A bicycle helmet does not prevent crashes, but in case of a fall or a crash, a helmet protects the head by the energy absorbing foam lining of the helmet. This foam lining crushes and reduces the impact on the brain. The foam lined interior is connected to the smooth hard exterior of the helmet. The smooth shell ensures little road surface friction and it can skid so as to prevent neck injuries. The hard shell ensures that the impact of the fall is spread over a broader surface area and that sharp objects cannot penetrate. The interior of the helmet is fitted with soft padding for comfort. The straps closing under the chin ensure the helmet stays on in case of a fall [1] [2]. Head rotation during a fall may be reduced by an extra layer in the helmet that moves along with the head (the so-called Multi-Directional Impact Protection Systems - MIPS) [3] [4]. Also see the question How may the protection offered by bicycle helmets be improved and are there any alternatives to bicycle helmets?
To achieve optimum protection the helmet should be a good fit and should be properly fastened [5] [6]. In case of a crash or fall, its effectiveness decreases as the impact speed increases. A helmet should at least comply with European technical standards (see the question Which requirements should a (good) helmet meet?).
For speed pedelecs a different, more robust helmet has been developed that protects a larger part of the head (also see the question Is helmet use mandatory for (speed) pedelecs?).
