Pneumatic tires

Abstract: A pneumatic tire comprises a tread provided with a plurality of circumferential grooves (1), a plurality of lateral grooves (2) and blocks (3) defined by these circumferential grooves and lateral grooves, in which (i) a plurality of wave-shaped sipes (4) are formed in each of the blocks (3) so as to have relatively high density at the central region of the block in the widthwise direction of the tire and to have relatively low density at both side regions thereof, and (ii) the amplitude of the wave-shaped sipes (4) in each side region of the block (3) is less than that in the central region of the block.

Description: This invention relates to pneumatic tires, and more particularly to a pneumatic studless tire having a tread provided with a plurality of circumferential grooves arranged at given intervals in the widthwise direction of the tire and extending in the circumferential direction of the tire or substantially in the circumferential direction, a plurality of lateral grooves arranged at given intervals in the circumferential direction and extending in the widthwise direction or substantially in the widthwise direction, and blocks defined by these circumferential grooves and lateral grooves and a plurality of wave-shaped sipes formed in each block and extending substantially in the same direction as the lateral grooves, for use on snow and ice road surfaces. In a studless tire used on snow and ice road surfaces, when a plurality of lateral sipes are formed in each of the blocks defined by plural circumferential grooves extending in the circumferential direction and plural lateral grooves crossing these circumferential grooves, water film is cut by these lateral sipes to improve the running performance on ice. However, if it is intended to further improve the running performance on ice by increasing the number of lateral sipes, the rigidity of the block itself considerably lowers and hence the running performance on ice inversely lowers. For this end, it is difficult to improve the running performance on ice of the studless tire above a certain level by the conventional technique. Attention is also drawn to the disclosures of US-A-4 598 747, which shows a tire with the features of the preamble of claim 1, and EP-A-0 654 366. It is, therefore, an object of the present invention to overcome the aforementioned problem of the conventional technique and to provide pneumatic studless tires having improved running performance on ice. According to the invention, there is provided a pneumatic tire comprising a tread provided with a plurality of circumferential grooves arranged at given intervals in the widthwise direction of the tire and extending in the circumferential direction of the tire or substantially in the circumferential direction, a plurality of lateral grooves arranged at given intervals in the circumferential direction and extending in the widthwise direction or substantially in the width-wise direction, and blocks defined by said circumferential grooves and lateral grooves, wherein a plurality of wave-shaped sipes arranged side by side and extending substantially in the same direction as the lateral grooves are formed in each of the blocks and are comprised by one-end opening sipes overlapping each other in the central region of the block with one end disposed at the right or left border side of the side region of the block in the widthwise direction of the tire, characterized in that the amplitude of the wave-shaped sipes in each side region of the block in the widthwise direction of the tire is less than that in the central region of the block in the widthwise direction of the tire, and in that the pitch of the wave-shaped sipes is relatively large at the central region of the block in the widthwise direction of the tire and relatively small at both side regions of the block. In preferable embodiments of the invention, the distance between the wave-shaped sipes at the central region of the block is 1.5-4.5 mm, more preferably 1.7-4.3 mm; and the amplitude of the wave-shaped sipes at the central region of the block is 1.25-10 times, more preferably 1.3-8 times of that at both side regions of the block; and the width of the central region of the block in the widthwise direction of the tire is 10-70%, more preferably 20-65% of the width of the block in the widthwise direction of the tire; and at least a part of a border line between the block and the lateral groove extends wavingly; and the tire is provided on a widthwise central region of the tread with at least two circumferential ribs each having a plurality of wave-shaped sipes extending in the widthwise direction of the tire or substantially in the widthwise direction, and the amplitude of said sipes is less than the amplitude of the wave-shaped sipe at both side regions of the block in the widthwise direction of the tire. The invention will be further described with reference to the accompanying drawings, wherein: Fig. 1 is a developed view of an embodiment of the tread pattern in a tire according to the invention; and Fig. 2 is a developed view of a tread pattern in a comparative example. In the tire according to the invention, wave-shaped sipes having a large amplitude are arranged in the central region of each block in the widthwise direction of the tire with a high density, whereby the edge component of the sipe is increased while lowering the block rigidity in the central region, and hence the effect of absorbing water in the central region of the block and the scraping effect through the edge of the sipe are enhanced to improve the running performance on ice of the tire. If the sipe arrangement in the central region of the block is applied to both side regions of the block, the block rigidity in both side regions is lower than that in the central region thereof. In the tire according to the invention, therefore, wave-shaped sipes having a large amplitude are arranged in the central region of each block in the widthwise direction of the tire with a high density as mentioned above, while wave-shaped sipes having a small amplitude are arranged in both side regions of the block in the widthwise direction of