A geared bicycle will typically feature two derailleurs -- front and back. Each derailleur is responsible for shifting the drive chain from one selected gear to another. Like other brand derailleurs, Shimano derailleurs, both front and rear, require an occasional adjustment in order to work properly. These adjustments include range-setting and cable tension. Range-setting keeps the derailleur from travelling too far to the left or right, while cable tension allows the derailleur to move within this range.
Shift the chain to the smallest rear cog and largest front chain ring.
Use a 5mm Allen wrench to loosen the rear derailleur cable anchor bolt, located beneath the derailleur.
Check the alignment of the upper derailleur pulley with the smallest rear cog. The two should be in line. If they are not, use a Phillips head screwdriver and turn the H-range screw at the back of the derailleur. On Shimano derailleurs, the screw is marked with an "H". Turning the screw clockwise moves the derailleur (and pulley) in toward the bike.
Turn the barrel adjuster at the back of the derailleur all the way into the body of the derailleur. Then back it out, in the opposite direction, two turns.
Pull the end of the cable taught with a pair of needle nose pliers, and tighten the anchor bolt over the cable.
Shift the chain to the next rear cog over. If chain response is sluggish, turn the barrel adjuster in toward the bike until the shift is complete.
Shift to the largest rear cog and smallest front chain ring. If the chain refuses to shift onto this cog or shifts past the cog, turn the L-range screw, on the back of the derailleur, until the upper derailleur pulley is directly beneath the cog. Turning the screw clockwise moves the derailleur away from the bike.
Shift the chain to the largest rear cog and smallest front chainring. Using your 5mm Allen wrench, loosen the front derailleur cable anchor bolt, located just above the derailleur.
Check the gap between the front derailleur and the chain. If the gap is more or less than 1mm, turn the L-range screw, atop the body of the derailleur, until the proper gap is achieved. Turning the screw clockwise moves the derailleur closer to the chain.
Turn the derailleur cable barrel adjuster all the way in toward the bike. Then back it out two turns. This barrel adjuster will be located on the downtube of the frame (road bikes) or on the side of the shifter (mountain bikes).
Pull the end of the cable taught with your needle nose pliers, and tighten the anchor bolt onto the cable.
Shift to smallest rear cog and largest front chainring. If the chain responds sluggishly, turn the barrel adjuster counterclockwise until the chain shifts cleanly.
Check the gap between the derailleur and the chain. Once again aim for 1mm. To adjust the gap, turn the H-range screw. The screw can found beside the L-range screw. Turning the screw clockwise moves the derailleur toward the chain.