Are collapsible bicycle helmets as protective against head injuries as conventional designs?

Collapsible bicycle helmets address portability as a barrier to helmet use, particularly for bicycle share scheme users, but whether they provide equivalent protection to conventional helmets is unknown. We determined whether head injury risk differs between designs and whether any difference is explained by mass and rotational protection technology. Thirty commercially available helmets, 24 conventional and 6 collapsible, were tested at 6.5 m/s onto a 45° anvil at four locations using an EN 17950:2024 headform. AIS4 + linear and AIS2 + rotational injury risks were estimated from peak linear acceleration and Brain Injury Criterion (BrIC). Linear mixed effects models (360 observations; design as fixed effect, helmet ID as random effect) assessed injury risk before and after adjustment for mass and rotational technology. Collapsible helmets had greater linear injury risk than conventional (0.242 ± 0.086 vs. 0.197 ± 0.051, p < 0.001), but this was non-significant after adjustment for mass and rotational protection. Rotational risk did not differ between designs. Collapsible helmets were heavier (461 ± 58 g vs. 294 ± 53 g), and mass predicted linear risk by + 0.002 per 10 g. Rotational technology reduced linear risk by - 0.026 and rotational risk by - 0.073 on average. Collapsible helmets exhibited greater linear injury risk than conventional designs, potentially through their greater mass and the absence of rotational protection technology. These findings guide safer collapsible designs that have the potential to facilitate helmet use and reduce head injuries, particularlyfor bicycle share scheme users.