Patent Description:
When a pneumatic tire having a plurality of sipes in a tread portion is vulcanized and molded, there is a problem that a tire mold and a tread rubber are bonded and poor appearance occurs in the vulcanized pneumatic tire. Such a bonding between the tire mold and the tread rubber is significant in winter tires in which a large number of sipes are disposed in a tread portion. In contrast, it has been proposed to reduce the number of sipes in a boundary region including a divided position of a sector and shorten a length of a sipe in a tire width direction (for example, see Patent Documents <NUM>, <NUM>).

However, reducing the number of sipes and shortening the length in the tire width direction not only changes the appearance of the tire, but also causes a problem that braking performance on wet road surfaces and snow-covered road surfaces based on the sipes cannot be sufficiently obtained.

<CIT>, <CIT> and <CIT> disclose different sipe depths in different regions of a tread.

An object of the present invention is to provide a pneumatic tire that allows enhancing braking performance on wet road surfaces and snow-covered road surfaces without changing an appearance of the tire, and reducing poor appearance.

A pneumatic tire to achieve the object includes a tread portion according to claim <NUM>.

In the embodiment of the present invention, the pneumatic tire includes the plurality of circumferential grooves, the plurality of rows of land portions, and the plurality of sipes. The circumferential grooves extend in the tire circumferential direction in the tread portion. The land portions are defined by the circumferential grooves. The sipes extend in the tire width direction in at least one row of the land portion. The tread portion is molded by the plurality of sectors divided in the tire circumferential direction. The sipes include at least one shallow bottom sipe and the plurality of regular sipes. The shallow bottom sipe is disposed in the boundary region including the dividing position of the sector. The regular sipes are disposed at the positions more separated from the dividing position of the sector than the shallow bottom sipe. The shallow bottom sipe is shallower than the regular sipe. Thus, while braking performance on wet road surfaces and snow-covered road surfaces is enhanced based on the sipes, bonding between a tire mold and a tread rubber can be suppressed based on the shallow bottom sipes in the boundary region, and poor appearance can be reduced. Additionally, by providing the shallow bottom sipes in the boundary region and changing the depth of the sipes, the appearance of the tire is not changed.

In the embodiment of the present invention, the boundary region is a region within <NUM> on one side in the tire circumferential direction with the dividing position of each of the plurality of sectors as a center. As a result, poor appearance can be effectively suppressed while the braking performance on wet road surfaces and snow-covered road surfaces is effectively enhanced.

In the embodiment of the present invention, the at least one shallow bottom sipe preferably has a depth in a range of from <NUM>% to <NUM>% to an average maximum depth of the plurality of regular sipes in each of the plurality of rows of land portions including the at least one shallow bottom sipe. As a result, the bonding between the tire mold and the tread rubber can be effectively suppressed while the braking performance on wet road surfaces and snow-covered road surfaces is effectively enhanced.

In the embodiment of the present invention, the shallow bottom sipes are disposed most in each of the plurality of rows of land portions located on an outer side in the tire width direction among the plurality of rows of land portions in the tread portion. As a result, a mold release property of the sipes can be enhanced, and the bonding between the tire mold and the tread rubber can be effectively suppressed.

In the embodiment of the present invention, an inclination angle of each of the plurality of sipes with respect to the dividing line of each of the plurality of sectors is defined as θ, the number of the shallow bottom sipes that satisfy <NUM>° ≤ θ ≤ <NUM>° is preferably larger than the number of the shallow bottom sipes that do not satisfy <NUM>° ≤ θ ≤ <NUM>°. As a result, the bonding between the tire mold and the tread rubber can be effectively suppressed.

Configurations of embodiments of the present invention will be described in detail below with reference to the accompanying drawings. <FIG> and <FIG> illustrate a pneumatic tire according to an embodiment of the present invention. In <FIG>, Tc indicates a tire circumferential direction and Tw indicates a tire width direction.

As illustrated in <FIG>, a pneumatic tire according to an embodiment of the present invention includes an annular tread portion <NUM> extending in the tire circumferential direction, a pair of sidewall portions <NUM>, <NUM> disposed on both sides of the tread portion <NUM>, and a pair of bead portions <NUM>, <NUM> disposed inward of the sidewall portions <NUM> in the tire radial direction.

