Due to increases in vehicle performance, there is currently a strong demand for pneumatic tires (hereinafter, referred to as “tires”) which achieve both of steering stability on a dry road surface and steering stability on a wet road surface to a high degree when traveling at high speed. In particular, there is tendency to provide tires emphasizing steering stability in which a rib of a land portion interposed between tire circumferential main grooves is widened in order to increase the ground contact area which comes into contact with the ground.
The tire of Japanese Unexamined Patent Application Publication No. 2010-215221A is an example of a tire provided with a tread pattern having a widened rib. This tire achieves both of steering stability on a dry road surface and steering stability on a wet road surface to a high degree.
Specifically, this pneumatic tire has four main grooves extending straight in the tire circumferential direction in the ground contact region of the tread surface, and has five land portions defined by the main grooves on the tread surface.
A groove width of the main groove positioned on the outermost side of one side (hereinafter, referred to as “first side”) in the tire width direction out of four main grooves is made narrower than a groove width of any of the groove widths of the other three main grooves, and a ratio Wmax/Wout of a maximum groove width Wmax of the three main grooves and a groove width Wout of the main groove positioned on the outermost side of the first side in the tire width direction is set within a range of 2.0 to 3.0.
Furthermore, only the land portion positioned on the outermost side of the first side in the tire width direction out of the five land portions is defined by inclined grooves arranged to be spaced at predetermined intervals in the tire circumferential direction to be formed into a row of blocks, and the other four land portions are formed into ribs which extend continuously in the tire circumferential direction.
A ratio Sin/Sout of a groove surface area ratio Sin of the other side (hereinafter, referred to as “second side”) in the tire width direction centered on the tire equator in the ground contact region and a groove surface area ratio Sout of the first side in the tire width direction is set within a range of 1.25 to 1.35, and the groove surface area ratios of the ground contact regions of the three land portions, which exclude the land portion on the outermost side of the second side in the tire width direction and the land portion on the outermost side of the first side in the tire width direction out of the five land portions, are greater on the second side in the tire width direction than on the first side in the tire width direction with the respective centerlines of the land portions set as a border.
In the known pneumatic tire described above, it is possible to achieve both of steering stability on a dry road surface and steering stability on a wet road surface to a high degree; however, there is a demand for a further improvement in the steering stability on a dry road surface.
FIG. 7 is a diagram illustrating an example of a shape of a ground contact patch of the tire described above. In the ground contact patch of the tire described above, each of the ground contact lengths of the land portions, which are three ribs surrounded by four tire circumferential main grooves, are short, and the ground contact area is small. In particular, in the center land portion through which the centerline passes, the ground contact length of the central portion of the land portions is shorter compared to the periphery thereof, which causes a large decrease in the ground contact area. In addition, the ground contact area of a land portion of a shoulder region (region B) is also small.
Since the ground contact area is decreased due to the decrease in the ground contact length of each of the land portions by increasing the rib width of the land portion, the effect of improving the steering stability is easily suppressed. For this reason, there is a problem in that it is not possible to efficiently obtain improvements in the steering stability in tires with land portions having a large rib width.