Chain rides off the tension pulley while riding

[aytchkay]

I have a SRAM Red 10 sp with a 28 tooth large cog. The chain is new; the cassette is about 2 years old. I change my chain each year regardless of wear.
Lately, while pedaling on the trainer, I noticed that the chain moves off the tension pulley and rides on it edge and the side of the cage. Sometimes it corrects itself and sometimes not. It seems to happen in any gear combo.

I don't want a worn carbon cage from this. Any ideas why this is happening?

 

[oldbobcat]

Most likely it's a bent derailleur hanger. If you don't have the tool for checking and straightening it, your local shop can do this for $10-20.

Other candidates are bent chain, stiff link, bent derailleur, or bad chainline due to improperly serviced crank or bottom bracket.

 

[aytchkay]

Sounds like a good list of possiblities..thanks for the reply!
I'll check em out at my LBS...

 

[CAMPYBOB]

Quote by OBC:
"Most likely it's a bent derailleur hanger."

This.

"stiff link"

Or that.

Possibly a bent rear derailleur mount bolt. Possibly a bent cage pivot bolt or twisted cage. Most likely it is the bent derailleur hanger. Your Mk II calibrated eyeball should be able to see an angled-from-vertical cage that's out of whack enough to cause the chain to run off the pulley.

If it looks good, move on to back-pedalling the chain slowly to see if you have a tight link in that new chain.