Patent Description:
In recent years, studless tires have been required to provide performance on snow and performance on ice in a compatible manner. In this type of studless tire, in order to improve performance on ice, a plurality of sipes are disposed on the tread surface of the land portion, and the sipes in the same block are disposed separated (divided) in the tire width direction so that the block rigidity is ensured and snow or ice clogging in the sipe is prevented.

On the other hand, in the configuration described above, there is a problem in that the ground contact pressure increases locally in a portion where the sipes are separated in the tire width direction, degrading the load durability performance. Therefore, a known configuration is proposed in which a circumferential groove extending in the tire circumferential direction is provided in the portion where the sipes are separated in the tire width direction (for example, see Patent Document <NUM>).

Incidentally, in tires where sipes in the same block are disposed so as to be separated in the tire width direction, there is room for further improvement for load durability performance and drainage performance.

In light of the foregoing, an object of the present invention is to provide a tire with improved load durability performance and drainage performance.

In order to solve the problems described above and achieve the object, a tire according to the present invention includes, in a tread portion, a plurality of circumferential main grooves extending in a tire circumferential direction; a plurality of lug grooves extending in a direction intersecting the circumferential main grooves; and a plurality of blocks defined by the circumferential main grooves and the lug grooves, a tread surface of the block including circumferential narrow grooves each extending in the tire circumferential direction and a plurality of sipes that are provided on both sides of the circumferential narrow grooves in a tire width direction and are separated from the circumferential narrow grooves, the plurality of sipes being disposed side by side in the tire circumferential direction and extending in the tire width direction, the circumferential narrow groove including a shallow bottom portion and a deep bottom portion having different depths from the tread surface, and the deep bottom portion having a depth from the tread surface deeper than a depth of the shallow bottom portion and being provided in a central portion of the block in the tire circumferential direction.

In the tire described above, the shallow bottom portion is preferably provided at each of both end portions of the circumferential narrow groove.

In the tire described above, a depth da of the shallow bottom portion, a depth db of the deep bottom portion, and a maximum depth dc of the sipe preferably satisfy the relationship da < db < dc.

Additionally, in the tire described above, the depth da of the shallow bottom portion and the maximum depth dc of the sipe preferably satisfy <NUM> ≤ da/dc ≤ <NUM>.

According to the invention, the depth db of the deep bottom portion and the maximum depth dc of the sipe satisfy <NUM> ≤ db/dc ≤ <NUM>.

Further, in the tire described above, in the block, a circumferential length L of the circumferential narrow groove and a circumferential length Lb of the deep bottom portion preferably satisfy <NUM> ≤ Lb/L ≤ <NUM>.

Additionally, in the tire described above, in the block, the circumferential length L of the circumferential narrow groove and a circumferential length La of the shallow bottom portion satisfy <NUM> ≤ La/L ≤ <NUM>.

Furthermore, in the tire described above, the circumferential narrow groove preferably has a stepped groove-like shape having a bent portion between the shallow bottom portion and the deep bottom portion.

Additionally, in the tire described above, at least one end of the circumferential narrow groove is preferably open to the lug groove.

Moreover, in the tire described above, the circumferential narrow groove is preferably provided in a plurality of shoulder blocks provided side by side in the tire circumferential direction on the outermost side in the tire width direction.

The tire according to an embodiment of the present invention can improve the load durability performance and the drainage performance.

Embodiments of the present invention will be described in detail below with reference to the drawings. However, the present invention is not limited to the embodiment. Constituents of the embodiment include elements that are essentially identical or that can be substituted or easily conceived by one skilled in the art.

A pneumatic tire according to the present embodiment will be described. In the following description, a tire radial direction refers to a direction orthogonal to a rotation axis of the tire, an inner side in the tire radial direction refers to a side toward the rotation axis in the tire radial direction, and an outer side in the tire radial direction refers to a side away from the rotation axis in the tire radial direction. In addition, a tire circumferential direction refers to a circumferential direction about the rotation axis as a center axis. Moreover, a tire width direction refers to a direction parallel to the rotation axis, an inner side in the tire width direction refers to a side toward a tire equatorial plane (tire equator line) in the tire width direction, and an outer side in the tire width direction refers to a side away from the tire equatorial plane in the tire width direction. Note that "tire equatorial plane" refers to the plane orthogonal to the rotation axis of the pneumatic tire, the plane passing through the center of the tire width.

