Originally Posted by
4500trek
how about the rear derailleur? should i be concerned with it? replace it? leave alone?
If it's healthy, keep using it. The main things to look for:
1. is it bent. This often ends with the derailleur getting snagged in the spokes and ripped up by the wheel. Put the derailleur in a middle gear and sight on it from behind, and the pulleys and their cage should be in the same plane as the bike, not aiming funny directions.
2. is it getting floppy in the parallelogram pivots. A little play may not be critical on 8-speed, but lots of play can interfere with the accuracy of the shifting. While looking at the derailleur from behind, hold the pulley cage at the bottom pulley and gently push it left & right with finger force while observing how much play there is. A new derailleur might have nearly none, a shot derailleur might have 1/2 inch.
3. are any parts damaged. The pulleys sometimes get teeth broken off in mishaps, and the adjuster barrel that the cable enters from the rear can get bent.
Beyond that, I'd make sure it's adjusted right so it doesn't bail the chain into the spokes, or jam it against the frame, or growl against the big cog in low gear. Park Tool has a guide here:
http://www.parktool.com/blog/repair-help