixed. Moreover, the carcass ply of the carcass layer <NUM> is made by covering a plurality of carcass cords made of steel or an organic fiber material (for example, aramid, nylon, polyester, rayon, or the like) with coating rubber and performing a rolling process on the carcass cords, and has a cord angle (defined as an inclination angle in a longitudinal direction of the carcass cords with respect to a tire circumferential direction) of <NUM> degrees or more and <NUM> degrees or less.

For example, in the configuration of <FIG>, the carcass layer <NUM> has a single layer structure formed of a single carcass ply, and a turned back portion <NUM> thereof extends along an outer circumferential surface of a body portion <NUM> and overlaps the belt layer <NUM> in the tire width direction. Additionally, a terminating end portion of the turned back portion <NUM> of the carcass layer <NUM> is located further on the outer side than the shoulder main grooves <NUM>, <NUM> described below in the tire width direction.

The belt layer <NUM> includes a plurality of belt plies <NUM> to <NUM> being layered, and is disposed by being wound around the outer circumference of the carcass layer <NUM>. The belt plies <NUM> to <NUM> include a pair of cross belts <NUM>, <NUM>, a belt cover <NUM>, and belt edge covers <NUM>.

The pair of cross belts <NUM>, <NUM> are made by covering a plurality of belt cords made of steel or an organic fiber material with coating rubber and performing a rolling process on the belt cords, and each have a cord angle with an absolute value of <NUM> degrees or more and <NUM> degrees or less. Further, the pair of cross belts <NUM>, <NUM> have cord angles (defined as inclination angles in a longitudinal direction of the belt cords with respect to the tire circumferential direction) of opposite signs relative to each other and are layered such that the longitudinal directions of the belt cords intersect each other (a so-called crossply structure). Furthermore, the pair of cross belts <NUM>, <NUM> are disposed layered on an outer side in the tire radial direction of the carcass layer <NUM>.

The belt cover <NUM> and the belt edge covers <NUM> are made by covering a plurality of belt cover cords made of steel or an organic fiber material with coating rubber, and each have a cord angle with an absolute value of <NUM> degrees or more and <NUM> degrees or less. Additionally, for example, a strip material is formed of one or a plurality of belt cover cords covered with coating rubber, and the belt cover <NUM> and the belt edge covers <NUM> are made by winding this strip material multiple times and in a spiral-like manner in the tire circumferential direction around outer circumferential surfaces of the cross belts <NUM>, <NUM>. Additionally, the belt cover <NUM> is disposed so as to completely cover the cross belts <NUM>, <NUM>, and the pair of belt edge covers <NUM> and <NUM> are disposed covering the left and right edge portions of the cross belts <NUM>, <NUM> from the outer side in the tire radial direction.

The tread rubber <NUM> is disposed on the outer circumference of the carcass layer <NUM> and the belt layer <NUM> in the tire radial direction and constitutes a tread portion of the tire. Additionally, the tread rubber <NUM> includes a cap tread <NUM>, an undertread <NUM>, and left and right wing tips <NUM>, <NUM>. Other details of the tread rubber <NUM> will be described below.

The pair of sidewall rubbers <NUM>, <NUM> are disposed on the outer side in the tire width direction of the carcass layer <NUM> and constitute left and right sidewall portions. For example, in the configuration of <FIG>, the end portion of the sidewall rubber <NUM> on the outer side in the tire radial direction is disposed in the lower layer of the tread rubber <NUM> and is sandwiched between the belt layer <NUM> and the carcass layer <NUM>. However, no such limitation is intended, and the end portion of the sidewall rubber <NUM> on the outer side in the tire radial direction may be disposed on the outer layer of the tread rubber <NUM> and exposed at a buttress portion (not illustrated).

The pair of rim cushion rubbers <NUM>, <NUM> extend from an inner side in the tire radial direction of the left and right bead cores <NUM>, <NUM> and turned back portions of the carcass layer <NUM> toward the outer side in the tire width direction to constitute rim fitting surfaces of the bead portions.

The innerliner <NUM> is an air permeation preventing layer disposed on the tire inner surface and covering the carcass layer <NUM>, and suppresses oxidation caused by exposure of the carcass layer <NUM> and also prevents leaking of the air in the tire. In addition, the innerliner <NUM> is constituted by, for example, a rubber composition with butyl rubber as a main component, thermoplastic resin, thermoplastic elastomer composition made by blending an elastomer component with a thermoplastic resin, and the like.

<FIG> is a plan view illustrating a tread surface of the pneumatic tire illustrated in <FIG>. The same drawing illustrates a tread surface of a summer tire. In reference to the same drawing, "tire circumferential direction" refers to the direction revolving about the tire rotation axis. Reference sign T denotes a tire ground contact edge, and a dimension symbol TW denotes a tire ground contact width.

As illustrated in <FIG>, the pneumatic tire <NUM> includes, in a tread surface: a plurality of circumferential main grooves <NUM> to <NUM> extending in the tire circumferential direction and a plurality of land portions <NUM> to <NUM> defined by the circumferential main grooves <NUM> to <NUM>.

