(1) Field of the Invention:
The present invention relates to heavy duty pneumatic tires, and more particularly the invention relates to attainment of both a block-chipping resistance and a traction performance of tires for rough road running, more specifically tires for wheel type loaders (hereinafter referred to as "wheel loader tires").
(2) Related Art Statement:
When wheel loader tires run on rough terrain, particularly on a rocky places, there often occur troubles called block-chipping in the case of laterally symmetrical tires shown in FIG. 2, that is. That a rubber is partially chipped at the tire surface from a tread portion to a shoulder portion on the surface outer side of the tire surface when the tire is mounted on a wheel (hereinafter referred to as "wheel-mounted outer side").
In order to prevent the block-chipping mentioned above, the rigidity of the tread on the wheel-mounted outer side has been increased by adopting a so-called semi-slick pattern. As shown in FIG. 3, grooves from the tread portion to the shoulder portion on the wheel-mounted outer side (on the left in FIG. 3) are diminished or the rigidity of the whole tread has been increased by adopting a so-called slick pattern in which all the grooves are diminished.
In the conventional countermeasures, the block-chipping resistance is largely improved by increasing the rigidity between the tread portion and the shoulder portion on the wheel-mounted outer side. However, since wheel loader tires run not only on hard and rough roads such as rocky places but also on relatively soft rough roads, traction performance, which is a force by which the tire grasps the road and drags a vehicle is required. In the case of tires having a semi-slick pattern or a slick pattern, the traction performance originating from the shearing resistance due to the pattern is greatly deteriorated so that a problem occurs that the tire is difficult to run on soft roads. Further, since the one side surface or the whole surface of the tread is designed as "slick", a greater amount of rubber is required for the tread and manufacturing cost unfavorably increases.