Pneumatic tire having excellent steering stability

Abstract: A pneumatic tire has a block pattern of a tread defined by a plurality of circumferential grooves (1,2) extending in a circumferential direction of the tread and a plurality of lateral grooves (3) each extending across the circumferential grooves to tread ends (E), wherein specified chamfered face (8,11), sipe (12) or fine groove (13) is formed in each block of all block rows to improve self-aligning torque in the high-speed running of a vehicle. For example, each block (7) in an outermost block row (5) in a widthwise direction is provided on its leading edge portion (7a) with a chamfered face (8) gradually decreasing the height of the block toward the leading edge, and each block (9,10) in the other block rows (4,6) is provided on its trailing edge portion (9a) with a chamfered face (11) gradually decreasing the height of the block toward the trailing edge, and a length (1) of a chamfered face forming region is not more than 1/3 of the circumferential length (L) of the block.

Description: This invention relates to a pneumatic tire having particularly an improved steering stability in the high-speed running of a vehicle by increasing a self-aligning torque of the tire. In the running of the vehicle, the steering stability is largely affected by disturbances such as road surface state, wind and the like, which becomes serious as the vehicle speed is increased. In the conventional pneumatic tire, however, examinations on the shape of the block, formation of sipe and the like are not made for sufficiently satisfying the steering stability in the high-speed running of the vehicle. Conventional techniques dealing with the problem of the residual self-aligning torque by varying the orientation of the groove inclination between adjacent block rows are known from e.g. EP-A-0 498 287 and EP-A-0 635 382, which correspond with the preamble of claim 1. It is, therefore, an object of the invention to provide pneumatic tires advantageously improving the steering stability in the high-speed running by directly increasing self-aligning torque of the tire as a result of various studies with respect to the formation of sipes therein. According to the invention, there is provided a pneumatic tire having a block pattern of a tread defined by a plurality of circumferential grooves extending in a circumferential direction of the tread and a plurality of lateral grooves each extending across the circumferential grooves to tread ends, wherein a plurality of sipes each extending in a widthwise direction of the tread and being symmetrical with respect to a line segment passing through a center of the block in the circumferential direction of the tread are formed in each block of the block rows so as to gradually change lengths of these sipes from an end of the block in the circumferential direction toward the other end thereof, and the sipes in each block of an outermost block row are longer at a leading side of the block and shorter at a trailing side thereof, and the sipes in each block of the other block rows are longer at a trailing side of the block and shorter at a leading side thereof. In this tire, each sipe formed in each block and extending in the widthwise direction of the tread can naturally develop an excellent water film cutting performance under its edge action but also particularly decreases a block rigidity at the leading edge portion in each block of the outermost block row. As a result, when a slip angle is applied to the tire, a force in a forward direction produced in the leading of the block is decreased to relatively increase a force in a backward direction, while a block rigidity at a trailing edge portion is decreased in each block of the other block rows to relatively increase a force in a forward direction. Even in this case, the self-aligning torque can effectively be increased based on force couple in the ground contact region of the tread, whereby excellent steering stability can be realized during the high-speed running of the vehicle. In a preferable embodiment of the invention, both ends of each sipe in each block are terminated in the block, whereby excessive lowering of the block rigidity is prevented to control the occurrence of uneven wear. The invention will be further described with reference to the accompanying drawing, wherein: Fig. 1 is a schematic partial developed view of an embodiment of the tread pattern of a tire according to the invention. In Fig. 1 is shown an embodiment of the tread pattern of a tire according to the invention. Moreover, an inner structure of the tire is the same as in the generally used radial tire. Such an inner structure itself is not directly related to the invention, so that the illustration of the inner structure is omitted. In the illustrated embodiment of the tread pattern according to the invention, a centerblock row 4 located on an equator of the tire, shoulder block rows 5 located at both outermost sides of a tread portion and