A carcass layer <NUM> is mounted between the pair of bead portions <NUM>, <NUM>. The carcass layer <NUM> includes a plurality of reinforcing cords extending in the tire radial direction and is folded back around bead cores <NUM> disposed in each of the bead portions <NUM> from a tire inner side to a tire outer side. A bead filler <NUM> having a triangular cross-sectional shape formed from rubber composition is disposed on the outer circumference of the bead core <NUM>.

A plurality of belt layers <NUM> are embedded on the outer circumferential side of the carcass layer <NUM> in the tread portion <NUM>. The belt layers <NUM> each include a plurality of reinforcing cords that are inclined with respect to the tire circumferential direction, the reinforcing cords being disposed between layers in a criss-cross manner. In the belt layers <NUM>, the inclination angle of the reinforcing cords with respect to the tire circumferential direction falls within a range of from <NUM>° to <NUM>°, for example. Steel cords are preferably used as the reinforcing cords of the belt layers <NUM>. To improve high-speed durability, at least one belt cover layer <NUM>, formed by arranging reinforcing cords at an angle of, for example, not larger than <NUM>° with respect to the tire circumferential direction, is disposed on an outer circumferential side of the belt layers <NUM>. Nylon, aramid, or similar organic fiber cords are preferably used as the reinforcing cords of the belt cover layer <NUM>.

Note that the tire internal structure described above represents a typical example for a pneumatic tire, and the pneumatic tire is not limited thereto.

The pneumatic tire is vulcanized and molded using a sectional tire mold. The tire mold includes an annular side mold for molding the tread portion <NUM>, and the side mold includes sectors plurally divided along the tire circumferential direction. The sectors divided into <NUM> to <NUM> in the tire circumferential direction are usually used. As illustrated in <FIG>, the tread portion <NUM> has a dividing position P of the sector, and includes a boundary region S including the dividing position P of the sector. The boundary region S is a region having a predetermined length on both sides in the tire circumferential direction with the dividing position P of the sector as its center.

Four circumferential grooves <NUM> extending in the tire circumferential direction are formed in the tread portion <NUM>. The circumferential grooves <NUM> includes a pair of inner circumferential grooves 9A located on both sides of a tire center line CL and a pair of outer circumferential grooves 9B located on the outermost side in the tire width direction. Land portions <NUM> are defined by the four circumferential grooves <NUM> in the tread portion <NUM>. The land portions <NUM> include a center land portion 10A located on the tire center line CL, a pair of intermediate land portions 10B located on the outer side of the center land portion 10A in the tire width direction, and a pair of shoulder land portions 10C located on the outer side of the respective intermediate land portions 10B in the tire width direction. A plurality of sipes <NUM> extending in the tire width direction are formed in at least one row of the land portion <NUM> of the land portions 10A to 10C. The sipes <NUM> can be formed in a linear or zigzag-like manner on a road contact surface of the tread portion <NUM>. The sipes <NUM> are narrow grooves having a groove width of <NUM> or less.

More specifically, a plurality of sipes 20A and a plurality of narrow grooves <NUM>, which are inclined in the same direction with respect to the tire width direction, are disposed at intervals in the tire circumferential direction in the center land portion 10A. One end of the sipe 20A communicates with the inner circumferential groove 9A, and the other end communicates with the narrow groove <NUM>. On the other hand, the narrow groove <NUM> is a groove having a larger groove width than that of the sipe 20A. One end of the narrow groove <NUM> communicates with the inner circumferential groove 9A, and the other end communicates with the sipe 20A. The sipes 20A and the narrow grooves <NUM> are disposed alternately in the tire circumferential direction such that the narrow grooves <NUM> are disposed in a staggered manner in the tire circumferential direction as a whole of the center land portion 10A.

A plurality of lug grooves <NUM> inclined in the same direction with respect to the tire width direction are disposed at intervals in the tire circumferential direction in the intermediate land portion 10B. While one end of the lug groove <NUM> opens to the outer circumferential groove 9B, the other end terminates within the intermediate land portion 10B. The lug groove <NUM> includes a bent portion 32A formed in an acute angle manner at a midway position between the one end and the other end. A plurality of sipes 20B extending in a direction intersecting with the lug grooves <NUM> are disposed at intervals in the tire circumferential direction. The sipes 20B are divided into a plurality of portions by intersecting with the lug grooves <NUM>, and the divided portions are disposed on the same straight lines. At least one end of the sipe 20B communicates with the outer circumferential groove 9B.

