The seat tube - bottom bracket junction is under extremely high stress, and that is where my Nishiki frame failed after about 20K miles of hard use. I was able to ride home just fine, with a groan coming out of the frame on every pedal downstroke, even though I geared pretty far down and took it easy. I never felt endangered, so I would say ride it until it inevitably fails there.
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069