Numerical Simulation of Stress Distribution in a Bicycle Crank Shaft Under Cyclic Loading Conditions

Fatigue failure of bicycle crank shafts is a common cause of sudden component fracture during pedalling, posing injury risks to riders who rely on bicycles as a primary transport mode across many mid-density Nigerian cities and peri-urban communities. This study performs a numerical simulation of stress distribution in a standard bicycle crank shaft under cyclic pedalling loads to identify critical failure-prone regions and inform design improvements. A detailed solid model of a crank shaft conforming to common commercial dimensions was created in SolidWorks and subjected to finite element analysis in ANSYS using a tetrahedral mesh refined at geometric stress concentrations. Cyclic loading equivalent to the pedalling force of a 75-kilogram rider at maximum effort was applied as a sinusoidal load function. Von Mises stress distributions were mapped under peak and minimum load conditions, and stress concentration factors were determined at the pedal thread junction, the crank arm mid-section, and the bottom bracket spindle interface. Peak cyclic stress was recorded at 148 MPa at the pedal thread root, exceeding the endurance limit of standard low carbon steel at this location. Fatigue life estimates based on the Goodman diagram indicated that failure at this location could occur within 200,000 cycles under maximum loading. Design modifications including a fillet radius increase at the thread root and local material upgrade to alloy steel are recommended. Keywords: fatigue analysis, bicycle crank shaft, finite element method, cyclic loading, stress concentration.