"Main groove" refers to a groove on which a wear indicator must be provided as specified by JATMA and has a groove width of <NUM> or more and a groove depth of <NUM> or more.

The groove width is measured as a distance between opposing groove walls at a groove opening portion when the tire is mounted on a specified rim, inflated to a specified internal pressure, and in an unloaded state. In a configuration in which the groove opening portion includes a notch portion or a chamfered portion, the groove width is measured with intersection points between an extension line of the tread contact surface and extension lines of the groove walls as measurement points, in a cross-sectional view parallel with the groove width direction and the groove depth direction.

The groove depth is the distance from the tread contact surface to the maximum groove depth position and is measured when the tire is mounted on a specified rim, inflated to the specified internal pressure, and in an unloaded state. Additionally, in a configuration in which the grooves include a partial recess /protrusion portion or sipes on the groove bottom, the groove depth is measured excluding these portions.

"Specified rim" refers to a "standard rim" defined by JATMA, a "design rim" defined by TRA, or a "measuring rim" defined by ETRTO. Additionally, "specified internal pressure" refers to a "maximum air pressure" defined by JATMA, to the maximum value in "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES" defined by TRA, or to "INFLATION PRESSURES" defined by ETRTO. Additionally, "specified load" refers to a "maximum load capacity" defined by JATMA, the maximum value in "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES" defined by TRA, or "LOAD CAPACITY" defined by ETRTO. However, in JATMA, in a case of a tire for a passenger vehicle, the specified internal pressure is an air pressure of <NUM> kPa, and the specified load is <NUM>% of the maximum load capacity at the specified internal pressure.

For example, in the configuration of <FIG>, the pneumatic tire <NUM> has a tread pattern that is left-right asymmetric with respect to the tire equatorial plane CL. However, no such limitation is intended, and, for example, the pneumatic tire <NUM> may have a tread pattern that is left-right axisymmetric with respect to the tire equatorial plane CL and may have a substantially point-symmetric tread pattern having a center point on the tire equatorial plane CL (not illustrated).

Furthermore, in the configuration of <FIG>, the left and right regions demarcated by the tire equatorial plane CL each have two circumferential main grooves <NUM>, <NUM>; <NUM>, <NUM>. Furthermore, the circumferential grooves <NUM>, <NUM>; <NUM>, <NUM> are disposed being substantially left-right symmetric with respect to the tire equatorial plane CL as a center. Five land portions <NUM> to <NUM> are defined by these circumferential main grooves <NUM>, <NUM>; <NUM>, <NUM>. In addition, one land portion <NUM> is disposed on the tire equatorial plane CL.

However, no such limitation is intended, and three, or five or more circumferential main grooves may be disposed, or the circumferential main grooves may be arranged asymmetrically with respect to the tire equatorial plane CL (not illustrated). Additionally, one circumferential main groove is disposed on a tire equatorial plane CL (not illustrated).

Additionally, of the circumferential main grooves <NUM>, <NUM>; <NUM>, <NUM> disposed in one region demarcated by the tire equatorial plane CL, the circumferential main grooves <NUM>, <NUM> on the outermost side in the tire width direction are defined as shoulder main grooves, and the other circumferential main grooves <NUM>, <NUM> are defined as center main grooves.

For example, in the configuration of <FIG>, a distance Dg1 from the tire equatorial plane CL to the groove center line of the shoulder main groove <NUM> (<NUM>) is in the range of <NUM>% or more and <NUM>% or less of a tire ground contact width TW. Additionally, a distance Dg2 from the tire equatorial plane CL to the groove center lines of the center main groove <NUM> (<NUM>) is in the range of <NUM>% or more and <NUM>% or less of the tire ground contact width TW.

The groove center line is defined as an imaginary line connecting the midpoints of the distance between the left and right groove walls. In a case where the groove center line of the main groove has a zigzag shape or a wavelike shape, a distance to the groove center line is defined using, as a measurement point, a straight line parallel to the tire circumferential direction extending through midpoints of the maximum amplitude positions on the left and right of the groove center line.

The tire ground contact width TW is measured as the maximum linear distance in the tire axial direction of a contact surface between the tire and a flat plate when the tire is mounted on a specified rim, inflated to a specified internal pressure, placed perpendicular to the flat plate in a static state, and loaded with a load corresponding to a specified load.

The tire ground contact edge T is defined as a maximum width position in the tire axial direction of the contact surface between the tire and a flat plate when the tire is mounted on a specified rim, inflated to a specified internal pressure, placed perpendicular to the flat plate in a static state, and loaded with a load corresponding to a specified load.

The land portions <NUM>, <NUM> located on the outer side in the tire width direction and defined in the shoulder main grooves <NUM>, <NUM> are defined as shoulder land portions. Each of the shoulder land portions <NUM> and <NUM> is a land portion on the outermost side in the tire width direction, and is located on the tire ground contact edge T. The other land portions <NUM> to <NUM> are defined as center land portions. Note that, in a configuration including four circumferential main grooves <NUM> to <NUM> as illustrated in <FIG>, a pair of shoulder land portions <NUM>, <NUM> and three center land portions <NUM> to <NUM> are defined.