A plurality of lug grooves <NUM> inclined in the same direction with respect to the tire width direction are disposed at intervals in the tire circumferential direction in the shoulder land portion 10C. The lug grooves <NUM> do not communicate with the outer circumferential groove 9B. A plurality of narrow grooves <NUM> that communicate with the lug grooves <NUM> and extend in the tire circumferential direction are formed. A plurality of sipes 20C inclined in the same direction with respect to the tire width direction are disposed at intervals in the tire circumferential direction. The sipes 20C do not communicate with the outer circumferential groove 9B. The sipes 20C are divided into a plurality of portions by intersecting with the narrow grooves <NUM>, and the divided portions are disposed on the same straight lines.

The sipe <NUM> (sipes 20A to 20C) described above includes at least one shallow bottom sipe <NUM> and a plurality of regular sipes <NUM>. In other words, all of the sipes <NUM> excluding the shallow bottom sipe <NUM> are the regular sipes <NUM>. The shallow bottom sipes <NUM> are disposed nearest to the dividing position P of the sector on both the respective sides of the dividing position P of the sector in the tire circumferential direction. The regular sipes <NUM> are disposed at positions more separated from the dividing position P of the sector than the shallow bottom sipes <NUM>.

The bottom of the shallow bottom sipe <NUM> is raised. Therefore, as illustrated in <FIG>, a depth d of the shallow bottom sipe <NUM> is shallower than a depth D of the regular sipe <NUM>. The shallow bottom sipe <NUM> shallower than the regular sipe <NUM> means that the shallow bottom sipe <NUM> is shallower than the average maximum depth of the regular sipe <NUM> in the land portion <NUM> including the shallow bottom sipe <NUM>. The average maximum depth of the regular sipe <NUM> is found by averaging the maximum depths of the regular sipes <NUM> included in the land portions <NUM> in the entire circumference of the tire. Note that when the sipe <NUM> has a plurality of step portions as illustrated in <FIG>, the depth (maximum depth) from the road contact surface of the tread portion <NUM> to the lowest step is measured as the depth of the sipe <NUM>.

The pneumatic tire described above includes the plurality of circumferential grooves <NUM>, the plurality of rows of the land portions <NUM>, and the plurality of sipes <NUM>. The circumferential grooves <NUM> extend in the tire circumferential direction in the tread portion <NUM>. The land portions <NUM> are defined by the circumferential grooves <NUM>. The sipes <NUM> extend in the tire width direction in at least one row of the land portion <NUM>. The tread portion <NUM> is molded by the plurality of sectors divided in the tire circumferential direction. The sipes <NUM> include at least one shallow bottom sipe <NUM> and the plurality of regular sipes <NUM>. The shallow bottom sipe <NUM> is disposed in the boundary region S including the dividing position P of the sector. The regular sipes <NUM> are disposed at the positions more separated from the dividing position P of the sector than the shallow bottom sipe <NUM>. The shallow bottom sipe <NUM> is shallower than the regular sipe <NUM>. Thus, while the braking performance on wet road surfaces and snow-covered road surfaces are enhanced based on the sipes <NUM>, the bonding between the tire mold and the tread rubber can be suppressed based on the shallow bottom sipes <NUM> in the boundary region S, and poor appearance can be reduced. Additionally, by providing the shallow bottom sipes <NUM> in the boundary region S and changing the depth of the sipes <NUM>, the appearance of the tire is not changed. Furthermore, this also contributes to enhancing the uniformity of the pneumatic tire.

In the pneumatic tire, the boundary region S is a region within <NUM> on one side in the tire circumferential direction with the dividing position P of the sector as its center, and more preferably a region within <NUM> on one side. By appropriately setting the boundary region S where the shallow bottom sipe <NUM> is disposed in this manner, poor appearance can be effectively suppressed while the braking performance on wet road surfaces and snow-covered road surfaces is effectively enhanced.

The depth of the shallow bottom sipe <NUM> is preferably in the range of from <NUM>% to <NUM>% to the average maximum depth of the regular sipe <NUM> in the land portion <NUM> including the shallow bottom sipe <NUM>, and more preferably in the range of from <NUM>% to <NUM>%. Appropriately setting the depth of the shallow bottom sipe <NUM> to the average maximum depth of the regular sipe <NUM> in this manner makes it possible to effectively suppress the bonding between the tire mold and the tread rubber while the braking performance on wet road surfaces and snow-covered road surfaces is effectively enhanced. Here, the ratio of the depth of the shallow bottom sipe <NUM> to the average maximum depth of the regular sipe <NUM> of less than <NUM>% increases an occurrence rate of poor appearance. Conversely, the ratio in excess of <NUM>% cannot sufficiently obtain the effect of enhancing the braking performance on wet road surfaces and snow-covered road surfaces.