<FIG> is a plan view of a tread surface of the pneumatic tire according to the present embodiment. In <FIG>, a reference sign CL denotes the tire equatorial plane, and reference signs T denote tire ground contact edges, respectively. Additionally, a pneumatic tire <NUM> according to the present embodiment (hereinafter, also referred to simply as "tire <NUM>") is specified in the mounting direction with respect to the vehicle, and in the example of <FIG>, it has a left-right asymmetric tread pattern centered on the tire equatorial plane CL. Note that in <FIG>, the region illustrated on the outer side in the tire width direction of the ground contact edge T includes a so-called sidewall portion.

The ground contact edge T is defined as a maximum width position in the tire axial direction of the contact surface between the tire <NUM> and a flat plate when the tire <NUM> 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.

"Specified rim" refers to an "applicable rim" defined by the Japan Automobile Tyre Manufacturers Association Inc. (JATMA), a "Design Rim" defined by the Tire and Rim Association, Inc. (TRA), or a "Measuring Rim" defined by the European Tyre and Rim Technical Organisation (ETRTO). Additionally, the specified internal pressure refers to a "maximum air pressure" specified by JATMA, the maximum value in "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES" specified by TRA, or "INFLATION PRESSURES" specified by ETRTO. Additionally, the specified load refers to a "maximum load capacity" specified by JATMA, the maximum value in "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES" specified by TRA, or "LOAD CAPACITY" specified by ETRTO. However, in the case of JATMA, for 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.

A tread portion <NUM> of the tire <NUM> is made of a rubber material (tread rubber) and is exposed on the outermost side of the tire <NUM> in the tire radial direction, with the surface thereof constituting the contour of the tire <NUM>. The surface of the tread portion <NUM> forms a tread surface <NUM> that is a surface that comes into contact with the road surface when a vehicle (not illustrated) on which the tire <NUM> is mounted is driven.

The tire <NUM> includes, in the tread surface <NUM>, a plurality of circumferential main grooves <NUM> to <NUM> extending in the tire circumferential direction, a plurality of land portions <NUM> to <NUM> defined by the circumferential main grooves <NUM> to <NUM>, a plurality of lug grooves <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> disposed in each of the land portions <NUM> to <NUM>, and a plurality of sipes <NUM> disposed in each of the land portions <NUM> to <NUM>. Here, "circumferential main groove" refers to a groove extending in the tire circumferential direction on which a wear indicator must be provided as specified by JATMA and typically has a groove width of <NUM> or more and a groove depth of <NUM> or more. "Lug groove" refers to a lateral groove extending in a direction intersecting the circumferential main groove (tire width direction) and typically having a groove width of <NUM> or more and a groove depth of <NUM> or more. Further, "sipe" refers to a cut formed in the tread surface and typically has a sipe width of less than <NUM> and a sipe depth of <NUM> or more, so that the sipe closes when the tire comes into contact with the ground. Accordingly, the tire <NUM> of the present embodiment is configured as a studless tire provided with the sipe <NUM> on the tread surface <NUM>.

A plurality (four in <FIG>) of circumferential main grooves <NUM> to <NUM> that extend in the tire circumferential direction are provided on the tread surface <NUM> at predetermined intervals in the tire width direction respectively. In the present embodiment, with the tire equatorial plane CL as the boundary, two circumferential main grooves <NUM>, <NUM> are provided on the inner side in the vehicle width direction, and two circumferential main grooves <NUM>, <NUM> are provided on the outer side in the vehicle width direction, respectively, as illustrated in <FIG>. Here, the inner side in the vehicle width direction and the outer side in the vehicle width direction are specified as orientations with respect to the vehicle width direction when the tire <NUM> is mounted on the vehicle. Additionally, two circumferential main grooves <NUM>, <NUM> on the outermost side in the tire width direction are defined as shoulder main grooves, and two circumferential main grooves <NUM>, <NUM> on the inner side in the tire width direction are defined as center main grooves.

