Originally Posted by
HillRider
The big and middle chainrings for a triple or the big and small chainrings for a double crank have to be at least 10 teeth different in size (52/42/X or more) for a 105 and 12T (52/40/X or more) for an Ultegra. This is required so the smaller ring is small enough in radius to clear the inner cage of the front derailleur when it is shifted to the big chainring.
There is also a maximum total tooth difference specified by the maker to assure the shifting is clean enough. For many Shimano triples that maximum is 22T (52/X/30) but it can be exceeded with some slight loss in shifting speed. I routinely fit 26T granny rings to Shimano triple cranks for a total tooth difference 26T (52-26) and they work fine.
Thanks, this is very helpful to me.
Another option I'm looking at is a Jamis Aurora Elite that comes stock with an FSA triple, the chainrings are 50-39-30; the rear cassette is 12-27. (The bike comes stock withi 105 STI shifters with a 105 FD and Ultegra RD.).
IMHO, that's too high of gearing for touring (at least for me)...the LBS is a little unsure whether I'd be happy with the gearing if I got them to swap out the 30T inner chainring for a 26 inner chainring (so the chainrings would be 50-39-26). . I'm accustomed to slow shifting on a triple so if all I have to do is be a bit careful when reaching for the inner chainring I'm OK.
That gives me really smooth 10 speed shifting for commuting, but a nice "bail out" set of gears for light touring.