In particular, the shallow bottom sipes <NUM> are disposed most in the land portions located on the outer side in the tire width direction among the land portions <NUM> in the tread portion <NUM>. In the case of the land portions 10A to 10C illustrated in <FIG>, the land portion <NUM> located on the outer side in the tire width direction refers to the shoulder land portion 10C located on the outermost side in the tire width direction. The bonding between the tire mold and the tread rubber is likely to occur in the shoulder land portion. Accordingly, by disposing the shallow bottom sipes <NUM> mostly in the land portion <NUM> located on the outer side in the tire width direction, the mold release property of the sipes <NUM> can be enhanced, and the bonding between the tire mold and the tread rubber can be effectively suppressed.

An inclination angle of the sipe <NUM> with respect to the dividing line of the sector is defined as θ (see <FIG>). The inclination angle θ of the sipe <NUM> is an angle formed by a straight line connecting groove width center positions of both ends in a longitudinal direction of the sipe <NUM> and the dividing line of the sector. At this time, the number of the shallow bottom sipes <NUM> that satisfy <NUM>° ≤ θ ≤ <NUM>° is preferably larger than the number of the shallow bottom sipes <NUM> that do not satisfy <NUM>° ≤ θ ≤ <NUM>°. In particular, the number of the shallow bottom sipes <NUM> that satisfy <NUM>° ≤ θ ≤ <NUM>° is more preferably larger than the number of the shallow bottom sipes <NUM> that do not satisfy <NUM>° ≤ θ ≤ <NUM>°. The closer to substantially parallel the sipes are disposed to the dividing line of the sector, the more likely the bonding between the tire mold and the tread rubber occurs. Accordingly, relatively increasing the shallow bottom sipes <NUM> satisfying the relationship of the inclination angle θ allows effectively suppressing the bonding between the tire mold and the tread rubber. Note that in <FIG>, the dividing position of the sector is parallel to the tire width direction.

While the embodiment of <FIG> illustrates an example in which the shallow bottom sipes <NUM> are provided on both sides of the dividing position P of the sector in the tire circumferential direction. However, the shallow bottom sipe <NUM> may be provided on only one side of the dividing position P of the sector in the tire circumferential direction. Additionally, while an example in which the shallow bottom sipes <NUM> are disposed at the positions separated from the dividing position P of the sector is illustrated, the shallow bottom sipes <NUM> is disposed on the dividing position P of the sector. In the embodiment of the present invention, even when the shallow bottom sipe <NUM> is disposed across the dividing position P of the sector, the bonding between the tire mold and the tread rubber can be suppressed based on the shallow bottom sipe <NUM> in the boundary region S.

In the embodiment of the present invention, it is sufficient that the shallow bottom sipe <NUM> is disposed to at least one dividing position P on the tire circumference, and the shallow bottom sipes <NUM> need not to be disposed to the dividing positions P of all sectors on the tire circumference. Additionally, it is sufficient that shallow bottom sipe <NUM> is disposed in at least one row of the land portion <NUM> in the tread portion <NUM>, and the shallow bottom sipes <NUM> need not to be disposed in all land portions <NUM> in the tread portion <NUM>.

The tire to which the sipe structure according to the embodiment of the present invention is applied can be a winter tire having a large number of sipes and provides a significant effect of suppressing the bonding between the tire mold and the tread rubber, in addition to an effect of enhancing the braking performance on wet road surfaces and snow-covered road surfaces.

In a pneumatic tire having a tire size of <NUM>/55R16 that includes a plurality of circumferential grooves extending in a tire circumferential direction in a tread portion, a plurality of rows of land portions defined by the circumferential grooves, a plurality of sipes extending in a tire width direction in at least one row of the land portion, and the tread portion molded by a plurality of sectors divided in the tire circumferential direction, the presence of the sipe in a boundary region, the presence of a shallow bottom sipe, the range of the boundary region, the proportion of raised bottom of the shallow bottom sipe, the comparison of the number of the shallow bottom sipes in each land portion, the inclination angle θ of the sipe in the center land portion, the inclination angle θ of the sipe in the intermediate land portion, and the inclination angle θ of the sipe in the shoulder land portion were set as in Table <NUM>, and tires of Conventional Example, Comparative Example, and Examples <NUM> to <NUM> were manufactured.

