Originally Posted by hueyhoolihan

a rear DR shifts into lower gears because it is springloaded, given any extra cable the spring is designed to be strong enough to collapse the cage and move the chain to the outside. most problems, IME, involving inability to shift to a lower gears is a result of too much friction somewhere in the system. could be lever, cable/housing or even the DR itself.

i might first disconnect the cable at the DR. and see if it will shift into the smallest cog. this test should show whether or not the RD cage spring is even strong enough to do the job. if not, the problem is probably in the RD. broken or bent or just gunked up.

then i'd remove the cable/housing completely and test it to see if the cable runs smoothly through the housing.