A tire comprises a tread portion provided with two circumferential grooves disposed one each side of the tire equator and extending continuously in the tire circumferential direction, and a crown land region between the two circumferential grooves. The crown land region is provided with first crown sipes extending from one of the circumferential grooves and terminating within the crown land region, and second crown sipes extending from the other circumferential groove and terminating within the crown land region.

TECHNICAL FIELD

The present invention relates to a tire, more particularly to a tread pattern suitable for sporty use such as circuit racing.

BACKGROUND ART

The following patent document 1 discloses a tire whose running performance during circuit running is improved, while securing wet performance. This tire is provided with two circumferential grooves one on each side of the tire equator, and oblique grooves disposed axially outside the circumferential grooves. Between the two circumferential grooves, there is formed a land region extending in the tire circumferential direction on the tire equator. Such tire has a problem such that the temperature of the land region of the tread portion is hard to increase and thereby the tire has low grip performance initially after starting to run.

Patent document 1:

Japanese Patent Application Publication No.2013-173394

SUMMARY OF THE INVENTION

Problems to be Resolved by the Invention

It is therefore, an object of the present invention to provide a tire capable of exerting high grip performance even immediately after starting to run.

According to the present invention, a tire comprises a tread portion provided with two circumferential grooves disposed one each side of the tire equator and extending continuously in the tire circumferential direction, and a crown land region formed between the two circumferential grooves, wherein

the crown land region is provided with

first crown sipes extending from one of the circumferential grooves and terminating within the crown land region, and second crown sipes extending from the other circumferential groove and terminating within the crown land region.

It is preferable that the tread portion comprises a shoulder land region positioned on each side of the crown land region and comprising a continuous part extending continuously in the tire circumferential direction.

It is preferable that the first crown sipes or the second crown sipes are inclined with respect to the tire axial direction.

It is preferable that the first crown sipes and the second crown sipes extend from the circumferential grooves while inclining to one side in the tire circumferential direction.

It is preferable that the tire has a tread pattern having an intended rotational direction, and the first crown sipes and the second crown sipes extend from the circumferential grooves while inclining to the opposite direction to the intended rotational direction (toward the toe-side).

It is preferable that the first crown sipes or the second crown sipes are each provided with

a major radial portion extending radially outwardly from the bottom of the sipe with a constant width, and

a radially outermost radial portion opened at the tread face of the tread portion with a width more than the constant width of the major radial portion.

It is preferable that the width of the major radial portion is 0.2 to 1.0 mm.

It is preferable that the first crown sipes and the second crown sipes each have an axially inner end within the crown land region, and

the distances in the tire circumferential direction between the axially inner ends of the first crown sipes and the axially inner ends of the second crown sipes are not more than 5 mm.

It is preferable that the first crown sipes and the second crown sipes each have an axially inner end within the crown land region, and

the distance in the tire axial direction between the axially inner ends of the first crown sipes and the axially inner ends of the second crown sipes is more than the length in the tire axial direction of the first crown sipes and more than the length in the tire axial direction of the second crown sipes.

It is preferable that the tread portion comprises a shoulder land region positioned on each side of the crown land region, and provided with oblique grooves inclined with respect to the tire axial direction and positioned in the tire circumferential direction so that

the circumferential extents of the oblique grooves do not overlap in the tire circumferential direction with the circumferential extents of the first and second crown sipes.

It is preferable that the pitch length in the tire circumferential direction of the oblique grooves is more than the pitch length in the tire circumferential direction of the first crown sipes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described in detail in conjunction with the accompanying drawings.

FIG. 1shows a part of the tread portion2of a tire1as an embodiment of the present invention.

The tire1in this embodiment is a street-legal high-performance pneumatic tire for passenger cars suitable for sporty use such as circuit racing.

In this embodiment, the tire1is provided in the tread portion2with a directional tread pattern having an intended tire rotational direction R.

Incidentally, the intended rotational direction R is indicated in the tire sidewall portion (not shown) by markings, characters, symbols and the like.

In this application, the expression “the toe-side in the intended tire rotational direction” means one side in the tire circumferential direction which is toward the opposite direction to the intended tire rotational direction.