In the example of <FIG>, the shoulder main grooves <NUM>, <NUM> each have straight shapes. In contrast, the center main grooves <NUM>, <NUM> oscillate in the tire width direction while extending in the tire circumferential direction to form zigzag shapes. In particular, the center main groove <NUM> on the inner side in the vehicle width direction has a zigzag shape that the groove wall on the tire equatorial plane CL side has a straight shape, while the groove wall on the ground contact edge T side oscillates in the tire width direction while extending in the tire circumferential direction. Note that the number of circumferential main grooves is not limited to the above, and three or five or more circumferential main grooves may be disposed on the tread surface <NUM>.

A plurality of (five in <FIG>) land portions <NUM> to <NUM> extending in the tire circumferential direction are defined and formed by the four circumferential main grooves <NUM> to <NUM> on the tread surface <NUM>. In the present embodiment, the land portions <NUM>, <NUM> defined on the outer side in the tire width direction by the shoulder main grooves <NUM>, <NUM> respectively are defined as shoulder land portions. Furthermore, the land portions <NUM>, <NUM> defined on the inner side in the tire width direction by the shoulder main grooves <NUM>, <NUM> are defined as second land portions. The second land portions <NUM>, <NUM> are adjacent to the shoulder land portions <NUM>, <NUM> with the above-described circumferential main grooves <NUM>, <NUM> disposed respectively therebetween. Additionally, a land portion <NUM> defined between the center main grooves <NUM>, <NUM> is defined as a center land portion. The center land portion <NUM> is provided extending on the tire equatorial plane CL.

Note that in the example of <FIG>, only the single center land portion <NUM> exists, but in a configuration with five or more circumferential main grooves, a plurality of center land portions are formed. Furthermore, in a configuration with three circumferential main grooves, the center land portion may also serve as the second land portion.

The left and right shoulder land portions <NUM>, <NUM> include a plurality of lug grooves <NUM>, <NUM>, respectively. Each of the lug grooves <NUM>, <NUM> has one end opening to the shoulder main grooves <NUM>, <NUM>, respectively, and extends in the outer side in the tire width direction, and has an other end opening in a region across the ground contact edge T. A plurality of lug grooves <NUM>, <NUM> are provided repeatedly in the tire circumferential direction in the shoulder land portions <NUM>, <NUM>, respectively. Accordingly, the shoulder land portions <NUM>, <NUM> are partitioned into a plurality of blocks B (shoulder blocks) by the lug grooves <NUM>, <NUM>, respectively. The blocks B include circumferential narrow grooves <NUM>, <NUM> each extending in the tire circumferential direction, and a plurality of sipes <NUM> extending in the tire width direction. In the example of <FIG>, the circumferential narrow grooves <NUM>, <NUM> are formed in a straight shape.

Additionally, the second land portion <NUM> on the inner side in the vehicle width direction includes two types and a plurality of lug grooves <NUM>, <NUM>, and a plurality of sipes <NUM> extending in the tire width direction. The lug groove <NUM> (first lug groove) has one end facing one end of the above-mentioned lug groove <NUM> and opening to the shoulder main groove <NUM>, and an other end terminating inside the second land portion <NUM>. Further, the lug groove <NUM> (second lug groove) has one end opening to the center main groove <NUM> and an other end terminating inside the second land portion <NUM>. In the example of <FIG>, one end of the lug groove <NUM> opens to a corner portion of the center main groove <NUM> having a zigzag shape that projects on the ground contact edge T side. Therefore, the lug grooves <NUM>, <NUM> have a semi-closed structure that does not cross the second land portion <NUM>. Additionally, the lug grooves <NUM>, <NUM> are disposed in a staggered manner (alternately) in the tire circumferential direction, and each extend so as to be inclined in opposite directions with respect to the tire circumferential direction, and overlap each other in the tire width direction. Accordingly, the second land portion <NUM> is formed as a rib R that is continuous in the tire circumferential direction without being divided in the tire circumferential direction by the lug grooves <NUM>, <NUM>.

