I feel like a monk emerging from a dusty corner of the St. Pachomius library with a fresh insight. I found something this weekend in the sacred writings of Sheldon Brown that solved a problem I've been having. Finding something useful in Sheldon's writings is, of course, not at all uncommon. What surprised me is that I discovered something hinted at in passing that I hadn't seen directly discussed before.
I know a lot of people on this forum, myself included, have put drop bars on mountain bikes. When you do that, there are two common problems that need to be solved: (1) the top tube length is usually too long, and (2) MTB front derailleurs aren't index-compatible with road shifters. What I found in Sheldon's writings is a new (yet old) solution to this second problem.
The problem with this incompatibility is two-fold: (1) Shimano MTB shifters and Shimano road shifters pull a different amount of cable, and (2) road front derailleurs are made for large chain rings so the curvature doesn't match MTB cranksets.
There are, of course, a lot of ways around this, mostly involving either using friction shifting or tolerating poor shifting. The solution I discovered in Sheldon's writings this weekend doesn't involve either of these. I found the critical clue in Sheldon's article on "
Upgrading Bicycle Gearing". There he says:
Shimano does make one front derailer, the R440 model, that is designed to work with upright bar shifters and a crankset with a 52-53 tooth big ring. They no longer make any front derailer that is compatible between drop-bar shifters and smaller chainrings.
(emphasis added)
If Shimano
no longer makes a front derailluer that is compatible between drop-bar shifters and smaller chainrings, that would imply that they once did, right?
As fate would have it, I was reading this particular entry because I was upgrading an '89 Rockhopper to modern 9-speed MTB components. I wanted to know in advance whether to expect the old MTB front derailleur to work with the new MTB shifters. It didn't.
So...I pulled the modern MTB front derailleur that had been giving me sub-par service on my 29er drop bar conversion bike and put it on the Rockhopper where, naturally, it performed perfectly. Then, just as I was about to put a Tiagra front derailleur on the 29er, the light bulb came on above my head.
Comparing the old M452 "Mountain LX" front derailleur to the Tiagra front derailleur and seeing that the arm where the cable attaches had a similar geometric relationship to the parallelogram of the derailleur, I decided to give it a try. On the stand, it seemed good. I tried it on the road today, and BINGO! It shifts perfectly.
Now, I realize that like the monk in my opening analogy I may discover that either I'm delusion or nobody cares or both, but I thought I would share, just in case this is useful information.