Pneumatic tire

Provided is a pneumatic tire including a tread portion, sidewall portions, and bead portions. The tread portion comprises a center main groove extending in the tire circumferential direction and a shoulder main groove located outward of the center main groove and extending in the tire circumferential direction. A land portion is defined between the center main groove and the shoulder main groove. The center main groove has a zigzag shape in the tire circumferential direction. Lug grooves extending inward from the shoulder main groove in the tire lateral direction and terminating without communicating with the center main groove are provided in the land portion. A bent portion bent toward a first side in the tire circumferential direction is formed at a terminating end of each of the lug grooves. Narrow grooves extending discontinuously in the tire circumferential direction without communicating with the bent portions are formed in the land portion.

TECHNICAL FIELD

The present technology relates to a pneumatic tire including a tread portion including a center main groove extending in the tire circumferential direction, shoulder main grooves located outward of the center main groove and extending in the tire circumferential direction, and land portions extending in the tire circumferential direction defined between the center main groove and the shoulder main grooves; and particularly relates to a pneumatic tire including a tread pattern with an appropriate configuration that can achieve good steering stability on dry road surfaces and steering stability on wet road surfaces, which have a negative correlation, in a compatible manner and furthermore can provide improved uneven wear resistance.

BACKGROUND ART

Pneumatic tires have a tread pattern in which a plurality of rows of land portions are defined by a plurality of main grooves disposed in a tread portion and extend in the tire circumferential direction (for example, see Japanese Unexamined Patent Application Publication No. 2012-228992). Such pneumatic tires are provided with a plurality of lug grooves extending in the tire lateral direction in each of the land portions in the tread portion and thus ensure excellent drainage performance with the lug grooves.

Unfortunately, if the number of the lug grooves in the tread portion is increased, the rigidity of the tread portion decreases and steering stability on dry road surfaces declines. Conversely, if the number of the lug grooves in the tread portion is decreased, drainage performance decreases and steering stability on wet road surfaces declines. In this way, steering stability on dry road surfaces and steering stability on wet road surfaces have a negative correlation with each other, and it is difficult to enhance both simultaneously.

If the tread portion is subdivided by the main grooves and the lug grooves, the tread portion is susceptible to uneven wear due to nonuniform rigidity. To prevent uneven wear, there are constraints on the arrangement of the main grooves and the lug grooves. Thus, it is more difficult to enhance uneven wear resistance as well as achieving good steering stability on dry road surfaces and steering stability on wet road surfaces in a compatible manner.

SUMMARY

The present technology provides a pneumatic tire including a tread pattern with an appropriate configuration that can achieve good steering stability on dry road surfaces and steering stability on wet road surfaces, which have a negative correlation, in a compatible manner and furthermore can provide improved uneven wear resistance.

A pneumatic tire according to the present technology includes a tread portion extending in a tire circumferential direction and formed into an annular shape; a pair of sidewall portions disposed on both sides of the tread portion; and a pair of bead portions disposed inward of the sidewall portions in a tire radial direction. The tread portion includes a center main groove extending in the tire circumferential direction and a shoulder main groove located outward of the center main groove and extending in the tire circumferential direction; a land portion is defined between the center main groove and the shoulder main groove; the center main groove has a zigzag shape in the tire circumferential direction; a plurality of lug grooves are provided in the land portion, the lug grooves extending inward from the shoulder main groove in a tire lateral direction and terminating without communicating with the center main groove; a bent portion is formed at a terminating end of each of the lug grooves, the bent portion being bent toward a first side in the tire circumferential direction; a plurality of narrow grooves are formed in the land portion, the narrow grooves extending discontinuously in the tire circumferential direction without communicating with the bent portions; and the narrow grooves are arranged substantially parallel with the center main groove having the zigzag shape.

According to the present technology, the zigzag center main groove is provided in the tread portion, and the lug grooves are provided in the land portion between the zigzag center main groove and the shoulder main groove. This configuration can ensure steering stability on wet road surfaces. Especially the zigzag center main groove contributes to improvement in steering stability on wet road surfaces with edge effect thereof. Furthermore, the lug grooves extending inward from the shoulder main groove in the tire lateral direction are provided with the bent portions, and the narrow grooves are provided discontinuously in the tire circumferential direction. This configuration can increase the effect of enhancing wet performance with edge effect thereof.

