Pneumatic tire

1. Field of the Invention

The present invention relates to a pneumatic tire improved in steering stability and noise performance while maintaining the drainage performance.

2. Description of the Related Art

A pneumatic tire having a tread portion provided with circumferentially extending main grooves is proposed. Such main grooves may drain the water between the tread portion and the road. Also, the larger the groove width of the main groove, the better the drainage performance is.

However, when the groove width of the main groove is too large, the noise performance is liable to deteriorate due to the air tube resonance therein. Moreover, when the tread portion has a small land ratio, the steering stability of the tire is liable to decrease due to the small rigidity of the tread portion.

JP-2003-285610-A1 discloses a pneumatic tire having a tread portion provided with a plurality of main grooves to form a rib-tread pattern. The tire above has a limited total groove width in a certain range for main grooves, and is neither provided any sipes nor slots which extend from the main grooves to increase the rigidity of ribs. The tire may improve the steering stability and noise performance. However, such a tire has an unsatisfactory drainage performance.

The present invention has been worked out in light of the circumstances described above, and has a main object of providing a pneumatic tire in which the steering stability and noise performance can be improved while maintaining the drainage performance.

In accordance with the present invention, there is provided a pneumatic tire having a designated installing direction to a vehicle, comprising a tread portion having an inboard tread edge and an outboard tread edge, the tread portion being provided with: a pair of circumferentially extending inboard and outboard center main grooves disposed both sides of a tire equator to define a center portion therebetween; a pair of circumferentially extending inboard and outboard shoulder main grooves disposed axially outside of the inboard and outboard center main grooves; a pair of middle portions which include an inboard middle portion defined between the inboard center main groove and the inboard shoulder main groove, and an outboard middle portion defined between the outboard center main groove and the outboard shoulder main groove; and a pair of shoulder portions which include an inboard shoulder portion defined the axially outside of the inboard shoulder main groove, and an outboard shoulder portion defined the axially outside of the outboard shoulder main groove, wherein each of the inboard center main groove, the outboard center main groove and the inboard shoulder main groove has a groove width in a range of from 10 to 20 mm, the outboard shoulder main groove has a groove width in a range of from 2 to 5 mm, the outboard shoulder portion is provided with a plurality of outboard shoulder lateral grooves each of which extends at least from the outboard tread edge to the axially inside of the tire and terminates without reaching the outboard shoulder main groove, the outboard middle portion is provided with a plurality of outboard middle lateral grooves each of which extends from the outboard shoulder main groove beyond a center in width direction of the outboard middle portion without reaching the outboard center main groove, the inboard middle portion is provided with a plurality of inboard middle sipes each of which extends from the inboard shoulder main groove beyond a center in width direction of the inboard middle portion without reaching the inboard center main groove, and the inboard shoulder portion is provided with a plurality of inboard shoulder lateral grooves each of which connects between the inboard tread edge and the inboard shoulder main groove.

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 is inflated to a standard pressure but loaded with no tire load.

The standard wheel rim is a wheel rim officially approved or recommended for the tire by standards organizations, the standard wheel rim is the “standard rim” specified in JATMA, the “Measuring Rim” in ETRTO, the “Design Rim” in TRA or the like, for example.

The standard pressure is the “maximum air pressure” in JATMA, the “Inflation Pressure” in ETRTO, and the maximum pressure given in the “Tire Load Limits at Various Cold Inflation Pressures” table in TRA or the like. In case of passenger car tires, however, the standard pressure is uniformly defined by 180 kPa.

FIG. 1 is a development view of a tread portion of a pneumatic tire showing an embodiment of the present invention.

FIG. 2 is a cross sectional view taken along the line A-A of FIG. 1 .

FIG. 3 is a partial enlarged view of an inboard of the tread portion in FIG. 1 .

FIG. 4 is a partial enlarged view of an outboard of the tread portion in FIG. 1 .

FIG. 5 is a development view of a tread portion of a pneumatic tire showing a Reference 1.

FIG. 6 is a development view of a tread portion of a pneumatic tire showing a Reference 2.

An embodiment of the present invention will be explained below with reference to the accompanying drawings.

