Originally Posted by Camilo
Bringing this to the top to see if there was any solution. I have what sounds like the same problem, same components. The best I can desribe it is a very pronounced, loud and "coarse" vibration - the vibration seems to be identical to what the teeth on the rear sproket would make as the chain passes over it. It is not the frequency of the front chain wheel cycle, but seems to be the frequency of the rear sprocket, if that makes sense.
For me, I know it is NOT: chain noise on front derailleur cage ... also NOT the noise that is caused by rear derailleur cable adjustment. I know how to adjust rear derailleur and have fiddled with that - no change. Shifting up and down is very crisp and smooth.
It could be rear derailleur alignment. If so, would that explain why it only happens with large front chain wheel and rear smallest two or three sprockets? (problem starts with 3rd smallest, get worse as they get smaller - therefore worst on smallest).
Is there an "idiot's guide" to fixing rear derailleur alignment - I don't want to just start trying to bend anything!
The drive train is very smooth and quiet on all other "normal" (i.e. not cross chained) combinations of rear and front gears.
OK, I thought I'd report back. I've decided the issue has three aspects:
1- Chain: I lubed it up real well and that seemed to decrease the noise and roughness somewhat. Not night and day, but it seemed a little smoother.
2 -Rear Derailleur: It indeed was slightly bent out of alignment. Shifting up and down was actually OK, but I had a pro mechanic check it out and he straightened it and it is a little smoother/quiter from that as well.
3 -But based on my discussion with him, most of what I'm feeling and hearing is normal. It's just a coarse "whirring" sound made by the chain links on the teeth. And the only time it's felt is when it's free spinning on the repair stand, not when underway (e.g. "under load").
I think that I'm just not used to the friction-free, easy motion of this high quality drive train. It is Ultegra/ DuraAce vs. my early 90s Exage. It is just the chain spinning the big 52t wheel and tiny 11t cog with very little resistence that is the main difference. I can definitely feel that it's free-er spinning. I think my old bike's drive train in essence operated "under load" at all times because it just wasn't as free running - dampening the action of the chain over the teeth. If that makes sense at all?
Anyway, the mechanic, whom I respect, said the drive train (chain, wheels, cogs) are in great shape and it sounded and felt normal to him. I feel good because (1) I was pretty sure the rear der. was a little bit out of alignment (but was afraid to mess with it) and (2) I was pretty sure all the teeth looked just fine. After cleaning everything up, I'd have to say they look almost like new, and the chain measured well within the tolerance for wear.
Thanks for the pointers everyone. Living and learning - and loving the nice "new" (used) bike!