Can you use a Shiman RD 6400 with an 8 speed cassettte? Specs say it is designed for a 7 speed. Since I never use my granny gear can I set it for the high 7 and call it a day?

Also, I keep throwing my chain off the derailleur cog? Is it bend? Anyone have a remedy?

Chain bounces. Off here.


Bent? Fixable?

Yes, all Shimano 7, 8, and 9 speed road and MTB rear derailleurs (RDs) (with exception of early Dura Ace) are interchangable as far as indexed shifting is concerned. But you need to verify that the RD can handle the largeest cog and has the total capacity for the cassette you are using. For your change, does the 8 speed cassette have a larger big cog than the 7 speed, If so, you may need to adjust the B-screw and add links to your chain.

As to why your RD keeps throwing a chain, can't tell from the photos if the RD is bent. The 2nd photo makes it look like the chain is coming off the tension pulley, but if the photo was from a slightly higher perspective, where you could see the chainrings, then it would be easier. I would think that if the chain is coming off the tension pulley, then it would be very visible that the RD is bent. Do you have a bad chain link?