Compatibility issue with derailleur and shifter
 

Do I need a SRAM rear derailleur with a SRAM twist grip shifter? My rear shimano derailleur moves two gears with the SRAM shifter. I'm assuming the ratio with the SRAM is 2:1. To complicate things, I found an old SRAM rear derailleur that says 1:1 ratio on the derailleur. Does this mean it will work with Shimano shifters? Will it work with SRAM shifters? I am hoping someone who knows will school me on these ratio questions. Thanks a million in advance. I almost wish I had a graphic to make this more appealing....

SRAM has both shifters and derailers that are Shimano compatible and those that aren't. So there's no guarantee that SRAM + SRAM will automatically work unless you keep track of the model.
If you already have another derailer, try it on. Can't remember offhand which are Shimano spec and which are SRAM spec.

I recently had this problem because the LBS misinformed me. I was running Sram X3 trigger shifters with Shimano altus derailleurs. Certain models of Sram shifters are compatible with Shimano der's. While some Sram shifters are only compatible with Sram der's. Its the cassettes/freewheels that are compatible across both brands (e.g. Sram der system with shimano freewheel) Can you give us what model shifters and derailleur is giving you the issue? Also here's the thread where I had the issue if it helps:
http://www.bikeforums.net/bicycle-me...l#post16601000

But your symptoms sound like mine and mine was a compatibility issue. Can't tell you for sure without knowing the models though.

And as stated above, just because both components are Sram doesn't mean they work. You may be using Sram shifters that are meant for Shimano derailleurs.
So you'll most likely need Sram compatible shifters to match your der's. OR replace the der's with something Shimano (since it sounds like the Shimano comp. shifters).

Yes you do. I ought a bike a long time ago for my wife that had twist grip. She didn't like it so I put a set of Shimano trigger shifters on it. i also had to change the rear derailleur to a Shimano to keep the ratio correct.