Thursday, November 10, 2011

Pivot Rivets and Marvin Bolts

The repair tag said, "Bike fell over on drive side. Trouble shifting to big ring." The bike was stopped, so it wasn't a serious slamming, sliding crash. There were no major or even minor impact marks.

The front derailleur cable felt a little slack. I thought maybe the cable had simply gone out of adjustment and the bike's fall had been a coincidence. Snugging the cable did not fix the shifting, but it did change the problem enough for me to notice that the front derailleur was not swinging in a smooth arc.
This picture was taken after I noticed that the rivet on which the derailleur cage pivots had backed out about three millimeters. When I tapped it back in, the derailleur then functioned properly. I have no idea how a minor topple could have caused this problem, but there it is: something else to check when normal adjustments aren't bringing the desired results. The rivet could have been backing out already and the minor fall either helped it the rest of the way or had nothing to do with it.
Shown at the center of this shot, the pivot rivet. Derailleur is Shimano 105.

Some older front derailleurs were held together with bolts that can loosen. Check your derailleur and snug these bolts every so often. The term Marvin bolts comes from the particular repair on which I failed to remember them soon enough.

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