The chainset for a three chainwheel Derailleur gearbox is shown in the photograph. The pedal crank (A) has a 'spider' that carries the set of chainwheels (B) that will expand the overall range of the Derailleur gearing. The spacers and washers (D) hold this

gearset together.

The length of the pedal crank is important as it determines the torque that the rider can generate about the crank spindle and governs the amplitude of the leg movement. A longer crank will cause the riders foot to move a larger distance in one revolution. For a 165 mm crank the foot travel per crank rotation is 0.518 m and for a 170 mm crank, 0.534 m. The longest cranks that are consistent with the leg length and comfort of the rider will provide the largest torque.

In mountain bike applications, the ground clearance of the pedals and chain wheels will also be an important consideration.

From: Sloane, "Sloane's Complete Book
of Bicycling," Simon & Schuster (1995)