A tire includes a tread portion bound with an intended tire rotational direction. The tread portion is provided a plurality of blocks. Each of the blocks includes a first ground contact edge, which is a ground contact edge on a heel side in the tire rotational direction, and a second ground contact edge, which is the ground contact edge on a toe side in the tire rotational direction. A second tire radius which is a tire radius at the second ground contact edge is larger than a first tire radius which is the tire radius at the first ground contact edge.

TECHNICAL FIELD

The present invention relates to a tire having a plurality of blocks formed in a tread portion bound with an intended tire rotational direction.

BACKGROUND ART

Conventionally, a tire with a tread portion bound with an intended tire rotational direction has been known. For example, the following Patent Literature 1 has proposed a tire that achieves both wear resistance and wet grip performance by keeping a ground contacting surface shape index in a certain range.

PRIOR ART DOCUMENT

Patent Document

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

However, in the tire of Patent Literature 1, a ground contact pressure on the heel side in the tire rotational direction is locally increased, therefore, a difference may be caused in the ground contact pressure between the heel side and the toe side in the tire rotational direction, which is a cause of decrease in frictional force. Therefore, there has been a demand for further improvements in traction performance, which is affected by frictional force.

The present invention was made in view of the above, and a primary object thereof is to provide a tire capable of improving the traction performance by making the ground contact pressure uniform during tire rotation.

Means for Solving the Problems

The present invention is a tire including a tread portion bound with an intended tire rotational direction, wherein the tread portion is provided with a plurality of blocks, each of the blocks includes a first ground contact edge, which is a ground contact edge on a heel side in the tire rotational direction, and a second ground contact edge, which is the ground contact edge on a toe side in the tire rotational direction, and a second tire radius which is a tire radius at the second ground contact edge is larger than a first tire radius which is the tire radius at the first ground contact edge.

In the pneumatic tire according to the present invention, it is preferred that the second tire radius is lamer than the first tire radius by 0.3 mm or more and 1.5 mm or less.

In the pneumatic tire according to the present invention, it is preferred that each of the blocks has a side wall extending inward in a tire radial direction from the second ground contact edge, and the side wall is parallel to a tire radial direction reference line which is a radial linear line passing through the first ground contact edge.

In the pneumatic tire according to the present invention, it is preferred that each of the blocks includes a protruding portion protruding radially outward from a reference plane defined by the first tire radius, the side wall is provided in the protruding portion, and the reference plane is a plane passing through the first ground contact edge and an imaginary line on a side surface on the toe side of the block, the imaginary line being arranged at a distance equal to the first tire radius from a tire center.

In the pneumatic tire according to the present invention, it is preferred that each of the blocks has a ground contacting surface extending in an arc shape in a tire circumferential direction between the first ground contact edge and the second ground contact edge, and a radius of curvature of the ground contacting surface is 0.5 times or more and 2.0 times or less the first tire radius.

In the pneumatic tire according to the present invention, it is preferred that each of the blocks has a rubber hardness of 55 degrees or more and 70 degrees or less.

In the pneumatic tire according to the present invention, it is preferred that a pitch length of the blocks adjacent to each other in the tire circumferential direction is 25 mm or more and 50 mm or less.

In the pneumatic tire according to the present invention, it is preferred that the tread portion includes a block row in which 50 to 80 blocks are arranged in a tire circumferential direction.

Effects of the Invention

In the pneumatic tire according to the present invention, each of the blocks includes the first ground contact edge which is the ground contact edge on the heel side in the tire rotational direction and the second ground contact edge which is the ground contact edge on the toe side in the tire rotational direction, and the second tire radius which is the tire radius at the second ground contact edge is larger than the first tire radius which is the tire radius at the first ground contact edge. In the tire configured as such, a local change in the ground contact pressure during rotation is suppressed, therefore, the ground contact pressure at the first ground contact edge and the ground contact pressure at the second ground contact edge can be uniformized. Thereby, in the tire of the present invention, it is possible that the traction performance is improved.

MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will now be described in conjunction with accompanying drawings.

FIG. 1is a schematic side view of a tire1of the present embodiment. As shown inFIG. 1, the tire1of the present embodiment is suitably used as a pneumatic tire to be mounted on a 4 WD vehicle and the like capable of running on rough terrain. The tire1is not limited to such an embodiment and can be used as appropriate for a variety of tires, such as pneumatic tires to be mounted on motorcycles, passenger cars, heavy load vehicles, and the like, and non-pneumatic tires that are not filled with pressurized air inside the tire, for example.

The Tire1in the present embodiment has a tread portion2bound with an intended tire rotational direction (R). A plurality of blocks3are formed in the tread portion2of the present embodiment. The tire1having the tread portion2configured as such is suitable for running on rough terrain, such as snowy roads, muddy roads, and the like.