As shown in FIG. 1 , a pneumatic tire (hereinafter it may simply be referred as “the tire”) 1 in accordance with the present invention has a designated installing direction to a vehicle. The installing direction may be indicated on the sidewall portion of the tire 1 using characters such as “inside” and “outside”, for example.

The tire 1 includes a tread portion 2 with a left-right asymmetry tread pattern including an inboard tread edge 2 i and an outboard tread edge 2 o defining a tread width TW therebetween. The tread edges 2 o and 2 i are defined as edges of the tread portion 2 if those are clearly identified in appearance. However if it is difficult, the tread edges 2 o and 2 i are defined as the axial outermost edges of the ground contacting patch which occurs under the normally inflated loaded condition when the camber angle of the tire is zero.

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 a standard tire load. Moreover, the standard tire load is the “maximum load capacity” in JATMA, the “Load Capacity” in ETRTO, and the maximum value given in the above-mentioned table in TRA or the like.

The inboard tread edge 2 i refers to one of the two tread edges which is intended to be positioned towards the center of the vehicle body. The outboard tread edge 2 o refers to the other tread edge which is intended to be positioned away from the center of the vehicle body. According thereto, in this application, the terms “outboard” and “inboard” are used toward the outboard tread edge 2 o and inboard tread edge 2 i , respectively, to refer relative positions in the tire axial direction.

The terms “axially inner”, “axially inward” and the like are used toward the tire equator C, and the terms “axially outer”, “axially outward” and the like are used toward the tread edge in order to refer relative positions in the tire axial direction.

The tread portion 2 of the pneumatic tire 1 is provided with a pair of circumferentially extending center main grooves 3 A, 3 B disposed both sides of the tire equator C, and a pair of circumferentially extending shoulder main grooves 3 C, 3 D disposed the axially outside of center main grooves 3 .

The center main grooves include: an inboard center main groove 3 A which is disposed in the side of the inboard tread edge 2 i than the tire equator C, and an outboard center main groove 3 B which is disposed in the side of the outboard tread edge 2 o than the tire equator C.

Also, the shoulder main grooves include: an inboard shoulder main groove 3 C which is disposed in the side of the inboard tread edge 2 i than the inboard center main groove 3 A, and an outboard shoulder main groove 3 D which is disposed in the side of the outboard tread edge 2 o than the outboard center main groove 3 B.

Hence, the tread portion 2 includes: a center portion 4 A between the outboard and inboard center main grooves 3 A and 3 B; an inboard middle portion 4 B defined between the inboard center main groove 3 A and the inboard shoulder main groove 3 C; an outboard middle portion 4 C defined between the outboard center main groove 3 B and the outboard shoulder main groove 3 D; an inboard shoulder portion 4 D defined between the inboard shoulder main groove 3 C and the inboard tread edge 2 i ; and an outboard shoulder portion 4 E defined between the outboard shoulder main groove 3 D and the outboard tread edge 2 o.

Preferably, in order to improve the noise performance while maintaining the drainage performance, the tread portion 2 has a land ratio Sb/Sa in a range of from 69 to 73%, wherein: “Sb” is the net ground-contacting area of the ground-contacting region of the tread portion 2 ; and “Sa” is the gross area of the ground-contacting region of the tread portion 2 including the grooved area.

If the land ratio Sb/Sa is less than 69%, the air tube resonance is liable to be produced due to large grooved area. On the other hand, if the land ratio Sb/Sa is more than 73%, the drainage performance is liable to deteriorate due to small grooved area.

Each of the inboard center main groove 3 A, the outboard center main groove 3 B, the inboard shoulder main groove 3 C and the outboard shoulder main groove 3 D is preferably formed as a straight groove extending along the circumferential direction of the tire to drain the water between the tread portion 2 and the road outside toward the tire circumferential direction when running. Referring to FIG. 2 , groove depths D 1 a , D 1 b , D 1 c and D 1 d of main grooves 3 A, 3 B 3 C and 3 D are preferably in a range of from 6 to 10 mm.

Referring to FIG. 1 , each of the inboard center main groove 3 A, the outboard center main groove 3 B and the inboard shoulder main groove 3 C has a groove width W 1 a , W 1 b and W 1 c in a range of from 10 to 20 mm, respectively. On the other hand, the outboard shoulder main groove 3 D has a groove width W 1 d in a range of from 2 to 5 mm.

