I am struggling to diagnose the cause of an intermittent side to side wobble of the cogs on a rear wheel. Here's what I know:
- Periodically (maybe every 10-15 minutes) during a ride, I experience slippage between gears not associated with shifting. A glance at the rear cassette while continuing to pedal reveals the cause: the rear cogs are all wobbling side to side, a few mm each way.
- The wobble is in sync with the rotation of the cogs: slow pedaling = slow wobble, fast pedaling = fast wobble, no pedaling = no wobble.
- If I stop pedaling, coast, and then resume pedaling, the wobble goes away. I can't say for certain whether the inverse is true: that the wobble always starts after coasting, by the time I notice it, I can't be sure exactly when it started. Nor can I say for certain that it goes away every time after I coast--I just know that by coasting (even briefly) and then pedaling, I can eliminate the wobble (for a while).
- With the wheel off the bike, I have not been able to reproduce the wobble. There does not appear to be any plan in the hub or the cassette. I have checked that the cassette is securely tightened to the freehub and it is. There is not noticeable play in either the freehub or the cassette.
- With the wheel on the bike and the bike in a stand, I haven't been able to reproduce the wobble either. I have tried moving the pedals slowly while watching the rear cassette, stopping the pedals, and re-starting and ran out of patience before seeing the wobble.
- I do not believe the whole wheel is wobbling. This is tough to know for sure since I have only witnessed the behavior on the bike, but to my distracted on-the-bike eye, the wheel is rim is rolling true while the cogs wobble.
- The hub is a Velocity Road Rear Hub, the cassette is a SRAM PG-970 9 Speed
What are the most likely causes of this behavior? What additional diagnostic or troubleshooting steps would you recommend?