Pneumatic tire

A pneumatic tire including a closed sipe 5 formed in a tread surface 1 that includes a small hole 2 extending along a tire radial direction and a plurality of cuts 4 extending in a radiation direction from the small hole 2 and that terminating in a land portion 3. A diameter of an inscribed circle of the small hole 2 is greater than a thickness of the cuts 4. The cuts 4 are provided with a twist in a depth direction centered on the small hole 2, and a twist angle θ from a top surface to a bottom surface of the land portion 3 is not less than 10° and less than 135°.

TECHNICAL FIELD

The present technology relates to a pneumatic tire, and particularly relates to a pneumatic tire configured so that braking performance when traveling on icy road surfaces is enhanced without hindering the steering stability of a tire having sipes formed in a tread surface thereof.

BACKGROUND ART

Sipes are commonly provided in a tread surface in order to enhance water absorption in order to enhance traveling performance on wet road surfaces and icy and snowy road surfaces. However, if an excessive number of sipes are disposed in the tread surface, the tread rigidity will decline, leading to steering stability and braking ability being negatively affected. Therefore, conventionally, various technologies have been proposed regarding the form and arrangement of the sipe (e.g. see Japanese Unexamined Patent Application No. H9-263111A and Japanese Unexamined Patent Application Publication No. 2006-27306A).

Of these, Japanese Unexamined Patent Application No. H9-263111A describes enhancing steering stability on ice while preventing damage such as the sipes cracking or chunks being taken out of the tire by disposing two or more sipes in a block face that extend in mutually differing directions so as to cross. Additionally, Japanese Unexamined Patent Application Publication No. 2006-27306A describes enhancing riding comfort while ensuring steering stability and wet braking performance by providing a sipe with a shape having a twist around a twisting axis that extends in a tire radial direction.

However, in the case of Japanese Unexamined Patent Application No. H9-263111A, while the water absorption of the tread surface is enhanced to a certain degree, there is a limitation in that it becomes difficult to maintain steering stability if further enhancements of the water absorption are attempted. Additionally, in the case of Japanese Unexamined Patent Application Publication No. 2006-27306A, due to the twist angle being set to a size reaching 135° and greater, there are problems such as releasability from a mold after the tire is vulcanization molded being negatively affected and the tread surface becoming easily damageable.

SUMMARY TECHNOLOGY

The present technology provides a pneumatic tire configured so that water absorption can be enhanced while suppressing a decline in tread rigidity in a tire having a sipe formed in a tread surface; braking performance on ice can be enhanced while maintaining steering stability performance; and releasability from a mold is not hindered.

The pneumatic tire of the present technology includes a closed sipe that is formed in a tread surface and includes a small hole extending in a tire radial direction and a plurality of cuts extending in a radiation direction from the small hole and terminating in a land portion. A diameter of an inscribed circle of the small hole is greater than a thickness of the cuts. The cuts are provided with a twist in a depth direction centered on the small hole, and a twist angle from a top surface to a bottom surface of the land portion is not less than 10° and less than 135°.

Furthermore, the configuration described above is preferably configured as described in (1) to (3) below.

(1) From 3 to 6 of the cuts are provided.

(2) The diameter of the inscribed circle of the small hole is configured so as to be from 1.5 to 20 times the thickness of the cuts.

(3) The closed sipe is disposed in the tread surface in combination with a sipe extending in a tire width direction. In this case, the closed sipe is preferably disposed along the tire width direction on a leading edge side and/or a trailing edge side of a block formed in the tread surface.

According to the present technology described above, a closed sipe is provided in a tread surface that is centered on a small hole extending in a tire radial direction, and has a plurality of cuts extending in a radiation direction from the small hole and that terminate in a land portion. The cuts are provided with a twist in a depth direction centered on the small hole, and a twist angle from a top surface to a bottom surface of the land portion is not less than 10° and less than 135°. Therefore, water absorption can be enhanced due to an increase in the cubic capacity of the closed sipe while suppressing a decline in tread rigidity without hindering releasability from a mold; and braking performance on ice can be enhanced while maintaining steering stability when traveling on dry and wet road surfaces.

Moreover, because the diameter of the inscribed circle of the small hole located in a center portion of the closed sipe is configured so as to be greater than the thickness of the cuts, the flow of water into the small hole when traveling on icy road surfaces is facilitated, and the water that flows into the small hole can be efficiently dispersed toward each of the cuts. This leads to rapid and efficient water absorption.

DETAILED DESCRIPTION

Detailed descriptions will be given below of a configuration of the present technology with reference to the accompanying drawings.FIG. 1is a partial plan view illustrating an example of a closed sipe formed in a tread surface of a pneumatic tire according to an embodiment of the present technology in a box-shaped frame.FIG. 2is a perspective view illustrating an outer wall shape of the closed sipe ofFIG. 1inFIGS. 7A and 7B.

