Pneumatic tires for passenger cars

Abstract: A pneumatic tire for passenger cars has a directional tread pattern defined by at least one circumferential rib (1) and a plurality of directional slant grooves (2, 3), in which the directional slant grooves include directional low-slant grooves (2) each having a smaller inclination angle with respect to the circumferential direction, and a block (41) among plural blocks (41, 42, 43) formed between the adjoining directional low-slant grooves (2) at given intervals in the circumferential direction is adjacent to the circumferential rib and has a relation such that a height of a preceding ground contact side is lower than a height of a later ground contact side during the forward rotation of the tire.

Description: This invention relates to a pneumatic tire for passenger cars, and more particularly to a high-performance pneumatic tire developing excellent running performances even during sport running and controlling pattern noise to a low level without sacrificing wet-running performance. In Fig. 3 of the accompanying drawings is shown a typical example of a conventional tread pattern in conventional high-performance pneumatic tires for passenger cars. That is, the conventional tire has a tread pattern defined by several circumferential grooves (four grooves in the illustrated embodiment, generally about 2-8 grooves) and many directional slant grooves arranged at given intervals in the circumferential direction. The term "circumferential groove" used herein means a straight groove continuously extending in the circumferential direction of the tire or a substantially straight groove, and the term "circumferential rib" used herein means a rib continuously or substantially continuously extending in the circumferential direction. The term "directional slant groove" used herein means a groove slantly extending with respect to the circumferential direction or a groove indicating the rotating direction of the tire when the tire is mounted onto a vehicle or forming a so-called directional tread pattern in which a portion of the slant groove near to an equatorial plane of the tire is first contacted with the ground during the running of the tire and a portion of the slant groove apart from the equatorial plane is contacted with the ground subsequently. In the conventional tire as mentioned above, in order to effectively enhance the steering stability and resistance to hydroplaning on wet road surface, it is frequently adopted to increase the number of grooves or the groove width to increase a negative ratio (ratio of groove surface area to area of ground contact region of tread), and particularly to increase the negative ratio of the directional slant grooves. However, it has been confirmed that the level of pattern noise undesirably increases in a pneumatic tire provided with a directional tread pattern having an improved wet-running performance by the above method. The pattern noise of the tire is comprised of various factors. A beat sound produced when the tire contacts with the ground during the running under load is one of the above factors. Particularly, it has been confirmed that pattern noise of the above beat sound component is extremely large in a tire having directional slant grooves at a high negative ratio. On the other hand, there is known a tire having a combination of relatively wide-width straight groove continuously extending in the circumferential direction for the improvement of drainage property and arranged in a central portion of the tread or so-called aqua channel and a plurality of directional slant grooves arranged at given intervals in the circumferential direction. This tire has excellent drainage property, but has poor steering stability at a slight steering angle such as straight running performance or the like, so that it is unsuitable as a high-performance tire having excellent running performances even during sport running. It is, therefore, an object of the present invention to provide a tread pattern used in high running-performance pneumatic tires for passenger cars developing excellent running performances even during sport running and having an excellent wet-running performance and controlling pattern noise to a low level. The present invention provides a pneumatic tire for passenger cars having a directional tread pattern defined by at least one circumferential rib and a plurality of directional slant grooves arranged on both sides of the circumferential rib at given intervals in the circumferential direction of the tire, the directional slant grooves comprising directional low-slant grooves each having a small inclination angle with respect to the circumferential direction at a side near to an equatorial plane of the tire and extending from each side of the circumferential rib toward a side end of a tread, the directional slant grooves further comprising directional high-slant grooves each having a large inclination angle with respect to the circumferential direction in addition to the directional low-slant grooves, and wherein plural blocks are formed between adjacent directional low-slant grooves at given intervals in the circumferential direction, and a block among said plural blocks is joined to the rib and has a surface such that a height of a preceding ground contact side of the block is lower than a height of a subsequent ground contact side of the block during the forward rotation of the tire, a trailing edge of the block is defined by a high-slant groove, the height of a portion of the block at said trailing edge of the block is substantially the same as the height of the at least one rib, and the surface of the block and a sidewall of the rib intersect so as to define a line which is inclined with respect to the radial direction of the tire. In a preferable embodiment of the invention, the height of the preceding ground contact side of the block is the same level as the bottom of the directional low-slant groove adjacent to the block. In another preferable embodiment of the invention, the directional low-slant groove and/or the directional high-slant groove are open at a side end of the tread. The inclination angle of each directional high-slant groove with respect to the circumferential direction is preferably not less than 45° and the inclination angle of each directional low-slant groove is preferably not more