Most bicycles these days have multiple gears that can vary between 15 to 27 speeds. Different gearing is achieved by mounting 1 to 3 sprockets at the crank and 5 to 9 cogs on the rear wheel. To determine the number of speeds your bicycle is equipped with, multiply the number of sprockets on the crank by the number of cogs on the rear wheel (3 x 8 = 24 speeds).
Shifting between all these gears is controlled by levers on the handlebars, connected by cables to the front and rear derailleurs. The derailleurs--devices through which the bike's chain passes--move from side to side to shift the chain from one sprocket to the next. There are two main factors that affect the derailleur's function: cable tension and derailleur alignment.
Begin by shifting the rear shifter to largest number indicated and place the chain on the smallest sprocket. Adjust the High limit screw so the guide pulley and the smallest sprocket are lined up vertically. Reconnect the cable, pull out any slack, and retighten the anchor bolt securely.
Shift through the gears, making sure each gear achieved is done quietly and without hesitation. If necessary, use the barrel adjuster to fine tune each gear by turning it the direction you want the chain to go. For example, turning clockwise will loosen the cable tension and move the chain away from the wheel, while turning counter-clockwise will tighten cable tension and direct the chain towards the wheel.
Shift the rear shifter to the gear one and place the chain on the largest cog. Adjust the Low limit screw in quarter turn increments until the guide pulley and the largest cog are aligned vertically. Again, shift through each gear several times, checking that each gear is achieved smoothly. It may take several attempts before the rear derailleur and cable is adjusted properly. Ensure all bolts are secured tightly and the chain does not fall off in either direction.
Shift both shifters to the smallest number indicated and place the chain on the corresponding cog and chainwheel. Disconnect the front derailleur cable from the cable anchor bolt. Check the position of the front derailleur; it should be parallel with the outer chainwheel and clear the largest chainwheel by 1-3mm when fully engaged.
With the chain on the smallest chainwheel in front and the largest cog in back, adjust the Low limit screw so the chain is centered in the front derailleur cage. Reconnect the cable, pull any slack out, and tighten the anchor bolt securely.
Shift the front shifter to the largest chainwheel. If the chain does not go onto the largest chainwheel, turn the high limit screw in 1/4 turn increments counter-clockwise until the chain engages the largest chainwheel. If the chain falls off the largest chainwheel, and into the pedals, you will need to turn the High limit screw in 1/4 turn increments clockwise until the chain no longer falls off.
Shift through every gear, using the barrel adjusters to fine tune each transition. The barrel adjuster for the front derailleur is located on the front shifter where the cable comes out of the shifter. Clockwise will loosen the cable tension and direct the chain closer to the frame while counter-clockwise will tighten the cable tension and direct the chain away from the frame.
Do not ride a bicycle that is not shifting properly. Overlooking proper adjustments may cause irreparable damage to the bicycle and/or bodily injury. Never move the shifter while pedaling standing up, or under heavy load, nor pedal backwards after having moved the shifter. This could jam the chain and cause serious damage to the bicycle and/or rider.