Design intentions and standards

There are two main types of helmet: hard shell and soft/micro shell (no-shell helmets are now rare). Both are intended to reduce acceleration to the head due to impact, as a stiff expanded polystyrene liner is crushed.[5][6] Standards involve the use of an instrumented headform which is dropped, wearing a helmet, onto various anvils. The speed of impact is designed to simulate the effect of a rider's head falling from approximately usual riding height, without rotational energy and without impact from another vehicle.[7][8]

Collision energy varies with the square of impact speed; a typical helmet is designed to absorb the energy of a head falling from a bicycle, an impact speed of around 12mph or 20 km/h. This will only reduce the energy of a 30 mph or 50 km/h impact to the equivalent of 27.5 mph or 45 km/h, and even this will be compromised if the helmet fails. As a subsidiary effect they should also spread point impacts over a wider area of the skull. Hard shell helmets may do this better, but are heavier and less well ventilated. They are more common among stunt riders than road riders or mountain bikers. Additionally, the helmet should reduce superficial injuries to the scalp. Hard shell helmets may also reduce the likelihood of penetrating impacts although these are very rare.

[edit] Criticism of current standards; new designs

Helmet liners may be too stiff to be effective. Some standards require the use of headforms heavier and more rigid than the human head; these are more capable of crushing foam than is the human head. [9] [10] In real accidents "very little crushing of the liner foam was usually evident... What in fact happens in a real crash impact is that the human head deforms elastically on impact. The standard impact attenuation test making use of a solid headform does not consider the effect of human head deformation with the result that all acceleration attenuation occurs in compression of the liner. Since the solid headform is more capable of crushing helmet padding, manufacturers have had to provide relatively stiff foam in the helmet so that it would pass the impact attenuation test... As the results in Figure 15 illustrate, the child skull is far from being solid and will deform readily on impact. This fact is well known in the medical field and is largely why a child who has had a rather modest impact to the head is usually admitted to hospital for observation. The substantial elastic deformation of the child head that can occur during impact can result in quite extensive diffuse brain damage."[11]

In real accidents, while broken helmets are common, it is extremely unusual to see any helmet that has compressed foam and thus may have performed as intended. “Another source of field experience is our experience with damaged helmets returned to customer service... I collected damaged infant/toddler helmets for several months in 1995. Not only did I not see bottomed out helmets, I didn’t see any helmet showing signs of crushing on the inside.” [12]

A new design of liner, the "cone-head", now being manufactured for motorcycle helmets but not yet available for bicycle helmets, has been designed in response to the 1987 study. It may provide better impact absorption.[13]

Most helmets provide no protection against rotational injury and may make it worse. "The major discovery is that the skull plays an important role in protecting against rotational acceleration," says Phillips. He says almost all head injuries involve not just a direct blow to the skull but also damage to blood vessels caused by the brain rotating within the skull.

In mechanical terms, the head is an elliptical spheroid with a single universal joint, the neck. It is therefore almost impossible to hit it without causing it to rotate. The head tries to dampen these forces using a combination of built-in defences: the scalp, the hard skull and the cerebrospinal fluid beneath it. During an impact, the scalp acts as rotational shock absorber by both compressing and sliding over the skull. This absorbs energy from the impact." [14]

The Phillips head protection system, also only available in motorbike helmets at present, is designed to reduce rotational injury.[15]

[edit] Proper fit

It is important that a helmet should fit the cyclist properly – according to research most helmets (well over 90 % [16]) 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 almost twofold[17].

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.

[edit]

Collision energy varies with the square of impact speed; a typical helmet is designed to absorb the energy of a head falling from a bicycle, an impact speed of around 12mph or 20 km/h.

"very little crushing of the liner foam was usually evident... What in fact happens in a real crash impact is that the human head deforms elastically on impact. The standard impact attenuation test making use of a solid headform does not consider the effect of human head deformation with the result that all acceleration attenuation occurs in compression of the liner. Since the solid headform is more capable of crushing helmet padding, manufacturers have had to provide relatively stiff foam in the helmet so that it would pass the impact attenuation test... As the results in Figure 15 illustrate, the child skull is far from being solid and will deform readily on impact. This fact is well known in the medical field and is largely why a child who has had a rather modest impact to the head is usually admitted to hospital for observation. The substantial elastic deformation of the child head that can occur during impact can result in quite extensive diffuse brain damage.

Anecdotal evidence

A common misunderstanding is to assume that a broken helmet has prevented some serious injury. "the main impact was to my head. So much so, that my helmet broke in two (as it is designed to do). Without the helmet, it would have been my head that was broken and I wouldn’t be writing this blog entry! I’d be dead..."[47] Helmets are designed to crush without breaking; expanded polystyrene absorbs little energy in brittle failure and once it fails no further energy is absorbed. To prevent overt fragmentation, the foam in most helmets is reinforced inside with plastic netting to keep the foam together.

Ranting about helmets isn't doing a thing - lobby for safe riding in the streets.