Resilient tires and wheels – Tires – resilient – Anti-skid devices
Reexamination Certificate
1996-05-28
2002-06-25
Maki, Steven D. (Department: 1733)
Resilient tires and wheels
Tires, resilient
Anti-skid devices
C152S209180, C152S902000, C152SDIG003
Reexamination Certificate
active
06408911
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a studless pneumatic tire having excellent travel performance on roads covered partly or completely by ice and/or snow (hereinafter referred to as “icy roads”). The present invention also relates to a mold used for manufacturing through a vulcanizing molding process a pneumatic tire having a block pattern, and to a pneumatic tire manufactured using the mold.
2. Description of the Related Art
Since the use of spiked tires became prohibited, various improvements have been made on tire treads so as to obtain studless tires having improved performance.
The tread of a studless tire used for travel on icy roads has a plurality (3-5) of main grooves extending in the circumferential direction and a large number of lug grooves intersecting the main grooves, thereby forming block-shaped island portions between them. In order to improve the performance of the tire on icy surfaces, many sipes are provided in the island portions, thereby increasing the cumulative length of edges (hereinafter referred to as the “edge component”) of the island portions.
However, when many lateral sipes traversing the island portions are provided so as to improve driving and braking performances of the tire on icy surfaces, the overall stiffness of the blocks decreases although the edge component of the island portions increases. This increases the possibility of chipping occurring in the blocks of heavy duty tires, such as tires for trucks and buses, which are used while subject to high internal pressure and a heavy load. In tires for passenger cars, island elements, which are portions of each block finely divided by sipes, tend to bend down while cornering, resulting in decreased driving stability due to a reduced effective ground contact area.
Therefore, it is necessary to maintain the stiffness of the island portions even when many sipes are formed. For example, a tire disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2-200503 employs laterally extending sipes each having one end terminated within the island portion (i.e., only one end of each sipe is opened to a main groove) so as to maintain the stiffness of each island portion even when the density of sipes increases.
However, even when only one end of each sipe is opened to the main groove, the stiffness of each island portion decreases undesirably if many sipes are formed so as to obtain a desired edge effect.
The present inventors studied in detail the stiffness of island portions having sipes, and found that when the number of sipes is increased to obtain a desired edge effect, the stiffness at the lateral end regions of each island portion decreases more than expected as compared with the central region of the island portion, and that even when the sipes are formed such that each extends from one lateral edge of its corresponding island facing one main groove to a point near the other lateral edge thereof facing another main groove, the decrease of the stiffness cannot be completely prevented.
SUMMARY OF THE INVENTION
The present invention has been conceived in view of the above-described drawbacks, and an object of this invention is to provide a studless pneumatic tire which maintains the stiffness of island portions even when many sipes are formed, thereby providing improved driving stability and performance of the tire on icy roads while preventing island portions from being chipped.
According to a first aspect of the present invention, a studless pneumatic tire is provided which has a plurality of block-shaped island portions formed on a cylindrical tread extending between a pair of sidewalls. The island portions have laterally extending sipes and are separated by a plurality of main grooves extending in the circumferential direction as well as by many lug grooves. The sipes include first sipes and second sipes. Each first sipe extending from a first lateral end facing a first main groove traverses a first edge region of each island portion in a substantially axial direction and reaches a first interface between the first edge region and the central region of the island portion where the first sipe branches into a plurality of branch segments, thereby forming a fork-like shape. Each of these branch segments traverses the central region of the island portion and reaches a point near a second interface between a second edge region and the central region of the island portion. Each second sipe extending from a second lateral end of the island facing a second main groove traverses the second edge region and extends into the central region of the island portion in the axial direction. These first and second sipes divide the central region of the island portion into a larger number of island elements compared to the edge regions of the island portion.
In the studless pneumatic tire according to the first aspect of the present invention, the first sipe branches into a plurality of branch segments, thereby forming a fork-like shape, at the first interface between the first edge region and the central region of the island portion, and each branch segment traverses the central region and reaches a point near the second interface between the second edge region and the central region. The total projected length of the sipes in the circumferential direction increases in the central region because of the existence of the branch segments. This increases the edge component of each island portion. Further, each edge region of the island portion which is divided by the corresponding edge segment in the circumferential direction can effectively maintain the stiffness of the island portion.
In the first aspect of the present invention, when the tip of the second sipe extending from a lateral end opposite to the lateral end from which the first sipe extends is terminated at a position such that the tip of the second sipe does not intersect the first sipe, or when the second sipe has a fork-like shape similar to that of the first sipe and the tip of the second sipe is terminated at a position such that the tips of the branch segments of the second sipe do not intersect the first sipe, island elements divided by the sipes continue from a portion contacting one lug groove to a portion contacting the other lug groove in a staggered manner in the central region of the island portion. In contrast, each edge region of the island portion is divided by the edge segment of the fork-shaped sipe only, so that a decrease i n the stiffness of the island portion can be minimized.
According to a second aspect of the present invention, a studless pneumatic tire is provided which has a plurality of block-shaped island portions formed on a cylindrical tread ex tending between a pair of sidewalls. The island portions have laterally extending sipes and are separated by a plurality of main grooves extending in the circumferential direction as well as by many lug grooves. The sipes include first sipes and second sipes. Each first sipe has an extension segment and a return segment. The extension segment of the first sipe extends from a first lateral end of the island portion facing a first main groove so as to traverse a first edge region and a central region of the island portion in the axial direction. The return segment has a tip which is turned back at the interface between the central region and the second edge region or in the vicinity thereof and is terminated within the central region. Each second sipe also has an extension segment and a return segment, which are the same as those of the first sipe. By these first and second sipes, the central region of each island portion is circumferentially divided into a larger number of island elements compared to the edge regions thereof.
In the studless pneumatic tire according to the second aspect of the present invention, each block-shaped island portion in the tread has hook-shaped first and second sipes which extend in opposite directions from both lateral ends facing corresponding main grooves which extend in the circumferential directi
Tanabe Chishiro
Yamaguchi Yuji
Bridgestone Corporation
Maki Steven D.
Oliff & Berridg,e PLC
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