The tread portion2is provided with two circumferential grooves3disposed on each side of the tire equator C and extending continuously in the tire circumferential direction. In this example, each of the circumferential grooves3is a straight groove parallel with the tire circumferential direction. However, wavy grooves or zigzag grooves or a combination of two of a wavy groove, a zigzag groove and a straight groove may be employed.

It is preferable that the axial distance L1from the tire equator C to the widthwise center line of each of the circumferential grooves3is set in a range from 0.05 to 0.20 times the tread width TW between the tread edges Te.

The tread edges Te are the axial outermost edges of the ground contacting patch of the tire which occurs under a normally inflated loaded condition when the camber angle of the tire is zero.

The tread width TW is the width measured under a normally inflated unloaded condition, as the axial distance between the tread edges Te determined as above.

The normally inflated unloaded condition is such that the tire is mounted on a standard wheel rim and inflate to a standard pressure but loaded with no tire load.

The normally inflated loaded condition is such that the tire is mounted on the standard wheel rim and inflated to the standard pressure and loaded with the standard tire load.

The standard wheel rim is a wheel rim officially approved or recommended for the tire by standards organizations, i.e. JATMA (Japan and Asia), T&RA (North America), ETRTO (Europe), TRAA (Australia), STRO (Scandinavia), ALAPA (Latin America), ITTAC (India) and the like which are effective in the area where the tire is manufactured, sold or used.

The standard pressure and the standard tire load are the maximum air pressure and the maximum tire load for the tire specified by the same organization in the Air-pressure/Maximum-load Table or similar list.

For example, the standard wheel rim is the “standard rim” specified in JATMA, the “Measuring Rim” in ETRTO, the “Design Rim” in TRA or the like. The standard pressure is the “maximum air pressure” in JATMA, the “Inflation Pressure” in ETRTO, the maximum pressure given in the “Tire Load Limits at various Cold Inflation Pressures” table in TRA or the like. The standard load is the “maximum load capacity” in JATMA, the “Load Capacity” in ETRTO, the maximum value given in the above-mentioned table in TRA or the like.

It is preferable that the circumferential grooves3each have a groove width w1of not more than 20% of the tread width TW. More preferably, the groove width w1is set in a range from 4.0% to 10.0% of the tread width TW.

when the tire1is for passenger cars, it is preferable that the circumferential grooves3have a groove depth of from 4 to 10 mm.

The tread portion2is axially divided by the two circumferential grooves3into tow shoulder land regions5and one crown land region4therebetween.

In this embodiment, each of the shoulder land regions5is not divided in the tire axial direction by a circumferentially continuously extending groove. In other word, each shoulder land region5is continuous between the circumferential groove3and the tread edge Te.
Further, each of the shoulder land regions5is continuous in the tire circumferential direction. More specifically, the shoulder land region5has a continuous part6extending continuously in the tire circumferential direction without any void such as groove and sipe. InFIG. 1, one of the two continuous parts6is shaded by thin dot pattern for easy understanding.
The continuous part6in this embodiment is formed between the circumferential groove3and the oblique grooves disposed in the shoulder land region5. Thereby, the shoulder land region5is provided with a high rigidity portion and helps to improve the steering stability.

In this application including the specification and claims, the term “sipe” means a narrow groove having a width not more than 1.5 mm inclusive of a cut having no substantial width. If a sipe has, in the tire radial direction, a widened portion whose width exceeds 1.5 mm, it is called “sipe” as long as its major portion has a width of not more than 1.5 mm.

It is preferable that the crown land region4is provided with first crown sipes26and second crown sipes27as shown inFIG. 2.

The first crown sipes26extend from one of the circumferential grooves3and terminate within the crown land region4.

The second crown sipes27extend from the other circumferential groove3and terminate within the crown land region4.

The crown land region4has a part extending continuously in the tire circumferential direction.

The first and second crown sipes can facilitate the temperature rise of the crown land region4immediately after starting to run, while suppressing the decrease in the rigidity of the crown land region4.

According to the present invention, therefore, the crown land region4can exert high grip even immediately after starting to run.

It is preferable that the axial length L10aof the first crown sipes26and the axial length L10bof the second crown sipes27are set in a range from 0.20 to 0.40 times the axial width w5of the crown land region4.

Such first and second crown sipes can increase the grip immediately after starting (hereinafter, referred to as the “initial stage grip”), while suppressing the decrease in the grip when the temperature of the crown land region4is sufficiently increased (hereinafter, referred to as the “maximum grip”).

