Rubber composition for reinforcing bead of tires which contains butadiene rubber with syndiotactic crystals, carbon black, sulfur and sulfenamide accelerator

A rubber composition suitable for reinforcing beads of tires which comprises

• (A) a rubber component containing 30 to 70% by weight of a butadiene rubber containing syndiotactic crystals and 70 to 30% by weight of other diene rubbers, the content of syndiotactic crystals in said butadiene rubber being 6 to 17% by weight
• (B) 60 to 75 parts of a carbon black having an iodine adsorption number of 65 to 130 mg/g and an oil absorption of 90 to 140 ml/100 g,
• (C) 1 to 2 parts of sulfur, and
• (D) 2 to 4 parts of a sulfenamide accelerator,
• the total amount of said sulfur (C) and said accelerator (D) being from 3 to 5 parts, and said parts all being parts by weight per 100 parts by weight of said rubber component (A),
• wherein the ratio of said sulfenamide accelerator to said sulfur is from 1 to 4 by weight, and wherein the rubber composition has a JIS A hardness of at least 75, a rate of rise in hardness by thermal aging of at most 10%, a permanent set under a constant stress 20 kgf/cm.sup.2 of at most 12% and a picoabrasion index of at least 240 measured according to ASTM D 2288.