Ice/snow road-running pneumatic tires

Abstract: A pneumatic tire is provided with a tread pattern including a wider circumferential straight groove (1) dividing the tread into right and left sides and having a groove width which is 7-25% of the width of the tread, a pair of circumferential ribs (2) provided adjacent to right and left sides of the wider circumferential straight groove (1), respectively and each having a groove width which is 2.5-15% of the width of the tread, and a pair of narrower circumferential straight grooves (3) provided adjacent to and axially outside the circumferential ribs (2), respectively, and each having a groove width being 0.5 to 5% of that of the tread and a groove depth being 50 to 100% of that of the wider circumferential straight groove (1), and wherein each of the circumferential ribs (2) is provided with a plurality of sipes (4) extending obliquely relative to the circumferential direction while being spaced in the circumferential direction, the depth of each of the sipes (4) being 25 to 100% of the groove depth of the wider circumferential straight groove (1).

Description: The present invention relates to ice/snow road-running pneumatic tires. Particularly, the invention relates to pneumatic tires which exhibit improved ice road-running performance and snow road-running performance. In Fig. 5 of the accompanying drawings is shown a conventionally typical tread pattern to be applied to ice/snow road-running pneumatic tires. As shown, this typical ice/snow road-running pneumatic tire has a tread pattern in which circumferentially zigzag grooves and circumferential straight grooves are alternatively arranged in the width direction of the tire; these circumferentially and continuously zigzag and straight grooves are laterally connected via a number of lateral grooves which are spaced in the circumferential direction; plural rows of blocks are defined by the circumferential grooves and the lateral grooves; and a plurality of laterally straight sipes are formed in each of the blocks. This tire exhibits both snow and ice running performance. In the present specification, (1) "circumferential straight groove" means a straight groove or a substantially straight groove which continuously extends in the circumferential direction; (2) "circumferential rib" means a straight rib or a substantially straight rib which continuously extends in the circumferential direction. (3) "circumferentially zigzag groove" means a groove which continuously extends zigzag in the circumferential direction. (4) "directionally inclined groove" means a groove which extends obliquely relative to the circumferential direction and which constitutes a so-called directional tread pattern. A directional tread pattern means that the turning direction is specified when the tire is fitted to a vehicle such that an inclined portion of the above groove near the tire equatorial plane first contacts the ground and then an inclined portion thereof remote from the tire equatorial plane contacts the ground later when the tire rotates in the forward direction. According to the conventionally typical ice/snow running pneumatic tire mentioned above, there is a tendency that, if the tire becomes flattened and widened, the ground contact length of the tread becomes shorter although its ground contact width becomes greater so that the traction performance on ice and snow is deteriorated. In view of this, if the ground contact length is increased to enhance the traction performance, there is a tendency that the ground contact width becomes smaller so that the cornering performance is deteriorated. Besides the conventional typical ice/snow road-running pneumatic tire mentioned above, a flat and wide tire is known in which drainage performance is intended to be improved by providing a wide circumferential straight groove in a central portion of a tread. However, rigidity of blocks adjacent to the wide circumferential straight groove is so insufficient that local wear may tend to occur. It is an object of the present invention to provide a pneumatic tire with an ice/snow road-running tread pattern which effectively improves both the traction performance and the cornering characteristic on ice and snow roads and is unlikely to cause local wear. The present invention provides a pneumatic tire comprising a tread pattern which includes a circumferential straight groove dividing the tread into right and left sides, a pair of circumferential ribs provided adjacent to right and left sides of the circumferential straight groove, respectively, and a pair of narrower circumferential straight grooves provided adjacent to and axially outside the circumferential ribs, respectively, wherein: (1) the groove width of the circumferential straight groove is 7 to 25% of the width of the tread; (2) the width of each of the circumferential ribs is 2.5 to 15% of the width of the tread; (3) each of said narrower circumferential straight grooves has a groove width being 0.5 to 5% of that of the tread and a groove depth being 50 to 100% of that of the circumferential straight groove; and (4) each of the circumferential ribs is provided with a plurality of sipes extending obliquely relative to the circumferential direction while being spaced in the circumferential direction, and the depth of each sipe is 25 to 100% of the groove depth of the circumferential straight groove. It is preferable that a number of directionally inclined grooves and a number of blocks are arranged between said narrower circumferential straight grooves and each of end portions of the tread while being spaced from one another in the circumferential direction, and each of the blocks is provided with a number of sipes spaced in the circumferential direction. In order to realize the braking traction performance