Geared bicycles feature a hand control, called a "shifter", which allows the cyclist to change from one gear to another. Completion of the shift relies on the derailleur moving the chain to the selected gear. When the cable that links the shifter to the derailleur becomes stretched from use, the derailleur struggles to shift the chain to the gear. The problem can often be fixed by restoring the right amount of tension to the bike cable.
1. Take the bicycle for a short ride in an area free of car traffic. If fixing front gears, shift down into the lowest front gear. If fixing rear gears, shift up into the highest rear gear. The lowest gears require the least effort to pedal. Higher gears require greater effort.
2. Stop the bicycle and climb off. Note the cable emerging from the side of the gear shifter and linking the shifter to the derailleur, at the other end. Along the cable there is a threaded nut called an "barrel adjuster." Depending on the design of the bicycle, the adjusting barrel will be located either at one end of the cable or halfway along the cable on the side of the bicycle.
3. Twist the barrel adjuster counterclockwise, but no more than a half-turn initially. The adjuster barrel allows tension to be added to the cable. Working in half-turns, and then checking the adjustment, prevents over-adjustment.
4. Ride the bicycle, and shift into the next gear. As tension is added to the cable, shifting should become more precise.
5. Add further tension to the cable as needed. Following each adjustment, ride the bicycle and shift the gears back and forth. Adjust until the chain is able to shift to each gear without stalling.