D and BB

Derailleurs:

A common maintenance problem is that one of the derailleurs either exceeds its full range, or fails to reach it, causing shifting to become sticky or, at worse, inoperable when the chain refuses to shift altogether.
Adjustment is routine and fairly simple.
Begin by flipping the bike upside-down and balancing it on the saddle and handle-bars. While slowly turning the cranks, shift through the range of gears and see where it's not shifting cleanly. If the chain won't go onto the larger cogs smoothly, increase the cable tension by screwing out the adjuster barrel, and vice versa if if won't shift to the smaller cogs. If the shifting becomes sloppy on the largest or smallest cogs, the limit screws need to be adjusted. These are two small screws located side by side on the derrailleur, and limit the travel of the chain on the edges of the cassette. The high limit screw, marked "H" applies to the smallest cog and the low limit screw, "L" applies to the largest. If the chain won't change onto the cog, screw out the appropriate limit screw a small bit at a time, around a 1/4 turn. If the chain skips over and leaves the cassette altogether, the limit screws need to be screwed in a small bit.

Bottom Bracket:

There is one sure-fire way to know if your bottom bracket (BB) is giving trouble and thats if your cranks wobble. In that case you'll have to do this immediatly or the BB will rip out the screw threads and make you frame unuseable. Another give-away is a loud annoying squeaking, or, even worse, crunching coming from the BB. Get it checked straight away.

Remove your cranks with a ratchet or 8 mm allen key and then use a crank extractor and BB tool to remove the bottom bracket. Replace and repack the bearings in fresh grease, then reinstall the BB. This may sound simple, but its not - you may want to seek professional help with this one.