InFIG. 1, a closed sipe5(seeFIG. 2) is formed in a tread surface1from a small hole2extending in a tire radial direction, and a plurality of cuts4(four cuts in the drawing) extending in a radiation direction from the small hole2and that terminate in a land portion3. The cuts4are each provided with a twist in a depth direction centered on the small hole2, and a twist angle θ from a top surface to a bottom surface of the land portion3is not less than 10° and less than 135° and preferably from 90° to 120°. In the drawings, the dotted lines represent the bottom surfaces of the cuts4. Note that in the closed sipe5of the present technology, as described hereinafter, a diameter of an inscribed circle of the small hole2is formed so as to be greater than a thickness of the cuts4.

As a result, water absorption can be enhanced by increasing a cubic capacity of the closed sipe5while declines in tread rigidity can be suppressed, without hindering releasability from a mold. Thus, braking performance on ice can be enhanced while maintaining steering stability when traveling on dry and wet road surfaces.

Moreover, because the diameter of the inscribed circle of the small hole2located in a center portion of the closed sipe5is configured so as to be greater than the thickness of the cuts4, the flow of water into the small hole2when traveling on icy road surfaces is facilitated, and the water that flows into the small hole2can be efficiently dispersed toward each of the cuts4. This leads to rapid and efficient water absorption.

If the twist angle θ is less than 10°, water absorption will not be sufficiently enhanced and braking performance on ice will not be sufficiently enhanced. If the twist angle θ is 135° or greater, releasability when removing the tire from a mold after vulcanization will be negatively affected and the tread surface1will become easily damageable.

Note that in the present technology, the twist angle θ of the cuts4need not be the same in each of the cuts4that form the closed sipe5. In other words, cases in which spacing varies between mutually adjacent cuts4in a single closed sipe5are allowable. Additionally, in addition to being configured so as to vary uniformly (linearly) with respect to the depth direction, the twist angle θ of the cuts4may also be configured so as to vary nonlinearly.

With the pneumatic tire of the present technology configured as described above, the form of the closed sipe5includes a plurality of the cuts4that are provided with a twist around the small hole2that extends in the tire radial direction. Therefore, when removing the tire from the mold after vulcanization, the tread surface1becomes easily damageable by the molding blades of the cuts4. From this perspective, freely rotating molding blades of the cuts4centered on a molding axis of the small hole2are preferably provided on an inner surface of the mold used for vulcanization molding the pneumatic tire of the present technology, and vulcanization molding is preferably performed using this mold.

In the embodiments illustrated inFIG. 1andFIG. 2, examples where four of the cuts4that form the closed sipe5are provided are illustrated, but the number of the cuts4on the present technology is not limited thereto, and preferably from 3 to 6 of the cuts4are provided. If the number of the cuts4is two or less, it will become difficult to obtain excellent braking performance on ice. If the number is seven or greater, there will be a risk of cracking in the tread surface1during traveling of the tire.

Additionally, a cross-sectional shape of the small hole2positioned in the center portion of the closed sipe5is not particularly limited but, as illustrated inFIG. 1, can be configured to have a substantially round shape or a rectangular shape. Alternately, as illustrated inFIGS. 3A and 3B, depending on the number of the cuts4that form the closed sipe5, the cross-sectional shape of the small hole2can be configured so as to form a polygonal or star shape that corresponds to the number of the cuts4.

As illustrated inFIG. 3C, with the closed sipe5of the present technology, a diameter d of an inscribed circle2zof the small hole2is configured so as to be a size from 1.5 to 20 times, and preferably from 1.5 to 5 times of a thickness w of the cuts4. As a result, water absorption when traveling on icy road surfaces can be enhanced while declines in tread rigidity can be suppressed, and braking ability on ice can be reliably enhanced.

Note that the thickness w of the cuts4is not particularly limited, but is preferably set to from about 0.5 to 2 mm. Furthermore, a depth of the closed sipe5is not particularly limited, but when the tire is provided with a platform which acts as an indication of specification limits as a winter tire, the depth is preferably set so as to reach a top surface of the platform. As a result, after use as a winter tire is finished, it will be possible to use the tire as a summer tire.

In the embodiment described above, an example is described in which the cuts4extend in a linear form in the radiation direction from the small hole2. However, the closed sipe5of the present technology can be formed so that a planar shape of the cuts4extend in a wavelike or zigzag manner in the radiation direction from the small hole2.

With the pneumatic tire of the present technology, the closed sipe5described above is preferably disposed so as to be dispersed throughout an entire surface of the land portion3formed in the tread surface1. In this case, from the perspectives of uniformly maintaining a rigidity distribution of the land portion3and suppressing uneven wear, the closed sipe5is preferably disposed so that the cuts4that form the closed sipe5are not near each other.

Furthermore, depending on the characteristics desired for the tire, as illustrated inFIGS. 4 and 5, the closed sipe5can be disposed in the tread surface1in combination with a sipe6extending in the tire width direction. As a result, the edge effects accompanying the disposal of the sipe6are increased, and a high degree of both steering stability and braking performance on ice can be achieved. Note that in this embodiment, both ends in the extending direction of the sipe6are open to a side surface in the tire width direction of a rib7, but this should not be construed to mean that non-open sipes are excluded.