Accordingly, the inboard center main groove 3 A, the outboard center main groove 3 B and the inboard shoulder main groove 3 C having large groove widths W 1 a , W 1 b and W 1 c are useful to effectively enhance the drainage performance.

Since the influence of the outboard shoulder main groove 3 D on the air tube resonance is great, such an outboard shoulder main groove 3 D having a comparatively small groove width W 1 d is useful for effectively preventing the air tube resonance therein, and thereby the noise performance is improved. Basically, a large side force is applied on the outboard of tread portion 2 when cornering. However, since such an outboard shoulder main groove 3 D having a comparatively small groove width W 1 d maintains the rigidity of the outboard tread portion 2 , the steering stability is also improved.

In order to further improve the steering stability and noise performance while maintaining the drainage performance, each groove width W 1 a , W 1 b and W 1 c is preferably in a range of from 12 to 18 mm.

In order to further prevent the air tube resonance while maintaining the drainage performance, the groove width W 1 d is preferably in a range of from 3 to 4 mm.

Preferably, the outboard center main groove 3 B has the largest groove width W 1 b in the main grooves 3 A to 3 C. Although drainage performance on the outboard tread portion comparatively tends to be lower, such an outboard center main groove 3 B having the largest groove width W 1 b may compensate for improving the drainage performance on the outboard of the tread portion 2 . Preferably, a groove width ratio W 1 b /W 1 a of the groove width W 1 b of the outboard center main groove 3 B to the groove width W 1 a of the inboard center main groove 3 A is in a range of from 102 to 106%.

Referring to FIG. 3 , the center portion 4 A is formed as a rib which straightly and continuously extends in the tire circumferential direction. Namely, the center portion 4 A is not divided perfectly in the tire circumferential direction by any grooves.

Such a center portion 4 A has a high rigidity in the tire circumferential direction, and thereby the stability in straight running is improved. Also, since the center portion 4 A has no lateral grooves which connect between the inboard and outboard center main grooves 3 A and 3 B, it does not produce the air tube resonance. Preferably, the maximum land width W 2 a of the center portion 4 A is in a range of from 8 to 13% of the tread width TW (shown in FIG. 1 ).

The center portion 4 A is provided with a plurality of center sipes 5 A at least one of which extends from the inboard center main groove 3 A and at least one of which extends from the outboard center main groove 3 B, respectively. Each center sipe 5 A has an axially inner end without reaching the tire equator C to improve the grounding-property of the center portion 4 A while increasing the sipe edges. Also, the straight running stability and initial response of cornering may be improved by such a center portion 4 A. Preferably, the center sipe 5 A has a length L 3 a in the tire axial direction in a range of from 15 to 25% of the maximum width W 2 a of the center portion 4 A, and has the depth of from 3 to 6 mm.

Preferably, the center sipe 5 A is inclined at angle α 3 a of from 50 to 70 degrees with respect to the tire circumferential direction to effectively improve the grounding-property of the center portion 4 A.

Preferably, a pitch P 1 between adjacent center sipes 5 A in the tire circumferential direction is larger than a pitch P 2 between adjacent inboard middle lateral grooves 6 B (it will describe later). Such an arrangement of center sipes 5 A is useful to enhance the rigidity of the center portion 4 A, and thereby the transient response in lane-change may be stable.

In order to further improve the effects above, the pitch P 1 of center sipes 5 A is preferably in a range of from 150 to 250%, and more preferably in a range of from 170 to 230% the pitch P 2 of inboard middle lateral grooves 6 B.

Similarly, the inboard middle portion 4 B is formed as a straight rib to improve the steering stability and the noise performance. Preferably, the maximum width W 2 b of the inboard middle portion 4 B is in a range of from 8 to 13% the tread width TW (shown in FIG. 1 ).

The inboard middle portion 4 B is provided with a plurality of inboard middle sipes 5 B each of which extends from the inboard shoulder main groove 3 C to the axially inside of the tire, and a plurality of inboard middle lateral grooves 6 B each of which extends from the inboard center main groove 3 A to the axially outside of the tire. The inboard middle sipes 5 B and the inboard middle lateral grooves 6 B are alternately arranged in the tire circumferential direction.

