Heavy-duty pneumatic radial tire tread with narrow groove near shoulder

Claim 1

A heavy-duty pneumatic radial tire comprised of a carcass of at least one layer, a reinforcing belt comprising at least two metal cord layers and provided on the outer periphery of the tread portions, a plurality of ribs formed in the main tread area of contact with the road and partitioned by a plurality of main grooves extending into the circumferential direction of the tire, narrow grooves provided in the ribs located in the shoulder portions and having a width narrower than that of the main grooves, and an auxiliary area of contact with the road provided at the edge of each of both shoulder portions and having a round shape, wherein said main area of contact with the road is formed with a circular arc, having a large radius of curvature, which crosses another circular arc having a small radius of curvature, Ra, to form an edge-shaped boundary, said radius of curvature, Ra, being larger than the depth of said main grooves and smaller than the value five times as much as the depth of said main grooves, wherein said narrow grooves each are linearly provided in the circumferential direction of the tire and at a distance of 1.5 to 5 mm away towards the center of the tread surface from said edge-shaped boundary and have a groove width in the range of 1.5 to 5 mm, the direction of said narrow grooves being inclined at an angle in the range of 1° to 5° relative to the perpendicular to the rotating shaft of the tire, the bottoms of said narrow grooves and the bottoms of said main grooves being present on an imaginary line drawn parallel to the circular arc of said main area of contact with the road, and wherein the widths of the partitioned ribs, located on the side of the center of the tread surface, among the partitioned ribs formed by partition with said narrow grooves, are equal to or larger than the maximum width of the remaining ribs which are present near the center of the tread surface as compared with said partitioned ribs.