How does impact velocity affect rotational protection of bicycle helmets?

Pedal-assisted electric bicycles are becoming increasingly popular and have transformed urban mobility worldwide. This shift has also led to an increase in the incidence of traumatic brain injury (TBI). Despite this, injury biomechanics research and safety standards for e-bike helmets remain limited. Current bicycle helmet certification relies on linear impact protocols, thereby failing to capture the rotational kinematics of real-world crashes that primarily drive TBI. Moreover, e-bikes travel at generally higher speeds than conventional bicycles, yet helmet performance under these elevated impact conditions remains poorly characterized. This study aimed to evaluate the performance of eight bicycle helmets at two velocities (i.e., 6.0 m/s and 7.6 m/s) under oblique impact conditions representative of pedelec accident severity. At each velocity, the helmet was tested at four impact locations, of which the impact kinematics were measured using a high-biofidelity EN 17,950 headform and applied to an anatomically detailed finite element head model to assess brain strain responses. The results showed that increasing the impact velocity disproportionately elevated brain injury risk across helmet designs: the average mTBI risk rose by 13.7% for helmets equipped with the Multi-Directional Impact Protection System (Mips), compared to 23.2% for helmets without Mips, while relative helmet rankings remained largely consistent between the two velocities (Kendall’s τ = 0.71–0.79). Mips-equipped helmets reduced rotational kinematics and tissue-level brain strain metrics by 16%–21% relative to helmets without Mips at the higher impact velocity ( p adj < 0.05). These findings offer biomechanical evidence to support future helmet standard development.