Led by the irrepressible pranksters at Shimano, proprietary shifting systems infected the bike industry starting in the 1980s and reached pandemic proportions in the 1990s after the introduction of the first Rapidfire shifters. That innovation has been responsible for more misery than anything, including the hard, narrow racing saddle.
Bike componentry has always invited some compatibility issues, with things like French thread, Italian bottom bracket shells, crank spindle lengths and whatnot. But the C word, Compatibility, took on a larger, more ominous meaning once shifters had to be perfectly coordinated with derailleurs and with the number of cogs in the whole system. The more gears you cram into the same space, the more precise everything needs to be.
To make matters worse, the Big S can't even keep its own products consistent from year to year.
A customer came in two days ago because the front derailleur on the Long Haul Trucker I'd built for him had ceased to function. It's a Tiagra 9-speed over a Deore LX two-piece crank. It had worked perfectly until the tiny tab that retains the return spring snapped off, sticking him in the large chain ring.
He stopped in on his way home from work. I had a Tiagra front derailleur in stock, so I told him I could slap it on in about ten minutes. It's true. I could. But Shi-no has changed the derailleur so it no longer swings out far enough to clear the chain in the highest gear.
An LX derailleur failed to work because Shi---!@#@!# road shifters aren't compatible with their own mountain derailleurs.
A two-piece crank can't be tweaked inward, only slightly outward. That won't help me here.
The plug-and-play turned into more than an hour of slogging before I sent him on his way with the chain stuck on one front chainring while I compile some parts to see if I can get this mess to work.
Shift in friction unless you're racing. Barcons are plenty convenient and won't give you ANY of these headaches.
SKRIT--!! UPDATE: 10:41 EDT
Looking at the bike I see two options. I was going to fashion eccentric shims to space the derailleur over, but I notice the BB has spacers I can stack in different (unsanctioned) configurations. Let's hope that doesn't spin up some worse problem. At least it's a place to start.
UPDATE II: 11:46 EDT -- Lesson 1: Look carefully at the complete assembly and all related structures. Lesson 2: Throw away the instructions.
Moving the shims seems to have worked. I've sent a test pilot who fits the bike better than I do and who is interested in purchasing one. He has not reported yet.
FINAL WORD: Test pilot reported full front derailleur function. Customer required one minor tweak. I should have caught the shim thing the first day, but I had put myself in a time crunch by offering to knock out what I thought would be a quick, simple fix. Those are the ones that always go out of control.