The center land portion <NUM> includes a plurality of lug grooves <NUM>. The lug groove <NUM> is formed extending in the tire width direction between the two center main grooves <NUM>, <NUM>, and both end portions are open to the center main grooves <NUM>, <NUM>, respectively. In the example of in <FIG>, one end of the lug groove <NUM> opens to a corner portion projecting on the tire equatorial plane CL side in the zigzag-shaped center main groove <NUM>, and extends along the extension direction of the short portion of the center main groove <NUM>. Further, the lug groove <NUM> is provided with respect to every other corner portion that forms the zigzag of the center main groove <NUM>. The center land portion <NUM> is partitioned into a plurality of blocks B by a plurality of lug grooves <NUM>, and a plurality of sipes <NUM> extending in the tire width direction are provided in each block B.

The second land portion <NUM> on the outer side in the vehicle width direction includes a plurality of lug grooves <NUM>. The lug groove <NUM> is formed extending in the tire width direction between the adjacent center main groove <NUM> and a shoulder main groove <NUM>, and one end opens to the center main groove <NUM>, and an other end opens to the shoulder main groove <NUM>. In the example of <FIG>, one end of the lug groove <NUM> opens to a corner portion projecting on the ground contact edge T side of the zigzag-shaped center main groove <NUM>, and an other end opens to the shoulder main groove <NUM> facing one end of the lug groove <NUM> described above. The second land portion <NUM> is partitioned into a plurality of blocks B by a plurality of lug grooves <NUM>. The blocks B include a circumferential narrow groove <NUM> and a plurality of sipes <NUM> each extending in the tire width direction. In the example of <FIG>, the circumferential narrow groove <NUM> is formed in a zigzag shape that oscillates in the tire width direction while extending in the tire circumferential direction.

Note that the pneumatic tire <NUM> according to the present embodiment has a meridian cross-section shape similar to that of a known pneumatic tire. Here, the meridian cross-section form of the pneumatic tire refers to the cross-sectional shape of the pneumatic tire as it appears on a plane normal to the tire equatorial plane CL. The tire <NUM> according to the present embodiment has a bead portion, a sidewall portion, a shoulder portion, and a tread portion <NUM> from the inner side to the outer side in the tire radial direction in a tire meridian cross-sectional view, not illustrated. Further, in the tire meridian cross-sectional view, for example, the tire <NUM> includes a carcass layer extending from the tread portion <NUM> to the bead portions on both sides and wound around a pair of bead cores, and a belt layer and a belt reinforcing layer provided in that order on the above-described carcass layer on the outer side in the tire radial direction.

Next, the tread pattern formed in the shoulder land portion is described in detail. <FIG> is a plan view illustrating a shoulder land portion of the tread pattern illustrated in <FIG>. <FIG> is a cross-sectional view taken along line A-A of <FIG>. <FIG> is a cross-sectional view taken along line B-B of <FIG>. In <FIG>, a part of the shoulder land portion on the outer side in the vehicle width direction is illustrated, and the same configuration is also provided in the shoulder land portion on the inner side in the vehicle width direction. As described above, the left and right shoulder land portions <NUM>, <NUM> located on the outermost side in the tire width direction are partitioned into a plurality of blocks B by a plurality of lug grooves <NUM>, <NUM>, respectively. The blocks B include the circumferential narrow grooves <NUM>, <NUM> each extending in the tire circumferential direction, and the plurality of sipes <NUM> provided on both sides of the circumferential narrow grooves <NUM>, <NUM> in the tire width direction.

As shown in <FIG>, the plurality of sipes <NUM> extend along the tire width direction and are provided side by side in the tire circumferential direction. Each of the sipes <NUM> is separated from the circumferential narrow groove <NUM> and extends in the tire width direction without intersecting the circumferential narrow groove <NUM>. Sipe <NUM> refers to a groove having, for example, a groove width of <NUM> or more and less than <NUM> and a groove depth of <NUM> or more and <NUM> or less. Additionally, the sipe <NUM> is formed in a zigzag shape in which an opening portion to the tread surface <NUM> is continuously bent a plurally of times. In this case, the sipe <NUM> may be a two-dimensional sipe, in which the shape of the tread portion <NUM> from the tread surface <NUM> toward the inner side in the tire radial direction is a zigzag shape along the zigzag shape of the tread surface <NUM>, or may be a three-dimensional sipe bent further in addition to the zigzag shape. Additionally, the sipes <NUM> do not communicate with the circumferential narrow groove <NUM>, but there is a configuration in which the sipes <NUM> communicate with the shoulder main groove <NUM> and a configuration in which the sipes <NUM> do not. According to this configuration, since the plurality of sipes <NUM> are disposed on the tread surface <NUM> so as to be separated (divided) in the tire width direction, the block rigidity is ensured and snow and ice clogging in the sipe <NUM> can be prevented. Therefore, it is possible to improve the performance on ice of the tire <NUM>.

