Cycles of all sorts

A bicycle helmet is a helmet intended to be worn while riding a bicycle. They are designed to attenuate impacts to the head of a cyclist in falls while minimizing side effects such as interference with peripheral vision. They are specified to withstand simple falls onto a flat surface 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.

The subject of cycle helmets is controversial. Some evidence suggests that helmeted riders are less likely to suffer head injury; other evidence suggests the opposite. Even when cycle helmet use has risen steeply due to laws that require it, it has not been demonstrated that there is a correlation between helmet use and reduction in head injuries. Recent analysis supports the conclusion of Spaite et al. that much of the effect attributed to helmets in case-control studies may be due to behavioural differences in the types of cyclists who choose to wear or not wear them.

In a low speed crash, a cyclist might benefit from wearing a helmet but the theory of risk compensation suggests that the fact of wearing one may subtly influence cyclists' riding by making them less careful.

In a more serious crash, especially if a motor vehicle is involved, it is unlikely that a helmet will make a significant difference. One study found that 16 of 20 cyclist fatalities whose primary cause of death was listed as head injury also had other fatal injuries. Of the remaining four, at least one rider had been helmeted at the time.

No cycle helmet is designed to be effective in high speed collisions or those involving motor vehicles, yet where helmet laws have been passed they have always been for road cycling only. This is considered by some to be victim blaming or a smokescreen to obscure the danger posed to cyclists by motor traffic. Helmets may mitigate some injuries during a collision, but cannot prevent collisions from happening.

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.

Proper fit

It is important that a helmet should fit the cyclist properly - according to research most (well over 90%) helmets have been found to be incorrectly fitted. Efficacy of incorrectly fitted helmets is reckoned to be much lower, one estimate states that risk is increased threefold.

Most manufacturers provide a range of sizes ranging from children's to adult with additional variations from small to medium to large. The correct size is important. Some adjustment can usually be made using different thickness foam pads. Helmets are held on the head with nylon straps, which must be adjusted to fit the individual. This can be difficult to achieve, depending on the design. Most helmets will have multiple adjustment points on the strap to allow both strap and helmet to be correctly positioned. Additionally, some helmets have adjustable cradles which fit the helmet to the occipital region of the skull. These provide no protection, only fit, so helmets with this type of adjustment are unsuitable for roller skating, stunts, skateboarding and unicycling.

The helmet should sit level on the cyclists head with only a couple of finger-widths between eyebrow and the helmet brim. The strap should sit at the back of the lower jaw, against the throat, and be sufficiently tight that the helmet does not move on the head. It should not be possible to insert more than one finger's thickness between the strap and the throat.