tandemnh

On our tandem when in the front gear 54 when I shift from 9 to 8 or 8 to 7, the bike shifts one extra gear and I have to shift back up to get to the gear I wanted to be in. Any advice to stop the double shirt?

Donegal

Either the timing of the rear derailleur is off by a click or two or the original timing setup was off. I had a Campy 10-speed that I couldn't get consistent in 5th-6th gears. Why would it only be off on the middle gears? We re-timed it starting from the outside sprocket with no slack and pulled the other gears in. Viola, perfection.

It's possible the cable is slightly tight, causing overshifting. It's also possible that the derailleur or derailleur hanger are slightly bent.

Al1943

I don't know much about tandems but this sounds like you need a cable tension adjustment or the rear derailleur hanger may be bent. When you shift from 9 to 8 does this mean you are shifting to a larger cassette (or freewheel) cog? If so you may have too much cable tension.

Al

bikinfool

Check conditions of your cable/housing, too.

DannoXYZ 

Excess cable-friction will do this too. The spring in the derailleur's not strong enough to overcome the friction until you do two clicks. Then it goes all at once and shift two gears. Then you have to shift back up one.