On the other hand, there is a problem in that in a portion where the above-described sipes <NUM> in the tread surface <NUM> are separated in the tire width direction, that is, in a region where the sipes are divided and not disposed, the ground contact pressure increases locally, degrading the load durability performance. Accordingly, in the present embodiment, the circumferential narrow groove <NUM> is provided in each block B. The circumferential narrow groove <NUM> is a narrow groove extending in the tire circumferential direction, and specifically, the groove width W is formed being <NUM> or more and <NUM> or less. Additionally, the circumferential narrow groove <NUM> is provided in a region where the sipes <NUM> that extend in the tire width direction are divided (separated) in the tire width direction, that is, in a central portion in the width direction of the shoulder land portion <NUM>. Specifically, the width W1 of the shoulder land portion <NUM> specified by the distance between the edge of the shoulder main groove <NUM> and the ground contact edge T and the width W2 between the edge of the shoulder main groove <NUM> and the groove center line of the circumferential narrow groove <NUM> have a relationship <NUM> ≤ W2/W1 ≤ <NUM>. According to this configuration, by providing the circumferential narrow grooves <NUM>, <NUM> in a region where the sipe <NUM> in the tread surface <NUM> is separated in the tire width direction, the ground contact pressure in each block B can be reduced, and the load durability performance can be improved. Additionally, in the configuration described above, the ground contact pressure in each block B is reduced by the circumferential narrow grooves <NUM>, <NUM>, and the ground contact pressure of the second land portions <NUM>, <NUM> is relatively increased. Accordingly, the effect of improving the performance on ice and the performance on snow by the second land portions <NUM>, <NUM> is efficiently obtained.

Additionally, at least one end 352A of the circumferential narrow groove <NUM> opens to the lug groove <NUM>. In the example of <FIG>, an other end 352B terminates inside the block B, while both ends 352A, 352B may open to the lug groove <NUM>. In this case, preferably, the block length P on the extension line of the circumferential narrow groove <NUM> and the length L of the circumferential narrow groove <NUM> satisfy <NUM> ≤ L/P ≤ <NUM>, and satisfy <NUM> ≤ L/P ≤ <NUM>. According to this configuration, one end 352A opens to the lug groove <NUM>, and the length L of the circumferential narrow groove <NUM> is <NUM>% or more of the block length P, so that the load durability performance and the drainage performance can be improved.

Additionally, in this configuration, as illustrated in <FIG>, the circumferential narrow groove <NUM> includes a shallow bottom portion <NUM> and a deep bottom portion <NUM> having different depths from the tread surface <NUM>. The depth of the deep bottom portion <NUM> from the tread surface <NUM> is formed deeper than the shallow bottom portion <NUM>. The circumferential narrow groove <NUM> is formed into a stepped groove-like shape having a bent portion 352C between the shallow bottom portion <NUM> and the deep bottom portion <NUM>, the portion located on the outer side in the tire radial direction of the bent portion 352C being defined as the shallow bottom portion <NUM>, and the portion located on the inner side in the tire radial direction being defined as the deep bottom portion <NUM>. Therefore, the deep bottom portion <NUM> is not limited to having a flat groove bottom, and may be U-shaped or V-shaped in a cross-sectional view, for example. Also, the shallow bottom portion <NUM> may be inclined toward the bent portion 352C.

