Originally Posted by
ThermionicScott
I've noticed that chainring makers often produce inner rings that are a tooth or two bigger than the minimum of the BCD -- 42T on a 144mm BCD, 39T on a 130mm BCD, 34T on a 110mm BCD, etc. I wonder if it's just to avoid headaches in manufacturing, since you have a little more tolerance that way...
That makes sense to me. Also, the true minimums usually have odd numbers of teeth, and chainring makers seem to have a religious aversion to odd numbers that aren't 39. And that exception itself lends weight to your hypothesis... when road doubles on stock bikes went from 52/42 to 53/39 around 1990, 52/38 would have made more sense for anybody slower than Sean Kelly... which is to say, anybody who pays for their bike. Plus it's only one new chainring, not two. We had to wait for Tyler Hamilton's broken collarbone in the 2003 Tour to allow roadies to ride cranks that made sense. I was working in a shop then, and we changed out a lot of 53/39s for 50/36s that year. (Of course, that requires a 110 BCD, but that should have been the standard for Japanese cranks all along... I think it's the smallest BCD that allows for a really good-looking 5-arm crank.)
--Shannon
PS - Yeah, I know, EPO, blood transfusions, the whole works. Tyler's ride in '03 was still the hardest of the hard. Dude's a stud, doped or not.