middle block rows 6 located between the center block row and the shoulder block row are defined on a surface of a tread by four circumferential grooves 1, 2 extending straightforward in a circumferential direction of the tread and symmetrically located with respect to the equator of the tire, and a plurality of lateral main grooves 3 extend across these circumferential grooves 1, 2 between side tread ends E and substantially in a direction perpendicular thereto. A plurality of sipes 12 each extending in a widthwise direction of the tread and being symmetrical with respect to a line segment passing through a center of the block in the circumferential direction of the tread are formed in each of the blocks 7, 9,10 so as to gradually change lengths of these sipes from an end of the block in the circumferential direction toward the other end thereof. The sipes 12 in the shoulder block 7 are longer at a leading side of the block 7 and shorter at a trailing side thereof, and the sipes 12 in each of the center blocks 9 and middle blocks 10 are longer at a trailing side of the block than at a leading side thereof. In the illustrated embodiment, both ends of all sipes 12 in each block are terminated in the block. Moreover, the term "sipe" used herein means a narrow groove having generally an opening width of about 0.2-0.4 mm, wherein opposed groove walls of the sipe contact with each other in a ground contact region during the running of the tire under loading. When a certain slip angle is applied to this tire, force directing backward in a running direction of the tire is created in the shoulder block 7 subjected to a large load based on the difference of the extending lengths of the sipes 12 between mutual blocks to cause couple force in the ground contact region of the tread, whereby the increase of the self-aligning torque in the tire can be realized. The following examples are given in illustration of the invention and are not intended as limitations thereof. Example 1 There is provided a pneumatic tire having a tire size of 195/60R15 88H, wherein a belt is comprised of two cross belt layers, cords of which layers are crossed with each other at a cord angle of 20° with respect to an equator of the tire and has a width of about 150 mm, and each block has a height of 5 mm and a length in circumferential direction L of 18.9 mm. This tire is mounted onto a rim of 6J and inflated under an air pressure of 220 kPa and rotated on a drum testing machine at a speed of 60 km/h under a load of 4067 N while applying a slip angle of 3° to measure a self-aligning torque. The tire has a tread pattern shown in Fig. 1, wherein four sipes having an opening width of 0.3 mm are formed in each of all blocks and lengths of the sipes in each of shoulder blocks located at a position apart from a tread end within 0.15 times a ground contact width of a tread successively change into 20 mm, 14 mm, 8 mm and 4 mm from a leading edge of the block and lengths of the sipes formed in the other blocks successively change into 20 mm, 14 mm, 8 mm and 4 mm from a trailing edge of the block. For comparison, there is provided a comparative tire wherein four sipes having a length of 10 mm are formed in all of the blocks. When the self-aligning torque is represented by an index on the basis that the comparative tire is 100, the index value of the example tire is 100.5, so that the self-aligning torque can be increased by about 0.5%. As mentioned above, according to the invention, the self-aligning torque of the tire can effectively be increased to advantageously improve the steering stability in the high-speed running of the vehicle.

Claims: A pneumatic tire having a block pattern of a tread defined by a plurality of circumferential grooves (1,2) extending in a circumferential direction of the tread and a plurality of lateral grooves (3) each extending across the circumferential grooves to tread ends (E), characterized in that a plurality of sipes (12) each extending in a widthwise direction of the tread and being symmetrical with respect to a line segment passing through a center of the block in the circumferential direction of the tread are formed in each block (7,9,10) of the block rows (5,4,6) so as to gradually change lengths of said sipes from an end of the block in the circumferential direction toward the other end thereof, and the sipes (12) in each block (7) of an outermost block row (5) are longer at a leading side of the block and shorter at a trailing side thereof, and the sipes (12) in each block (9,10) of the other block rows (4,6) are longer at a trailing side of the block and shorter at a leading side thereof. A pneumatic tire as claimed in claim 1, characterized in that both ends of each sipe (12) in each block (7,9,10) are terminated in the block.

Inventors: FUJITA KAZUTO, MATSUMOTO HIROYUKI, TAKASHI SUZUKI, FUJITA, KAZUTO, MATSUMOTO, HIROYUKI, TAKASHI, SUZUKI
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 2000
Main classification: B60C11
IPC codes: B60C11