In this embodiment, the first crown sipes26and second crown sipes27are disposed line-symmetrically about the tire equator C, and the first crown sipe26and second crown sipe27are structurally symmetrical. Thus, although the following description is made mainly on the first crown sipe26, the description is applicable to the second crown sipe27, namely, applicable to both of the first crown sipes26and the second crown sipes27.

The crown sipes26are inclined with respect to the tire axial direction. It is preferable that the first crown sipes26and the second crown sipes27are extend from the circumferential grooves3while inclining to one side in the tire circumferential direction.

In this embodiment, the crown sipes26extend from the circumferential groove3, while inclining toward the opposite direction to the intended tire rotational direction R.

Preferably, the crown sipes26have an angle θ6of not more than 10 degrees with respect to the tire axial direction.

As shown inFIG. 3Awhich is a cross sectional view taken along line A-A ofFIG. 2, the crown sipe26in this embodiment comprises a major portion28and a radially outermost portion29.

The major portion28extends radially outwardly from the bottom with a constant width.

The radially outermost portion29extends radially outwardly from the major portion28to opened at the tread surface of the tread portion2with a larger width than the constant width of the major portion28.

Such crown sipe26helps to suppress uneven wear of the crown land region4.

It is preferable that the major portion28has a width w6of from 0.2 to 1.0 mm. It is preferable that the radially outer portion29has a width w7of from 1.0 to 2.5 mm.

It is preferable that the depth d2of the radially outer portion29is in a range from 0.10 to 0.30 times the depth d1of the crown sipe26.

Such crown sipes26can speed up the temperature rise of the crown land region4, while maintaining the above described maximum grip of the crown land region4.

It is preferable that the depth d1of the crown sipes26is set in a range from 0.40 to 0.80 times the depth of the circumferential grooves3.

As shown inFIG. 2, the first crown sipes26and the second crown sipes27each have axially inner end within the crown land region4.

It is preferable that the distances L8(not shown) in the tire circumferential direction between the axially inner ends26iof the first crown sipes26and the axially inner ends27iof the second crown sipes27are not more than 5 mm.

By arranging the axially inner ends26iclose to the axially inner ends27i, the temperature of the crown land region4becomes more easily to raise. Consequently, it becomes possible to improve the above described initial stage grip.

It is preferable that the distance L9in the tire axial direction between the axially inner ends26iof the first crown sipes26and the inner ends27iof the second crown sipes27is more than the axial length L10aof the first crown sipes26and more than the axial length L10bof the second crown sipes27. More specifically, it is preferable that the distance L9is in a range from 0.40 to 0.50 times the width w5of the crown land region4. Such arrangement of the crown sipes can allow the crown land region4to maintain the necessary rigidity for good grip performance.

In order to effectively derive the above advantageous effect, it is preferred that the axial width w5of the crown land region4is set in a range from 0.10 to 0.30 times the tread width TW.

As shown inFIG. 3Bwhich is a cross sectional view of the crown land region4taken along line B-B ofFIG. 2, it is preferable that the radially outer surface of the crown land region4has a radially outwardly convex arched profile in its cross section.

Such configuration of the crown land region4can even the ground pressure to provide good grip performance.

As shown inFIG. 1, the shoulder land regions5are each provided with oblique grooves10inclined with respect to the tire axial direction. In this embodiment, each of the oblique grooves10extends from its axially inner end to outer end, while inclining toward the opposite direction to the intended tire rotational direction R. The oblique grooves10are however not limited to such inclining direction.

It is preferable that, in the tire circumferential direction, the circumferential extents of the respective oblique grooves10do not overlap with the circumferential extents of the first and second crown sipes26and27as shown inFIG. 1. Thereby, uneven wear between the land regions4and5can be suppressed.

The oblique grooves10include first oblique grooves11and second oblique grooves12. The first oblique grooves11have axially inner ends11i, and the second oblique grooves12have axially inner ends12ipositioned axially outside the inner ends11iof the first oblique grooves11.

In this embodiment, the first oblique grooves11and the second oblique grooves12are arranged alternately in the tire circumferential direction in each shoulder portion.