than 30°. The invention will be further described with reference to the accompanying drawings, wherein: Fig. 1a is a diagrammatic developed view of a tread pattern in a first embodiment of a pneumatic tire for a passenger car according to the invention; Fig. 1b is a side view taken along a line Ib-Ib viewing from a directional slant groove; Fig. 2a is a diagrammatic developed view of a tread pattern in a second embodiment of a pneumatic tire for a passenger car according to the invention; Fig. 2b is a side view taken along a line IIb-IIb viewing from a directional slant groove; and Fig. 3 is a diagrammatic developed view of a tread pattern in a conventional pneumatic tire. In the pneumatic tire for passenger cars according to the invention, many directional low-slant grooves each having a small inclination angle with respect to the circumferential direction are arranged at given intervals in the circumferential direction as mentioned above, so that the wet-running performance is excellent. However, when the tread pattern is formed only by the directional low-slant grooves, it is difficult to ensure sufficient block rigidity and hence the steering stability is adversely affected. According to the invention, the tread pattern is formed by at least one circumferential rib in addition to the above directional slant grooves, so that desired rigidity is given to the tread pattern by the circumferential rib and hence a response to the steering wheel at a slight steering angle becomes higher to enhance so-called "firm feeling" given to a driver, whereby the straight running performance is improved and also pattern noise is reduced owing to smooth contacting with the ground. The circumferential rib is preferably a so-called center rib arranged in a central region of the tread, but may be located within a range from the tread center to a position corresponding to about 1/3 of the tread width in the case of an unsymmetrical tread pattern. Alternatively, two circumferential ribs may be arranged in the central region of the tread to form a so-called center groove. Furthermore, in the tread pattern according to the invention, blocks are formed between adjacent directional low-slant grooves at intervals in the circumferential direction and a height of a block at least adjacent to the circumferential rib among these blocks is gradually increased from a preceding ground contact side toward a later ground contact side during the forward running of the tire, so that the wet-running performance can be further improved while ensuring the rigidity of the rib. Particularly, the circumferential rib is supported by the block adjacent thereto among the blocks arranged between the adjacent directional low-slant grooves at intervals in the circumferential direction, so that the width of the circumferential rib can be narrowed as compared with that in a conventional tread pattern of a high-performance pneumatic tire for passenger cars (shown in Fig. 3 as a typical example). The invention will be described with respect to the following examples by reference to the accompanying drawings. In Figs. 1a and 2a are shown tread patterns of Examples 1 and 2 of pneumatic tires for passenger cars according to the invention, respectively. These tires have a tire size of 225/50R16 and a tread width (TW) of about 200 mm. The pneumatic tire of Example 1 shown in Fig. 1 has a tread pattern comprising one circumferential rib 1 arranged in a central region of a tread and a plurality of directional slant grooves 2, 3 arranged on both sides of the circumferential rib 1 at given intervals in the circumferential direction of the tire. These directional slant grooves comprise many directional low-slant grooves 2 each having a small inclination angle of 10-30° with respect to the circumferential direction and many directional high-slant grooves 3 each having a large inclination angle of 60-80° with respect to the circumferential direction. These directional slant grooves are curvedly extended so that the inclination angle with respect to the circumferential direction is made smaller in a region near to an equatorial plane than in a region apart from the equatorial plane, and first contacted at a portion near to the equatorial plane with the ground and later contacted at a portion apart from the equatorial plane with the ground when the tire is mounted onto a vehicle in a forward direction of the tire shown by an arrow D, whereby a so-called directional tread pattern is formed. Thus, when the mounted state of the tire onto the vehicle is viewed from the front, the tread is provided with a plurality of directional slant grooves each bordering around a central region of the tread and gradually enlarging from downward toward upward. Blocks 41, 42, 43 are formed between the adjacent directional low-slant grooves 2 at intervals in the circumferential direction. Among these blocks, the block 41 adjacent to the circumferential rib 1 has a relation such that a height of a preceding ground contact side is lower than a height of a later ground contact side during the forward rotation of the tire and the height of the preceding ground contact side of the block is the same level as the bottom of the directional low-slant groove adjacent to the block as shown in Fig. 1b. In other words, the surface of the block 41 is slantly formed so that the height of the preceding ground contact side of the block is equal to the groove depth of the directional low-slant groove. The directional high-slant grooves 3 are open at a side end of the tread TE to serve as a discharge port of water between the ground contact region of the tire tread and the road surface. The pneumatic tire of Example 2 shown in Fig. 2 is basically the same as the pneumatic tire of Example 1 shown in Fig. 1 except that a part of the directional high-slant groove 3 is narrowed in form of a slit and the directional high-slant grooves 3 are arranged in the vicinity of the side end TE of the tread apart from the central region of the tread so as to form somewhat longitudinally long blocks 41, 43. That is, this tire aims to improve the pattern noise by slightly sacrificing the drainage property as compared with the pneumatic tire of Example 1. A pneumatic tire of a Conventional Example shown in Fig. 3 is a typical embodiment of a conventional tread pattern comprising five circumferential grooves 61, 62, 