Hi,

I changed the chainrings on our tandem FSA Gossamer Triple crankset and went from a 52/42/30 to 52/39/30. Using the adjustment barrel I was able to adjust the front derailleur to shift up from the small to middle chain rings but unless the cranks are spinning quickly and freely I can't reliably shift down to the small ring. I adjusted the lower limit to allow the derailleur to move way in, even to the point that once on the small ring it will rub on the outside of the derailleur cage in the largest cog, but it still won't shift into the small ring when there's moderate pressure on the chain. The shifters are Dura-Ace 9 speed and the derailleur I think is Shimano XTR.

Any suggestions?

Doug

XTR, being for mountain bikes was not anticipating those larger chainrings..

By running friction shift levers, I get to re center the cage around the chain.

I run older FD with narrower cages , from the 80's before compact triples became common for MTB's.

now it seems STI goes ka thunk once for each chainring, then relies on cage shape,
to clear the chain in not crosschain combinations , for the crankset it was to be paired with..

you might be better off with a road triple FD for that crankset chainring combination..

perhaps going to the 50's tandem setup, the final crossover drive crankset
Up front, on the captain's BB,
so the gear changes on the back are making a lesser angle change..

+ you can see if the FD is dragging, looking down, and make the fine tuning adjustments,
assuming the feature, is available.

Modern FD cages have highly shaped and stepped side plates based on assumptions of the chainrings they're being used with. Sometimes changing the rings is detrimental if they no longer matched the shapes of the cage.

But I suspect, that your problem may be that your 39t ring was made as an inner of a double. That means that it won't have any shift gates (cut down teeth) to start the derailing process. If that's the case, it's now a question of your confidence in your own skills with a file. Remove the ring, and compare it to the 42t you removed. Start by lining up the timing mark (if any), then use a file to make shift gates using the other as a model. This isn't that complicated, but for your first effort, go at it by degrees, mounting and testing as you go along. One place to give careful attention to the first tooth after the gate. This must shaped with the point somewhat inboard (toward the bike) compared to the rest, so it can pick up a chain that's coming from an inside angle.

Good luck.

Are you sure it's an XTR front derailleur? Fietsbob is correct that MTB front derailleurs are not designed for large chainrings but since yours worked with a 52T ring in the past I'm doubtful it's an MTB fd.

FBinNY is almost certainly right that your new 39T chainring was intended as the inner ring of a double, not the middle of a triple. So, if you are willing to "customize" it as he recommends, that should reduce or eliminate the problem. Shimano does make a triple crank that uses a 39T middle ring, the FC-5603 105 triple, and, if you can locate a replacement 39T chainring (part # Y-1GF 98020) it should work without any other modification. However, this is a 10-speed chainring and may cause chain rubbing if used with a 9-speed chain. Shimano's 9-speed 105 triple cranks (FC-5504) only came with a 42T middle ring.

Thanks for the advice. I think you're right, both that the front derailleur is probably not MTB (I don't have the bike with me but will double check this weekend), and the 39 is probably designed for a double. Fortunately I kept the 42 so I'll compare and file accordingly.

Cheers,
Doug

Based post #1 the reason you are having trouble shifting from the middle chainring to the smallest chainring is excess cable tension. Try turning the barrel adjuster clock wise about a 1/2 turn to let some cable out. Adjusting the limit screw was likely the wrong thing to do. The ramps and pins built into a triple middle ring are to assist the chain in climbing onto the middle ring when shifting up from the smallest ring.
Most 9-speed D-A shifters are for double cranks, 7700, but some were built for triples, 7703. D-A triple front derailleurs were designed to work with 53-39-30 chainrings. But we don't know which FD you have.

Made some adjustments this weekend but still having trouble. First, the front derailleur is a Shimano Durace Triple (couldn't find the P/N). I believe the middle chainring is designed for a triple, the side edge of some of the teeth are filed down to facilitate shifting. The chainring is labeled 39/30, is this an indication that it's designed to work with a 30T chainring?

After talking with a mechanic at a LBS I tried bending the cage a bit. I also screwed back in the limit screw to the point where the it does not rub in the first three cogs. Lastly I turned the barrel adjuster about 1/4 to 1/2 turn. All this may have resulted in a little better shifting (no problem while on the stand) but still won't shift when there's medium pressure on the chain, i.e., when climbing. I'll continue to adjust the barrel screw and maybe try tweaking the cage a little more.

Doug

Yes, you do have a 39t triple ring, so that's not the problem. Possibly there isn't a real problem except your habits.

FDs are different tan RDs in that they operate on the drive side of the chain. That means that they're very sensitive to the chain tension resulting from pedal pressure. As you start to shift, there are two forces in play where the chain engages. The tension is pulling it down onto the teeth, and keeping the chain straight. The FD return spring is trying to flex the chain sideways far enough that a plate will snag the tip of a tooth and cause the chain to climb up to where it can jump off.

FD springs have limited force, so if you tense the chain with pedal pressure, the FD cannot force it sideways and start the process. You have to help by relaxing momentarily until the shift begins, or better yet, until the chain is engaged on the smaller sprocket.

There will be differences in how chains shift from various chainrings based on chain line, since the angle the chain is coming from factors into how much the FD must deflect it to start a shift.

Try anticipating your needs and shifting earlier in climbs while you still have good cadence at fairly low pedal pressure. Otherwise you'll need to take your chances and back off pedal pressure and hope you still have enough momentum to keep rolling through the shift.

Another thing to check is the outer cage pivot angle. FDs vary in effectiveness based on there the FD cage contacts the chain to derail it to a different chainring. When the rings were 52-42-30, the difference allowed the FD cage to probably contact the chain on the middle ring just barely and push it inside onto the small ring when down-shifting. But the 39t middle ring is now a tiny bit lower and so the cage isn't exerting as much inward push on the chain to derail it. You may need to carefully check the FD. If there is allowable clearance, lower it half to 1 mm down so it still clears the big ring, and if possible, loosen the clamp (if it's not a braze on) and pivot the FD clockwise a tiny bit when looking down the seat tube toward the BB. This should cause the cage to contact the chain just a bit sooner and with more force possibly to derail it properly. Test the up-shift to make sure that this doesn't harm the performance going the other way.