A Structural Evaluation of an Affordable BMX’s Swaged Three-piece Crank Arm: Under Regular Cycling and Stunt Impact

This report evaluates the structural integrity of an affordable BMX’s swaged 3-piece crank arm under two in-service load cases: regular cycling and novice-level stunt performance. The arm comprises three areas vulnerable to failure: the crank arm root, the axle boss and the boss weldment. Cranks convert pedal loads into rotational motion, while experiencing significant accelerations (6.05 g) when performing simple stunts, such as the bunnyhop. This study characterises these load cases and performs static load analysis to assess the stresses within the crank’s vulnerable areas. Subsequently, the integrity is assessed through factor of safety analysis, elastic-plastic fracture mechanics, and fatigue life analysis. A literature review demonstrates how pedal force data from biomechanical studies on regular cycling can be utilised for structural evaluation. However, a literature gap exists regarding the structural performance of the crank arm under stunt impact, which the author deems a significant safety-critical design oversight. This helped inform the project's objectives: to assess loading from regular cycling using pedal force data from the literature, and to conduct accelerometer experiments to approximate impact forces for structural analysis. The methodology evaluates minimum and maximum loads applied at the pedals for both load cases. Load paths are then determined, and static load analysis is utilised to calculate bending moments and reactions at the axle boss, allowing the author to define stress types within the crank arm. The study determines: • The combined bending and uniaxial stresses at the root of the tubular swaged arm • The resultant shear stress in the weldment, • The combined interference and Hertzian stresses within the axle boss. These stresses are used to perform FOS analysis, EPFM and fatigue analysis under varying mean stresses. Allowing comparisons to be made between LC1 and LC2. The analysis concludes that stunt loading increases internal stresses by up to 55% compared to those produced in regular cycling, a direct result of the greater magnitude of the pedal loads generated. This impacts the FOS against yielding, reducing the margin of safety by up to 55% in both the crank arm root and weldment. EPFM, in the presence of a flaw, concludes that all vulnerable areas are structurally sound with all reserve factors ≥1.4. Fatigue analysis reveals that although LC2 reduces fatigue life, for all analysis cases, components exceed the 1 x 10^7 cycles infinite lifetime value. Consequently, affordable swaged cranks maintain their structural integrity during novice-level stunts, with fatigue life remaining largely unaffected.