Each inboard middle sipes 5 B has an axially inner end which goes beyond a center in width direction of the inboard middle portion 4 B and terminates within the inboard middle portion 4 B without reaching the inboard center main groove 3 A.

Accordingly, the grounding-property of the center portion 4 A is improved by such an inboard middle sipe 5 B while increasing the sipe edges. Preferably, the axial length L 3 b of the inboard middle sipe 5 B is in a range of from 60 to 70% the maximum width W 2 b of the inboard middle portion 4 B, and the sipe depth thereof is preferably set the same range with the center sipe 5 A.

The inboard middle sipe 5 B has an angle (an obtuse angle) α 3 b with respect to the circumferential direction of the tire which is gradually decreasing toward the tire equator C. Such an inboard middle sipe 5 B improves grounding-property of the inboard middle portion 4 B, and increases the edge thereof. Since the inboard middle sipe 5 B may cause the pressure fluctuation in the inboard shoulder main groove 3 C, the air tube resonance in the inboard shoulder main groove 3 C is also prevented. Preferably, the angle α 3 b is in a range of from 120 to 155 degrees.

In order to improve the effects above, the angle α 3 b is still further preferably in a range of from 130 to 150 degrees.

The inboard middle lateral groove 6 B extends from the inboard center main 3 A groove to the axially outside of the tire. The inboard middle lateral groove 6 B has an axially outer end which terminates within the inboard middle portion 4 B without going beyond the center in width direction of the inboard middle portion 4 B. Such an inboard middle lateral groove 6 B may improve the steering stability and the drainage performance. Preferably, the axial length L 4 b of the inboard middle lateral groove 6 B is in a range of from 15 to 25% the maximum width W 2 b of the inboard middle portion 4 B, the groove width W 4 b thereof is preferably in a range of from 1 to 3% the tread width TW (shown in FIG. 1 ), and the groove depth D 4 b (shown in FIG. 2 ) thereof is preferably in a range of from 3 to 6 mm.

Preferably, the inboard middle lateral groove 6 B has an angle α 4 b of from 50 to 70 degrees with respect to the circumferential direction of the tire to improve the drainage performance.

Referring to FIG. 4 , the outboard middle portion 4 C is also formed as a straight rib to improve the steering stability and noise performance. Preferably, the maximum width W 2 c of the outboard middle portion 4 C is in a range of from 10 to 15% the tread width TW (shown in FIG. 1 ).

The outboard middle portion 4 C is provided with a plurality of outboard middle lateral groove 6 C each of which extends from the outboard shoulder main groove 3 D to the axially inside and terminates within the outboard middle portion 4 C. Each outboard middle lateral groove 6 C has an axially inner end which goes beyond a center in width direction of the outboard middle portion 4 C.

Since the outboard middle lateral groove 6 C has a large groove length in the tire axial direction, the drainage performance on the outboard tread portion is improved. Preferably, the axial length L 4 c of the outboard middle lateral groove 6 C is in a range of from 55 to 65% the maximum width W 2 c of the outboard middle portion 4 C, the groove width W 4 c thereof is preferably in a range of from 1 to 3% the tread width TW (shown in FIG. 1 ), the groove depth D 4 c (shown in FIG. 2 ) thereof is in a range of from 2 to 7 mm.

The outboard middle lateral groove 6 C has an angle α 4 c of from 50 to 70 degrees with respect to the circumferential direction of the tire to improve the drainage performance.

Referring to FIG. 3 , the inboard shoulder portion 4 D is provided with a plurality of inboard shoulder lateral grooves 6 D each of which extends from the inboard shoulder main groove 3 C to the inboard tread edge 2 i to form a plurality of inboard shoulder blocks 7 D.

The inboard shoulder lateral groove 6 D has an angle α 4 d gradually decreasing toward the inboard shoulder main groove 3 C to smoothly drain the water between the inboard shoulder portion 4 D and the road, and thereby the drainage performance is further improved. Preferably, the inboard shoulder lateral groove 6 D has an angle α 4 d of from 50 to 90 degrees, the groove width W 4 d thereof is preferably in a range of from 1.5 to 3% the tread width TW (shown in FIG. 1 ), and the groove depth D 4 d thereof (shown in FIG. 2 ) is in a range of from 2 to 7 mm.

