I disagree to a degree with some of the replies. I see that some Shimano rear ders parallelogram springs get soft over time. Whether it's the actual spring or the pivots getting 'tight' i don't know. I do know that sometimes the der won't pull the cable (even after new casing/inners) hard enough to have a shift into the smallest rear cog consistently. Disconnecting the cable from the der solves this. A sensitive hand can feel this lack of returnability. Install a new der and the returnability is stronger and the problem is solved. Andy.