Pneumatic tire

A pneumatic tire comprises a tread portion 2 provided with shoulder axial grooves 10 extending from a shoulder main groove 3 to a tread edge Te while inclining to a first-side in the tire circumferential direction. The shoulder axial groove 10 has a first-side groove edge, a second-side groove edge, a first-side groove-sidewall 14 extending radially inward from the first-side groove edge 12, and a second-side groove-sidewall 15 extending radially inward from the second-side groove edge 13. The shoulder axial groove 10 has a part where the inclination angle β of the first-side groove-sidewall 14 with respect to a normal line passing through the first-side groove edge 12 perpendicularly to the tread surface 2s is more than the inclination angle α of the second-side groove-sidewall 15 with respect to a normal line passing through the second-side groove edge 13 perpendicularly to the tread surface 2s.

BACKGROUND OF THE INVENTION

The present invention relates to a pneumatic tire, more particularly to a tread groove configuration capable of improving uneven wear without sacrificing wet performance.

usually, a passenger tire is provided in the tread portion with axial grooves inclined with respect to the tire axial direction in order to improve wet performance. In such tire, tread blocks circumferentially divided by the inclined axial grooves are liable to wear unevenly starting from their acute-angled corners and suffer from so-called heel-and-toe wear.

In Japanese Patent Application Publication No. 09-002019, in order to improve such heel-and-toe wear, oblique grooves constituting a unidirectional tread pattern symmetrical with respect to the tire equator are each configured such that the inclination angle of the toe-side groove-sidewall is larger than the inclination angle of the heel-side groove-sidewall in connection with the intended tire rotational direction.

Such groove-sidewall configuration, however, can not fully reduce the heel-and-toe wear especially in the tread shoulder regions. Further, it can not be employed in a bidirectional tread pattern because the heel-and-toe wear are accelerated.

SUMMARY OF THE INVENTION

It is therefore, an object of the present invention to provide a pneumatic tire, in which uneven wear such as heel-and-toe wear can be effectively improved without sacrificing wet performances.

According to the present invention, a pneumatic tire comprises:

a tread portion having a tread surface and provided with a pair of circumferentially continuously extending axially outermost shoulder main grooves, and

a pair of shoulder land portions axially outside the respective shoulder main grooves,

each of the shoulder land portions is provided with a plurality of shoulder axial grooves extending from the shoulder main groove to the tread edge, while inclining to a first-side in the tire circumferential direction,

the above-mentioned plurality of shoulder axial grooves each has a first-side groove edge, an opposite second-side groove edge, a first-side groove-sidewall extending radially inwardly from the first-side groove edge, and a second-side groove-sidewall extending radially inwardly from the second-side groove edge, and

the shoulder axial groove has a part where the inclination angle β of the first-side groove-sidewall with respect to a normal line passing through the first-side groove edge perpendicularly to the tread surface is more than the inclination angle α of the second-side groove-sidewall with respect to a normal line passing through the second-side groove edge perpendicularly to the tread surface,

wherein the inclination angles α and β are measured in the cross section of the shoulder axial groove perpendicular to the widthwise center line of the shoulder axial groove.

Further, the pneumatic tire according to the present invention may be provided with the following features (1)-(3):

(1) the tread portion comprises a middle land portion axially inside the shoulder main grooves, provided with middle axial grooves extending axially inwardly from the shoulder main grooves, while inclining to the above-mentioned first-side in the tire circumferential direction or a second-side in the tire circumferential direction which is the opposite side to the first-side,

the middle axial grooves each have a first-side groove edge, a second-side groove edge, a first-side groove-sidewall extending radially inwardly from the first-side groove edge, and a second-side groove-sidewall extending radially inwardly from the second-side groove edge,

the middle axial groove has a part where the inclination angle δ of the first-side groove-sidewall with respect to a normal line passing through the first-side groove edge perpendicularly to the tread surface is more than the inclination angle γ of the second-side groove-sidewall with respect to a normal line passing through the second-side groove edge perpendicularly to the tread surface,

wherein the inclination angles δ and γ are measured in the cross section of the middle axial groove perpendicular to the widthwise center line of the middle axial groove;

(2) the difference β−α of the inclination angle β from the inclination angle α is more than the difference β−γ of the inclination angle δ from the inclination angle γ;

(3) each of the shoulder axial grooves comprises an axially outer wide main part having a constant groove width, an axially inner narrower part having a constant groove width less than the width of the main part, and

a transitional part connecting therebetween and having a groove width gradually decreasing toward the axially inside,

the narrower part is such that the inclination angle α of the first-side groove-sidewall is equal to the inclination angle β of the second-side groove-sidewall, and

the narrower part is positioned on the second-side of an extended line of the widthwise center line of the main part.