It is preferable that the pitch length P1(shown in FIG.4) in the tire circumferential direction of the oblique grooves11is set to be more than the pitch length P2(shown inFIG. 2) in the tire circumferential direction of the first crown sipes26. It is preferable that the pitch length P1of the oblique grooves11is set in a range from 1.5 to 2.5 times the pitch length P2of the first crown sipes26.

As shown inFIG. 5, the first oblique groove11has, on one of the groove edges, a vertex of curve15farthest from a straight line14drawn between the axially inner end11iand the axially outer end11o. And the vertex of curve15is positioned axially outside the axially inner end12iof the second oblique groove12. Accordingly, in the tire axial direction, the axially inner ends12iof the second oblique grooves12are positioned between the axially inner ends11iand the vertexes of curve15of the first oblique grooves11.

The axially inner end11iand the vertex of curve15of the first oblique groove11and the axially inner end12iof the second oblique groove12are liable to become start positions of deformation of the tread portion2. By setting their positions as described above, the ground contact of the shoulder land region5can be improved. Thereby, the tire1can exert good road grip even if the ground pressure of the tread portion2is relatively low.

It is preferable that, as shown inFIG. 4, all the first oblique grooves11are curved toward the same direction, and the groove edges of the first oblique grooves11on the same side in the tire circumferential direction each have no vertex of curve other than the above-said vertex of curve15.

In this embodiment, the first oblique groove11is curved such that the angle with respect to the tire axial direction becomes decreased from the axially inner end11itoward the axially outer end11o. Preferably, the angle is continuously decreased. The first oblique groove11is however not limited to such configuration. It may be possible that the first oblique groove11is partly curved and other part is linear.

It is preferable that the axially inner ends11iof the first oblique grooves11terminate within the respective shoulder land regions5. It is preferable that the axially outer ends11oof the first oblique grooves11are positioned axially outside the respective tread edges Te. Such first oblique grooves11can improve the wet performance, while providing good grip performance by maintaining the rigidity of the shoulder land regions5.

It is preferable that, as shown inFIG. 5, the distance L2in the tire axial direction from the tire equator C to the axially inner ends11iof the first oblique grooves11is set in a range from 0.10 to 0.30 times the tread width TW, and the distance L3in the tire axial direction from the tire equator C to the vertex of curve15is set in a range from 0.25 to 0.45 times, more preferably 0.30 to 0.40 times the tread width TW.

Such first oblique grooves11can improve the wet performance and grip performance on dry roads in good balance.

As shown inFIG. 4, the first oblique groove11comprises an axially inside portion16on the axially inside of the vertex of curve15, and an axially outside portion17on the axially outside of the vertex of curve15.

The axially inside portion16preferably has an angle θ1of from 25 to 45 degrees with respect to the tire axial direction. The axially outside portion17preferably has an angle θ2of from 0 to 30 degrees with respect to the tire axial direction.

It is preferable that, as shown inFIG. 5, a straight line14drawn between the axially inner end11iand the axially outer end11ois inclined at an angle θ3of from 20 to 30 degrees with respect to the tire axial direction.

It is preferable that the distance L4from the straight line14to the vertex of curve15of the first oblique groove11is less than the maximum groove width w2(shown inFIG. 4) of the first oblique groove11. More specifically, the distance L4is preferably set in a range from 0.60 to 0.90 times the groove width w2. Such configuration of the first oblique groove11can effectively discharge the water in the groove toward the axially outward during running in wet conditions.

In this embodiment, in each shoulder portion5, the second oblique grooves12are inclined to the same direction as the first oblique grooves11.

It is preferable that, similarly to the first oblique groove11, the second oblique groove12has, on one of the groove edges, a vertex of curve19farthest from a straight line18drawn between the axially inner end12iand the axially outer end12o.

It is preferable that the second oblique grooves12are curved toward the same direction, and the angle with respect to the tire axial direction of the second oblique grooves12becomes gradually decreased toward the axially outer end12ofrom the axially inner end12i. Preferably, the angle is continuously decreased.

Preferably, the second oblique grooves12extend substantially parallel with the first oblique grooves11.

The axially inner ends12iof the second oblique grooves12terminate within the respective shoulder land regions5.

It is preferable that the distance L5in the tire axial direction from the tire equator C to the axially inner ends12iof the second oblique grooves12is set in a range from 0.20 to 0.40 times the tread width TW. Thereby, the ground contacting area of a region on the axially inside of the second oblique grooves12are secured to provide good grip performance.

