Originally Posted by
HillRider
Your analysis is correct. It's not just the indexing that differs between road and MTB front derailleurs. MTB fronts have a deeper inner cage plate intended to work with chainring of smaller diameter and greater tooth difference than road derailleurs. Your crank has a 10 tooth big to middle chainring difference so if the outer cage plate is set properly above the big ring, the inner plate is too deep to clear the middle chainring. Basically, you have the wrong front derailleur.
Also, it depends on which generation Tiagra triple you have. If it's a 4600 or older, yes the granny bolt circle is 74 mm and you can fit as small as a 24T chainring. If it a 4700 , the granny chainring is bolted directly to the middle chainring with a 92 mm bolt circle and 30T is the smallest that will fit.
thanks alot for that feedback. i should have perhaps specified that i am using bar-end shifters, so the front is friction, not indexed. and the question on the granny gear change was for a different crankset (50-39-30) on another bike. no problems with that one at all, just wanted to drop that granny down a bit for my wife !