When adjusted properly, a bicycle's derailleur ensures smooth, easy shifting. But a derailleur is a complicated mechanism, with a range of adjustments necessary to ensure proper shifting. Because of its complexity, adjusting a derailleur is a painstaking operation. When you adjust your derailleur, you must take into account not only the adjustments on the derailleur itself, but also how the mechanism works with the shifters and the shift cables. Though daunting, repairing derailleurs is an enviable skill to have as a bike owner.
Adjusting Front Derailleurs
Check the height and alignment of the front derailleur. The derailleur's cage, which is the metal piece that moves the chain on the gears, should be parallel with the chain rings. The derailleur should hang between 1 and 2mm above the gears.
Shift the derailleur into the smallest gear, so it is closest to the frame. Look at the derailleur cage while rotating the pedals with your hands. If the chain hits the cage, use a flat head or Phillips screwdriver (this varies based on the type of derailleur) to loosen the screw marked "L" on the derailleur. This will back the cage away from the chain. Do this until 1mm of space is between the cage and the side of the chain. Likewise, if the cage is more than 1mm from the chain, tighten the limit screw until the cage is 1mm from the chain.
Shift into the next highest gear. On some bikes, this is the highest gear, while on others its the middle. If the chain has problems shifting to the next gear, loosen the barrel adjuster on either the derailleur, shifter or frame. This is a barrel-shaped bolt that the derailleur cable threads through, and can be located on a variety of places on the bike. Thread it outward until the chain shifts onto the next gear.
Shift to the next highest gear, if you have one. Check the outside spacing of the chain and the derailleur cage. If the chain rubs on the outside of the cage, spin the screw marked "H" on the derailleur outward. If the chain shifts off the gear, spin the screw inward. As with the lower gear, the spacing between the chain and the outside of the cage should be 1mm. If this is the spacing on the derailleur, and it still won't shift onto the next gear, loosen the barrel adjuster until the chain jumps onto the next gear.
Shift through the gears. If it had trouble shifting down into a gear, tighten the barrel adjuster slowly until it drops into the gear.
Adjusting Rear Derailleurs
Shift into the easiest gear, which has the largest cog. Using your hand, push the derailleur toward the spokes. If the chain falls off the gear toward the spokes, use your screwdriver to tighten the screw marked "L." Tighten the screw a quarter turn, then check to see whether the derailleur still shifts into the spokes. Be careful not to overtighten the screw because that will prevent the derailleur from shifting into the largest gear.
Shift the derailleur into the harder, or smaller, gears, until you reach the smallest gear. If it hesitates while shifting upward, spin the barrel adjuster on the derailleur, frame or shifter further inward.
Look at the alignment of the smallest gear and the top pulley of the derailleur. The pulleys are the wheels that hold the chain in the derailleur. The top pulley, which is closest to the gear, should be roughly parallel with the smallest cog. If the pulley is too far out, tighten the screw labeled "H." If it is too far in, loosen the screw.
Shift back through the gears, toward the easiest gear. If the derailleur has trouble getting into any gear, loosen the barrel adjuster until it shifts into the gear.