<FIG> is a cross-sectional view illustrating the tread portion of the pneumatic tire illustrated in <FIG>. The same drawing illustrates the half region demarcated by the tire equatorial plane CL. <FIG> is an enlarged view illustrating the shoulder land portion of the tread portion illustrated in <FIG>. <FIG> is an enlarged view illustrating the center land portion of the tread portion illustrated in <FIG>. Note that <FIG> and <FIG> respectively illustrate cross-sectional views of the shoulder land portion <NUM> and the center land portion <NUM> in the outer region in the vehicle width direction. On the other hand, the cross-sectional structure of the tread portion in the outer region in the vehicle width direction is left-right symmetrical with respect to the cross-sectional structure of the inner region in the vehicle width direction except for the configuration of the grooves, notch portions, and sipes in the tread pattern (see <FIG>).

In the configuration in <FIG>, the tread rubber <NUM> includes the cap tread <NUM>, the undertread <NUM>, and the left and right wing tips <NUM>, <NUM> as described above.

The cap tread <NUM> is formed of a rubber material that is excellent in ground contact characteristics and weather resistance, and is exposed at the tread surface all across the tire ground contact surface to form an outer surface of the tread portion. In addition, the loss tangent tanδ_cap of the cap tread <NUM> is in the range of <NUM> or more and <NUM> or less. Additionally, the rubber hardness Hs_cap of the cap tread <NUM> is in the range of <NUM> or more and <NUM> or less.

The loss tangent tanδ is measured using a viscoelastic spectrometer available from Toyo Seiki Seisaku-sho Ltd. at a temperature of <NUM>, a shear strain of <NUM>%, an amplitude of ±<NUM>%, and a frequency of <NUM>.

Rubber hardness is measured in accordance with JIS K <NUM>.

The undertread <NUM> is formed of a rubber material having a lower hardness than the cap tread <NUM> and being excellent in heat resistance, and is sandwiched between the cap tread <NUM> and the belt layer <NUM> to constitute the base portion of the tread rubber <NUM>. Additionally, the loss tangent tanδ_ut of the undertread is less than <NUM>.

Additionally, the loss tangent tanδ_ut of the undertread is preferably smaller than the loss tangent tanδ_cap of the cap tread <NUM> (tanδ_ut < tanδ_cap). Specifically, the difference between the loss tangent tanδ_cap of the cap tread <NUM> and the loss tangent tanδ_ut of the undertread is preferably <NUM> or more.

Additionally, the rubber hardness Hs_ut of the undertread is preferably in the range of <NUM> or more and <NUM> or less. Additionally, the rubber hardness Hs_ut of the undertread is preferably smaller than the rubber hardness Hs_cap of the cap tread <NUM> (Hs_ut < Hs_cap). Additionally, a difference between the rubber hardness Hs_cap of the cap tread <NUM> and the Hs_ut of the undertread is preferably <NUM> or more.

The wing tip <NUM> is disposed on each of the left and right ends of the cap tread <NUM> in the tire width direction to form a buttress portion of the tire.

Additionally, it should be noted that in <FIG>, the shoulder land portion <NUM> and the center land portion <NUM> are located adjacent to each other as a set with the shoulder main groove <NUM> sandwiched therebetween. At this time, the maximum gauge UT_ce of the undertread <NUM> in the center land portion <NUM> preferably has the relationship <NUM> ≤ UT_ce/UT_sh ≤ <NUM> and more preferably has the relationship <NUM> ≤ UT_ce/UT_sh ≤ <NUM>, with respect to the maximum gauge UT_sh of the undertread <NUM> in the ground contact region of the shoulder land portion <NUM>. Accordingly, the gauge of the undertread <NUM> is set being relatively thick in the center land portion <NUM> and relatively thin in the shoulder land portion <NUM>.

The rubber gauge in the tread portion is measured on an imaginary line perpendicular to the tread contact surface.

For example, in the configuration of <FIG>, the thickness of the undertread <NUM> takes the maximum gauge UT_sh in a region around the edge portion on the shoulder main groove <NUM> side of the shoulder land portion <NUM> and takes a maximum gauge UT_ce at the central portion of the center land portion <NUM>. The maximum gauge UT_sh of the undertread <NUM> in the ground contact region of the shoulder land portion <NUM> is set being smaller than the maximum gauge UT_ce of the undertread <NUM> in the center land portion <NUM>.

Additionally, in the configuration of <FIG> and <FIG>, the groove configuration of the shoulder land portion <NUM> in the inner region in the vehicle width direction is different from the groove configuration of the shoulder land portion <NUM> in the outer region in the vehicle width direction in that the shoulder land portion <NUM> further includes a circumferential narrow groove <NUM>. However, in the cross-sectional structure of the tread portion, the outer region and the inner region in the vehicle width direction are left-right symmetric except for the configuration of the grooves, notch portions, and sipes of the tread pattern (see <FIG>). Additionally, as illustrated in <FIG>, the maximum gauge of the undertread <NUM> in the left and right shoulder land portions <NUM>, <NUM> is set being smaller than the maximum gauge of the undertread <NUM> in all of the center land portions <NUM> to <NUM>. However, no such limitation is intended, and it is sufficient that the relationship between the maximum gauge UT_ce of the cap tread <NUM> and the maximum gauge UT_sh of the undertread <NUM> described above be satisfied for the set of the shoulder land portion <NUM>; <NUM> and the center land portion <NUM>; <NUM> located adjacent to each other with the shoulder main groove <NUM>; <NUM> sandwiched therebetween.

