Join Date: Jul 2000
Bikes: 1959 Capo; 1980 Peugeot PKN-10; 1981 Bianchi; 1988 Schwinn KOM-10;
Mentioned: 3 Post(s)
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With 53-39 / 12-25 and a short-cage racing-style derailleur, you may be challenging your rear derailleur's take-up capacity (53+25-39-12=27 teeth). Ideally, you want the chain to be long enough to accommodate a shift into the 53/25 cross (which you solemnly swear never to use) safely and smoothly, but short enough not to sag severely in the 39/12 cross (which of course you also solemnly swear never to use). If you cannot achieve both objectives simultaneously, err towards excessive length, unless you trust yourself NEVER to attempt a large-large crosschain shift.
I use a short-cage SunTour Cyclone II on the PKN-10, whose 48-45-34/13-23 gearing requires the derailleur's full rated 24-tooth wrap capacity. I have just a hint of chain slack in the 34/13 combo, and everything works very smoothly.
"Early to bed, early to rise. Work like hell, and advertise." -- George Stahlman
Capo [dschaw'-poe]: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger, S/N 42624
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1981 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069