In this application including specification and claims, various dimensions, positions and the like of the tire refer to those under a normally inflated unloaded condition of the tire unless otherwise noted.

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 undermentioned 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. In case of passenger car tires, however, the standard pressure and standard tire load are uniformly defined by 180 kPa and 88% of the maximum tire load, respectively.

The tread edges Te are the axially outermost edges of the ground contacting patch of the tire (camber angle=0) in the normally inflated loaded condition.

The tread width TW is the axial distance between the tread edges measured in the normally inflated unloaded condition of the tire.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

In the drawings, pneumatic tire1according to the present invention is designed as a passenger car tire having a bidirectional tread pattern which is symmetrical with respect to any point on the tire equator.

The pneumatic tire1is provided in the tread portion2with circumferentially continuously extending main grooves which include a central main groove4extending along the tire equator C, and a shoulder main groove3disposed on each side of the tire equator C as the axially outermost main groove.

In the embodiments shown inFIG. 1andFIG. 2, the shoulder main grooves3and central main groove4are each formed as a straight groove whose edges are parallel with the tire circumferential direction to minimize the resistance to the drainage flow.

The width W1of the shoulder main grooves3and the width W2of the central main groove4may be arbitrarily determined. However, if the groove widths and depths are small, wet performance is deteriorated.

If the groove widths and depths are large, the ground contacting area and the rigidity of the tread portion are decreased, and the steering stability is liable to deteriorate.

Therefore, the width W1of the shoulder main grooves3is preferably set in a range of from 4.5 to 7.5% of the tread width TW.

The width W2of the central main groove4is preferably set in a range of from 3.0% to 6.0% of the tread width TW.

The depth of the shoulder main grooves3and the depth of the central main groove4are preferably set in a range of from 6.0 to 12.0 mm.

In the tread portion2, there are formed a pair of shoulder land portions5axially outside the shoulder main grooves3, and

a pair of middle land portions6between the shoulder main grooves3and the central main grooves4.

The axial width W3of the middle land portion6is set in a range of from 13.0% to 20.0% of the tread width TW.

The axial width W4of the shoulder land portion5is set in a range of from 22.0% to 28.0% of the tread width TW.

On each side of the tire equator, the shoulder axial grooves10extend from the shoulder main groove3to the tread edge Te, while inclining to one tire circumferential direction in order to facilitate the drainage toward the axially outside and improve the wet performance.

In the embodiments shown inFIGS. 1 and 2, the shoulder axial grooves10(10L) on one side (left side) of the tire equator are included to one circumferential direction (upward), but the shoulder axial grooves10(10R) on the other side (right side) of the tire equator are included to the other circumferential direction (downward).

The shoulder axial groove10has two groove edges11which are a first-side groove edge12and a second-side groove edge13,

The term “first-side” means the side toward a circumferential direction to which the shoulder axial groove10is inclined when regarding the groove extends from the main groove3to the tread edge Te. The term “second-side” means the opposite side.

This, in the examples shown inFIG. 1andFIG. 2, the upper groove edge11and the lower groove edge11of the shoulder axial groove10L disposed in the left shoulder land portion5L are the first-side groove edge12and the second-side groove edge13, respectively.

The lower groove edge11and the upper groove edge11of the shoulder axial groove10R disposed in the right shoulder land portion5R are the first-side groove edge12and the second-side groove edge13, respectively.

In each side of the tire equator C, the terms “first-side” and “second-side” are used in the same senses to express one circumferential direction and the other circumferential direction. But, between both sides of the tire equator C, the meanings are reversed.

In addition to the first-side groove edge12and the second-side groove edge13, the shoulder axial groove10has

a first-side groove-sidewall14extending radially inwardly from the first-side groove edge12, and

a second-side groove-sidewall15extending radially inwardly from the second-side groove edge13.

In the cross section of the groove, the groove-sidewall14,15is straight and continued to the groove bottom through a small arc.

The shoulder axial groove10has a part where the inclination angle β of the first-side groove-sidewall14with respect to a normal line7apassing through the first-side groove edge12perpendicularly to the tread surface2sis more than the inclination angle α of the second-side groove-sidewall15with respect to a normal line7bpassing through the second-side groove edge13perpendicularly to the tread surface2s.

When the tread portion is provided with axial grooves inclined with respect to the axial direction, usually, a rigidity difference occurs between a part on the first-side and a part on the second-side of the axial groove and as a result uneven wear or heel-and-toe wear is liable to occur.

In the present invention, however, since the inclination angles of the groove-sidewalls14and15of the shoulder axial groove10are specifically defined, the rigidity difference is reduced, and uneven wear, especially heel-and-toe wear can be effectively reduced.

