Front derailleur over and under shifting

[weaponsgrade]

I'm having some trouble adjusting the front derailleur on my mtn bike. It's a random mix of parts so maybe that's it. I can't seem to get the derailleur adjusted so that it shifts from the middle ring onto the outer ring without overshifting. With the chain on the outer ring, I adjust the high limit screw so that the outer FD cage is close, but not touching the chain. But, then I'm not able to shift from the middle ring to the outer ring. I then back out the high limit screw, but then the chain over shifts the outer ring. I can't seem to get the high limit screw to that sweet spot where it shifts from the middle to the outer without overshifting.

Any ideas? I've tried adjusting the derailleur height and setting it at an angle, but it's not working.

Like I said, this has a random mix of parts. The FD is an old XT, cranks are old square taper XTs (the chunky looking type), the bottom bracket is probably a bit short because I've got the lower limit screw backed all the way out so I can shift onto the inner ring - and I've also had to set the FD at a slight angle with the back of the cage angled in so that I can shift on to the inner ring. The middle ring is a 34T from my old Campy Centaur cranks. The outer ring is a 46T Shimano that I think is original to the crank arms.

[oliver1850]

Using indexed or friction front shifter?

[weaponsgrade]

Friction - an old XT thumbshifter to be exact.

[oliver1850]

That ruins my cable tension theory. Sounds like it's not lifting the chain with the amount of FD travel that should make it shift. I would try straightening it out so it's parallel to the big ring, and see if it will shift without losing the chain. Maybe your FD is just not compatible with a ring that big. M735 was the last FD before the compact cranks came out. M737 or newer was for 22-32-42 or 22-32-44 cranks.

[Ken Robb]

I know that rings have ramps and pins to "encourage" the chain to shift with a certain amount of derailer movement and presumably they might be interdependent for shift quality. Could the odd middle ring be the problem?

[weaponsgrade]

That ruins my cable tension theory. Sounds like it's not lifting the chain with the amount of FD travel that should make it shift. I would try straightening it out so it's parallel to the big ring, and see if it will shift without losing the chain. Maybe your FD is just not compatible with a ring that big. M735 was the last FD before the compact cranks came out. M737 or newer was for 22-32-42 or 22-32-44 cranks.

Good call oliver. The FD I was using was an M737. I dug up an M735 FD and I'm now able to shift through all three rings. The one slight area of concern is the bottom bracket spindle is still a bit short. It's a Phil BB which allows some adjustability via the mounting rings. The drive-side mounting ring sticks out maybe 6-7 millimeters and Phil recommends no more than 5 millimeters sticking out. I'm going to keep an eye on it and hopefully it doesn't loosen up.