It looks pretty scary: cables, gears, chains, grease, and springs. Who wants to mess with that? But the majority of shifting issues (jumping chains, rough shifts, clacking) can be fixed fast with zero tools. So next time your shifting goes, follow these steps from cycling guru Lennard Zinn, a former U.S. National Team rider and the author of several books on bicycle maintenance:
(1) Determine whether you need to tighten or loosen the cable. Shift up until the chain is on the smallest cog (a) in the rear, then downshift one gear. If the chain doesn't climb to the next cog, or if it does so slowly, you need to increase tension. If it overshifts, you need to reduce it.*
(2) Find the barrel adjuster (b). It'll be at the rear derailleur (most likely) or by the handlebars.
(3) To increase tension, turn the barrel adjuster counterclockwise (when viewed from the end where the cable housing enters the adjuster). To reduce tension, turn clockwise. The barrel adjuster will click as you turn it. Go one or two clicks and recheck the shifting. Repeat until the chain moves easily in both directions.
*This holds true for most derailleurs. But some, called "low normal," work in the opposite direction, with cable tension moving the chain away from the wheel, instead of toward it.To determine which kind you have, pull on the cable and watch which way the derailleur moves. If you have an LN, start on the largest cog and reverse this whole process.