Originally Posted by
Saddle Up
Do you really think it matters to the derailleur how many gears it shifts? I'm using a road 10 speed 105 derailleur on a 9 speed drivetrain, I could use 10 speed Ultegra for that matter. The derailleur itself is not indexed, the shifters are. No reason it would not work. The overall width of the cassette and how far the dearilleur reaches or extends would determine if it would work.
This is true for Shimano, where mountain and road components are generally cross-compatible, but SRAM has chosen to use a 1:1 ratio in their MTB line, and a 2:1 ratio in their road line. The mountain derailleur will move twice as far per shifter click as the road derailleur if both are mated to a SRAM road shifter.
The OP may want to look into a shiftmate or a respaced cassette with a Shimano der. if he/she is bent on using Rival shifters.