what derailleurs for loaded touring, suggestions?

[artemidorus]

Along the same lines as this thread. I have an Ultegra GS RD (9 speed 6500) on my wife's Giant CRX (flat bar road) and I'm trying to put an XT 11-34 cassette on it. It has a double 50/36 front, rapid9 shifters (all shimano) Is there any way to get it to work? I tried fitting it all last night, but the top jockey wheel rubs on the cassette, the RD body seems to be just a bit too high. Do I have to run a MTB RD with this cassette, or can I maybe use a longer RD hanger? I've only just read that the RD6500 has a max sprocket of 27, why is it so different to a MTB RD GS? Any ideas?
Cheers, Jason.

MTB derailleurs do not only have longer cages, they also have a different parallelogram geometry to allow more clearance between the largest cassette sprocket and the jockey wheel. Only an MTB RD will accommodate a 34T cassette sprocket. You will not be able to make the Ultegra RD work with that cassette, whatever you do.