Join Date: Apr 2009
Location: New Rochelle, NY
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
Mentioned: 20 Post(s)
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Start by focusing your efforts in the right place. The RD has absolutely nothing to do with chainring issues when pedaling forward. The chain is coming off the top of the rear sprocket which totally insulates it from whatever the RD is doing at the bottom.
Disregard this post and clarify your problem of the chain falls off only when backpedaling.
Your problem most likely relates to FD issues. Start by checking the height, alignment and outer limit setting. Correct adjustment should keep the chain on.
From your description of this as a long term problem, I suspect that you may also have a chainline issue, with the chainrings lining up somewhat inboard of the center of your cassette creating greater angle when using your outboard sprockets.
You can check this simply enough, with a straightedge about 15-18" long. Measure from the outside of your outer chainring to the center of the space between the rings. This should be about 8mm or so. Now lay the edge of your straight edge against the outer ring on a secant, to get a line and extend it back to the cassette. It should line up the same 8mm or so distance to the outside of the space between the 5th and 6th sprockets. If it's inboard center by more than 2-3 millimeters it explains why your chain won't feed properly coming from the 11t sprocket.
BTW- when you're ready for your next chain, give it a careful eyeball exam. You want to find one with more than average bellmouth formed into the inner plates. Bellmouth on the inner plates acts like a funnel picking up sprocket teeth and guiding them into the chain as they engage.
An ounce of diagnosis is worth a pound of cure.
“Never argue with an idiot. He will only bring you down to his level and beat you with experience.”, George Carlin
“One accurate measurement is worth a thousand expert opinions”
- Adm Grace Murray Hopper - USN
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