This invention relates to a pneumatic tire having an asymmetric tread pattern. More particularly, it relates to a pneumatic tire capable of improving driving performance on a wet road surface and reducing noise during driving while keeping excellent driving performance on a dry road surface.
A pneumatic tire having an asymmetric tread pattern wherein ribs continuing in a tire circumferential direction are disposed at a shoulder end portion on a tread surface on the outer side of a vehicle when the tire is fitted to the vehicle, and block lines each comprising a plurality of blocks are disposed at a shoulder end portion on the inner side of the vehicle exhibits excellent driving performance on a dry road surface because its outer shoulder end portion is constituted by ribs having high rigidity. It is known generally that such a pneumatic tire exhibits excellent wear resistance even under a critical state such as in circuit driving because the shoulder end portion on the outer side of the vehicle comprises the ribs having high rigidity and the tire does not have relatively sharp block edges.
However, though this pneumatic tire has no problem of draining property on a wet road surface having a relatively small depth of water, it involves inferior drainability when the depth of water is great because the shoulder portion on the outer side of the tire with respect to the vehicle consists of the ribs. Various attempts have been made so far to solve these problems by, for example, increasing the void volume of the tread, or by employing an asymmetric profile as disclosed in Japanese patent application Kokai publications No. 57-147901 and No. 61-98601. However, these methods have not been entirely satisfactory because they invite the drop of other items of tire performance.
Also, lately it has been increasingly strongly demanded from the viewpoint of the environmental integrity that noise generation occurring when vehicles are run should be suppressed, and the above described tires having remarkable dry performance are not excluded in this respect. Tires the tread pattern of which is asymetrical and comprises a rib at the shoulder end on the outer side in the condition in which they are mounted to wheels of a vehicle have a feature such that they do not have a continuous lug groove in the outer rib in contrast to such tires of which the tread pattern comprises a block-based pattern over a whole tread area, and basically such feature ought to be advantageously influential upon supression of the noise generation. However, a pair of tires, one for a left and the other for a right wheels of a vehicle, are produced by a same molding die, so that in mounting such tires to the vehicle, either of the two tires has to be reversed in the mounting-position relative to the other so that the outer rib of each tire is on the outer side of the vehicle. If the tire mounting is made in that way, then a basic direction of inclination of sub-grooves extending in the widthwise direction of the tires falls to be identical in connection with the left tire and the right tire on the vehicle, whereby a difference in the block rigidity is produced between the left tire and the right tire, possibly resulting in a large difference in the noise generation between the left side and the right side of the vehicle.