Originally Posted by
Road Fan
To check whether a given BB combination will work, I agree you have to look at clearance between the inner chainring and the chainstay, but you also have to look at the chainline, i.e. the distance from the center of the downtube or seat tube and the center of the middle chainring.
Right, the correct chainline would be something like 43 mm between the center of the middle ring and the center of the seat tube. But clearance is easier to measure, and in my experience getting the clearance right also means that the chainline will be right. Shamelessly empirical, as I said, but there's no need for a high degree of precision with this.