on ice roads, it is preferable that the depth of each of the sipes provided in the circumferential ribs is less than the groove depth of the narrower circumferential straight grooves. In order to realize the braking traction performance on snow roads, it is preferable that the depth of each of the sipes provided in the circumferential ribs is greater than the groove depth of each of the narrower circumferential straight grooves. Since the pneumatic tire according to the present invention has the above-mentioned tread pattern, the groove width of the circumferential straight groove is as wide as 7 to 25% of that of the tread and, as shown in Fig. 4 of the accompanying drawings, the ground-contacting profile of the tread is divided into right and left portions being each an elliptical shape elongated in the circumferential direction. Thereby, a tire in which the ground contacting width of the tread is large and its ground contact length is large can be obtained so that the traction performance and the cornering characteristic on ice and snow roads can be both effectively improved. Since the conventional typical ice/snow road-running pneumatic tire mentioned above has an integrated ground contacting shape without being divided into right and left portions, the ground-contacting pressure near the central portion of the tread is maximum. On the other hand, the maximum ground-contacting pressure area of the pneumatic tire according to the present invention is divided into the two right and left portions so that the ice braking performance may be enhanced. In order that the ground-contacting shape of the tread may be divided into the right and left portions each having the elliptical shape elongated in the circumferential direction, it is necessary that the groove width of the circumferentially straight groove is not less than 7% of the width of the tread. If this groove width is more than 25 % of the width of the tread, the ground contact width of each of the right and left elliptical shapes elongated in the circumferential direction becomes narrower, so that the cornering characteristic is deteriorated. According to the pneumatic tire of the invention, the width of each of the circumferential ribs is 2.5 to 15% of the width of the tread; each of said narrower circumferential straight grooves has a groove width being 0.5 to 5% of that of the tread and a groove depth being 50 to 100% of that of the wide circumferential straight groove; and each of the circumferential ribs is provided with a plurality of sipes extending obliquely relative to the circumferential direction while being spaced in the circumferential direction; and the depth of each sipe is 25 to 100% of the groove depth of the wide circumferential straight groove. According to the pneumatic tire of the present invention, a pair of circumferential ribs are continuously and circumferentially provided adjacent to right and left sides of the wide circumferential straight groove, respectively, and the width of each of the circumferential ribs is 2.5 to 15% of the width of the tread. Therefore, the tread has sufficient rigidity so that occurrence of local wear near the wide circumferential straight groove can be prevented. Since each of the circumferential ribs is provided with a number of sipes extending obliquely relative to the circumferential direction while being spaced in the circumferential direction, the width of the circumferential rib needs to be not less than 2.5% of that of the tread so that the tread may have sufficient rigidity. If the width of the circumferential rib is more than 15% of the tread width, the negative ratio of the central portion of the tread decreases to deteriorate the traction performance on snow roads. Further, since each of the circumferential ribs is provided with a number of sipes extending obliquely relative to the circumferential direction while being spaced in the circumferential direction, the braking performance and the traction performance on ice roads can be improved by edge components of the sipes near the central portion of the tread. A plurality of sipes provided in each circumferential rib, extending obliquely relative to the circumferential direction while being spaced in the circumferential direction, have a depth of not less than 25% of the groove depth of the wide circumferential straight groove so as to obtain effective ice/snow performance, and a depth of not more than 100% of the groove depth of the wide circumferential straight groove so as to ensure the durability. When the depth of the sipes provided in the circumferential ribs is less than the groove depth of the narrow circumferential straight grooves, local wear resistance and braking traction performance are enhanced. On the other hand, when the depth of the sipes provided in the circumferential ribs is greater than the groove depth of the narrow circumferential straight grooves, the braking traction performance on ice roads is improved. According to the pneumatic tire of the present invention, each of said narrower circumferential straight grooves has a groove depth of 50 to 100% of that of the wide circumferential straight groove. As the groove depth of the narrower circumferential straight grooves decreases, the rigidity of the tread near the central portion becomes higher so that local wear resistance may be better. In order to ensure the running performance on ice/snow roads until at least 50% tread rubber is worn, the groove depth of the narrower circumferential straight grooves needs to be at least 50% of that of the wide circumferential straight groove. On the other hand, as the groove depth of the narrow circumferential straight grooves increases, the volume of the groove increases so that performance on ice and hydroplaning characteristic on wet roads may be