Free Sample
The Complete Handbook of Bicycle Cranks
A practical reference for mechanics, retailers, builders, and riders who need to specify, install, and maintain bicycle cranks with confidence.
by Alumigogo Books
Chapter 1: Understanding Bicycle Cranks
The Crank's Job Is Torque Transfer, Not Appearance
A bicycle crank is the part that turns your pedaling force into rotary motion at the bottom bracket. In engineering terms, it is a torque-transfer device: your foot pushes down on a pedal, the pedal mount eyelet at the end of each arm converts that linear force into a moment, and the arm's spindle connection transmits that moment into the bottom bracket axle. Almost everything about a crank's design - arm length, arm shape, spindle diameter, interface splines, even the bolt you tighten to hold it in place - exists to manage this load path effectively without bending, slipping, or breaking.
Consider a typical pedaling force: a serious rider produces an average torque of 50-80 N·m during steady climbing, with peak instantaneous forces over 200 N·m when sprinting out of the saddle. That peak is applied to the end of a crank arm (typically 165-175mm from the spindle center), which means the arm itself experiences a moment of roughly 40-70 N·m. The pedal spindle alone must transmit that force into the arm through a 10mm diameter steel axle. Then the arm body must carry that load across its length without noticeable flex, and the spindle - whether square taper or splined - must hold the arm's angular position relative to the drive side, so the left and right arms stay exactly 180° opposite each other. Any slack, wear, or plastic deformation in that system shows up as