I have a bike with 6600 triple. I wanted much lower gears than a 27t cog in the back. I now have a 10sp 12-36 cassette and a 9sp XT Shadow rear der. (10sp rear der will not work for this.) It works great.
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Gunnar Sport
Specialized Ruby
Salsa Vaya Ti
Novara Randonee x2
Motobecane Fantom CXX
Jamis Dakar XCR