The depth da of the shallow bottom portion <NUM> and the depth db of the deep bottom portion <NUM> refer to the maximum depth from the tread surface <NUM> of the shallow bottom portion <NUM> and the deep bottom portion <NUM>, respectively. The depth da of the shallow bottom portion <NUM> and the depth db of the deep bottom portion <NUM> satisfy the relationship da< db < dc with the maximum depth dc of the sipe <NUM>. By satisfying the above-described relationship, a decrease in block rigidity and clogging of snow and ice in the sipe <NUM> can be suppressed, which makes it possible to improve the performance on ice and the performance on snow of the tire <NUM>. Additionally, satisfying the relationship described above allows for the load durability performance and the drainage performance of the tire <NUM> to be provided in a compatible manner. In the present embodiment, the shallow bottom portion <NUM> is set to <NUM> ≤ da ≤ <NUM>, and the deep bottom portion <NUM> is set to <NUM> ≤ db ≤ <NUM>. Further, the maximum depth dc of the sipe <NUM> is set to <NUM> ≤ de ≤ <NUM>.

Here, the depth da of the shallow bottom portion <NUM> of the circumferential narrow groove <NUM> and the maximum depth dc of the sipe <NUM> preferably satisfy <NUM> ≤ da/dc ≤ <NUM>. When da/dc < <NUM>, the depth da of the shallow bottom portion <NUM> is not sufficient, and the ground contact pressure increases locally, degrading the load durability performance. Further, when da/dc > <NUM>, the block rigidity decreases. In the present embodiment, the depth da of the shallow bottom portion <NUM> and the maximum depth dc of the sipe <NUM> satisfy <NUM> ≤ da/dc ≤ <NUM>, and thus the block rigidity can be maintained and the load durability performance can be improved.

Additionally, the depth db of the deep bottom portion <NUM> of the circumferential narrow groove <NUM> and the maximum depth dc of the sipe <NUM> prefered satisfy <NUM> ≤ db/dc ≤ <NUM>. When db/dc < <NUM>, the ground contact pressure at the central portion in the tire circumferential direction of the block B where the deep bottom portion <NUM> is formed cannot be sufficiently reduced, and the load durability performance degrades. The drainage performance through the circumferential narrow groove <NUM> decreases. Additionally, when db/dc > <NUM>, the rigidity of the block B decreases. In the present embodiment, the depth db of the deep bottom portion <NUM> and the maximum depth dc of the sipe <NUM> satisfy <NUM> ≤ db/dc ≤ <NUM>, and thus the block rigidity can be maintained and the load durability performance and the drainage performance can be provided in a compatible manner.

Also, the shallow bottom portion <NUM> is provided at both end portions in the tire circumferential direction of the circumferential narrow groove <NUM>, and the deep bottom portion <NUM> is provided in the central portion in the tire circumferential direction between both shallow bottom portions <NUM>. Specifically, the length La of each shallow bottom portion <NUM> with respect to the length L of the circumferential narrow groove <NUM> satisfies <NUM> ≤ La/L ≤ <NUM>. The length La of the shallow bottom portion <NUM> is the length from each end 352A, 352B to the bent portion 352C. According to this configuration, the deep bottom portion <NUM> can be provided in the central portion of the block B, and thus the central portion in the tire circumferential direction where the ground contact pressure tends to concentrate in the block B can be relatively deep, a local increase in ground contact pressure can be suppressed, and the load durability performance can be effectively improved. Moreover, the circumferential narrow groove <NUM> includes the deep bottom portion <NUM>, and thus the drainage performance can be improved.

Further, the length Lb of the deep bottom portion <NUM> with respect to the length L of the circumferential narrow groove <NUM> satisfies <NUM> ≤ Lb/L ≤ <NUM>. The length Lb of the deep bottom portion <NUM> is the length between the bent portions 352C. Here, when <NUM> > Lb/L, the distance of the deep bottom portion <NUM> is not sufficient, and sufficient effect on the drainage performance (wet performance) is not obtained. Furthermore, when Lb/L > <NUM>, the block rigidity decreases and the load durability performance degrades. According to this configuration, the length Lb of the deep bottom portion <NUM> with respect to the length L of the circumferential narrow groove <NUM> satisfies the range described above, and thus the load durability performance and the drainage performance can be improved.