In order to effectively derive this effect, the angle difference β−α is preferably set in a range of not less than 3 degrees, more preferably not less than 8 degrees, but not more than 15 degrees, more preferably not more than 12 degrees.

Each of the shoulder land portions5is provided with axially extending shoulder sipes19such that one shoulder sipe19is disposed between every two adjacent shoulder axial grooves10.

The shoulder axial groove10comprises

an axially outer wide main part16,

an axially inner narrower part17and

The wide main part16has a constant width W5in a range of not less than 2.0 mm, preferably not less than 4.0 mm, but not more than 8.0 mm, more preferably not more than 6.0 mm in order to provide a good drainage in the shoulder land portion5.

The narrower part17has a constant width W6(less than W5) in a range of not less than 0.3 mm, preferably not less than 0.5 mm, but not more than 1.0 mm, preferably not more than 0.8 mm in order to provide rigidity for the shoulder land portion5and thereby to control uneven wear and improve the steering stability.

It is preferable that the narrower part17is not perpendicular to the shoulder main groove3, and

the angle θ1of the narrower part17with respect to the tire circumferential direction is preferably set in a range of not less than 60 degrees, more preferably not less than 65 degrees, but not more than 80 degrees, more preferably not more than 75 degrees.
Such narrower parts17can reduce uneven wear in the axially inner part5aof the land portion where the narrower parts17are formed, while maintaining drainage by the shoulder axial grooves10.

It is preferable that the narrower part17is positioned on the second-side of an extended line16eof the widthwise center line16cof the main part16. Thereby, the rigidity decrease of a part of the land portion on the first-side of the shoulder axial groove10is controlled and uneven wear is prevented.

As shown inFIG. 5, it is preferable for the narrower part17that the inclination angle β of the first-side groove-sidewall14is equal to the inclination angle α of the second-side groove-sidewall15. Such narrower part17can increase the rigidity of the axially inner edge part5iof the shoulder land portion5and improve the steering stability of the tire1, while maintaining the drainage of the shoulder axial groove10.

As shown inFIG. 3, the groove width of the transitional part18is gradually decreased toward the axially inside in order to prevent the occurrence of uneven wear.

The shoulder sipes19has a groove width of from 0.3 to 1.0 mm.

The shoulder sipes19in the embodiments are slightly curved convexly toward the first-side.

The axially inner ends19iof the shoulder sipes19are positioned axially inside the transitional parts18of the shoulder axial grooves10.

The axially outer ends19oof the shoulder sipes19are positioned axially inside the tread edge Te.

Such shoulder sipes19improve the wet performance of the tire without sacrificing the rigidity of the shoulder land portion5, and control the uneven wear.

Each of the middle land portions6is provided with middle axial grooves20and axially extending middle sipes21which are arranged alternately in the circumferential direction.

In the embodiment shown inFIG. 1, the middle axial grooves20extend axially inwardly from the shoulder main groove3while inclining to the first-side, namely, to the inclining direction of the shoulder axial grooves10.

More specifically, the middle axial groove20L disposed in the left middle land portion6L extends axially inwardly from the shoulder main groove3while inclining upward.

The middle axial groove20R disposed in the right middle land portion6R extends axially inwardly from the shoulder main groove3while inclining downward.

In the embodiment shown inFIG. 2, the middle axial grooves20extend axially inwardly from the shoulder main groove3while inclining to the second-side, namely, to the opposite direction to the inclining direction of the shoulder axial grooves10.

More specifically, the middle axial groove20L disposed in the left middle land portion6L extends axially inwardly from the shoulder main groove3while inclining downward.

The middle axial groove20R disposed in the right middle land portion6R extends axially inwardly from the shoulder main groove3while inclining upward.

As shown inFIG. 6, the middle axial groove20has two groove edges22which are a first-side groove edge23and a second-side groove edge24.

The lower groove edge and the upper groove edge of the middle axial groove20disposed in the right middle land portion6R are the first-side groove edge23and the second-side groove edge24, respectively.

As shown inFIG. 7, the middle axial groove20has

a first-side groove-sidewall25extending radially inwardly from the first-side groove edge23, and

a second-side groove-sidewall26extending radially inwardly from the second-side groove edge24.

Further, the middle axial groove20has a part where the inclination angle δ of the first-side groove-sidewall25with respect to a normal line7cpassing through the first-side groove edge23perpendicularly to the tread surface2sis more than the inclination angle γ of the second-side groove-sidewall26with respect to a normal line7dpassing through the second-side groove edge24perpendicularly to the tread surface2s.

The rigidity difference caused by the inclination of the middle axial groove20can be reduced by specifically limiting the inclination angles of the groove-sidewalls25and26, therefore, the occurrence of uneven wear especially heel-and-toe wear is effectively prevented.