In the configuration described above, (<NUM>) the undertread <NUM> of the center land portions <NUM>, <NUM> has a low loss tangent tanδ_ut and a thick maximum gauge UT_ce, and thus heat build-up in the tread portion center region during rolling of the tire is suppressed, improving tire rolling resistance. Additionally, (<NUM>) the undertread <NUM> of the shoulder land portions <NUM>, <NUM> has a thin maximum gauge UT_sh, thus relatively ensuring the gauge of the cap tread <NUM> corresponding to high breakdown resistance. This suppresses cracks and/or fractures in the shoulder land portions <NUM>, <NUM> caused by the significant effect of repeated strain during rolling of the tire, improving the durability of the tire. This is advantageous in that both tire rolling resistance performance and durability performance are provided in a compatible manner.

According to present invention, the maximum gauge UT_sh of the undertread <NUM> in the shoulder land portion <NUM> has the relationship <NUM> ≤ UT_sh/Ga_sh and more preferably has the relationship <NUM> ≤ UT_sh/Ga_sh, with respect to the total gauge Ga_sh of the tread rubber <NUM> at the measurement point of the maximum gauge UT_sh. The upper limit of the ratio UT_sh/Ga_sh is not particularly limited, but is subject to restrictions by other conditions (particularly the relationship with the imaginary line Lw described below). Additionally, the maximum gauge UT_sh of the undertread <NUM> is preferably in the range <NUM> ≤ UT_sh ≤ <NUM>.

Additionally, the maximum gauge UT_ce of the undertread <NUM> in the center land portions <NUM> to <NUM> preferably has the relationship <NUM> ≤ UT_ce/Ga_ce and more preferably has the relationship <NUM> ≤ UT_ce/Ga_ce, with respect to the total gauge Ga_ce of the tread rubber <NUM> at the measurement point of the maximum gauge UT_ce. The lower limit of the ratio UT_ce/Ga_ce is not particularly limited, but is subject to restrictions by the other conditions (particularly the relationship with the imaginary line Lw described below). Additionally, the maximum gauge UT_ce of the undertread <NUM> is preferably in the range <NUM> ≤ UT_ce ≤ <NUM>.

Additionally, as illustrated in <FIG> and <FIG>, in the configuration including the plurality of center land portions <NUM> to <NUM>, the ratio of the maximum value to the minimum value of the maximum gauge UT_ce of the undertread <NUM> in all of the center land portions <NUM> to <NUM> is preferably in the range of <NUM> or more and <NUM> or less, and is preferably in the range of <NUM> or more and <NUM> or less. Accordingly, the gauge of the undertread <NUM> is set being uniform among the plurality of center land portions <NUM> to <NUM>.

Additionally, in the configuration of <FIG>, the shoulder land portion <NUM> includes a plurality of shoulder lug grooves <NUM> and a plurality of sipes (reference sign is omitted in the drawings). Additionally, each of the shoulder lug grooves <NUM> has a so-called semi-closed structure, and at one end portion opens to the tire ground contact edge T and at the other end terminates within the shoulder land portion <NUM>. Additionally, the groove width (dimension symbol omitted in drawings) of the shoulder lug groove <NUM> is in the range of <NUM> or more and <NUM> or less, and a groove depth H11 (see <FIG>) is in the range <NUM> ≤ H11/Hg1 ≤ <NUM> with respect to the groove depth Hg1 of the shoulder main groove <NUM>. In addition, the sipe at one end portion opens to the shoulder main groove <NUM> and at the other end portion terminates within the shoulder land portion <NUM>. Additionally, the sipe has a sipe width of <NUM> or less and a sipe depth of <NUM> or more and <NUM> or less (dimension symbol omitted in drawings). "Sipe" refers to a cut formed in a tread contact surface and is distinguished from lug grooves in that due to the above-described sipe width and sipe depth, the sipe closes when the tire comes into contact with the ground.

At this time, as illustrated in <FIG>, a distance D2 from the edge portion of the shoulder main groove <NUM> side of the shoulder land portion <NUM> to the maximum gauge position of the undertread <NUM> in the shoulder land portion <NUM> preferably has the relationship <NUM> ≤ D2/D1 ≤ <NUM> and more preferably has the relationship <NUM> ≤ D2/D1 ≤ <NUM>, with respect to the distance D1 from the edge portion on the shoulder main groove <NUM> side of the shoulder land portion <NUM> to the terminating end portion of the shoulder lug groove <NUM>. The gauge of the undertread <NUM> gradually decreases from the maximum gauge position toward the tire ground contact edge T.