In order to increase the ground contact of the shoulder land regions5, the distance L6in the tire axial direction between the axially inner end12iof the second oblique groove12and the vertex of curve15of the first oblique groove11is preferably not less than 0.10 times, more preferably not less than 0.15 times, but preferably not more than 0.30 times, more preferably not more than 0.25 times the axial width w3of the shoulder land region5.

It is preferable that, as shown inFIG. 4, the axially outer ends12oof the second oblique grooves12are positioned axially outside the tread edges Te.

It is preferable that the angle θ4with respect to the tire axial direction of the second oblique grooves12is set in a range from 0 to 45 degrees.

It is preferable that, as shown inFIG. 5, the angle θ5with respect to the tire axial direction of a straight line18drawn between the axially inner end12iaxially outer end12oof the second oblique groove12is in a range from 15 to 25 degrees.

It is preferable that the difference between the angle θ5of the straight line18and the angle θ3of the straight line14drawn between the axially inner end11iand the axially outer end11oof the first oblique groove11, is not more than 10 degrees. Such configuration of the second oblique groove12can effectively discharge the water in the groove toward the axial outside during running in wet conditions.

It is preferable that the distance L7from the straight line18to the vertex of curve19of the second oblique groove12is set in a range from 0.90 to 1.10 times the distance L4from the straight line14to the vertex of curve15of the first oblique groove11. Thereby, uneven wear in the vicinities of the vertexes of curve of the oblique grooves can be prevented.

It is preferable that, as shown inFIG. 4, the maximum groove width w4of the second oblique groove12is less than the maximum groove width w2of the first oblique groove11.

More specifically, the groove width w4of the second oblique groove12is preferably set in a range from 0.70 to 0.90 times the groove width w2of the first oblique groove11.

Such second oblique grooves12can improve the wet performance and the grip performance in good balance.

It is preferable that, for the same reason, the second oblique groove12has a groove depth less than that of the first oblique groove11.

In this embodiment, the shoulder land regions5are preferably provided with auxiliary grooves20.

Each of the auxiliary grooves20is positioned, for example, so as to overlap with an extension toward the tire equator C of one of the second oblique grooves12.

Each of the auxiliary grooves20has an axially inner end20iand outer end20owithin the shoulder land region5. Preferably, the axially inner end20iis disposed axially inside the axially inner end11iof the first oblique groove11, and the axially outer end20ois disposed axially outside the axially inner end11iof the first oblique groove11.

Such auxiliary grooves20can improve the wet performance, while maintaining the rigidity of the continuous part6of the shoulder land region5.

In this example, a part of the groove edge of the auxiliary groove20is aligned with an axial groove edge12eon the toe-side in the rotational direction R, of the second oblique groove12. Thereby, uneven wear at the groove edges of the second oblique groove12and auxiliary groove20can be prevented.

While detailed description has been made of preferable embodiments of the present invention, the present invention can be embodied in various forms without being limited to the illustrated embodiments.

Comparison Tests

Based on the tread pattern shown inFIG. 1, pneumatic tires of size 205/55R16 (rim size 16×6.5J) were experimentally manufactured as test tires including: working examples Ex.1-Ex.10, comparative example Ref.1 whose crown land region (a) was devoid of a sipe as shown inFIG. 6A, and comparative example Ref.2 whose crown land region (b) was provided with only sipes (c) extending across the entire width of the crown land region as shown inFIG. 6B. The construction of the shoulder land regions was the same for all the test tires.

The test tires were tested for the initial stage grip and the maximum grip as follows, using a test car (2000cc front engage rear drive passenger car with test tires mounted on all wheels and inflated to 230 kPa).

<Initial Stage Grip and Maximum Grip>

The test car was run on a dry road surface, and the test driver evaluated the grip immediately after starting (initial stage grip) and the grip when the temperature of the tires was sufficiently increased (maximum grip).

The test results are indicated in Table 1 by an index based on Ref.1 being 100, wherein the larger the index number, the better the performance (namely, initial stage grip and maximum grip).

From the test results, it was confirmed that the tires according to the present invention can exert high grip performance immediately after starting, without sacrificing the maximum grip.

REFERENCE SIGNS LIST