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Old 07-17-19, 11:03 PM
  #23  
scarlson 
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Bikes: Ron Cooper touring, 1959 Jack Taylor 650b ladyback touring tandem, Vitus 979, Joe Bell painted Claud Butler Dalesman, Colin Laing curved tube tandem, heavily-Dilberted 1982 Trek 6xx, René Herse tandem

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I can't believe nobody's gone and referred to St. Sheldon's article on this.

Originally Posted by Sheldon Brown
The usual cause of the problem, though, believe it or not, is the cable guide that the derailer or hub shifter cable uses to get around the bottom bracket. As you pedal the bike, the frame flexes from side to side. This causes the gear cable to get tighter and looser with every other pedal stroke.

If the bottom-bracket cable guide has too much friction, it can act as a one-way clutch, pulling the cable down from the lever, but not allowing it to retract on the opposite pedal stroke. In many cases, greasing the cable guide is all that is required.

In one particularly bad case, that of a large, strong racer with an old steel bike, I had to use more heroic measures. I installed a Sturmey-Archer pulley that clamped onto the bottom of the seat tube in place of the original cable guide. This eliminated the problem.
I couldn't have said it better myself. This does explain above posts. Plastic cable guides help because they lower friction. Same with having a nice smooth rust-free cable. Tightening the shifter also helps because if the shifter is tighter than the grip the cable has on the bottom bracket guide, it won't move the shifter. Putting a fixed run of cable there is like completely locking the shifter in place. The cable gets tighter and moves the derailleur, then it slackens up and gets pulled back by the spring in the derailleur. No net movement in either direction.

So grease your cable, check for rust (even that powdery corrosion that occurs on galvanized cables), and go buy a Sturmey Archer pulley if you still can't make it stop.

My Vitus 979 is really bad. Nuovo Record levers and an alu cable guide under the bottom bracket, with an old Campagnolo cable. Sometimes it works to my advantage. Sometimes not. I keep it like that for a good laugh.
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