Originally Posted by terry b
I have 4 carbon bikes, 2 with braze-on and two without - so much for rules of thumb.
It's still a "rule of thumb", it just isn't universally true. Carbon frames made with round carbon tubes and either lugs or bonded joints can use a clamp-on fd since the seat tube is (wait for it...) round. However, most molded and a lot of bonded carbon frames have flaired, aero or other shaped seat tubes and must have a "braze-on" tab. And, it isn't universally true that metal frames have round seat tubes either so some of them have braze-on tabs out of necessity.
Older high-line steel frame bikes, even though they had round seat tubes, often came with braze-on (in this case it really was brazed on) fd tabs because braze-on fittings in general were the sign of a superior frame and the more the better. Also there was a minute weight savings with a braze-on fd.