the tire with a low density, whereby the lowering of the block rigidity in both side regions is prevented to improve the running performance on ice of the tire. In the tire according to the invention, it is preferable that the amplitude of the wave-shaped sipes located at the central region of the block in the widthwise direction of the tire is 1.25-10 times, more preferably 1.3-8 times, of the amplitude of the wave-shaped sipes located at both side regions of the block as mentioned above. When the amplitude of the wave-shaped sipes in the central region of the block is less than the lower limit, the effect of preventing the lowering of the block rigidity in both side regions of the block might not be obtained, while when it exceeds the upper limit, the block rigidity in the central region of the block is lowered and there is apt to occur a phenomenon of collapsing of the block and hence the running performance on ice of the tire is degraded. In the tire according to the invention, it is preferable that the distance between the wave-shaped sipes in the central region of the block in the circumferential direction of the tire is 1.5-4.5 mm, more preferably 1.7-4.3 mm, as mentioned above. When the distance between the wave-shaped sipes is less than the lower limit, collapsing of the block is apt to be caused upon contacting with the ground and it is difficult to manufacture the tire, while when it exceeds the upper limit, the running performance on ice of the tire is degraded. In the tire according to the invention, it is preferable that the width of the central region of the block in the widthwise direction of the tire is 10-70%, more preferably 20-65%, of the width of the block in the widthwise direction of the tire. When the width of the central region of the block is less than the lower limit, the edge effect and the water absorbing effect are considerably degraded, while when it exceeds the upper limit, the block rigidity in both side regions in the widthwise direction of the tire is lowered. A tire of an example according to the invention and a tire of a comparative example will be described in detail with reference to the drawings. These tires have a tire size of 185/70R14. In Fig. 1 is developmentally shown a tread pattern in the tire according to the invention. As shown in Fig. 1, the tire of the example according to the invention comprises a tread provided with four circumferential grooves 1 arranged at given intervals in the widthwise direction of the tire and extending in the circumferential direction of the tire or substantially in the circumferential direction, a plurality of lateral grooves 2 arranged at given intervals in the circumferential direction and extending substantially in the widthwise direction, and blocks 3 defined by these circumferential grooves 1 and lateral grooves 2. A greater part of a border line between the block 3 and the lateral groove 2 extends wavingly. Seven wave-shaped sipes 4 extending substantially in the same direction as the lateral grooves 2 are arranged in each of these blocks 3, in which the number of sipes is 7 at the central region of the block in the widthwise direction of the tire and 3 or 4 at both side regions of the block in the widthwise direction of the tire so as to make the sipe density relatively high in the central region and relatively low in both side regions. Furthermore, the amplitude of the wave-shaped sipes 4 in both side regions of the block 3 is 1 mm, while the amplitude of the wave-shaped sipes 4 in the central region of the block 3 is 2 mm. Therefore, the amplitude of the sipes in the central region of the block is 2 times the amplitude of the sipes in both side regions of the block. Also, the pitch of the wave-shaped sipes 4 is 4mm at the central region of the block in the widthwise direction of the tire and 2 mm at both side regions thereof. The distance between the wave-shaped sipes 4 is 2.5 mm at the central region of the block in the circumferential direction of the tire. Moreover, the width of the central region of the block 3 in the widthwise direction of the tire is about 50% of the width of the block in the widthwise direction. In the tire of the example, two circumferential ribs 5 are provided on a central portion of the tread as shown in Fig. 1, in which a plurality of wave-shaped sipes 6 extending substantially in the widthwise direction of the tire are arranged in the circumferential ribs 5 and the amplitude of the wave-shaped sipes 6 in the circumferential ribs 5 is less than the amplitude of the wave-shaped sipes 4 at the central region of the block in the widthwise direction of the tire. As shown in Fig. 2, the tire of the comparative example has substantially the same tread pattern as in the tire of the example except that the amplitude and pitch of the wave-shaped sipes 4 at both side regions of the block 3 in the widthwise direction of the tire are the same as those of the wave-shaped sipes 4 at the central region of the block 3 in the widthwise direction of the tire. A test for the evaluation of the running performance on ice is made with respect to the tires of the example and the comparative example. Each of these tires is mounted on front and rear wheels of a vehicle and inflated under an internal pressure of 1.8 kgf/cm 2 and then run on an ice road surface of a test course, during which a feeling performance on ice is evaluated by total feeling with respect to braking, traction, straight running and cornering,and a braking performance on ice is evaluated by measuring a braking distance when the tire is fully braked from a state of running at a speed of 20 km/h on the ice road surface. When the evaluation results of these performances are represented by an index value on the basis that the tire of the comparative example is 100, the index values of the feeling performance on ice and the braking performance on ice of the tire of the example are 103 and 105, respectively. Moreover, the larger the index value, the better the performance. As seen from the above results, the tire of the example according to the invention has improved running performance on ice as compared with the tire of the comparative example.

