I currently run a 21-speed 48-40-24 / 13-26 "1.5-step plus granny" with old SunTour XC derailleurs on my mountain bike, having recently replaced my original 28T inner ring with the 24. To obtain a smooth, fast 24-40 gear change, I do have to pedal gently while paying attention to how far I move the shift lever, and I need to avoid trying to shift across too many rear cogs at the same time. In practice, this has been a problem only when I frist start off after storing the bike in the (minimum-tension) small-small combination. In contrast, the 28-40 upshift was almost idiot-proof.
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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