The present invention relates to a heavy duty tire, more particularly to a tread reinforcing belt having a three-ply structure capable of reducing its weight and capable of increasing the retreadability.
In the heavy duty tires such as truck/bus tires, heretofore, in order to secure safety and dynamic performance, the tread portion is reinforced by a belt made up of four plies of steel belt cords as shown in FIG. 5(A). Particularly, in order to improve the dynamic performance, the radially innermost first belt ply A1 is provided with a cord angle θ1 of a relatively higher value of from 40 to 60 degrees with respect to the tire equator C. And in order to increase the tire binding force of the belt and thereby to secure safety, the second, third and fourth belt plies A2, A3 and A4 disposed radially outside the first belt ply A1 are provided with cord angles θ2, θ3 and θ4 of relatively lower values of from 15 to 30 degrees with respect to the tire equator C.
In recent years, on the other hand, in order to reduce tire weight, there has been proposed a heavy duty tire in which the number of belt plies constituting the belt is reduced to three, as disclosed in Japanese patent application publication JP-A-2005-212742. In this heavy duty tire, as shown in FIG. 5(B), the cord angles θ1 to θ3 of the first to third belt plies A1 to A3 are set in a range of 16 to 22 degrees, and with respect to the tire equator, the belt cords of the first belt ply A1 are inclined oppositely to those of the second and third belt plies A2 and A3.
In this belt structure, although the belt cords of the first belt ply A1 cross those of the second belt ply A2, since the cord angles θ1 to θ3 are 16 to 22 degrees, namely, relatively small, the in-plane rigidity of the belt becomes relatively low, therefore, there is a problem with deterioration in the dynamic performance.
On the other hand, when a worn tire is reused by retreading, usually the tread rubber is removed together with the outermost belt ply, and then replaced with new ones.
In the case of the above-mentioned heavy duty tire provided with the three-ply belt structure, the outermost third belt ply A3 and the inner second belt ply A2 are inclined towards the same direction with respect to the tire equator, and the difference between their cord angles θ2 and θ3 are small. As a result, in the used heavy duty tire, the belt cords of the third belt ply A3 are very liable to sink between the belt cords of the second belt ply A2, in other words, the third belt ply A3 and second belt ply A2 become united, and it is very difficult to separate or remove the outermost third belt ply A3 only. Thus, the retreading of such tire is difficult.
If the third and second belt plies A3 and A2 are removed together, the retreading becomes almost impossible due to the following reasons. In the retread process, the tire is needs to be inflated. Since the two belt plies A3 and A2 have been removed from the tire, namely, the tire has only the innermost belt ply A1, the tire binding force of the belt is insufficient. And the remaining belt cords are oriented in one direction which is inclined with respect to the tire circumferential direction. Accordingly, when the tire is inflated, the tire gets distorted. Thus, the retreading is very difficult or almost impossible.