Are helmets supposed to provide protection against all impacts?
No. Sharp, high speed objects are likely to penetrate helmets particularly those with many vent openings. Also, helmet tests monitor the effect of linear force but not rotational force. A blow which is not square on centre, i.e. not linear, will rotate the head. Diffuse injuries – the most serious and common type of brain injuries – result from rotational stresses on the brain. Linear force on the other hand, result in focal or localized injuries rather than diffuse injuries. It has not been ruled out that the added mass, size and surface texture of a helmet may make the rotational effect more severe. A “safety” device which has been shown undeniably to assist in rotation and increase the risk of diffuse brain injury is the headrest on car seats in rear-end crashes. Nothing has been shown one way or another though, for bicycle helmets. When acquiring new helmets, buyers should consider helmets which are spherical in shape as they are more likely to minimize rotational effect than the t