In a configuration in which the edge portion of the land portion includes a chamfered portion, an intersection point between the extension line of the road contact surface of the land portion and the extension line of the groove wall surface of the main groove is used as a measurement point for measurement of the distance from the edge portion of the land portion.

Furthermore, in <FIG>, the distance D1 of the terminating end portion of the shoulder lug groove <NUM> preferably has the relationship <NUM> ≤ D1/Wr_sh ≤ <NUM> and more preferably has the relationship <NUM> ≤ D1/Wr_sh ≤ <NUM>, with respect to the ground contact width Wr_sh of the shoulder land portion <NUM>.

Additionally, in <FIG>, a curved line Lw is defined that extends through a point at a distance of <NUM> from the maximum groove depth position of the shoulder main groove <NUM> and parallel to the tread profile. The curved line Lw corresponds to the position of the top surface of a wear indicator. At this time, all of the undertread <NUM> in the ground contact region of the shoulder land portion <NUM> is located further on the inner side than the curved line Lw in the tire radial direction. This prevents the undertread <NUM> from being exposed at the tread contact surface at the terminal stages of wear.

Additionally, in <FIG>, the gauge of the cap tread at the maximum groove depth position of the shoulder lug groove <NUM> is preferably <NUM> or more.

Additionally, in <FIG>, the gauge UT_e of the undertread <NUM> at the end portion of the wider cross belt <NUM> preferably has the relationship <NUM> ≤ UT_e/Ga_sh ≤ <NUM> with respect to the total gauge Ga_sh of the tread rubber at the measurement point of the gauge UT_e. Additionally, the gauge UT_e is preferably in the range <NUM> ≤ UT_e. In addition, the gauge UT_e is required to be less than the maximum gauge UT_sh of the undertread <NUM> in the shoulder land portion <NUM> (UT_e < UT_sh).

Additionally, in the configuration of <FIG>, the turned back portion <NUM> of the carcass layer <NUM> overlaps the cross belts <NUM>, <NUM> in the tire width direction. In addition, the end portion <NUM> of the turned back portion <NUM> is located within the ground contact width Wr_sh of the shoulder land portion <NUM>, and is located further on the outer side than the maximum gauge position of the undertread <NUM> in the tire width direction.

Additionally, in <FIG>, gauges UT_a, UT_b, and UT_c of the undertread <NUM> are defined that correspond to positions of <NUM>%, <NUM>%, and <NUM>% of the ground contact width Wr_ce of the center land portion <NUM> from one edge portion of the center land portion <NUM>. In this case, the ratio between the maximum value and the minimum value of these gauges UT_a, UT_b, and UT_c is preferably in the range of <NUM> or more and <NUM> or less and more preferably in the range of <NUM> or more and <NUM> or less. Accordingly, the gauges of the undertread <NUM> in the central portion of the center land portion <NUM> are preferably uniform.

Additionally, the maximum gauge UT_ce of the undertread <NUM> in the center land portion <NUM> is preferably in the region of <NUM>% or more and <NUM>% or less of the ground contact width Wr_ce of the center land portion <NUM> from one edge portion of the center land portion <NUM>. Thus, the maximum gauge position of the undertread <NUM> is in the central portion of the center land portion <NUM>.

Additionally, in <FIG>, the curved line Lw is defined that extends through a point at a distance of <NUM> from the maximum groove depth position of the shoulder main groove <NUM> and parallel to the tread profile, as is the case with <FIG> described above. In this case, the maximum gauge position of the undertread <NUM> in the center land portion <NUM> is located further on the inner side of the curved line Lw in the tire radial direction. This prevents the undertread <NUM> from being exposed at the tread contact surface at the terminal stages of wear.

Additionally, in <FIG>, the gauge UT_g of the undertread <NUM> at the position of the maximum groove depth of the shoulder main groove <NUM> and the center main groove <NUM> preferably has the relationship <NUM> ≤ UT_g/Ga_g ≤ <NUM> with respect to the groove bottom gauge Ga_g of the tread rubber <NUM>. Additionally, the gauge UT_g of the undertread <NUM> is preferably in the range of <NUM> or more and <NUM> or less.

The groove bottom gauge is measured when the tire is mounted on a specified rim, inflated to the specified internal pressure, and in an unloaded state. In this case, for example, the following measurement method is used. First, a tire unit is applied to the imaginary line of a tire profile measured by a laser profiler and fixed with tape or the like. Then, the gauge to be measured is measured with a caliper or the like. The laser profiler used here is a tire profile measuring device (available from Matsuo Co.

<FIG> is an explanatory diagram illustrating a method of manufacturing the pneumatic tire illustrated in <FIG>. The same drawing illustrates a cross-sectional view in the width direction of an unvulcanized tread rubber <NUM>'.