Note that the configuration of the circumferential narrow grooves <NUM>, <NUM> and the sipes <NUM> included in each block B is best provided in the shoulder land portions <NUM>, <NUM> (shoulder blocks) in terms of load durability performance and drainage performance, but the load durability performance and the drainage performance can also be improved even when it is provided in other land portions (blocks). For example, in the example of <FIG>, each block of the second land portion <NUM> includes the circumferential narrow groove <NUM> and the sipe <NUM>, and thus the central portion (short portion, for example) in the tire circumferential direction of the circumferential narrow groove <NUM> can be formed as the deep bottom portion described above.

Next, another embodiment will be described. <FIG> is a plan view illustrating a tread surface of a pneumatic tire according to another embodiment. Components that are the same as those of the above-described embodiment have the same reference sign and the description thereof is omitted. In the above-described embodiment, the tire <NUM> has a configuration including four circumferential main grooves <NUM> to <NUM> extending in the tire circumferential direction in the tread surface <NUM>, but a difference in this another embodiment is that a pneumatic tire 1A (hereinafter referred to simply as the tire 1A) includes five circumferential main grooves 21A to 25A in the tread surface 12A.

Specifically, as illustrated in <FIG>, with the tire equatorial plane CL as the boundary, two circumferential main grooves 21A, 22A are provided on the inner side in the vehicle width direction, and two circumferential main grooves 23A, 24A are provided on the outer side in the vehicle width direction, respectively, and one circumferential main groove 25A is provided on the tire equatorial plane CL. Similar to the embodiment described above, the circumferential main grooves 21A, 24A on the outermost side in the tire width direction are defined as the shoulder main grooves, and the circumferential main grooves 22A, 23A on the inner side in the tire width direction of the shoulder main groove are defined as the second main grooves. Furthermore, the circumferential main groove 25A is defined as the center main groove.

In this embodiment, six land portions <NUM> to <NUM> extending in the tire circumferential direction are defined and formed by the five circumferential main grooves 21A to 25A in the tread surface 12A. In this embodiment, the new center land portion <NUM> is formed in addition to the center land portion <NUM> by the two second main grooves 22A, 23A and the center main groove 25A. The center land portion <NUM> includes a plurality of lug grooves <NUM>. The lug groove <NUM> is formed extending in the tire width direction between the second main groove 22A and the center main groove 25A, and both end portions are open to the second main groove 22A and the center main groove 25A, respectively. The center land portion <NUM> is partitioned into a plurality of blocks B by a plurality of lug grooves <NUM>, and a plurality of sipes <NUM> extending in the tire width direction are provided in each block B.

Also in this another embodiment, the tread surface 12A of each block B provided in the shoulder land portions <NUM>, <NUM> includes the circumferential narrow grooves <NUM>, <NUM> each extending in the tire circumferential direction and a plurality of sipes <NUM> that are provided on both sides of the circumferential narrow grooves <NUM>, <NUM> in the tire width direction so as to be separated from the circumferential narrow grooves <NUM>, <NUM> and are disposed side by side in the tire circumferential direction and extend in the tire width direction, and the circumferential narrow grooves <NUM>, <NUM> are each provided with the deep bottom portion whose depth from the tread surface 12A is deeper than that of the shallow bottom portion in the central portion of the block B in the tire circumferential direction, and thus the load durability performance and the drainage performance of the tire can be improved.

<FIG> is a table showing the results of performance tests of tires according to the present embodiment. In the performance tests, the (<NUM>) load durability performance and (<NUM>) wet braking performance as drainage performance were evaluated for a plurality of types of test tires. Furthermore, the test tires having a tire size of <NUM>/65R15 91Q were assembled on specified rims having a rim size of <NUM>×<NUM> J, and a specified air pressure was applied to the test tires. Further, the test tires were mounted on all wheels of a test vehicle being a front-engine front-drive (FF) vehicle with an engine displacement of <NUM> cc.

In the evaluation related to load durability performance, each tire was inflated to an air pressure of <NUM> kPa, and while a circumferential temperature was controlled at <NUM> ± <NUM>, the tire was loaded with a load equivalent to <NUM>% of the maximum load (maximum load capacity) specified by JATMA and driven for <NUM> hours at a speed of <NUM>/h, then the load was increased by <NUM>% every two hours, and the running time when the tire broke was measured using an indoor drum testing machine (drum diameter: <NUM>). On the basis of the measurement results, the evaluation is expressed as index values with the value of Conventional Example being assigned the reference <NUM>. In the evaluation, larger index values indicate superior load durability performance.

