1. Field of the Invention
The present invention relates to prevention of shoulder drop wearing at shoulder portions in heavy duty pneumatic tires. The invention also relates to an improvement for the simultaneous prevention of both shoulder drop uneven wearing at shoulder portions and river-like uneven wearing at a second rib and a center portion of a tread in heavy duty pneumatic tires. Further, the present invention relates to prevention of shoulder drop wearing at shoulder portions and one side wearing in heavy duty pneumatic tires.
2. Related Art Statement
In conventional heavy duty pneumatic tires, particularly, in heavy duty pneumatic radical tires, shoulder drop uneven wearing has frequently occurred at the shoulder portion on the outer side of a front wheel. According to the conventional heavy duty pneumatic tires, the radius of curvature is laterally symmetric, and the ground contact pressure at tread edge portions is increased while the lateral symmetry is maintained. Thereby, shoulder drop uneven wearing is suppressed.
The uneven wearing is developed mainly at the tread edge of the outer side of a tire mounted to a wheel, not on the opposite sides of the tread. This results from the fact that roads include not only straight portions but also curved portions, that is, curves. When a vehicle turns along a curve, it rolls owing to a centrifugal force. Therefore, a load increases and a lateral force concentrates at the tread edge portion on the vehicle outer side in the tread of the tire mounted on the outside in turning, so that uneven wearing is likely to be produced at the tread edge portion on the outer side in the outwardly mounted tire. In order to restrain uneven wearing, the ground contact pressure at the tread edge portions has been increased by enlarging the radius of the curvature in the laterally symmetric fashion. However, this causes an increase in cost due to an increased tread volume. Further, since on the mounting outer side is faster, there is a problem that one sided wearing is likely to be produced.
In addition to shoulder drop uneven wearing at the shoulder portions of the treads of the heavy duty pneumatic tires, particularly, radial tires, in the steering wheels, a river-like uneven wearing frequently occurs at the second rib and the center portion of the tread. As a countermeasure to prevent these uneven wears, an NR base rubber or an SBR base rubber have been selectively used as a tread rubber of the tire paying attention to either one of the shoulder drop uneven wearing and the river-like wearing. Further, as shown in FIG. 1, according to the conventional heavy duty pneumatic tires, the radius of curvature of the crown portion of the tread is laterally symmetric, and wear resistance has been partially improved by varying the radius of curvature, R.sub.1, while the lateral symmetry being maintained.
However, with respect to the uneven wearing in the tread of the heavy duty pneumatic tire, the shoulder drop uneven wearing at the shoulder portions and the rib punching and the river-like uneven wearing in the second rib and the center portion often simultaneously proceed in both the steering wheel and non-steering wheel. This is because the uneven wearings proceed while both wearing due to the side force which is produced when the vehicle is turning and wearing due to the shearing force owing to the difference in diameter which is produced when the vehicle straightly runs are always combined together.
Despite these circumstances, as mentioned above, countermeasures have been made only against either one of the shoulder drop uneven wearing and the river-like uneven wearing, in which the radius of curvature of the tread is changed depending upon the input conditions while the radius of curvature is set laterally symmetric or the whole tread is selectively formed from a single rubber of the NR base rubber or the SBR base rubber alone as a tread rubber. Although the laterally symmetric curvature of the tread provides a tentatively positive effect against the wearing at the "mounting outer side" where an input is large (at a portion locating on the outer side when the tire is mounted to the wheel), the wear resisting performance of the whole tire is ultimately hardly obtained. Furthermore, selection of the rubber material can merely control the uneven wearing at either one of the shoulder portion and the second rib and the center portion. Therefore, there is a problem that both of the uneven wearings are not simultaneously restained.