As illustrated in <FIG>, in a step for molding the green tire, two-color extrusion molding is performed using a rubber member <NUM>' corresponding to the cap tread <NUM> and a rubber member <NUM>' corresponding to the undertread <NUM>, the rubber members <NUM>' and <NUM>' being layered, thus forming the unvulcanized tread rubber <NUM>'. At this time, the tread rubber <NUM>' is formed such that at the arrangement position Pg of the main grooves <NUM> to <NUM>, the rubber member <NUM>' of the undertread <NUM> has a small gauge, whereas the rubber member <NUM>' of the cap tread <NUM> has a large gauge. Accordingly, in the tire after vulcanization molding, the gauge ratio UT_g/Ga_g of the undertread <NUM> at the maximum groove depth position of the shoulder main grooves <NUM> is set being relatively small.

Additionally, in the configuration of <FIG>, the pair of center land portions <NUM>, <NUM> defined by the left and right shoulder main grooves <NUM>, <NUM> are provided with a plurality of chamfered sipes <NUM>; <NUM>, and short sipes (reference sign is omitted in the drawings). The chamfered sipes <NUM>, <NUM> at one end portion open to the shoulder main grooves <NUM>; <NUM>, and at the other end portion terminates within the center land portions <NUM>; <NUM>. Additionally, none of the center land portions <NUM> to <NUM> include lug grooves that extend through the center land portions <NUM> to <NUM> in the tire width direction.

<FIG> is an explanatory diagram illustrating a chamfered sipe of the center land portion illustrated in <FIG>. The same drawing illustrates a cross-sectional view perpendicular to the extension direction of the chamfered sipe <NUM>.

As illustrated in <FIG>, the chamfered sipe <NUM> includes a sipe portion <NUM> and a chamfered portion <NUM>, and has an opening width W21 of <NUM> or more and <NUM> or less and a depth H21 of <NUM> or more and <NUM> or less. Due to having a width of <NUM> or more and <NUM> or less (dimension symbol omitted in drawings), the sipe portion <NUM> closes when the tire comes into contact with the ground. The chamfered portion <NUM> has a depth Hc of <NUM> or more and <NUM> or less, and is formed at the connection portion between the wall surface of the sipe portion <NUM> and the road contact surface of the land portion <NUM> and extends along the longitudinal direction of the chamfered sipe <NUM>. The chamfered portion <NUM> increases the opening width W21 of the chamfered sipe <NUM> to improve drainage properties.

The width of the chamfered sipe is measured as the maximum opening width of in the tread contact surface, with the tire mounted on a specified rim, inflated to a specified internal pressure, and in an unloaded state.

The depth of the chamfered sipe is measured as the distance from the tread contact surface to the sipe bottom, with the tire mounted on a specified rim, inflated to a specified internal pressure, and in an unloaded state.

As described above, in the configuration of <FIG>, the center land portions <NUM> to <NUM> include no lug grooves but include only the chamfered sipes <NUM>, <NUM>, and normal sipes (reference sign is omitted in the drawings). Such a configuration is preferable in that the rigidity of the center land portion <NUM> to <NUM> is ensured, improving steering stability performance of the tire.

However, no such limitation is intended, and the center land portions <NUM>, <NUM> may include lug grooves instead of the chamfered sipes <NUM>, <NUM> (not illustrated). In this case, the lug grooves preferably have a groove depth of less than <NUM>. Additionally, the lug grooves preferably do not penetrate the center land portions <NUM>, <NUM> as is the case with the chamfered sipes <NUM> and <NUM> described above. The rigidity of the center land portion <NUM> to <NUM> is ensured.

As described above, the pneumatic tire <NUM> includes a carcass layer <NUM>, a pair of cross belts <NUM>, <NUM> disposed on the outer side of the carcass layer <NUM> in the radial direction, a tread rubber <NUM> that includes a cap tread <NUM> and an undertread <NUM> that are layered and that is disposed on the outer side of the cross belts <NUM>, <NUM> in the radial direction, a pair of shoulder main grooves <NUM>, <NUM> and at least one center main groove <NUM>, <NUM> that are formed in the tread surface, and a pair of shoulder land portions <NUM>, <NUM> and two or more center land portions <NUM>, <NUM> that are defined by the shoulder main grooves <NUM>, <NUM> and the center main grooves <NUM>, <NUM> (see <FIG> and <FIG>). Additionally, the loss tangent tanδ_cap of the cap tread <NUM> at <NUM> has the relationship tanδ_ut < tanδ_cap with respect to the loss tangent tanδ_ut of the undertread <NUM> at <NUM>. In addition, the maximum gauge UT_ce of the undertread <NUM> in the center land portions <NUM>, <NUM> has the relationship <NUM> ≤ UT_ce/UT_sh ≤ <NUM> with respect to the maximum gauge UT_sh of the undertread <NUM> in the ground contact regions of the shoulder land portions <NUM>, <NUM> (see <FIG>).

In such a configuration, (<NUM>) the undertread <NUM> of the center land portions <NUM>, <NUM> has a low loss tangent tanδ_ut and a thick maximum gauge UT_ce, and thus heat build-up in the tread portion center region during rolling of the tire is suppressed, improving tire rolling resistance. Additionally, (<NUM>) the undertread <NUM> of the shoulder land portions <NUM>, <NUM> has a thin maximum gauge UT_sh, thus relatively ensuring the gauge of the cap tread <NUM> corresponding to high breakdown resistance. This suppresses cracks and/or fractures in the shoulder land portions <NUM>, <NUM> caused by the significant effect of repeated strain during rolling of the tire, improving the durability of the tire. This is advantageous in that both tire rolling resistance performance and durability performance are provided in a compatible manner.