The center main groove has a zigzag shape, and the lug grooves formed in the land portion adjacent to the center main groove terminate in the land portion. This configuration ensures sufficient rigidity of the land portion and achieves good steering stability on dry road surfaces and steering stability on wet road surfaces in a compatible manner at a higher level. Furthermore, the narrow grooves arranged substantially parallel with the zigzag center main groove make the rigidity of the land portion uniform and effectively prevent generation of uneven wear.

According to the present technology, an inclination angle α of the lug grooves including the bent portions with respect to the tire circumferential direction preferably ranges from 25° to 75°. By setting the inclination angle α of the lug grooves with respect to the tire circumferential direction in this range, the effect of enhancing steering stability on dry road surfaces can be sufficiently ensured.

A depth Ds of the narrow grooves and a depth Dc of the center main groove having the zigzag shape preferably satisfy a relationship of 0.10×Dc≤Ds≤0.50×Dc. A depth Ds of the narrow grooves in this range can effectively enhance steering stability on dry road surfaces and uneven wear resistance.

A distance d1between the narrow grooves and the center main groove having the zigzag shape in a tire axial direction and a width d2of the land portion in the tire axial direction preferably satisfy a relationship of 0.10×d2≤d1≤0.40×d2. A distance d1between the narrow grooves and the zigzag center main groove in this range can exhibit the effect of enhancing uneven wear resistance to the utmost.

Preferably, the tread portion includes four main grooves including a pair of center main grooves extending in the tire circumferential direction and a pair of shoulder main grooves located outward of the center main grooves and extending in the tire circumferential direction; and at least one of the paired center main grooves has a zigzag shape in the tire circumferential direction. This arrangement of the main grooves can enhance steering stability on dry road surfaces, steering stability on wet road surfaces, and uneven wear resistance in a compatible manner.

DETAILED DESCRIPTION

Configurations according to the present technology is described below in detail with reference to the accompanying drawings.FIGS. 1 to 3illustrate a pneumatic tire according to an embodiment of the present technology.

As illustrated inFIG. 1, a pneumatic tire of the present embodiment includes an annular tread portion1extending in the tire circumferential direction, a pair of sidewall portions2,2disposed on both sides of the tread portion1, and a pair of bead portions3,3disposed inward of the sidewall portions2in the tire radial direction.

A carcass layer4is mounted between the pair of bead portions3,3. The carcass layer4includes a plurality of reinforcing cords extending in the tire radial direction, and is folded back around a bead core5disposed in each of the bead portions3from a tire inner side to a tire outer side. A bead filler6having a triangular cross-sectional shape formed from rubber composition is disposed on a periphery of the bead core5.

On the other hand, a plurality of belt layers7are embedded on an outer circumferential side of the carcass layer4in the tread portion1. The belt layers7include a plurality of reinforcing cords that incline with respect to the tire circumferential direction, and the directions of the reinforcing cords of the different layers intersect each other. In the belt layers7, an inclination angle of the reinforcing cords with respect to the tire circumferential direction is set in a range from, for example, 10° to 40°. Steel cords are preferably used as the reinforcing cords of the belt layers7. For the purpose of improving high-speed durability, at least one layer of a belt cover layer8formed by arranging reinforcing cords at an angle of, for example, not greater than 5° with respect to the tire circumferential direction, is disposed on an outer circumferential side of the belt layers7. Nylon, aramid, or similar organic fiber cords are preferably used as the reinforcing cords of the belt cover layer8.

Note that the tire internal structure described above is exemplary of a pneumatic tire, but is not limited thereto.

As illustrated inFIG. 2, four main grooves11to14extending in the tire circumferential direction are formed in the tread portion1. That is, a pair of center main grooves11,12located on both sides of a tire equator CL and a pair of shoulder main grooves13,14located outward of the center main grooves11,12in the tire lateral direction are formed in the tread portion1. The center main groove12has a zigzag shape in the tire circumferential direction, whereas the other main grooves11,13,14are linear. These four main grooves11to14define, in the tread portion1, a center land portion21located on the tire equator CL, an intermediate land portion22located on a first side of the center land portion21in the tire lateral direction, an intermediate land portion23located on a second side of the center land portion21in the tire lateral direction, a shoulder land portion24located on the first side of the intermediate land portion22in the tire lateral direction, and a shoulder land portion25located on the second side of the intermediate land portion23in the tire lateral direction.