In order to effectively derive this effect, the angle difference δ−γ is preferably set in a range of not less than 2 degrees, more preferably not less than 4 degrees, but not more than 10 degrees, more preferably not more than 8 degrees.

In general, heel-and-toe wear is liable to occur in the shoulder land portion than in the middle land portion.

Therefore, it is preferable that the angle difference β−α is more than the angle difference δ−γ.

Thus, such shoulder axial grooves10and the middle axial grooves20even the rigidity between the shoulder land portion5and the middle land portion6, and the occurrence of uneven wear (heel-and-toe wear) is effectively prevented.

The middle axial groove20comprises

a wide part27extending from the shoulder main groove3, and a narrow part28extending from the wide part27to the central main groove4.

The groove width W7of the wide part27is gradually increased from the axially inside to the axially outside in order to increase the drainage and thereby improve the wet performance.

In the embodiment shown inFIG. 1, the second-side groove edge24of the wide part27is curved so as to further increase the groove width W7near the connection27jwith the shoulder main groove3in order to further improve the drainage.

If the groove width W7of the wide part27is small, the wet performance is deteriorated. If the groove width W7is large, there is a possibility that uneven wear is caused in the middle land portion6.

Therefore, the groove width W7of the wide part27is preferably set in a range of not less than 2.0 mm, more preferably not less than 4.0 mm, but not more than 8.0 mm, more preferably not more than 6.0 mm.

The angle θ2between the wide part27and the shoulder main groove3is preferably set in a range of not less than 50 degrees, more preferably not less than 55 degrees, but not more than 70 degrees, more preferably not more than 65 degrees in order to improve the drainage while preventing the uneven wear of the middle land portion6.

The narrow part28extends straight and has a constant groove width W8less than the width of the wide part27. The width W8of the narrow part28is preferably set in a range of from 0.3 to 1.0 mm in order to improve the steering stability while maintaining the rigidity of the middle land portion6.

It is preferable that the narrow part28is positioned on the first-side of an extended line27eof the widthwise center line27cof the wide part27. Thereby, the decrease in the rigidity of a part of the land portion on the first-side of the middle axial groove20can be controlled, and the uneven wear is prevented.

As shown inFIG. 8, it is preferable for the narrow part28that the inclination angle δ of the first-side groove-sidewall25is equal to the inclination angle γ of the second-side groove-sidewall26in order to increase the rigidity of the axially inner edge6iof the middle land portion6, while maintaining the drainage of the middle axial groove20, and improve the steering stability of the tire.

As shown inFIG. 9, it is preferable that an axially outer end potion20oof the middle axial groove20is provided with a tie bar29rising from the groove bottom20dand extending substantially between the opposed groove walls in order to prevent the decrease in the rigidity of the tread portion2due to the provision of the middle axial groove20.

If the height h1of the tie bar is small, it is difficult to prevent the decrease in the rigidity. If the height h1of the tie bar is large, the drainage performance of the middle axial groove20is decreased.

Therefore, the height h1of the tie bar is preferably set in a range of not less than 1.0 mm, more preferably not less than 2.0 mm, but not more than 3.5 mm, more preferably not more than 2.5 mm. The height h1may be obtained by taking the groove depth d2at the position of the tie bar29from the maximum depth d1of the middle axial groove20.

Similarly, the axial width W9of the tie bar29is preferably set in a range of not less than 0.10 times, more preferably not less than 0.20 times, but not more than 0.40 times, more preferably not more than 0.30 times the axial width W3of the middle land portion6.

Further, it is preferable that the tie bar29is provided with a sipe30extending along the middle axial groove20to improve the wet performance and control the pumping sound generated from the middle axial groove20during running.

It is preferable that the above-mentioned middle sipes21extend to the shoulder main groove3.

The middle sipe21has a groove width of from 0.3 to 1.0 mm. The middle sipe21is slightly curved convexly toward the second-side.

The axially inner ends21iof the middle sipes21are positioned axially outside the axially inner ends27iof the wide parts27of the middle axial grooves20in order to optimize the rigidity distribution of the middle land portion6and thereby control the uneven wear of the middle land portion6.
Comparison Tests

Passenger radial tire of size 195/65R15 (rim size 15×6JJ) having specifications shown in Table 1 were manufactured and tested for uneven wear.

In the test, the tires were installed on the rear wheels of a Japan-made 1800 cc FF passenger car and run for 20,000 km. (tire pressure 230 kPa) Then, the difference between the wear amount at the first-side edge and the wear amount at the second-side edge of the shoulder axial groove was measured as uneven wear (heel-and-toe wear).

The results are shown in Table 1 by an index based on comparative example tire Ref. 1 being 100 wherein the smaller index number is better.

From the test results, it was confirmed that, according to the present invention, uneven wear can be remarkably improved.