Originally Posted by
jimmuller
...So I guess I would argue with dddd's requirement. I'd say instead that the ratio of medium to small should be equal to (or greater than) the ratio of large to medium. Equal would be also the mathematical relationship that makes the tops of the three rings form a straight line so that the medium ring is not masked.
I stated: "The cause I would expect would be that the difference in tooth count between the small and middle ring is as small or smaller than the difference in tooth count between the middle and big ring." And I mentioned a possible derailer issue as well.
I simply stated what I expected the causes of the poor shifting to be.
There is no definite "requirement" per se, since many old Shimano Triple setups such as FC-M730 with no pickup pins and 28-38-48t chainrings shifts marvelously (under tremendous authority of the shift lever) using Shimano's period "UG Narrow" or newer HG chain.
And Shimano's newer offerings, even at the very lowest end of their lineup, likewise shift flawlessly using 24-32-42t rings!
But each step in the "wrong" direction (toward a relatively smaller middle ring) increases the requirements of chain and chainring design, and of a more suitable front derailer. For example, what works really well with HG chain may barely work at all with, say, Sedisport chain.
As I've said, sometimes I couldn't get my own selected parts to quite work well without going to the extreme or last-ditch route of removing the middle ring and bending every third tooth inward, as when I finally got an old 1970's Cyclone front derailer to properly index-shift via an Alivio shifter over a set of three selected rings (34, 42 and 52t iir) that I mounted to an old Miyata tourer's SR APEX 84bcd crank.
