Originally Posted by
robabeatle
Update: still not fixed!
Something is definitely wrong as my road bike (Ultegra) does not exhibit this behavior. I have backed out the pulley screws (previously quick cleaned them) to make sure they weren't overtightened to no avail. The spring seems to operate correctly as far as I can tell. Im starting to wonder about the derailleur though. I double checked the big big length method of the chain and so what else could it be?
I am starting to get like another poster on BF: simply swapping chains should be a 10 min job, which for me takes well now hours/days and still not resolved! To think, I used to rebuild engines but now I simply don't have the patience/time.
The bike needs a new mechanic, not new parts.
You're failing diagnostics 101 and listening to too many people. Chainring auto-shifting which is what you originally described is NEVER caused by the RD (except sometimes when backpedaling) because the upper loop runs directly from the top of the rear sprocket to the chainring.
So, go back to my my first post (no.2) and review, ft derailleur adjustment, or look for a loose BB, or a damaged tooth on the chainring for starters.
One thing you might look at is how the chain meshes with the teeth. Look down from the top through the FD cage and the chain to see if the tips of the about to engage teeth are coming up into the center of the plate gap, or touching the sides of the inner plates. Think of a ferryboat coming smoothly into the slip vs. coming in at an angle and bumping down the sides of the slip, and think of the sprocket's teeth the same way.
Another possible factor, is chain lube. Well oiled chains will slide smoothly down the sides of the teeth even when there's some rubbing, but poorer lube can make a chain not engage smoothly, and once it begins to ride up on the teeth it's derail.
So my advice is simple, look for the problem where it is (at the chainrings) and stop fretting about unrelated stuff at the RD.