InFIG. 4, an example is illustrated in which a plurality of the closed sipe5is disposed in parallel in a surface of the rib7formed in the tread surface1and the sipes6, having a zigzag shape extending in the tire width direction, are disposed alternately in a tire circumferential direction T. However, the form of the sipe6and the arrangement thereof are not limited thereto, and can be arbitrarily modified depending on the form of the tread pattern. Examples of the form of the sipe6include substantially linear or wavelike forms, forms that constitute three-dimensional forms, and the like.

Additionally, when forming a block8in the tread surface1, as illustrated inFIG. 5, the block8is partitioned by the sipe6that extends in the tire width direction. The closed sipe5is preferably disposed in parallel in a tire width direction along at least one side of front/back edges (corresponding to a trailing edge side and/or a leading edge side of the tire) of the block8.

Particularly, to an extent possible, the sipe6extending in the tire width direction is not formed in regions of the leading edge and/or the trailing edge corresponding to about 30% or less of a length of the block8in the tire-circumferential direction. Rather, the closed sipe5of the present technology is preferably disposed in parallel along the tire width direction. As a result, block rigidity in the leading edge and/or the trailing edge of the block8can be ensured, steering stability can be enhanced while efficiently suppressing uneven wear, water absorption can be enhanced, and braking performance on ice can be enhanced.

As described above, the pneumatic tire of the present technology includes a closed sipe that is formed in a tread surface and includes a small hole extending in a tire radial direction and a plurality of cuts extending in a radiation direction from the small hole and terminating in a land portion. The cuts are provided with a twist in a depth direction centered on the small hole, and a twist angle from a top surface to a bottom surface of the land portion is not less than 10° and less than 135°. Therefore, braking performance when traveling on icy road surfaces can be enhanced while maintaining steering stability without hindering releasability from a mold. Thus, superior effects can be provided while realizing a simple construction and, therefore, the pneumatic tire of the present technology can be advantageously applied to a studless tire that requires braking performance on ice.

EXAMPLES

Tires of the present technology (Working Examples 1 to 8) and comparison tires (Comparative Examples 1 to 4) were fabricated having a tire size of 195/65R15 91Q and the tire pattern illustrated inFIG. 6. Note that inFIG. 6, an example is illustrated in which three of the cuts4of the closed sipe5are provided.

In the Working Examples, the closed sipe5had a form wherein the cuts4extend in a radiation direction from a center axis that extends in the tire radial direction, and terminate in the land portion; and the small hole2was formed on the center axis. Each of the closed sipes5had a common depth of 7 mm and each of the cuts4had a common thickness w of 0.5 mm. Tires of Working Examples 1 to 8 and Comparative Examples 1 and 2 were fabricated wherein the number of cuts4, the twist angle of the cuts4, the diameter d of the inscribed circle of the small hole2, d/w, the presence of the sipe6extending in the tire width direction, and whether the closed sipe5is disposed on the leading edge side and the trailing edge side of the block were varied according to the configurations shown in Table 1 inFIGS. 7A and 7B.

Additionally, tires of Comparative Examples 3 and 4 were fabricated as follows. That is, the closed sipe5was obtained by forming the cuts4that extend in a radiation direction from a center axis extending in the tire radial direction and that terminate in the land portion without forming the small hole2on the center axis. In particular, tires of Comparative Examples 3 and 4 were fabricated by varying the number of the cuts4and the twist angle of the cuts4according to the configurations shown in Table 1. Each of the closed sipes5had a common depth of 7 mm and each of the cuts4had a common thickness w of 0.5 mm.

Each of these 12 types of tires was evaluated for braking performance on ice according to the test method described below, and releasability when removing the tire from a mold after vulcanization was also evaluated. In evaluating the releasability when removing the tire from a mold after vulcanization, tires where the tread surface was not damaged were shown as “◯” and tires where the tread surface was damaged were shown as “x” in Table 1.

Braking Performance on Ice Test

Each tire was assembled on a 15×6JJ rim, inflated to an air pressure of 230 kPa, and mounted on the front and back wheels of a passenger car (made in Japan) having an engine displacement of 2,000 cc. A braking test from an initial speed of 40 km/hr was performed on an icy road surface, and braking performance on ice was evaluated based on the inverse of the stopping distance following application of the brakes. The results were indexed and Comparative Example 1 was assigned an index value of 100. The results are shown in Table 1. A larger index value indicates superior braking performance on ice.

The tires of Working Examples 1 to 8 of the present technology have the small hole2formed on the center axis and are configured so as to have a twist angle θ with respect to the tire radial direction in a range of not less than 10° and less than 135°. It is clear from Table 1 that these tires of Working Examples 1 to 8, in comparison with the comparison tires (tires of Comparative Examples 1 to 4), have superior braking performance on ice and have excellent releasability from a mold. Particularly, it is clear that the tires of Working Examples 7 and 8, where from 3 to 6 cuts4were provided, and a ratio (d/w) of the diameter d of the inscribed circle of the small hole2to the thickness w of the cuts4was in a range of 1.5 to 20, had exceptionally superior braking performance on ice.