FIG. 2is an enlarged view of part (A) ofFIG. 1. As shown inFIG. 2, each of the blocks3of the present embodiment includes a first ground contact edge4, which is a ground contact edge on the heel side in the tire rotational direction (R), and a second ground contact edge5, which is the ground contact edge on the toe side in the tire rotational direction (R). Here, the ground contact edges of the blocks3are edges in a tire circumferential direction of the blocks3, and when the edges extend in a tire axial direction, the ground contact edges are formed in a linear shape, and when the edges extend obliquely with respect to the tire axial direction, the ground contact edges are formed in a dot shape.

In the present embodiment, a second tire radius (r2), which is the tire radius at the second ground contact edge5, is larger than a first tire radius (r1), which is the tire radius at the first ground contact edge4. In the tire1configured as such, a local change in the ground contact pressure during rotation is suppressed, therefore, it is possible that the ground contact pressure at the first ground contact edge4and the ground contact pressure at the second ground contact edge5are made uniform. Thereby, in the tire1of the present embodiment, it is possible that the traction performance is improved.

As a more preferred embodiment, the second tire radius (r2) is greater than the first tire radius (r1) by 0.3 mm or more and 1.5 mm or less. In other words; it is preferred that a difference (h) between the second tire radius (r2) and the first tire radius (r1) is 0.3 mm or more and 1.5 mm or less. When the difference (h) between the second tire radius (r2) and the first tire radius (r1) is 0.3 mm or more, the locally high ground contact pressure at the first ground contact edge4can be suppressed. When the difference (h) between the second tire radius (r2) and the first tire radius (r1) is 1.5 mm or less, the locally high ground contact pressure at the second ground contact edge5can be suppressed.

As shown inFIG. 1, it is preferred that the tread portion2includes a block row6in which 50 to 80 blocks3are arranged in the tire circumferential direction. The tread portion2, which includes the block row6configured as such, can exert excellent rough terrain running performance.

When the number of the blocks3included in the block row6is N, a pitch angle θ of the blocks3adjacent to each other in the tire circumferential direction can be expressed by the following Expression 1.

Here, if the block row6has more than one pitch angle θ, the pitch angle θ expressed in Expression 1 is the average pitch angle θ of them.

Further, in this case, a pitch length (P) of the blocks3adjacent to each other in the tire circumferential direction on a reference plane (B), which is defined by the first tire radius (r1), can be expressed by the following Expression 2. The reference plane (B) is a plane passing through the first ground contacting edge and an imaginary line on a wall surface (8a), which is a side surface on the toe side of the block, arranged at a distance from the tire center equal to the first tire radius (r1).

In the tread portion2of the present embodiment, the pitch length (P) of the blocks3adjacent to each other in the tire circumferential direction is 25 mm or more and 50 mm or less. When the pitch length (P) is 25 mm or more, the rigidity of each of the blocks3can be improved. When the pitch length (P) is 50 mm or less, excellent rough terrain running performance can be maintained.

As shown inFIG. 2, each of the blocks3of the present embodiment has a ground contacting surface7which extends in the tire circumferential direction between the first ground contact edge4and the second ground contact edge5and a side wall8which extends inward in a tire radial direction from the second ground contact edge5. The ground contacting surface7includes a linear section (7a) extending linearly between the first ground contact edge4and the second ground contact edge5, for example. The side wall8is provided on the wall surface (8a) arranged on the toe side in the tire rotational direction (R) and including the second ground contact edge5, for example. The side wall8may be firmed as the wall surface (8a) or may be a line portion (8h) provided on the wall surface (8a), for example.

Each of the blocks3of the present embodiment includes a protruding portion9protruding radially outward from the reference plane (B) defined by the first tire radius (r1). The ground contacting surface7of the present embodiment is provided in the protruding portion9. It is preferred that a length (L) in the tire circumferential direction of the ground contacting surface7is smaller than the pitch length (P) of the blocks3. Here, the length (L) in the tire circumferential direction of the ground contacting surface7is the length measured along the ground contacting surface7.

It is preferred that the length (1) in the tire circumferential direction of the ground contacting surface7in the present embodiment is 20 mm or more and 40 mm or less. When the length (L) of the ground contacting surface7is 20 mm or more, the rigidity of each of the blocks3can be improved. When the length (L) of the ground contacting surface7is 40 mm or less, excellent rough terrain running performance can be maintained.

It is preferred that the side wall8is formed at least in the protruding portion9. The side wall8of the present embodiment continuously extends from the protruding portion9to the radially inner side of the blocks3. The side wall8configured as such is helpful for improving the frictional force during acceleration.

A height (h) of the protruding portion9in the present embodiment is the difference (h) between the second tire radius (r2) and the first tire radius (r1). The height (h) of the protruding portion9can be expressed by the following Expression 3 by an angle σ between the ground contacting surface7and the reference plane (B) and the length (L) of the ground contacting surface7.

