Low car diet
Join Date: Aug 2007
Location: Corvallis, OR, USA
Bikes: 2006 Windsor Dover w/105, 2007 GT Avalanche w/XT, 1995 Trek 820 setup for touring, 201? Yeah single-speed folder, 199? Huffy tandem.
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Maybe you mean the seat tube is 33 mm? That's where the FD attaches. Front derailleur bands come in 28.6 mm (1-1/8"), 31.8 mm (1-1/4") and 34.9 mm (1-3/8"). 33 mm is right between the two larger sizes. It might be printed on the inside of the band on the old FD (or at least measure the band; it's more accurate).
Not really necessary to know right now, but rather than "distance from the outerface of the tube to the outerface of the largest ring", chainline is a more common measurement (center of seat tube to middle ring on a triple).
Not sure what "chainset being 17.5mm over the 3 rings" is referring to.
I think 7 and 8 speed chains are the same width, so any 7 or 8 speed Shimano MTB front derailleur will work (8-speed will give you better options). Be sure to match whether it is top pull or bottom pull (refers to whether the cable comes from the top or bottom of the seat tube).
Also be aware that using a 34.9 mm clamp FD, may require a slightly longer bottom bracket. My crank called for either a 110 or 113 square-taper BB. Tried the 110 mm for tighter chainline, but my 34.9 mm FD (w/shim for 31.8 ST), couldn't get to the inner ring.
I don't know if they make front derailleurs specifically for a 38T max ring, so you may have to get one for 42T or 44T max ring. (Common MTB chainrings are 22/24, 32, 42/44)
Generally, you should set up the FD so that it clears the teeth of the large ring by a mm or 2, but if your large ring is smaller than its rating, you may have to set it up a little higher, making room as if your large ring was that big. My new FD called for a 44T large ring, when I set it up with my 42T ring, the inside of the cage actually hit my inner ring when I tried shifting into the large ring.
I vaguely remember your old thread (or one similar), perhaps your problem is similar to mine (FD too low on a smaller outer ring? Like the FD is meant for a 42T.).
Last edited by JiveTurkey; 12-04-07 at 01:13 AM.