Recommend a rear derailleur, please. needed urgently.
On my way home from work today, my chain started binding. I backpedaled a bit but the problem didn't go away. i pull over and see that i've lost one of the cogs on my rear derailleur. I spent a good 15 minutes trying to hunt it down but didn't have any luck.
Suddenly I'm derailleur shopping.
I have a mountain bike with all shimano components that i use to commute to work. Its a 21 speed so i need a 7 (or 8?) speed derailleur. The previous derailleur was a Acera. The only 7 speed that nashbar has is $6 - I think I want something better.
Derailleurs don't have "speeds," the speeds are in the number of cogs on the cassette or freewheel and the number of clicks in the shifters. The Deore derailleur in the above link will work fine, and it's on sale for a very good price.
So all derailers move the chain exactly the same given the same cable pull?
If not, you won't be able to shift cleanly without friction shifters.
All index-compatible Shimano rear derailleurs use the same cable pull ratio, except pre-nine speed Dura Ace. The amount of cable that's actually pulled per click is in the shifters, not the derailleur. So if you've got a seven speed Shimano-compatible cassette and seven speed Shimano shifters, for example (and they're not Dura Ace), as long as you're using a Shimano derailleur (and it's not pre-nine speed Dura Ace), it will be compatible. This doesn't take into account the largest cog compatibility or chain wrap capacity of the derailleur, but those are separate issues. For the OP's needs, the "8/9 speed" Deore derailleur will work fine. The "8/9 speed" terminology in this case is pretty much marketing-