Differences in Impact Performance of Bicycle Helmets During Oblique Impacts

Abstract

Cycling is a leading cause of sport-related head injuries in the U.S. Although bicycle helmets must comply with standards limiting head acceleration in severe impacts, helmets are not evaluated under more common, concussive-level impacts, and limited data are available indicating which helmets offer superior protection. Further, standards evaluate normal impacts, while real-world cyclist head impacts are oblique-involving normal and tangential velocities. The objective of this study was to investigate differences in protective capabilities of ten helmet models under common real-world accident conditions. Oblique impacts were evaluated through drop tests onto an angled anvil at common cyclist head impact velocities and locations. Linear and rotational accelerations were evaluated and related to concussion risk, which was then correlated with design parameters. Significant differences were observed in linear and rotational accelerations between models, producing concussion risks spanning >50% within single impact configurations. Risk differences were more attributable to linear acceleration, as rotational varied less between models. At the temporal location, shell thickness, vent configuration, and radius of curvature were found to influence helmet effective stiffness. This should be optimized to reduce impact kinematics. At the frontal, helmet rim location, liner thickness tapered off for some helmets, likely due to lack of standards testing at this location. This is a frequently impacted location for cyclists, suggesting that the standards testable area should be expanded to include the rim. These results can inform manufacturers, standards bodies, and consumers alike, aiding the development of improved bicycle helmet safety.

Development of the STAR Evaluation System for Assessing Bicycle Helmet Protective Performance

“…2,14 A previous oblique impact study using the HIII neck required greater normal velocities to produce the same PLAs as the present study. 1 It should thus be noted that the present kinematics and risks may be larger compared to tests with a neck. As advanced ATD necks are developed with improved biofidelity in axial compression, their effects in oblique impacts should be investigated.…”

“…40 Conversely, an additional study demonstrated higher PRA for MIPS helmets compared to non-MIPS helmets. 1 The latter study incorporated a HIII neck, which likely influenced results. Further, only two MIPS helmets were included, and both produced greater PLAs, suggesting that additional design factors led to reduced impact protection overall.…”

“…No anthropomorphic test device (ATD) neck or effective torso mass was used, similar to a number of previous cyclist oblique head impact studies. 2,26,27,40 Although some studies have used a Hybrid III (HIII) neck for this type of testing, 1,22 oblique impacts may subject the neck to considerable axial compression for certain impact locations, a scenario in which the HIII neck is known to be overly stiff. 2,28,38 It has been further suggested that human necks have minimal effect on initial head impact response during head-first impacts due to the high loads and short durations involved.…”

“…Several oblique rigs have already been developed for bicycle helmet testing. 1,26,27 Establishing an oblique impact protocol for bicycle helmets could help manufacturers design around these common conditions as well as standards-prescribed conditions.…”

“…The existing objective impact data comparing helmet models are either limited to standards testing or only evaluate a relatively small subset of helmets on the US market. 1,3,26,40 While most helmets are designed with similar materials, typically including a polycarbonate shell and expanded polystyrene (EPS) liner that permanently crushes to absorb energy upon impact, the many possible styles of bicycle helmets produce wide ranging design features, and previous research has demonstrated considerable differences in the ability of commercially-available helmets to reduce head injury risk. 1,3,26,40 To help reduce incidence of concussion in cycling and to stimulate improved helmet design for both mild and severe injury, consumers should have access to biomechanical data differentiating helmet performance that is informed by real-world cyclist impact conditions and injury mechanisms.…”