Join Date: May 2004
Location: Wilmington, DE
Bikes: 1984 Trek 660, 2003 Specialized Hardrock, 2004 LOOK KG386i, 2005 Iron Horse Warrior Expert, 2009 Pedal Force CX1, 2014 Islabikes CNOC 14 (son's)
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Most likely, you have too much tension on the chain while you are trying to shift. Unlike the rear derailler, the front derailler works on the tensioned side of the chain. In order to derail properly, the chain needs to have some slack in it, enough so that the derailler's spring can deflect the chain enough to cause it to come off the chainring.
If you are having problems getting into your small ring, try shifting into it before you actually need it, i.e. before you are standing on the pedals and barely moving. Shifting habits vary as much as chain lube opinions but if I know I'm going to be needing my granny ring, I'll wait until about in the middle of my cassette, downshift into the granny and then upshift a cog or two on the rear. I've kept my gear ratio about the same but since I'm in the smaller chainring, I've got plenty of lower gears to use. By shifting while I still have a few more cogs in the rear to use, I can always downshift a few more times in the rear if I for some reason start to lose speed and can't get my shift completed.
The other possibility is that your low limit screw is set too far in. This could also make downshifting into the granny ring difficult unless the chain was extremely slack. If my first bit of advice does not work, trying turning the low limit screw out a 1/4 to 1/2 turn and see if that helps. Be careful though because if you go too far, you'll start dropping the chain on downshifts.