This invention relates to a pneumatic tire having improved in wet performance.
In general, known rib type pneumatic tires, for example, disclosed in FIG. 1 have often been used for trucks and buses. The tire of this type has a tread surface 1 including a plurality of main grooves 2 extending zigzag and substantially circumferentially. With such a tire running on a wet road on which water exists, the water entering in the proximity of a tread surface of the tire contacting the road is exhausted through the main grooves 2 of the tire. However, as the main grooves 2 are zigzaged, the water collides with sidewalls of the zigzag grooves many times and is obliged to turn its flowing directions. As a result, the flowing efficiency of the water flowing through the main grooves 2 is low. Therefore, the main grooves 2 are filled with the water. As there is no space for receiving water between the tread surface and the road, the water further enters between the tread surfaces and the road. From this fact, the rib type tire exhibits only narrow surface area contacting the road and poor wet performance. The wet performance means a faculty of a tire running on a road on which water exists.
In order to improve the wet performance, a pneumatic tire has been proposed, which is as disclosed in specification U.S. Pat. No. 4,299,264. As shown in FIG. 2, this proposed tire has a tread surface 3 including a plurality of main grooves 4 circumferentially extending and a plurality of main grooves 5 circumferentially spaced apart from each other and inclined to an equatorial plane 6 of the tire to ensure passages for causing water to flow laterally.
With such a pneumatic tire, the water entering within a zone of the tread of the tire is exhausted through both the main grooves 4 and 5 of the tire. However, as these main grooves 4 and 5 intersect with each other at a great number of positions, the water flowing through the main groove 4 and 5 collides with each other at these intersections so a to be disturbed with each other to lower the drainage faculty of the tire. As a result, the main grooves 4 and 5 are under a condition filled with the water which does not flow very well, and the water between the tread and the road does not have any space into which the water escapes. Moreover, part of water within the zone of the tread is exhausted through the main grooves 4 into immediate front of the tread, so that the water increases at the location immediately before the tire in a running direction. From the above fact, the water enters deeply between the tread and the road to make narrow the surface area of the tire contacting the road, so that the wet performance could not be improved very much. This is particularly acute in a flat tire whose contacting length is short.