Hard Plastic

A bicycle helmet is designed to attenuate impacts to the head of a cyclist in falls while minimizing side effects such as interference with peripheral vision. more...

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Bicycle helmets are intended for use by pedal cyclists on ordinary roads, to give protection in the kind of accident in which the rider falls onto the road without other vehicles being involved.

A cycle helmet should be light in weight and should provide adequate ventilation, because cycling can be an intense aerobic activity which significantly raises body temperature and the head in particular needs to be able to regulate its temperature.

About helmets

How they work

There are two main types of helmet: hard shell and soft/micro shell (no-shell helmets are now rare). In both types impact energy is absorbed as a stiff foam liner is crushed, up to the point where the liner is crushed to its minimum thickness, or the helmet shatters, after which no further energy is absorbed. Collision energy varies with the square of impact speed: a typical helmet will absorb the energy of a fall from a stationary or slow-moving bicycle, an impact speed of around 12mph, but will only reduce the energy of a 30 mph impact to 27.5 mph, and even this will be compromised if the helmet fails. This energy calculation is based on the standards, which take no account of the weight of the rider's body.

As a subsidiary effect they also spread point impacts over a wider area of the skull. Hard shell helmets do this rather better, but they tend to be heavier and less well ventilated so are more common among stunt riders than road riders or mountain bikers. Additionally, the helmet (like any good hat) will reduce superficial injuries to the scalp. Hard shell helmets can also reduce the likelihood of penetrating impacts although these are said to be very rare.

The key component of most modern bicycle helmets is a layer of expanded polystyrene (EPS), essentially the plastic foam material used to make inexpensive picnic coolers. This material is sacrificed in an accident, being crushed as it absorbs a major impact. Bicycle helmets should always be discarded after any accident.

Helmets are most effective in straight line, or linear, blows to the head at moderate speed. Helmets are not well designed to deal with high speed impacts or rotational stresses (crashes that are not centred, and involve rotation of the head). They are not designed to provide adequate protection for a collision involving another moving vehicle, (e.g. a car).

A common misunderstanding is to assume that a broken helmet has prevented some serious injury. Helmets are designed to crush without breaking; EPS absorbs little energy in brittle failure and once it fails no further energy is absorbed.

Read more at Wikipedia.org