Originally Posted by
SkyDog75
Shimano indexed road and mountain front derailleurs use a different cable pull ratio, and that's gonna be a problem for you: MTB derailleurs don't exist with proper cage radius for a 52-tooth chainring. Road derailleurs do, but they won't have the correct cable pull to index with your MTB shifters.
In addition, while there are top pull road derailleurs for cyclocross bikes, I'm not aware of any that have a shaped cage for a triple. (If the derailleur has a big enough swing and a long enough cage, you may be able to make a double front derailleur work.)
The spacing between the rings on the road crank is the same as the MTB crank. 7mm. Besides, the problem is not shifting, the problem is rubbing from small rear cog to large rear cog, and not enough drop to low on the crank. It shifts just fine. Now medium and high rings work fine, just the low ring drags.
I realized I had checked the wrong crank to get the MTB tooth counts. I have updated the calcs above to reflect the MTB is 28,38,48T. With the MTB crank, I felt I was pedling too fast at my common flats speeds of 10 to 15 mph. This 52T slows the cadence down a few rotations and I feel about the same as when riding my old 2x5 road bike.
The change in Gear Inches (48 vs 52 with 14 cog) is 90.7 to 97.42. Change in Cadence for 10 & 15mph is 37,56 to 35,52. Guess that is not a really big change, but it sure does make my 59 yr old knees feel better.
Funny thing is if you read Shimano Derailleur setup tech document, it says that rubbing is normal when at the extreme gears. So, seeing as rings 2 and 3 are running quiet, and its only ring 1 that drags the chain off the tail of the cage, I'll look into modifying the cage to be longer .