better. However, if the groove depth of the narrow circumferential straight grooves is greater than 100 % of that of the wide circumferential straight groove, local wear resistance is extremely deteriorated. According to the pneumatic tire of the present invention, it is preferable that a number of directionally inclined grooves and a number of blocks are arranged between said narrower circumferential straight grooves and each of end portions of the tread, while being spaced from one another in the circumferential direction, and each of the blocks is provided with a number of sipes spaced in the circumferential direction. The invention will be further described with reference to the accompanying drawings, wherein: Fig. 1 is a plan view of a tread pattern of a pneumatic tire according to the present invention; Fig. 2 is a plan view of another tread pattern of a pneumatic tire according to the present invention; Fig. 3 is a plan view of a further tread pattern of a pneumatic tire according to the present invention; Fig. 4 is a view illustrating a ground contacting profile of the pneumatic tire according to the present invention; and Fig. 5 is a plan view of the tread pattern of a conventionally typical pneumatic tire. Figs. 1 through 3 are tread patterns of passenger pneumatic tires according to the present invention as Examples 1 to 3, and their tire size is 225/50R16 in each case. The tread pattern of the pneumatic tire according to the present invention as Example 1 shown in Fig. 1 includes a wide circumferential straight groove 1 dividing the tread into right and left sides, a pair of circumferential ribs 2 provided adjacent to right and left sides of the circumferential straight groove 1, respectively, and a pair of narrower circumferential straight grooves 3 provided adjacent to and axially outside the circumferential ribs 2, respectively, wherein the width of the tread is 214 mm and the negative percentage is 40%. The groove width of the circumferential straight groove 1 is 20 mm (9.3% of the width of the tread). The width of each of the circumferential ribs 2 is 12 mm (5.6% of the width of the tread). Each of said narrower circumferential straight grooves 3 has a groove width of 3.3 mm (1.5% of the width of the tread) and a groove depth of 7 mm (70% of the groove depth (10 mm) of the circumferential straight groove 1). Each of the circumferential ribs 2 is provided with a number of 0.3 mm wide sipes 4 extending obliquely relative to the circumferential direction, and the depth of each sipe is 6 mm (60% of the groove depth (10 mm) of the wide circumferential straight groove). A number of directionally inclined grooves 5 and a number of blocks 6 are arranged between said narrower circumferential straight grooves 3 and each of end portions TE of the tread while being spaced from one another in the circumferential direction, and each of the blocks 6 is provided with a number of sipes 7 spaced in the circumferential direction. A so-called directional pattern is formed in which the rotating direction of the tire is specified when the tire is fitted to a vehicle such that an inclined portion of each of the directionally inclined grooves 5 near the tire equatorial plane first contacts the ground and then an inclined portion thereof remote from the tire equatorial plane contacts the ground later when the tire rotates in the forward direction. In Fig. 2 is shown a tread pattern of a pneumatic tire according to the present invention as Example 2. This tire is provided with a wide circumferential straight groove 1 dividing the tread into right and left sides, a pair of circumferential ribs 2 provided adjacent to right and left sides of the circumferential straight groove 1, respectively, and a pair of narrower circumferential straight grooves 3 provided adjacent to and axially outside the circumferential ribs, respectively. The width of the tread is 210 mm, and the negative ratio is 43.5%. The groove width of the circumferential straight groove 1 is 20 mm(9.5% of the width of the tread). The width of each of the circumferential ribs 2 is 10 mm (4.8% of the width of the tread), and each of said narrower circumferential straight grooves 3 has a groove width of 2 mm (1.0% of that of the tread) and a groove depth of 7 mm (70% of that of the wide circumferential straight groove). Each of the circumferential ribs is provided with a plurality of sipes 4 extending obliquely relative to the circumferential direction while being spaced in the circumferential direction, the depth of each sipe being 7 mm which is 70% of the groove depth (10 mm) of the wide circumferential straight groove. Most of the sipes 4 have a width of 0.3 mm, and other sipes 4 each having a width of 1 mm are arranged at the same pitch as that of the right and left inclined grooves. The remaining structure of the tire of Example 2 is fundamentally the same as that of the pneumatic tire of Example 1. In Fig. 3 is shown a tread pattern of a pneumatic tire according to the present invention as Example 3. This tire is provided with a wide circumferential straight groove 1 dividing the tread into right and left sides, a pair of circumferential ribs 2 provided adjacent to right and left sides of the circumferential straight groove 1, respectively, and a pair of narrower circumferential straight grooves 3 provided adjacent to and axially outside the circumferential ribs, respectively. The width of the tread is 210 mm, and the negative ratio is 42.8%. The groove width of the circumferential straight groove 1 is 20 mm(9.5% of the width of the tread). The width of each of the circumferential ribs 2 is 10 mm(4.8% of the width of the tread), and each of said narrower circumferential straight grooves 3 has a groove width of 3 mm (1.4% of that of the tread) and a groove depth of 7 mm (70% of that (10 mm) of the circumferential straight groove 1). Each of the circumferential ribs 2 is provided with a number of 0.3 mm wide sipes 4 extending obliquely relative to the circumferential direction while being spaced in the circumferential direction, the depth of each sipe being 9 mm which is 90% of the groove depth (10 mm) of the wide circumferential straight groove. The remaining structure of the tire of Example 3 is fundamentally the same as that of the pneumatic tire of Example 1. As shown in Fig. 5 by way of example, a conventional pneumatic tire is provided with a tread pattern in which zigzag grooves and straight grooves continuously extend in the circumferential direction of the tire and are alternatively arranged in the tire width direction; plural rows of blocks are formed by these circumferentially extending grooves and a number of lateral grooves spaced in the circumferential direction and connecting these circumferential grooves; and a plurality of lateral straight sipes are formed in each block. The tire size is 225/50R16, and the width TW of the tread is about 170 mm. The pneumatic tires according to the present invention shown in Figs. 1 through 3 as Examples 1 to 3 as well as the above conventional pneumatic tire shown in Fig. 5 were subjected to evaluation tests for feeling characteristic on snow, braking performance on snow, feeling characteristic on ice, and braking performance on ice. The test conditions were that the internal pressure of front wheel tires was 2.3 Kg/cm 2 , and that of rear wheel tires was 2.5 Kg/cm 2 . The feeling characteristic on snow was totally evaluated based on brakability, starting characteristic, straight running performance, and cornering performance on a test course with a compacted snow road. The braking performance on snow was evaluated based on a braked distance when a vehicle was fully braked from a running state at 40 Km/h on compacted snow. The feeling characteristic on ice was totally evaluated based on the brakability, starting characteristic, straight running performance and cornering performance on ice. The braking performance on ice was an evaluation result when the vehicle was fully braked from a running state at 20 Km/h on ice. Evaluation results are shown by index, while those of the conventional pneumatic tire were taken as 100. The higher the figure, the better the tire performance. The evaluation results are summarized in Table 1. Conventional Example Example 1 Example 2 Example 3 Feeling characteristic on snow 100 115 115 120 Braking performance on snow 100 110 110 115 Feeling characteristic on ice 100 120 120 120 Braking performance on ice 100 125 120 115 From the results shown in Table 1, the pneumatic tires according to the present invention are better than the conventional one with respect to the snow performance and ice performance. According to the tread pattern of the pneumatic tire of the present invention, since the wide circumferential straight groove dividing the tread into right and left sides and having a width of 7-25% of the width of the tread, a pair of circumferential ribs provided adjacent to right and left sides of the wide circumferential straight groove, respectively, and each having a width of 2.5-15% of the width of the tread, and a pair of narrower circumferential straight grooves provided adjacent to and axially outside the circumferential ribs, respectively, and having a width of 0.5 to 5% of the width of the tread are appropriately combined, an ice/snow road-running pattern which enhances both the traction performance and the cornering characteristic on ice and snow can be provided.

Claims: A pneumatic tire provided with a tread pattern, said tread pattern including a circumferential straight groove (1) dividing the tread into right and left sides, a pair of circumferential ribs (2) provided adjacent to right and left sides of the circumferential straight groove (1), respectively, and a pair of narrower circumferential straight grooves (3) provided adjacent to and axially outside the circumferential ribs (2), respectively, wherein: (1) the groove width of the circumferential straight groove (1) is 7 to 25% of the width of the tread; (2) the width of each of the circumferential ribs (2) is 2.5 to 15% the width of the tread; (3) each of said narrower circumferential straight grooves (3) has a groove width being 0.5 to 5% of that of the tread and a groove depth being 50 to 100% of that of the circumferential straight groove (1); and (4) each of the circumferential ribs (2) is provided with a plurality of sipes (4) extending obliquely relative to the circumferential direction while being spaced in the circumferential direction, and the depth of each sipe (4) is 25 to 100% of the groove depth of the circumferential straight groove (1). A pneumatic tire as claimed in claim 1, characterized in that a number of directionally inclined grooves (5) and a number of blocks (6) are arranged between said narrower circumferential straight grooves (3) and each of end portions (TE) of the tread while being spaced from one another in the circumferential direction, and each of the blocks (6) is provided with a number of sipes (7) spaced in the circumferential direction. A pneumatic tire as claimed in claim 1 or 2, characterized in that the depth of each of the sipes (4) provided in the circumferential ribs (2) is less than the groove depth of the narrower circumferential straight grooves (3). A pneumatic tire as claimed in claim 1 or 2, characterized in that the depth of each of the sipes (4) provided in the circumferential ribs (2) is greater than the groove depth of each of the narrower circumferential straight grooves (3).

Inventors: OCHI NAOYA, OCHI, NAOYA
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1995
Main classification: B60C11
IPC codes: B60C11