Join Date: Jul 2000
Bikes: 1959 Capo; 1980 Peugeot PKN-10; 1981 Bianchi; 1988 Schwinn KOM-10;
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Allowable chain angles are a function of lateral chain flexibility and chainring tooth profile. I agree that one should avoid the large/large and small/small combinations on almost any modern derailleur bicycle, but which (if any) other combinations to avoid depends also on chainstay length, chainring size difference, cogset width, and chainring-to-cog alignment. If you can make the chain run quietly and if it stays on the chainring when you backpedal, you are in a usable gear.
Both of my road bikes, which use 8-tooth chainring drops (50-42 and 52-44) and mid-size 7-speed freewheels (13-26 and 14-28), work well in all combinations except large/large. On my 3/7 mountain bike (48-40-24 / 13-26), I avoid 24/13, 24/15, and 48/26.