This invention relates to pneumatic tires and more particularly to a pneumatic tire having a wide reinforcing breaker or belt extending into an annular protuberance in the buttress region of the tire.
The tread reinforcement component commonly referred to as a belt or breaker is disposed between the tread and the carcass in the crown region of the tire. Typically, a breaker or belt is constructed of one or more plies of rubberized cord fabric. Each cord in the fabric is generally of a substantially non-extensible material such as metallic wires or strands, glass filaments, or rayon filaments. The cords within the breaker plies are oriented parallel to each other and substantially parallel to the plane of the beads and thus circumferentially of the tire, or at a small bias angle. In a single ply breaker construction, the small bias angle can be 0.degree.. Where the breaker is a multi-ply structure, however, the small bias angle is in the range of about 15.degree.-30.degree. with respect to the median equatorial plane of the tire, and may be oppositely disposed in successive plies.
The development of a reinforcing belt or breaker for use in pneumatic tires added a number of benefits to tires including considerable improvement to tire durability. On the other hand, the breaker also introduced some new problems of its own. The lateral edges of a breaker in a typical tire, due to their location between the tire tread and carcass at a position of continually changing stress, tend to separate from the carcass or themselves. Improper positioning of the breaker results in the uniformity problems of increased lateral force variation and conicity.
Another benefit of a reinforcing breaker is improved tread wear due to the greater stiffness imparted to the tire tread. Of course, this benefit is enhanced by utilizing a "flat" tread having no convex cross-sectional curvature so that the relative movement between the center portion of the tread and the axially outward portions is minimized thereby resulting in an even wear pattern. A serious drawback of a tread designed to have no convex cross-sectional curvature lies in the fact that a tire which is vulcanized with such a shape, when cured, may inflate to a concave tread cross-section rather than a flat cross-section. Elimination of this problem can be accomplished by constructing and vulcanizing the tire such that the tread has at least a small convex cross-sectional curvature, that is, a large crown radius ratio, when the cured tire is inflated to its rated inflation pressure.
In the detailed description of the invention following hereinafter, the cross-sectional curvature of the tread and breaker of the tire are discussed in terms of "crown radius ratio" and "crown radius." The term "crown radius" as used herein either with respect to the tread or breaker, means the radius of the arc of a circle which best approximates the arc of curvature of the tread surface or the breaker as the case may be of a tire. The crown radius is commonly determined by the formula ##EQU1## where "leg set" is the axial distance measured along a line tangent to the tread or breaker at its midpoint between the shoulders of the tread or edge of the breaker, or predetermined points axially equidistance inward of the shoulders or breaker edges. Shoulder drop is the perpendicular distance from the aforementioned tangent line to the tread shoulders or said predetermined points. In the case of a breaker, shoulder drop may be considered as the perpendicular distance between the line tangent to the breaker at its midpoint and the edges of the breaker. The "crown radius ratio" is the ratio of the crown radius of either the tread or breaker to the nominal cross-sectional width of the tire and as used herein is expressed in terms of percent.
Either a belted radial or a belted bias tire construction is suitable for use in the present invention. The radial type tire is, however, the preferred embodiment. A radial tire is inclusive of various tire constructions which typically comprise a carcass or body having one or more reinforcement plies of cord fabric extending from bead to bead wherein the cords in each ply are substantially radial in orientation, that is, the cords are oriented substantially normal to the beads and the crown centerline of the tire. In a single ply radial tire construction, the carcass cords normally have a 90.degree. bias angle, that is, in the unshaped carcass they extend perpendicular to the planes of the beads. In a two-ply radial tire construction, the cords in each carcass ply are parallel to the cords in the other carcass ply. However, the cords in each ply may be oriented at oppositely disposed angles of 70.degree. or more with respect to the median equatorial plane of the tire and thus the angle between cords in different plies is between 0.degree. and 40.degree.. In more than two ply radial ply tire constructions, similar cord arrangements in successive carcass plies is usually employed.
With regard to the prior art, U.S. Pat. No. 3,450,182 discloses a tire having a flat tread cross-section, wide protuberances extending beyond the width of the tire sidewalls, a tread surface in normal engagement with the ground between the lateral edges of the protuberances and a breaker extending laterally beneath the tread to positions adjacent the edges of the protuberances. This patent does not, however, disclose a breaker having its lateral edges removed from the stress area of contact by the tread with the ground or a convex cross-sectional tread curvature facilitating the manufacture of a tire. Moreover, the extreme lateral extension of the protuberances beyond the tire sidewall will result in a high degree of tread shoulder wear and breaker separation due to centrifugal force moving the protuberances radially outward during tire rotation.
In U.S. Pat. No. 2,477,754, an aircraft tire is shown in which a tread and breaker have a substantial convexly curved cross-section when the tire is inflated and the breaker has edges positioned within protuberances extending from the side of the tire. When this tire is loaded (but not impacted), the tread-ground contact area is in the crown area of the tread only and consequently the tread wear and traction of the tire will be relatively poor. Moreover, when the tire is subject to the normal type of impaction and very heavy loading which aircraft tires receive, the lateral edges of the breaker rotate through the tread-ground contact area to cause edge separation of the breaker. A further defect in the tire of U.S. Pat. No. 2,477,754 is that the circumferential breaker cords along the lateral sides of the breaker will carry the entire breaker load under deflection so that these breaker cords will break after a small amount of such use.