Cassette lockring came loose
 

My cassette lockring came loose. (And the chain overshifted, got caught between the low gear and the spokes, and torn up. On the good side, the derailleur doesn't look bent and the spokes look OK -- at least on first glance.)

What are the most likely causes of the lockring on my commuter's Shimano 9-speed cassette coming loose? I'd put it down simply to a failure to properly tighten the lockring, but this is the second time. (The first time was after a couple thousand miles after the bike was built by the shop and the second about 500 miles later.)

I've also had this problem a couple of times with the same cassette on different wheels. It didn't cause any problems except for making extra noise when shifting and sloppy shifting because the cogs were able to move around a bit. Basically, the "lockring" doesn't seem to be fulfilling it's 'lock' function. I've now tightened the lockring beyond the recommended torque, and hopefully this should help. However, if anyone has other advice then I'd be eager to hear it.

The recommended torque for Shimano lockrings is 40 Nm which is a LOT tighter than most riders would tighten them without a torque wrench or prior experience.

If this continues to happen, I would also check the threads on the freehub body and lockring. I've thrown out a couple of lock rings that had rounded threads.

Also, be sure all the threads are perfectly clean and oiled. Clean lubricated threads give much more accurate torque.

Is there a thin flat paper-thin washer between the lockring and the 1st cassette cog? For the cogsets that come with type of lockring, you DO NOT want to remove that washer. It allows the lockring to slide over the serrated surface of the cog and not dig in to get the proper torque. Without it, most people stop too early before developing enough tension on the lockring to keep it in place.

If you have a MAVIC hub it may require a 2 mm spacer behind the cassette. If missing the lockring will not tighten on the cassette properly.

Thanks
 

Thanks for your responses, everyone.

No washer or spacer in the cassette, so it seems that the most likely cause is a failure to adequately tighten the lockring. I cleaned, inspected and greased the threads and tightened the lockring with a torque wrench, so hopefully I won't have this problem again.

I'm also going to try to remember to inspect the lockring whenever the wheel is off the bike and whenever I notice a missed shift. (A missed shift a mile before I overshifted off of the cassette was the only sign that the lockring had come loose.)