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Old 09-18-11, 01:26 PM   #1
pstock
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rear derailleur jumps right on backpedal. what does it mean?

was looking down and back this weekend on a ride an noticed that my 8-speed Campy Record rear jumps a bit to the right (and forward) when I backpedal.

(My wife's similar setup stays steady on the same maneuver.)

I have some suspect that something down there is out of true. Derailleur hanger, chain line, derailleur?
anyone familiar with this glitch and can save me some time in diagnosis?
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Old 09-18-11, 02:11 PM   #2
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cross chaining or dont back pedal
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Old 09-18-11, 02:18 PM   #3
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thanks
I should have clarified that this was observed on the small front ring and about the middle of the rear cassette, so I do not think it was from crosschaining.
and Yes, if I didn't backpedal I would not notice this. but I still suspect something is out of line and it may may an affect (component wear? poor shifting?) that I can't see but would still like to avoid.
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Old 09-18-11, 02:20 PM   #4
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It could be that the chain is first moving toward center since it's unguided when back pedaling, then is no longer on the sprocker under which the RD is positioned. This is very common at either end of the cassette.

The other possibility is that the RD is riding a bit high and there's not enough clearance between the pulley and sprocket.

This is easy to check. Shift to the problem gear, and look at the pulley, chain and sprocket. There should be a gap between the pulley and sprocket with free chain between them. (compare to the other bike). If you suspect that's the problem, tighter the "B" screw at the back of the upper RD body to push it down and away from the cassette, and establish the necessary clearance.
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Old 09-18-11, 08:08 PM   #5
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tight link?
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Old 09-18-11, 08:26 PM   #6
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I suppose you could try adding a little tension to the rear shift cable (counter-clockwise on the barrel adjuster).
I also wondered about a tight link in the chain.
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Old 09-19-11, 05:33 AM   #7
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If the system works OK otherwise and FB's B-screw adjustment checks out I'd leave well enough alone. Or just don't backpedal.
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Old 09-19-11, 07:41 AM   #8
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and the winner is....

B-Screw! (mostly) I always wondered what that was for. Now I know.
Indeed the pulley was up against the sprocket.
A B adjustment plus an adjustment of the cable tension has mostly cleared it up.

Many thanks again Bike Forums
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