This invention relates to a heavy duty pneumatic radial tire for use in trucks, buses or the like, and more particularly to a heavy duty pneumatic tire whose bead portions have improved durability.
In general, a pneumatic radial tire for use in trucks, buses or the like has a carcass consisting of a pair of bead portions and a steel cord layer and extending between the bead portions. When the pneumatic radial tire is filled with pneumatic pressure or inner pressure, the configuration of the carcass changes to be larger so that an outer surface of the tire becomes larger together therewith into a configuration similar to that of the enlarged carcass.
With the heavy duty pneumatic radial tire, particularly, the enlarging change in the carcass configuration or carcass line tends to be large because it is used with high inner pressure. The amount of enlargement is an important factor in designing a tire.
FIG. 1 illustrates one example of the enlarging change of a carcass line of a prior art heavy duty pneumatic radial tire 1. In FIG. 1, solid lines show a tire 2 prior to filling with inner pressure and broken lines 3a denote outer surfaces of the tire 3 after filled with inner pressure. A thick solid line 5b-5c illustrates a carcass line 5 before it is filled with the air pressure. A thick broken line 7b-7c shows a carcass line 7 after filled with a predetermined maximum inner pressure. In the heavy duty pneumatic radial tire of the prior art, as shown in FIG. 1, an enlargement amount d5 of the carcass line 5b-5c in normal directions N thereof and an enlargement amount d2 of the outer surfaces 2a of the tire 2 are substantially uniform, respectively, from a crown portion 8 to bead portions 9. In other words, such a tire is designed in a so-called "natural equilibrium configuration".
In the tire designed in the natural equilibrium configuration according to the prior art, however, the bead portions do not exhibit enough durability to be used under severe conditions particularly recently required or repeatedly used as a base tire, so that the tire tends to cause failure in the bead portions for a short period of time.
It has been known that large tensile strains .epsilon..sub.1 occur in the rubber in the proximity of an outer end of a turn-up end 5a of the carcass 5 in the bead portion 9 as shown in FIG. 2 and repeatedly acts on the rubber in continuous running of a vehicle to cause fatigue of the rubber, which in turn causes separation of the rubber from cords resulting in the failure of the bead portion.
In order to protect the turn-up end 5a of the carcass 5, a wire chafer 11 is sometimes provided extending radially outwardly of the turn-up end 5a as shown in FIG. 3 illustrating a prior art tire bead portion. With this case, however, large tensile strains .epsilon..sub.2 occur end 11a of the wire chafer 11 so that continuous running of a vehicle finally causes a failure of the bead portion in the same manner as in the above description.
In order to avoid such troubles of the bead portions, it has been proposed to modify the constitution of the chafer in the bead portion or to replace rubber materials in the proximity of the turn-up end 5a of the carcass 5 or the end 11a of the wire chafer 11. However, such modifications do not reduce the strains themselves when filled with the inner pressure, so that these proposals do not fundamentally solve the problems and do not achieve satisfactory results.
In the case of that shown in FIG. 3 providing the wire chafer 11 extending radially outwardly of the turn-up end, the phenomenon occurring in the turn-up end 5a of the carcass shown in FIG. 2 still occurs in the end 11a of the wire chafer instead of the turn-up end.