A plurality of lug grooves31A,31B,33A,33B,34A,34B extending from the main grooves11,13,14, except the zigzag center main groove12, toward both sides in the tire lateral direction and terminating in the land portions21to25are formed at intervals in the tire circumferential direction in the tread portion1.

Specifically, the lug grooves31A each communicate with the center main groove11at a first end and terminate within the center land portion21at a second end, and the lug grooves31B each communicate with the center main groove11at a first end and terminate within the intermediate land portion22at a second end. The lug grooves33A each communicate with the shoulder main groove13at a first end and terminate within the intermediate land portion22at a second end, and the lug grooves33B each have a first end communicating with the shoulder main groove13and a second end terminating in the shoulder land portion24. The lug grooves34A each communicate with the shoulder main groove14at a first end and terminate within the intermediate land portion23at a second end, and the lug grooves34B each have a first end communicating with the shoulder main groove14and a second end terminating in the shoulder land portion25.

Note that the lug grooves31A and the lug grooves31B are preferably arranged facing each other but may be arranged, for example, offset in position in the tire circumferential direction to alleviate pattern noise. Such a relationship is also applied to the arrangement of the lug grooves33A and the lug grooves33B and the arrangement of the lug grooves34A and the lug grooves34B.

The lug grooves34A extending inward from the shoulder main groove14in the tire lateral direction each include a bent portion34C at the terminating end in the intermediate land portion23. The bent portion34C is bent into a hook shape toward a first side in the tire circumferential direction. In the intermediate land portion23in which the lug grooves34A including the bent portions34C are formed, a plurality of narrow grooves41are formed extending discontinuously in the tire circumferential direction without communicating with the bent portions34C. The narrow grooves41have a groove width of 3.0 mm or less and contain so-called sipes. The narrow grooves41are arranged substantially parallel with the zigzag center main groove12.

It is not necessary that the narrow grooves41are exactly parallel with the center main groove12. The narrow grooves41and the center main groove12can be regarded as being substantially parallel with each other when (d1max−d1min)/d1max≤0.1 is satisfied, where d1is a distance between the narrow grooves41and the center main groove12in the tire axial direction, d1min is the minimum value of the distance d1, and d1max is the maximum value of the distance d1.

A circumferential auxiliary groove42extending in the tire circumferential direction is formed in the shoulder land portion24. The circumferential auxiliary groove42has a groove width ranging from 0.8 mm to 3.0 mm. A plurality of shoulder lug grooves43extending inward from an end portion of the tread portion1in the tire lateral direction are formed at intervals in the tire circumferential direction in the shoulder land portion24. The shoulder lug grooves43intersect the circumferential auxiliary groove42and terminate without reaching the shoulder main groove13.

A plurality of shoulder lug grooves44extending inward from an end portion of the tread portion1in the tire lateral direction are formed at intervals in the tire circumferential direction in the shoulder land portion25. The shoulder lug grooves44terminate without reaching the shoulder main groove14. A plurality of sipes45extending outward from tip portions of the lug grooves34B in the tire lateral direction are formed in the shoulder land portion25.

The above-described pneumatic tire is provided, in the tread portion1, with the zigzag center main groove12and the lug grooves34A in the intermediate land portion23between the zigzag center main groove12and the shoulder main groove14. Thus, steering stability on wet road surfaces can be ensured. In particular, the zigzag center main groove12with its edge effect contributes to improvement in steering stability on wet road surfaces. Furthermore, the lug grooves34A extending inward from the shoulder main groove14in the tire lateral direction are provided with the bent portions34C, and the narrow grooves41are provided discontinuously in the tire circumferential direction. This configuration can increase the effect of enhancing wet performance with the edge effect thereof.

