There are few things more frustrating in life than taking a once stellar road racing bike, trying to update it with modern parts, only to find that one stupid part that isn’t made anymore gums up an otherwise perfect project. Enter the humble, bumble front derailleur mount…
If you’ve got a Trek 5200 (or a 5500 or a bunch of other brands, too, this isn’t just a Trek problem) and you’ve tried to switch from a 52 or 53 tooth chainring to a 50 tooth, you may have run into trouble in paradise: you can’t find a front derailleur mount that’ll allow the derailleur bolt to slide down far enough to hit the optimum height for the derailleur cage above the big ring teeth (about 2 millimeters). If you look at mine, that’s about 5 mm. That was an initial setup, though, I’m down to about 4 mm currently and that’s as low as I can go without taking a file to the derailleur mount hole.
My 5200 is a 1999 5200 T. “T” for triple. I swapped out the 52/42/30 triple for a 50/34 double drivetrain. I had some trouble with the chain dropping into the bottom bracket until recently, and that’s likely why you’ve landed here – because they don’t make a front derailleur hanger bracket for this. Don’t fret – it’s almost fixable without messing with the derailleur mount (though taking a file the hole is an option if you’re CAREFUL). First things first, lower the derailleur all the way down on the mount. Once you’re as low as you can go, line the cage up so it’s running on exactly the same line as the big chainring. Next, you’re going to have to get the front derailleur dialed in perfectly with the set screws.
I’m running a 2013 10-speed Shimano 105 system on my 1999 Trek and it works fine, after some tinkering, but there’s a trick…
Now, before we get into this, we all know you never touch the set screws. Because it’s never the set screws unless you change cranks or the derailleur (I did both, and even though I knew it was going to come down the the set screws, you MUST investigate everything else first – because it’s never the set screws). I started with the barrel adjuster first. Nope, still dropped into the bottom bracket – almost, but in the biggest cog, and 50% of the time in the second (on the cassette), it’d drop. Then the derailleur cage. Alignment was good and I was at the lowest setting on the bolt-on bracket. Now we move on to the set screws – and certain death. Once you mess with the set screws, unless you know EXACTLY what you’re doing, you’re… um… screwed…
I held my breath…
Don’t worry, I knew exactly what I was doing. You want the chain to be in the baby ring up front and the big cog in the back and set your derailleur cage with the low set screw so the cage is less than 1 mm from the chain. Normal suggestion is 1 mm… you’re going to take it till the chain rubs the derailleur cage and back it off the chain ever so slightly.
These are directly from Shimano’s 105, Ultegra and Dura Ace front derailleur manual (for my 10 speed setups):
On that second diagram, we’re going to overlook the fact we have 4-5 mm gap between the teeth and derailleur cage and just make sure we’ve got the inside (closest to the bike) of the cage just barely off the chain so it doesn’t rub when the pedals go ’round. Also, you’ll want to make sure your chain is within tolerance and your chainrings and cassette aren’t worn out, all of which contribute to poor shifting.
Here’s the trick, though: This isn’t perfect. You can’t shift to the little ring from the big ring from the biggest (easiest) cog on the back. You can’t do it. The chain will drop. EVERY time. Even the second cog is a little risky – but will work just fine if you shift properly, taking a little pressure off the pedals when shifting. The third cog or any cog thereafter? Perfection.
Now, some will say if it can’t operate perfectly, it ain’t right, so something else must be done. I respect that. I’m just not going to live by it. Shifting to the little ring from all but the biggest cog in the back is just fine with me. I only use that gear twice a year anyway – and if I can’t anticipate a gear change better than that, I’ve got bigger problems than dropping the chain into the bottom bracket.
Fit Recovery 