The inboard shoulder lateral groove 6 D, for example, includes: a wide portion 6 Da extending from the inboard tread edge 2 i toward the inboard shoulder main groove 3 C; and a narrow portion 6 Db having a groove width smaller than that of the wide portion 6 Da and connecting between the wide portion 6 Da and the inboard shoulder main groove 3 C. The narrow portion 6 Db is provided with a tie-bar 11 (shown in FIG. 2 ) which has a protruded groove bottom so as to have a small groove depth to enhance the rigidity of inboard shoulder blocks, and thereby the steering stability may be improved.

The inboard shoulder block 7 D has a substantially rectangular shape having the length L 5 d in the tire circumferential direction and the width W 5 d in the tire axial direction which is about equal to the length L 5 d . Since the inboard shoulder block 7 D has a high circumferentially rigidity, the steering stability is effectively improved. Preferably, the length L 5 d and the width W 5 d of the inboard shoulder block 7 D is in a range of from 15 to 20% the tread width TW (shown in FIG. 1 ).

Each inboard shoulder block 7 D is provided with a inboard shoulder sipe 5 D extending from the inboard shoulder main groove 3 C to the axially outside of the tire without reaching the inboard tread edge 2 i . The inboard shoulder sipe 5 D has an angle α 3 d of from 70 to 80 degrees with respect to the tire circumferential direction.

The inboard shoulder sipe 5 D softens the rigidity of the inboard shoulder block 7 D so that the grounding-property is improved. Moreover, the inboard shoulder sipe 5 D increases edges in the inboard shoulder block 7 D so that the cornering performance is improved. Preferably, the axial length L 3 d of the inboard shoulder sipe 5 D is in a range of from 75 to 85% the width W 5 d of the inboard shoulder block 7 D, and the sipe depth thereof (not shown) is preferably in the same range with the center sipe 5 A.

Referring to FIG. 4 , the outboard shoulder portion 4 E is formed as a straight rib to improve the steering stability and the noise performance. Preferably, the maximum width W 2 e of the outboard shoulder portion 4 E is in a range of from 18 to 24% the tread width TW (shown in FIG. 1 ).

The outboard shoulder portion 4 E is provided with a plurality of outboard shoulder lateral grooves 6 E and an outboard shoulder sub groove 8 E. Each outboard shoulder lateral grooves 6 E extends from the outboard shoulder tread edge 2 o to the axially inside of the tire and has an axially inner end 6 Ei without reaching the outboard shoulder groove 3 D. The outboard shoulder sub groove 8 E extends in the tire circumferential direction so as to connect each inner end 6 Ei of the outboard shoulder lateral grooves 6 E.

The outboard shoulder groove 6 E has an angle α 4 e with respect to the tire circumferential direction gradually decreasing toward the axially inside of the tire so that the drainage performance and the steering stability are improved.

In order to further improve the effects above, the axial length L 4 e of the outboard shoulder lateral groove 6 E is preferably in a range of from 65 to 75% the maximum width W 2 e of the outboard shoulder portion 4 E, the groove width W 4 e of thereof is in a range of from 1.5 to 3% the tread width TW (shown in FIG. 1 ), the groove depth D 4 e thereof is in a range of from 2 to 7 mm, and the angle α 4 e is in a range of from 70 to 90 degrees.

The outboard shoulder sub groove 8 E straightly extends along the tire circumferential direction to improve the drainage performance on the outboard shoulder portion 4 E.

Preferably, the outboard shoulder sub groove 8 E has the groove width W 6 e of from 0.3 to 1% the tread width TW (shown in FIG. 1 ), and the groove depth D 6 e (shown in FIG. 2 ) thereof is preferably in a range of from 1 to 3 mm to improve the noise performance and steering stability.

The present invention is more specifically described and explained by means of the following Examples and References. It is to be understood that the present invention is not limited to these Examples.

Comparison Test

Pneumatic tires with basic tread patterns of FIG. 1 except for details shown in Table 1 were made and tested. As shown in FIG. 5 , the Reference 1 which has an outboard shoulder main groove 3 D having the same groove width with the inboard shoulder main groove 3 A was also tested. As shown in FIG. 6 , the Reference 2 which has outboard shoulder lateral grooves, outboard middle lateral grooves, inboard middle sipes and inboard shoulder lateral grooves each of which is connected to main grooves was also tested. Major common specifics and test method are as follows.