Claims: A pneumatic tire comprising a tread provided with a plurality of circumferential grooves (1) arranged at given intervals in the widthwise direction of the tire and extending in the circumferential direction of the tire or substantially in the circumferential direction, a plurality of lateral grooves (2) arranged at given intervals in the circumferential direction and extending in the widthwise direction or substantially in the width-wise direction, and blocks (3) defined by said circumferential grooves (1) and lateral grooves (2), wherein a plurality of wave-shaped sipes (4) arranged side by side and extending substantially in the same direction as the lateral grooves (2) are formed in each of the blocks (3) and are comprised by one-end opening sipes overlapping each other in the central region of the block with one end disposed at the right or left border side of the side region of the block in the widthwise direction of the tire, characterized in that the amplitude of the wave-shaped sipes (4) in each side region of the block (3) in the widthwise direction of the tire is less than that in the central region of the block in the widthwise direction of the tire, and in that the pitch of the wave-shaped sipes (4) is relatively large at the central region of the block (3) in the widthwise direction of the tire and relatively small at both side regions of the block in the widthwise direction of the tire. A pneumatic tire as claimed in claim 1, characterized in that the distance between the wave-shaped sipes (4) at the central region of the block (3) in the circumferential direction of the tire is 1.5-4.5 mm. A pneumatic tire as claimed in claim 2, characterized in that said distance between the wave-shaped sipes (4) is 1.7-4.3 mm. A pneumatic tire as claimed in any of claims 1 to 3, characterized in that the amplitude of the wave-shaped sipes (4) at the central region of the block (3) is 1.25-10 times that at both side regions of the block. A pneumatic tire as claimed in claim 4, characterized in that the amplitude of the wave-shaped sipes (4) at the central region of the block (3) is 1.3-8 times that at both side regions of the block. A pneumatic tire as claimed in any of claims 1 to 5, characterized in that the width of the central region of the block (3) in the widthwise direction of the tire is 10-70% of the width of the block in the widthwise direction of the tire. A pneumatic tire as claimed in claim 6, characterized in that said width of the central region of the block (3) is 20-65% of the width of the block. A pneumatic tire as claimed in any of claims 1 to 7, characterized in that at least a part of a border line between the block (3) and the lateral groove (2) extends wavingly. A pneumatic tire as claimed in any of claims 1 to 8, characterized in that the tire is provided on a widthwise central region of the tread with at least two circumferential ribs (5) each having a plurality of wave-shaped sipes (6) extending in the widthwise direction of the tire or substantially in the widthwise direction. A pneumatic tire as claimed in claim 9, characterized in that the amplitude of the wave-shaped sipes (6) formed in the circumferential rib (5) is less than the amplitude of the wave-shaped sipes (4) at the central region of the block (3) in the widthwise direction of the tire.

Inventors: OCHI NAOYA, SHINOHARA KAZUNORI, OCHI, NAOYA, SHINOHARA, KAZUNORI
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1996
Main classification: B60C11
IPC codes: B60C11