According to present invention, in the pneumatic tire <NUM>, the maximum gauge UT_sh of the undertread <NUM> in the shoulder land portions <NUM>, <NUM> has the relationship <NUM> ≤ UT_sh/Ga_sh with respect to the total gauge Ga_sh of the tread rubber <NUM> at the measurement point of the maximum gauge UT_sh (see <FIG>). This is advantageous in that the maximum gauge UT_ce of the undertread <NUM> is made proper.

Additionally, in the pneumatic tire <NUM>, the maximum gauge UT_ce of the undertread <NUM> in the center land portions <NUM>, <NUM> has the relationship <NUM> ≤ UT_ce/Ga_ce with respect to the total gauge Ga_ce of the tread rubber <NUM> at the measurement point of the maximum gauge UT_ce (see <FIG>). This is advantageous in that the gauge of the undertread <NUM> in the center land portions <NUM>, <NUM> is ensured, ensuring the effect of improving the rolling resistance of the tire which is produced by the undertread <NUM>.

Additionally, in the pneumatic tire <NUM>, the maximum gauge UT_ce of the undertread <NUM> in the center land portions <NUM>, <NUM> is in the range <NUM> ≤ UT_ce ≤ <NUM>. This is advantageous in that the maximum gauge UT_ce of the undertread <NUM> is made proper.

Additionally, in the pneumatic tire <NUM>, the ratio between the maximum value and the minimum value of the maximum gauge UT_ce of the undertread <NUM> in the plurality of center land portions <NUM>, <NUM> is in the range of <NUM> or more and <NUM> or less. In such a configuration, the gauge of the undertread <NUM> is set being uniform between the plurality of center land portions <NUM>, <NUM>, and this is advantageous in that the difference in rigidity between the center land portions <NUM>, <NUM> can be reduced.

Additionally, in the pneumatic tire <NUM>, the shoulder land portions <NUM>; <NUM> include shoulder lug grooves <NUM>; <NUM> at one end portion opening to the tire ground contact edge T and at the other end portion terminating within the shoulder land portions <NUM>; <NUM> (see <FIG>). In addition, the distance D2 from an edge portion on the shoulder main groove <NUM>; <NUM> side of the shoulder land portion <NUM>; <NUM> to the measurement point of the maximum gauge UT_sh of the undertread <NUM> in the shoulder land portion <NUM>; <NUM> has the relationship <NUM> ≤ D2/D1 ≤ <NUM> with respect to the distance D1 from an edge portion of the shoulder main groove <NUM>; <NUM> side of the shoulder land portion <NUM>; <NUM> to the terminating end portion of the shoulder lug groove <NUM>; <NUM> (see <FIG>). This is advantageous in that the above-described lower limit properly ensures the gauge of the undertread <NUM> in the shoulder land portions <NUM>, <NUM> and in that the above-described upper limit ensures the gauge of the cap tread <NUM> at the groove bottoms of the shoulder lug grooves <NUM>, <NUM>.

Additionally, in the pneumatic tire <NUM>, the distance D1 of the terminating end portion of the shoulder lug groove <NUM>; <NUM> has the relationship <NUM> ≤ D1/Wr_sh ≤ <NUM> with respect to the ground contact width Wr_sh of the shoulder land portion <NUM>; <NUM> (see <FIG>). This is advantageous in that the extension length of the shoulder lug groove <NUM>; <NUM> is properly set and in that the wet performance and the dry performance of the tire are ensured.

Additionally, in the pneumatic tire <NUM>, the gauge UT_e of the undertread <NUM> at the end portion of the wider cross belt <NUM> has the relationship <NUM> ≤ UT_e/Ga_sh ≤ <NUM> with respect to the total gauge Ga_sh of the tread rubber <NUM> at the measurement point of the gauge UT_e. This is advantageous in that the gauge UT_e of the undertread <NUM> at the end portion of the cross belt <NUM> is properly set.

Additionally, in the pneumatic tire <NUM>, the gauge of the cap tread <NUM> at the maximum groove depth position of the shoulder lug groove <NUM>, <NUM> is <NUM> or more (not illustrated). This is advantageous in that the gauge of the cap tread <NUM> at the maximum groove depth position is ensured.

Additionally, in the pneumatic tire <NUM>, gauges UT_a, UT_b, and UT_c of the undertread <NUM> are defined that correspond to positions of <NUM>%, <NUM>%, and <NUM>% of the ground contact width of the center land portion <NUM>; <NUM>; <NUM> from one edge portion of the center land portion <NUM>; <NUM>; <NUM>, and the ratio between the maximum value and the minimum value of the gauges UT_a, UT_b, and UT_c of the undertread <NUM> is in the range of <NUM> or more and <NUM> or less (see <FIG>). This is advantageous in that the gauges of the undertread <NUM> in the central portions of the center land portions <NUM>; <NUM>; <NUM> are made uniform, ensuring the volume of the undertread <NUM>.