The center main groove12has a zigzag shape, and the lug grooves34A formed in the land portion23adjacent to the center main groove12terminate in the land portion23. This configuration ensures sufficient rigidity of the land portion23and achieves good steering stability on dry road surfaces and steering stability on wet road surfaces in a compatible manner to a high degree. Furthermore, the narrow grooves41arranged substantially parallel with the zigzag center main groove12make the rigidity of the land portion23uniform and effectively prevent generation of uneven wear.

The pneumatic tire is provided, in the tread portion1, with the lug grooves31A,31B,33A,33B,34A,34B extending from the main grooves11,13,14, except the zigzag center main groove12, toward both sides in the tire lateral direction and terminating in the land portions21to25and can thus ensure excellent drainage performance while minimizing a decrease in the rigidity of the tread portion1. That is, the lug grooves31A,31B,33A,33B,34A,34B do not completely divide the land portions21to25while exhibiting effective drainage performance by guiding water on road surfaces to the main grooves11,13,14, so that the high rigidity of the tread portion1can be maintained. This configuration achieves good steering stability on dry road surfaces and steering stability on wet road surfaces in a compatible manner to a high degree.

As illustrated inFIG. 3, in the pneumatic tire, an inclination angle α of the lug grooves34A including the bent portions34C with respect to the tire circumferential direction is preferably in a range from 25° to 75°. By setting the inclination angle α of the lug grooves34A with respect to the tire circumferential direction in this range, the effect of enhancing steering stability on dry road surfaces can be sufficiently ensured. An inclination angle α of less than 25° forms acute portions in the land portion23and thus locally decreases the rigidity, resulting in an adverse effect on steering stability on dry road surfaces. Conversely, an inclination angle α of greater than 75° causes both sides of the lug grooves34A to exhibit different behavior and thus decreases the rigidity as a pattern, resulting in an adverse effect on steering stability on dry road surfaces. Note that the inclination angle α of the lug grooves34A refers to the angle of a straight line connecting groove width center positions P1, P2on both longitudinal sides of the lug grooves34A except the bent portion34C, with respect to the tire circumferential direction.

As illustrated inFIG. 4, in the pneumatic tire, a depth Ds of the narrow grooves41and a depth Dc of the zigzag center main groove12preferably satisfy a relationship of 0.10×Dc≤Ds≤0.50×Dc. A depth Ds of the narrow grooves41in this range can effectively enhance steering stability on dry road surfaces and uneven wear resistance. A ratio Ds/Dc of less than 0.10 decreases the effect of enhancing uneven wear resistance, and conversely, a ratio Ds/Dc of greater than 0.50 decreases the rigidity of the land portion23, resulting in an adverse effect on steering stability on dry road surfaces.

In the pneumatic tire, the distance d1between the narrow grooves41and the zigzag center main groove12in the tire axial direction and a width d2of the land portion23in the tire axial direction preferably satisfy a relationship of 0.10×d2≤d1≤0.40×d2. A distance d1between the narrow grooves41and the zigzag center main groove12in this range can exhibit the effect of enhancing uneven wear resistance to the utmost. A ratio d1/d2outside this range does not sufficiently make the rigidity of the land portion23uniform, resulting in a decrease in the effect of enhancing uneven wear resistance. Note that the width d2of the land portion23refers to the minimum width of the land portion23adjacent to the zigzag center main groove12and that if the distance d1between the narrow grooves41and the center main groove12varies, the distance d1refers to the average value of the minimum value d1min and the maximum value d1max of the distance d1.

The above-described embodiment has exemplified the case in which the pair of center main grooves11,12and the pair of shoulder main grooves13,14are disposed in the tread portion1, and the center main groove12among the main grooves has a zigzag shape in the tire circumferential direction. However, in the present technology, both the paired center main grooves11,12may have zigzag shapes in the tire circumferential direction. For example, the configuration of the right side of the tire equator CL in the tread pattern inFIG. 2can be applied to the left side of the tire equator CL in a mirror or point symmetrical manner. Alternatively, the configuration of the left side of the tire equator CL in the tread pattern can be selected as desired as long as the right side has the configuration illustrated inFIG. 2.

