BIKE MECHANIC
Join Date: Jun 2005
Location: Dayton, Ohio
Posts: 185
Bikes: 1975 Full Campy N.R. Centurian Super Lemans,1984 Focus Vintage pre susp. mountain, hardtail,suntour xc sport, many treks, diamondbacks, and, 1950' crusier J.C.Higgins,triex (road) and kakakura silk (road)
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Sounds like a slightly out of true chain ring under tension, not on bike stand but under full force, the crank is flexing a little bit causing the chain to literally be shifted off to the bb side.Wow!
I would start by looking at the length of the bottom bracket spindal, is it too long?, then i would check the tightness of the crank arm bolts, and rotate the crank to check for true. Adjust your inner limit screw on the front derailler a little on the tight side as a stop or preventative measure, to prevent chain from coming off, and lastly, maybe the chain is too stretched or worn out , replacement might be necessary. Good luck!