1. Field of the Invention
The present invention relates to a pneumatic tire comprising a tread surface including a land portion defined by a main groove extending in a circumferential direction of the tire. The pneumatic tire of the present invention is particularly useful for a heavy duty pneumatic tire.
2. Description of the Related Art
In heavy duty pneumatic tires used for trucks and buses, repeated use may cause partial wear generated notably in shoulder portion on land portion. For example, in the case of tires having blocks formed as land portion in shoulder portion, partial wear tends to be generated in a side edge portion and a trailing side portion of the block (shadowed in FIG. 8). Therefore, in heavy duty pneumatic tires, there is a problem of a partial wear, so-called heel and toe wear or the like; that is, a wear difference is generated in the trailing side and the leading side of the block as viewed in a circumferential direction of the tire.
The inventor of the present invention focused its interest on the following technique as means for preventing partial wear of the land portion including blocks. That is, a wear sacrifice portion is provided by forming sipes in a side edge portion of the land portion in order to locally reduce the stiffness of the land portion. In this case, the wear energy is concentrated on the sacrifice portion. Therefore, the wear energy is reduced at the trailing side, and thus the heel and toe wear is prevented from being generated in the entire of the land portion. However, in order to concentrate the wear energy, the stiffness of the land portion between the sipes has to be reduced by increasing the volume of sipes. Accordingly, clacks may be generated starting at the bottom of the sipe, and then it may lead to a break of land portion.
Patent document 1 (Japanese Unexamined Patent Publication No. 2000-211321) teaches a pneumatic tire described below. That is, in the sipe formed in the block of the tread surface, the width of the sipe is formed to be narrower at the bottom side than at the tread side and sipe wall surface located at the trailing side is formed in a step-like shape. This pneumatic tire employs in particular both-side open sipes which have openings at both sides of block in order to increase snow/water ejecting performance. However, this tire is formed with sipes in the entire area of the block as viewed in the width direction thereof, but no wear sacrifice portion is formed for absorbing local wear that is generated locally in the block. Therefore, the entire of the block is not prevented satisfactorily from being generated with heel and toe wear. Further, since sipes are formed in the entire area of the block as viewed in the width direction, small blocks formed between the sipes have extremely reduced stiffness and clacks are easily generated.
Patent document 2 (Japanese Unexamined Patent Publication No. 2003-182313) teaches a pneumatic tire as described below. That is, the tire is formed with sipes in a block of tread surface, and a step portion is formed on wall of small block formed between the sipes; thereby, compared to the thickness at the base side of the small block, the thickness at the tread side of the small block is formed to be thinner. In this pneumatic tire, in particular both side open sipes are employed in order to increase on-ice traction performance and on-snow traction performance. However, same as the pneumatic tire set forth in Japanese Unexamined Patent Publication No. 2000-211321, sipes are formed in full width direction of the block. But no wear sacrifice portion is formed for absorbing local wear that is generated locally in the block. Therefore, the entire of the block is not prevented satisfactorily from being generated with heel and toe wear. Further, since sipes are formed in the entire area of the block as viewed in the width direction, small blocks formed between the sipes have extremely reduced stiffness and clacks are easily generated.
Patent document 1; Japanese Unexamined Patent Publication No. 2000-211321
Patent document 2; Japanese Unexamined Patent Publication No. 2003-182313