EXAMPLES

Tires of Examples 1 to 7 were manufactured with a tire size of 215/55R17. The pneumatic tires each included a tread portion, a pair of sidewall portions, and a pair of bead portions. As illustrated inFIG. 2, the pneumatic tire was provided, in the tread portion, with four main grooves including a pair of center main grooves extending in the tire circumferential direction and a pair of shoulder main grooves located outward of the center main grooves and extending in the tire circumferential direction; these main grooves defined five rows of land portions; one of the center main grooves had a zigzag shape in the tire circumferential direction, and the other main groove had a straight shape; a plurality of lug grooves extending inward from the shoulder main groove in the tire lateral direction and terminating without communicating with the center main groove were provided in the land portion located between the zigzag center main groove and the shoulder main groove; the lug grooves included bent portions bent toward the first side in the tire circumferential direction at the terminating ends; a plurality of narrow grooves extending discontinuously in the tire circumferential direction without communicating with the bent portions were formed in the land portion; and the narrow grooves were arranged substantially parallel with the zigzag center main groove.

A tire of Conventional Example was prepared for comparison. The tire was provided, in the tread portion, with four main grooves including a pair of center main grooves extending in the tire circumferential direction and a pair of shoulder main grooves located outward of the center main grooves and extending in the tire circumferential direction; these main grooves defined five rows of land portions; all the main grooves had a straight shape; and a plurality of lug grooves communicating with the main grooves on both sides thereof were provided between the main grooves.

A tire of Comparative Example 1 was prepared that had the same configuration as the tire of Example 1 except that all the main grooves had a straight shape and that the bent portions of the lug grooves and the narrow grooves were not provided. A tire of Comparative Example 2 was prepared that had the same configuration as the tire of Example 1 except that the bent portions of the lug grooves and the narrow grooves were not provided.

For Examples 1 to 7 and Comparative Examples 1 and 2, the inclination angle α of the lug grooves including the bent portions, the depth Ds of the narrow grooves, the depth Dc of the center main groove, the distance d1between the narrow grooves and the center main groove, and the width d2of the land portion including the narrow grooves were set as shown in Table 1.

These test tires were evaluated for steering stability on dry road surfaces, steering stability on wet road surfaces, and uneven wear resistance according to the following test methods. The results thereof are shown in Table 1.

Steering Stability on Dry Road Surfaces:

The test tires were assembled on wheels having a rim size of 17×7.5J and mounted on a front wheel drive vehicle of 2400 cc engine displacement, and the air pressure (F/R) after warm-up was set to 230 kPa/220 kPa. After the vehicle traveled on a dry road surface, panelists conducted sensory evaluation. Evaluation results were expressed as index values with the Conventional Example being defined as 100. Larger index values indicate superior steering stability on dry road surfaces.

Steering Stability on Wet Road Surfaces:

The test tires were assembled on wheels having a rim size of 17×7.5J and mounted on a front wheel drive vehicle of 2400 cc engine displacement, and the air pressure (F/R) after warm-up was set to 230 kPa/220 kPa. The lap time was measured under a rainy condition on a paved test course. The evaluation results were expressed, using the reciprocal of the measured value, with the value of the Conventional Example being defined as 100. Larger index values indicate superior steering stability on wet road surfaces.

The test tires were assembled on wheels having a rim size of 17×7.5J and mounted on a front wheel drive vehicle of 2400 cc engine displacement, and the air pressure (F/R) after warm-up was set to 230 kPa/220 kPa. After the vehicle traveled 10000 km in the market, the amounts of wear of the center main grooves and the shoulder main grooves were measured, and the difference between the amounts was calculated. The evaluation results were expressed, using the reciprocal of the difference, as index values, with the value of the Conventional Example being defined as 100. Larger index values indicate superior uneven wear resistance.

As can be seen from Table 1, in the tires of Examples 1 to 7, steering stability on dry road surfaces and steering stability on wet road surfaces were enhanced simultaneously, and furthermore uneven wear resistance was enhanced, in comparison with the tire of Conventional Example. The tires of Examples 1 to 7 also achieved excellent results in comparison with the tire of Comparative Example 1 that was not provided with the zigzag center main groove, the lug grooves including the bent portions, and the narrow grooves, and the tire of Comparative Example 2 that was not provided with the lug grooves including the bent portions and the narrow grooves.