Tire size: 225/45 R17 Rim size: 17×7.0JJ Tread width TW: 198 mm Groove depth of main groove D 1 a , D 1 b , D 1 c , D 1 d: 8.2 mm Maximum width W 2 a of center portion: 20.47 mm Ratio W 2 a /TW: 10.3% maximum width W 2 b of inboard middle portion: 21.46 mm Ratio W 2 b /TW: 10.8% Maximum width W 2 c of outboard middle portion: 26.65 mm Ratio W 2 c /TW: 13.5% Maximum width W 2 e of outboard shoulder portion: 42.33 mm Ratio W 2 e /TW: 21.4%

Inboard Shoulder Block

width W 5 d: 36.15 mm Ratio W 5 d /TW: 18.3% Circumferential length L 5 d: 54.8 mm Ratio L 5 d /TW: 27.7%

Center Sipe

Length L 3 a: 3.4 mm Ratio L 3 a /W 2 a: 1.7% Depth: 4.4 mm Angle α 3 a: 60 degrees

Inboard Middle Sipe

Length L 3 b: 12.5 mm Ratio L 3 b /W 2 b: 6.3% Depth: 4.6 mm

Inboard Middle Lateral Groove

Length L 4 b: 5.5 mm Ratio L 4 b /W 2 b: 2.8% Groove width W 4 b: 2.4 to 3.0 mm Ratio W 4 b /TW: 1.2 to 1.5% Groove depth D 4 b: 3 to 5.4 mm Angle α 4 b: 60 degrees

Outboard Middle Lateral Groove

Length L 4 c: 15.2 mm Ratio L 4 c /W 2 c: 7.7% Groove width W 4 b: 2.4 to 3.0 mm Ratio W 4 b /TW: 1.2 to 1.5% Groove depth D 4 c: 3 to 6.6 mm Angle α 4 c: 60 degrees

Inboard Shoulder Lateral Groove

Groove width W 4 d: 2.4 to 3.3 mm Ratio W 4 b /TW: 1.2 to 1.6% Groove depth D 4 d: 3.0 to 6.0 mm Angle α 4 d: 80 to 85 degrees

Inboard Shoulder Sipe

Length L 3 d: 28.9 mm Ratio L 3 d /W 5 b: 14.6% Depth: 2.0 to 5.9 mm Angle α 3 d: 80 degrees

Outboard Shoulder Lateral Groove

Length L 4 e: 30.55 mm Ratio L 4 e /W 2 e: 15.4% Groove width W 4 e: 3.3 mm Ratio W 4 e /TW: 1.6% Groove depth D 4 e: 6.0 mm Angle α 4 e: 80 to 85 degrees

Outboard Shoulder Sub Groove

Groove width W 6 e: 3.0 mm Ratio W 6 e /TW: 1.5% Groove depth D 6 e: 2.0 mm

Noise Performance Test:

According to JASO C-606, a Japanese FF passenger car with a displacement of 2,400 cc provided on all the wheels with test tires was coasted on a smooth asphalt road, and the pass-by noise was measured with a microphone fixed at a position 7.5 meter sideways from the center line of the running course. The tires were mounted on a rim and inflated to an inner pressure of 220 kPa. The performance was evaluated by the reciprocal number of the noise. Test results are indicated by an index of based on Reference 1 being 100. The larger the value, the better the noise level is.

Drainage Performance Test:

The test car above was driven on a wet asphalt road with 5 mm depth of water and suddenly braked at a speed of 60 km/hr so that the ABS worked. The reciprocal number of the braking distance of each tires were measured. The results are shown with an index of 100 representing a value in Reference 1. The larger the value, the better the performance is.

Steering Stability Test:

A professional test driver drove the test car above on a test course having a dry asphalt road, and evaluated steering stability such as the steering response during cornering, stiffness and cornering grip according to his feeling. The results are shown with a score of 100 representing a value in Reference 1. The larger the value, the better the performance is.

Test results are shown in Table 1

From the test results, it was confirmed that Example tires in accordance with the present invention can be effectively improved the steering stability and noise performance while maintaining the drainage performance.