The Yokohama Rubber Co., Ltd.

Masaru Hirai

Masatoshi Kozima

Yukio Tozawa

Akinori Tokieda

Tadanobu Nagumo

Yasuhiro Mizumoto

Disclosed is a high hardness rubber composition having a hardness of at least 92, which comprises 100 parts by weight of a rubber, 55 to 95 parts by weight of soft black having an average particle size of 40 to 80 millimicrons, 5 to 30 parts by weight of a thermosetting resin and a small amount of a curing agent, the sum of the amounts of the soft black and thermosetting resin being 60 to 100 parts by weight per 100 parts by weight of the rubber. This high hardness rubber composition is advantageously used as a tire reinforcement for the rim cushion portion or for both the rim cushion portion and bead filler.

Rubber composition employing soft black and a thermosetting resin and

A pneutmatic radial-ply tire in which the rubber portion of at least one of a carcass ply and an undertread constituting the tire is made of a rubber which has a viscoelastic property of loss tangent, tan.delta., of 0.2 or lower (tan.delta..ltoreq.0.2) and storage modulus, G, of 120 Kg/cm.sup.2 or higher (G.gtoreq.120 kg/cm.sup.2), so that the rolling resistance of the tire is reduced without being accompanied by degradation of braking performance, control stability, comfortableness and wear resistance of the tire.

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Yokohama, Japan

Masaru Hirai