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Technical FAQ: ‘Auto-shifting’ with vintage parts

Dear Lennard,
I have a mid 80’s Columbus steel frame made by Dave Moulton with a Campy Nuovo Record group. I have had problems with an “auto shift” every once in awhile – usually during a climb the chain will jump from, say, first to second gear. Will worn derailleur pulleys cause this or is my 1983 derailleur weakening?
-Dan

Dear Dan,
I would guess that one of three things, or a combination of them, is happening.

One cause could be flex in your chainstays causing the bike to autoshift as the chainline changes by virtue of the dropouts moving laterally inboard out of the plane of the front triangle. This will cause the chain to drop to a smaller cog.

Another cause with a derailleur like this is that the frictional shift lever is slipping. Tighten the lever screw, or take it apart, clean up the parts, and put it back together, cinching it up sufficiently to keep the lever from slipping back in response to the cable pull.

Another could be worn pivots in the rear derailleur making it sloppy and loose and unable to control the chain closely. Worn pulleys could play a role as well, yes.
-Lennard