I have an older Trek 800 mountain bike that I use as a general purpose utility bike.
I upgraded the drivetrain last year so I could have some lower gears while I tow things (mostly kids in trailers or bike seat) around the hills of New Hampshire.
It has a Bontrager mountain bike crankset (22-32-44) and a Shimano "Mega Range" 6 speed freewheel (13, 15, 18, 21, 24, 34).
I am having occasional problems when I am trying to shift from the middle front chain ring to the little chain ring. The problems usually occur when the drivetrain is under some heavier load (for example, if I'm climbing up a hill). Occasionally, one of two things happens,
1. The chain does not shift from the middle ring to the inner ring. The front derailleur just rubs against the chain but there is not enough force exherted by the derailleur to move the chain (which is under load) off of the middle ring and onto the little ring.
2. At other times, the front derailleur will move the chain off of the middle ring, and it will overshoot and the chain misses the inner ring and falls off of the chain ring completely (falls inside of the little ring and against the frame).
I've tried to adjust the high and low limits for the front derailleur. I have it so the chain just clears the derailleur in the front at the low and high gear combos. I've also tried adjusting the front derailleru cable tension. It seems to index well, but I still have problems when I shift under load from the middle to inner chain ring.
Any ideas on what I should try next?