Here, it is preferred that the angle σ between the ground contacting surface7and the reference plane (B) satisfies the following Expression 4.

The blocks3each having the protruding portion9configured as such can uniformize the ground contact pressure at the first ground contact edge4and the ground contact pressure at the second ground contact edge5, which is helpful for improving the traction performance of the tire1.

As the blocks3of the present embodiment, those having a rubber hardness of 55 degrees or more and 70 degrees or less are suitably used. Here, the rubber hardness is the hardness measured by a type-A durometer in an environment of 23 degrees Celsius in accordance with Japanese Industrial Standard JIS-K6253. The blocks3configured as such can suppress partial chipping while improving frictional force.

FIG. 3is an enlarged view of the part (A) according to another embodiment. As shown inFIG. 3, each of the blocks3in this embodiment has the ground contacting surface7extending in an arc shape in the tire circumferential direction between the first ground contact edge4and the second ground contact edge5. The ground contacting surface7includes an arc portion (7b) extending in an arc shape between the first ground contact edge4and the second ground contact edge5, for example. The ground contacting surface7configured as such suppresses the local increase in the ground contact pressure at the second ground contact edge5, therefore, the ground contact pressure of the ground contacting surface7can be uniformized.

It is preferred that a radius of curvature (R1) of the ground contacting surface7is 0.5 times or more and 2.0 times or less the first tire radius (r1). Here, the radius of curvature (R1) of the ground contacting surface7is the value measured in a cross section taken along the tire circumferential direction. When the radius of curvature (R1) is 0.5 times or more the first tire radius (r1), the local increase in the ground contact pressure of the ground contacting surface7can be suppressed. When the radius of curvature (R1) is 2.0 times or less the first tire radius (r1), the local increase in the ground contact pressure at the second ground contact edge5can be suppressed.

It is preferred that the length (L) in the tire circumferential direction of the ground contacting surface7in the present embodiment is 20 mm or more and 40 mm or less. When the length (L) of the ground contacting surface7is 20 mm or more, the rigidity of each of the blocks3can be improved. When the length (L) of the ground contacting surface7is 40 mm air less, excellent rough terrain running performance can be maintained.

Each of the blocks3of this embodiment is provided with the side wall8extending radially inward from the second ground contact edge5. It is preferred that the side wall8is parallel to a tire radial direction reference line (BL) which is a radial linear line passing through the first ground contact edge4. Each of the blocks3configured as such suppresses the chipping of the second ground contact edge5, which tends to occur during deceleration, therefore, the traction performance can be maintained high for a long period of time.

Each of the blocks3of this embodiment includes the protruding portion9protruding radially outward from the reference plane (B) defined by the first tire radius (r1). The ground contacting surface7and the side wall8of this embodiment are formed in the protruding portion9.

The height (h) of the protruding portion9of this embodiment can be expressed by the following Expression 5 by the angle c which is defined by the Expression 4 and the length (L) of the ground contacting surface7.

Each of the blocks3having the protruding portion9configured as such can uniformize the ground contact pressure at the first ground contact edge4and the ground contact pressure at the second ground contact edge5, which is helpful for improving the traction performance of tire1.

While detailed description has been made of an especially preferred embodiment of the present invention, the present invention can be embodied in various forms without being limited to the illustrated embodiment.

As the tire having the blocks ofFIG. 1, Example 1 having the blocks ofFIG. 2and Example 2 having the blocks ofFIG. 3were made by way of test: As Reference, a tire having no protruding portion was made by way of test. The test tires were tested for the traction performance, braking performance and block chipping. Common specifications and test methods were as follows.

By using a bench testing machine, the traction performance of the test tires during acceleration was measured under a dry road condition and a wet road condition. The results are indicated by an index based on the Reference being 100, wherein the larger the numerical value, the larger the frictional force is, which shows more excellent traction performance.

By using a bench testing machine, the braking performance of the test tires during deceleration was measured under a dry road surface condition and a wet road surface condition. The results are indicated by an index based on the Reference being 100, wherein the larger the numerical value, the larger the frictional force is, which shows more excellent braking performance.

After the braking performance test under the dry road surface condition was repeated 10 times, it was confirmed whether or not the blocks were chipped. The results are represented by the presence or absence of the chipping, wherein the absence of chipping indicates that the traction performance and the braking performance of the initial stage be maintained for a long period of time.

The test results are shown in Table 1.

From the test results, it was confirmed that the tires in the Examples had excellent traction performance compared with the tires in the Reference. Therefore, it was confirmed that the fraction performance was improved in the tires in the Examples compared with the tires in the Reference. Further, it was confirmed that more excellent traction performance can be maintained for a long period of time in the tires in the Example 2 compared with the tires in the Example 1.

DESCRIPTION OF REFERENCE SIGNS

1tire2tread portion3block4first ground contact edge5second ground contact edge