Additionally, in the pneumatic tire <NUM>, a curved line Lw is defined that extends through a point at a distance of <NUM> from the maximum groove depth position of the shoulder main groove <NUM>, <NUM> and parallel to the tread profile, and all of the undertread <NUM> in the center land portions <NUM> to <NUM> is located further on the inner side than the curved line Lw in the tire radial direction (see <FIG>). This is advantageous in that the undertread <NUM> is prevented from being exposed at the tread contact surface at the terminal stages of wear.

Additionally, in the pneumatic tire <NUM>, the gauge UT_g of the undertread <NUM> at the maximum groove depth position of the shoulder main groove <NUM> has the relationship such that <NUM> ≤ UT_g/Ga_g ≤ <NUM> with respect to the groove bottom gauge Ga_g of the tread rubber <NUM> at the maximum groove depth position (see <FIG>). This is advantageous in that the above-described lower limit ensures the gauge UT_g of the undertread <NUM> in the groove bottom portion of the shoulder main groove <NUM>. The above-described upper limit is advantageous in that the gauge of the cap tread <NUM> at the groove bottom portion of the shoulder main groove <NUM> is ensured, suppressing possible cracks in the groove bottom portion caused by repeated strain.

Additionally, in the pneumatic tire <NUM>, the loss tangent tanδ_cap of the cap tread <NUM> is in the range <NUM> ≤ tanδ_cap. This is advantageous in that the physical properties of the cap tread <NUM> are made proper.

Additionally, in the pneumatic tire <NUM>, the rubber hardness Hs_cap of the cap tread <NUM> has the relationship Hs_ut < Hs_cap with respect to the rubber hardness Hs_ut of the undertread <NUM>. This is advantageous in that the physical properties of the cap tread <NUM> are made proper.

<FIG> is a table showing results of performance tests of pneumatic tires according to the embodiment of the invention.

In the performance tests, (<NUM>) rolling resistance performance and (<NUM>) durability performance were evaluated for a plurality of types of test tires. Furthermore, the test tires having a tire size of <NUM>/65R15 <NUM> were assembled on rims having a rim size of <NUM>×6J, and an internal pressure and a load specified by JATMA were applied to the test tires.

The test tires of the Examples have the configuration illustrated in <FIG>, and the gauges of the undertread <NUM> in the center land portions <NUM> to <NUM> are all set being greater than the gauge of the undertread in the ground contact region of the shoulder land portion. In addition, the loss tangent tanδ_cap of the cap tread <NUM> at <NUM> is <NUM>, and the loss tangent tanδ_ut of the undertread <NUM> at <NUM> is <NUM>. Additionally, the maximum gauge Ga_sh of the tread rubber <NUM> in the shoulder land portions <NUM>, <NUM> is <NUM>, and the maximum gauge Ga_ce of the tread rubber <NUM> in the center land portions <NUM>, <NUM> is <NUM>. Additionally, the groove depth of the circumferential main grooves <NUM> to <NUM> is <NUM>. The distance D1 of the terminating end portion of the shoulder lug groove <NUM>, <NUM> has the relationship such that D1/Wr_sh = <NUM> with respect to the ground contact width Wr_sh of the shoulder land portion.

In the configuration of <FIG>, the test tires of Conventional Examples <NUM> and <NUM> have the same maximum gauges UT_ce and UT_sh of the undertread <NUM> in all of the land portions <NUM> to <NUM>.

Claim 1:
A pneumatic tire (<NUM>), comprising:
a carcass layer (<NUM>);
a pair of cross belts (<NUM>, <NUM>) disposed on an outer side of the carcass layer (<NUM>) in a radial direction;
a tread rubber (<NUM>) comprising a cap tread (<NUM>) and an undertread (<NUM>) that are layered, the tread rubber (<NUM>) being disposed on the outer side of the cross belts (<NUM>, <NUM>) in the radial direction;
a pair of shoulder main grooves (<NUM>) and at least one center main groove (<NUM>) that are formed in the tread surface; and
a pair of shoulder land portions (<NUM>, <NUM>) and two or more center land portions (<NUM>, <NUM>) that are defined by the shoulder main grooves (<NUM>) and the center main grooves (<NUM>), and
a maximum gauge UT_ce of the undertread (<NUM>) in the center land portion (<NUM>, <NUM>) having a relationship <NUM> ≤ UT_ce/UT_sh ≤ <NUM> with respect to a maximum gauge UT_sh of the undertread (<NUM>) in a ground contact region of the shoulder land portion (<NUM>, <NUM>),
characterized by a loss tangent tanδ_cap of the cap tread (<NUM>) at <NUM> having a relationship tanδ_ut < tanδ_cap with respect to a loss tangent tanδ_ut of the undertread (<NUM>) at <NUM>,
wherein the maximum gauge UT_sh of the undertread (<NUM>) in the shoulder land portion (<NUM>, <NUM>) has a relationship <NUM> ≤ UT_sh/Ga_sh with respect to a total gauge Ga_sh of the tread rubber (<NUM>) at a measurement point of the maximum gauge UT_sh.