63, 64, 65 and many directional slant grooves 71, 72, 73, 74, 75, 76 arranged at intervals in the circumferential direction. This tire has a tire size of 225/50R16 and a tread width TW of about 200 mm, which are the same as those of the above examples 1 and 2. The circumferential groove 63 arranged in the center of the tread is a narrow groove having a width of 4 mm, and a pair of circumferential grooves 62, 64 having a width of 11 mm are arranged on both sides of the groove 63, and further a pair of circumferential grooves 61, 65 having a width of 10 mm are arranged so as to extend between a side end of the tread and the tread center at positions corresponding to about 1/4 of the tread width. In the tire, the four thick circumferential grooves (61, 62, 64, 65) and many directional slant grooves (71 to 76) largely contribute together to improve the drainage property when the tire is run on a wet road surface. Tests for evaluating the resistance to hydroplaning, pattern noise and steering stability on a dry road surface are made with respect to each of the pneumatic tires for passenger cars of Examples 1 and 2 and the Conventional Example under an internal air pressure of 2.3 kgf/cm 2 . The resistance to hydroplaning during straight running is evaluated by measuring a remaining area of ground contact region when the tire is run at a speed of 80 km/h or 90 km/h on a wet road surface having a water depth of 5 mm, and the resistance to hydroplaning during cornering is evaluated by measuring a limit lateral G when the tire is cornered on a wet road surface having a water depth of 5 mm, and the pattern noise is evaluated by a test driver in respect of internal sound when the tire is inertially run on a straight smooth course after stopping the engine at a speed of 100 km/h, and the steering stability on dry road surface is evaluated by a test driver when the tire is run on a circuit course at a dry state under various sport running modes. The evaluated results are shown in Table 1, in which each result is represented by an index value on the basis that the conventional tire is 100. The larger the index value, the better the result. Conventional Example Example 1 Example 2 Resistance to hydroplaning: straight running 100 120 115 Resistance to hydroplaning: cornering 100 115 110 Pattern noise 100 105 110 Steering stability on dry road surface 100 105 105 As seen from Table 1, the pneumatic tires of Examples 1 and 2 according to the invention have improved resistance to hydroplaning, pattern noise and steering stability on dry road surface as compared with the conventional pneumatic tire. As mentioned above, the invention can provide pneumatic tires for passenger cars having excellent drainage property and steering stability and low pattern noise because the directional low-slant grooves and the circumferential rib are suitably arranged in the tread and blocks are formed between the adjacent directional low-slant grooves at intervals in the circumferential direction and the block adjacent to the circumferential rib among these blocks has a relation such that the height of the preceding ground contact side is lower than the height of the later ground contact side in the forward direction of the tire.

Claims: A pneumatic tire for passenger cars having a directional tread pattern defined by at least one circumferential rib (1) and a plurality of directional slant grooves (2,3) arranged on both sides of the circumferential rib at given intervals in the circumferential direction of the tire, the directional slant grooves comprising directional low-slant grooves (2) each having a small inclination angle with respect to the circumferential direction at a side near to an equatorial plane of the tire and extending from each side of the circumferential rib toward a side end of a tread (TE), the directional slant grooves further comprising directional high-slant grooves (3) each having a large inclination angle with respect to the circumferential direction in addition to the directional low-slant grooves (2), and wherein plural blocks (41,42,43) are formed between adjacent directional low-slant grooves (2) at given intervals in the circumferential direction, and a block (41) among said plural blocks is joined to the rib (1) and has a surface such that a height of a preceding ground contact side of the block (41) is lower than a height of a subsequent ground contact side of the block during the forward rotation of the tire, a trailing edge of the block (41) is defined by a high-slant groove (3), the height of a portion of the block (41) at said trailing edge of the block is substantially the same as the height of the at least one rib, and the surface of the block (41) and a sidewall of the rib (1) intersect so as to define a line which is inclined with respect to the radial direction of the tire. A pneumatic tire as claimed in claim 1, characterized in that the height of the preceding ground contact side of the block (41) is the same level as the bottom of the directional low-slant groove (2) adjacent to the block. A pneumatic tire as claimed in claim 1 or 2, characterized in that the directional low-slant groove (2) and/or the directional high-slant groove (3) are open at a side end of the tread (TE). A pneumatic tire as claimed in any of claims 1 to 3, characterized in that the inclination angle of each directional high-slant groove (3) with respect to the circumferential direction is not less than 45° and the inclination angle of each directional low-slant groove (2) is not more than 30°. A pneumatic tire as claimed in any of claims 1 to 4, characterized in that the inclination angle of each directional high-slant groove (3) is in the range of 60-80°. A pneumatic tire as claimed in any of claims 1 to 5, characterized in that the inclination angle of each directional low-slant groove (2) is in the range of 10-30°. A pneumatic tire as claimed in any of claims 1 to 6, characterized in that the directional high-slant grooves (3) have a narrowed portion. A pneumatic tire as claimed in claim 7, characterized in that the high-slant grooves (3) intersect with the low-slant grooves (2) and the narrowed portion is adjacent to the intersection of the low-slant and high-slant grooves.

Inventors: HIMURO YASUO, HIMURO, YASUO
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1995
Main classification: B60C11
IPC codes: B60C11