my rear gears wobble
 

I put my Trek 820 on a stand today. When I turned the pedals and got the rear wheel spinning I noticed that the rear gears wobble as a unit. This would be in coast mode - that is, the wheel is spinning but the gears are not. They wobble at a much slower rate than the wheel spins. In fact, the wobble rate is very slow, while I had the wheel spinning at a pretty good rate.
The bike is about a year old and is in good shape. I don't imagine this is anything out of the usual. Just wondering if anyone else has noticed anything like this.

(channaling sydney) NORMAL.

Have noticed, and it is normal, nothing to worry about

Thanks

Assuming the OP refers to the teeths, then yes, that's perfectly normal. It's part of the derailleur's shifting mechanism. However if the body of the cogs that's wobbling, then that's bad.

I had the same problem a few months ago. I took my bike to several places, and they all said the cassette wobble was due to the crappy hubs on my crappy wheels. I was riding stock Ritchies that had maybe 1200 miles total on them. I broke down and bought new Mavics shortly after and there is absolutely no wobble now; the cassette stays perfectly still when the rear wheel is spinning

Even though I didn't notice any symptoms from the wobble, there must have been some cost, however minor, in terms of pedal power and efficiency, since the chain was not running through the cogs as smoothly as it does when there is no wobble.

It's pretty common. When the wheel is spinning but the cogs are not, it means that the the freehub is rotating backwards (hence that clicking sound you hear). If the freehub does not rotate perfectly on the axis of the hub, then the gears will wobble.

It's not ideal, though neither is it a big problem. If the freehub isn't rotating perfectly on the axis of the hub, it means that the cogs don't stay perfectly in the same plane. Now, most of the cog motion is up-and-down (whereas side-to-side motion would affect the quality of the indexed shifting) so it's not much of a worry.

There are two main reasons that I can think of for the freehub not rotating perfectly on the axis of the hub.
1) freehub mechanism has slightly mis-aligned bearing because of manufacturing process - nothing you can do, probably not a problem anyway
2) the freehub unit itself isn't seated perfectly in a plane with respect to the hub
2a) this could be caused by the manufacture of the hub shell itself
2b) or it could be that the freehub needs to be loosened from the hub, re-connected and tightened down in proper alignment.

I noticed this same thing happening on a wheel that I'd recently rebuilt with an old hub last year. I disassembled the hub and removed the freehub body from the hub shell (using a 10mm allen wrench), cleaned the contact area and then tightened the freehub body back down onto the hub. Afterwards, alignment was perfect, and the cogs didn't wobble at all when the wheel was turning but the cogs weren't. I don't know why the freehub body wasn't quite perfectly aligned, but this took care of it.

Actually, it will be a bit in both directions: good and bad. For half the rotation, the cog will be approaching an improved chainline, and for half, it will be approaching a worse chainline -- unless the cog would normally offer a perfect chainline, but there's only one of those per chainring -- the other 9 cogs (or 8 or 7...) are out of the chainline.

Either way, it's really minor, and really common. Your Mavics will probably exhibit this behavior at some point too as the freehub bearings wear and deform.

How can the wobble be occurring at a slower rate than the wheel spin rate?