In the method for evaluating wet braking performance, the above-described test vehicle mounted with test tires inflated to an air pressure of <NUM> kPa for the front tires and <NUM> kPa for the rear tires was driven on the test course on wet road surfaces, and the professional test driver performed a filling evaluation with respect to braking performance. The evaluation is expressed as index values with the value of the Conventional Example being defined as the reference <NUM>. In the evaluation, larger index value indicates superior wet braking performance (drainage performance).

The performance evaluation tests were performed on <NUM> types of pneumatic tires including a tire according to Conventional Example as an example of a known tire, and Examples <NUM> to <NUM> corresponding to the tires <NUM> according to the present invention. All of these tires of Conventional Example and Examples <NUM> to <NUM> are provided with circumferential narrow grooves and sipes on the tread surface of the shoulder land portion. The groove width W of the circumferential narrow groove is set to <NUM>. Of these, in Conventional Example, the circumferential narrow groove is formed at a constant depth and includes no deep bottom portions.

In contrast, in all of Examples <NUM> to <NUM>, which are examples of the tire according to the present invention, the circumferential narrow groove has the deep bottom portion, and the deep bottom portion is provided in the central portion in the tire circumferential direction of the block. Furthermore, the tires according to Examples <NUM> to <NUM> are each different in a ratio of the depth da of the shallow bottom portion with respect to the maximum depth dc of the sipe (da/dc), a ratio of the depth db of the deep bottom portion with respect to the maximum depth dc of the sipe (db/dc), and a ratio of the circumferential length Lb of the deep bottom portion with respect to the circumferential length L of the circumferential narrow groove (Lb/L), and whether one end of the circumferential narrow groove is open to the lug groove.

As a result of performing the performance evaluation tests by using the tires <NUM>, it was revealed as indicated in <FIG> that compared with Conventional Example, the tires according to Examples <NUM> to <NUM> improve load durability performance and wet braking performance (drainage performance). In other words, the tires according to Examples <NUM> to <NUM> can provide load durability performance and drainage performance in a compatible manner.

Claim 1:
A tire (<NUM>, 1A), comprising:
in a tread portion (<NUM>),
a plurality of circumferential main grooves (<NUM>, 21A, <NUM>, 22A, <NUM>, 23A, <NUM>, 24A, 25A) extending in a tire circumferential direction;
a plurality of lug grooves (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) extending in a direction intersecting the circumferential main grooves (<NUM>, 21A, <NUM>, 22A, <NUM>, 23A, <NUM>, 24A, 25A); and
a plurality of blocks B defined by the circumferential main grooves (<NUM>, 21A, <NUM>, 22A, <NUM>, 23A, <NUM>, 24A, 25A) and the lug grooves (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>),
a tread surface (<NUM>, 12A) of the block B comprising circumferential narrow grooves (<NUM>, <NUM>, <NUM>) each extending in the tire circumferential direction and a plurality of sipes (<NUM>) that are provided on both sides of the circumferential narrow grooves (<NUM>, <NUM>, <NUM>) in a tire width direction and are separated from the circumferential narrow grooves (<NUM>, <NUM>, <NUM>), the plurality of sipes (<NUM>) being disposed side by side in the tire circumferential direction and extending in the tire width direction,
the circumferential narrow groove (<NUM>, <NUM>, <NUM>) comprising a shallow bottom portion (<NUM>) and a deep bottom portion (<NUM>) having different depths from the tread surface (<NUM>, 12A), and
the deep bottom portion (<NUM>) having a depth from the tread surface (<NUM>, 12A) deeper than a depth of the shallow bottom portion (<NUM>) and being provided in a central portion of the block B in the tire circumferential direction,
characterized in that
a maximum depth dc of the sipe (<NUM>) is set to <NUM> ≤ dc ≤ <NUM>, wherein a depth db of the deep bottom portion (<NUM>) and the maximum depth dc of the sipe (<NUM>) satisfy <NUM> ≤ db/dc ≤ <NUM>.