Construction
Most bicycles use two sets of multiple gears that are adjusted to alter the force required to drive the bicycle. The front gear set is attached to the pedal crank. The back gear set is part of the rear wheel axle assembly. The drive chain connects these two gear sets. As the bike is pedaled, two devices, called the front and rear gear shifters, can be operated to change the drive chain to different sized gears on the front or rear gear sets.
Shifter Handles
The shifter handles are located on the frame near the handlebars. They are levers that can be manually adjusted to move cables, which, in turn, operate the front and rear derailers to change gears.
Cables
Wound wire cables run from the shifter handles along the frame of the bicycle and are held in place by eyelets connected to the frame. Each cable is attached to and operates one of the derailers, also known as a derailleur.
Shifting the Front Gear Assembly
The derailers are the devices on the bicycle that move the drive chain from one gear to another on the gear sets. The front derailer is attached to the frame of the bicycle just above the front gear set. The drive chain is threaded through the front derailer. As the rider operates the front gear shift handle, the derailer will move back and forth across the top of the front gear set, pushing the chain up or down the gear set to change gears as they turn.
Shifting the Rear Gear Assembly
The rear derailer is a bit more complex. It is attached to the frame near the rear axle and consists of a geared, hinged armature that extends downward from the frame. The derailer is spring-loaded to provide constant tension on the drive chain. The chain is threaded through the gears in the rear derailer, then up onto the rear gear set. As the rider operates the rear gear shift handle, The derailer moves inward or outward, guiding the drive chain up or down the rear gear set to change gears.
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