Crank Length

Definition

Crank length is the distance from the center of the bottom bracket spindle to the center of the pedal axle. It defines the leverage arm for each pedal stroke.

Analysis

Crank length sets how far the rider's foot travels through each pedal revolution. Together with cadence and force, it determines power output, pedal speed, and the geometry of the rider's leg motion.

Context

Crank length is reported in millimeters, typically stamped on the inside face of each crank arm. Standard increments are 2.5 mm, with 165, 170, 172.5, and 175 mm being most common.

Function

Longer cranks give more leverage at the cost of higher knee bend and lower pedal-strike clearance. Shorter cranks reduce knee bend, improve pedal clearance, allow higher cadence, and open the hip angle for aero positions.

Variation

Crank length traditionally scaled mildly with frame size, though modern fitting practice often recommends shorter cranks for most riders regardless of size. Mountain bikes have trended toward shorter cranks for pedal clearance.

Common Ranges/Values

Adult crank lengths range from 160 to 180 mm. Road bikes commonly use 170 to 175 mm; mountain bikes 170 to 175 mm; track and time-trial bikes have trended shorter to 165 to 170 mm. Children's cranks range from 100 to 152 mm.

Common Practices & Evolution

There has been a clear industry trend over the past five to ten years toward shorter cranks, even for tall riders, driven by pedaling biomechanics research and the rise of aero positions. Mountain bikes have moved to 170 mm or shorter to reduce pedal strikes.

Specifics

Crank length affects effective bottom bracket clearance: a 175 mm crank has a pedal that sits 5 mm closer to the ground than a 170 mm crank at the bottom of the stroke. Shorter cranks subtly increase saddle height to maintain leg extension.

Impact

Crank length influences pedaling biomechanics, pedal-strike risk, and the rider's ability to hold an aerodynamic forward position.

Pros & Cons

Longer cranks provide more leverage for grinding low cadences and slow climbing, but increase knee bend, reduce pedal clearance, and limit aero positions. Shorter cranks improve pedal clearance, reduce knee strain, allow higher cadence and tighter aero positions, but provide less leverage at low cadence.

Relations

Crank length pairs with bottom bracket height and pedal type to determine pedal clearance. It interacts with saddle height and chainstay length to define pedaling geometry.