Note that the tire of Conventional Example has the reduced number of sipes in the boundary region, and structures without the sipes are disposed in the boundary region. The tire of Comparative Example has a structure in which the shallow bottom sipes are not disposed in the boundary region, and only regular sipes are disposed.

In Table <NUM>, the range of the boundary region indicates a distance to one side in the tire circumferential direction with the dividing line of the sector as its center. The proportion of raised bottom of the shallow bottom sipe is the proportion of the depth of the shallow bottom sipe to the average maximum depth of the regular sipe in the land portion including the shallow bottom sipe. In the comparison of the numbers of shallow bottom sipes in the respective land portions, "Equivalent" means that the numbers of the shallow bottom sipes disposed in the respective land portions are the same, "Shoulder land portion" or "Intermediate land portion" means that the land portion has the largest number of shallow bottom sipes.

These test tires were evaluated for the braking performance on wet road surfaces, the braking performance on snow-covered road surfaces, and a poor appearance occurrence rate by the following test methods. Table <NUM> shows the results.

The test tires were each mounted on a wheel with a rim size of <NUM>×<NUM> J, inflated to an air pressure of <NUM> kPa, and mounted on a test vehicle having an engine displacement of <NUM> cc. Braking was performed from a traveling condition at a speed of <NUM>/h on wet road surfaces, and a braking distance until the vehicle came to a complete stop was measured. The evaluation results were expressed, using the reciprocal of the measurement values, as index values with the value of the Conventional Example being defined as <NUM>. Larger index values indicate superior braking performance on wet road surfaces.

The test tires were each mounted on a wheel with a rim size of <NUM>×<NUM> J, inflated to an air pressure of <NUM> kPa, and mounted on a test vehicle having an engine displacement of <NUM> cc. Braking was performed from a traveling condition at a speed of <NUM>/h on snow-covered road surfaces, and a braking distance until the vehicle came to a complete stop was measured. The evaluation results were expressed, using the reciprocal of the measurement values, as index values with the value of the Conventional Example being defined as <NUM>. Larger index values indicate superior braking performance on snow-covered road surfaces.

<NUM> of each of the test tires were manufactured, chips and cracks of the land portion in each boundary region on the tire circumference were visually confirmed, and the number of boundary regions where the chips and the cracks occurred was counted. The evaluation results show the proportion of the number of boundary regions where chips and cracks occurred in each of the test tires. Larger proportions indicate a superior suppressing effect on poor appearance.

As can be seen from Table <NUM>, the tires of Examples <NUM> to <NUM> enhanced the braking performance on wet road surfaces, the braking performance on snow-covered road surfaces, and the poor appearance occurrence rate in a well-balanced manner as compared to those of Conventional Example.

Claim 1:
A pneumatic tire comprising a tread portion (<NUM>) molded by a plurality of sectors divided in a tire circumferential direction, the pneumatic tire comprising:
a plurality of circumferential grooves (<NUM>) extending in the tire circumferential direction in the tread portion (<NUM>);
a plurality of rows of land portions (10A, 10B, 10C) defined by the plurality of circumferential grooves (<NUM>); and
a plurality of sipes (<NUM>) extending in a tire width direction in at least one row of the plurality of rows of land portions (10A, 10B, 10C),
the plurality of sipes (<NUM>) including at least one shallow bottom sipe (<NUM>) and a plurality of regular sipes (<NUM>), the at least one shallow bottom sipe (<NUM>) being disposed in a boundary region (S) comprising a dividing position (P) of each of the plurality of sectors, the plurality of regular sipes (<NUM>) being disposed at positions more separated from the dividing position (P) of each of the plurality of sectors than the at least one shallow bottom sipe (<NUM>), the at least one shallow bottom sipe (<NUM>) being shallower than the plurality of regular sipes (<NUM>),
wherein the shallow bottom sipe (<NUM>) is shallower than an average maximum depth of the regular sipe (<NUM>) in the land portion (10A, 10B, 10C) including the shallow bottom sipe (<NUM>),
wherein the boundary region (S) is a region within <NUM> on one side in the tire circumferential direction with the dividing position (P) of each of the plurality of sectors as a center,
characterized in that
the shallow bottom sipes (<NUM>) are disposed most in a shoulder land portion (10C) located on the outermost side in tire width direction among the pluralities of rows of land portions (10A, 10B, 10C).