Bicycle helmet: Encyclopedia II - Bicycle helmet - About helmets

Bicycle helmet - About helmets

Bicycle helmet - 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 reduce the energy of a 30mph impact to only 27.5mph, 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 such as cuts and grazes 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 (E.P.S.), 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) on the head. They are not designed to provide adequate protection for a collision involving another moving vehicle, (e.g. a car), though they do provide some protection. The reason for this is that nobody has been able to draw up a practical design providing such a protection.

A cycle helmet should not be too heavy and provide adequate ventilation, because cycling can be an intense aerobic form of exercise which significantly raises body temperature. Thus, most helmets are constructed from lightweight materials pierced by strategically placed ventilation holes.

Bicycle helmet - History

Prior to the mid-1970s, the dominant form of helmet was the leather "hairnet" style. This offered minimal impact protection and acceptable protection from scrapes and cuts. Two of the first modern bicycle helmets were made by MSR, a manufacturer of mountaineering equipment, and Bell Sports, a manufacturer of helmets for auto racing and motorcycles. These helmets were a spinoff from the development of expanded polystyrene (EPS) foam liners for motorcycling and motorsport helmets, and had hard polycarbonate plastic shells. Ironically, the bicycle helmet arm of Bell was split off in 1991 as Bell Sports, having completely overtaken the motorcycle and motor sports helmet business.

The first commercially successful purpose-designed bicycle helmet was the Bell Biker, a polystyrene-lined hard shell released in 1975. At the time there was no appropriate standard; the only applicable one, from Snell, would be passed only by a light open-face motorcycle helmet. Over time the design was refined and by 1983 Bell were making the V1-Pro, the first polystyrene helmet intended for racing use. In 1984 Bell produced the Li'l Bell Shell, a no-shell children's helmet. These early helmets had little ventilation.

1985 saw the introduction of Snell B85, the first widely-adopted standard for bicycle helmets; this has subsequently been refined into B90 and B95 (see Standards below). At this time helmets were almost all either hard shell or no-shell (perhaps with a vacuum-formed plastic cover). Ventilation was still minimal due mainly to technical limitations of the foams and shells in use.

Around 1990 a new construction technique was invented: in-mould microshell. A very thin shell was incorporated during the moulding process. This rapidly became the dominant technology, allowing for larger vents and more complex shapes than hard shells.

Hard shells declined rapidly among the general cyclist population during the 1990s, almost disappearing by the end of the decade, but remain popular with BMX riders as well as inline skaters and skateboarders.

The late 1990s and early 2000s saw advances in retention and fitting systems, replacing the old system of varying thickness pads with cradles which adjust quite precisely to the rider's head. This has also resulted in the back of the head being less covered by the helmet; impacts to this region are rare, but it does make a modern bike helmet much less suitable for activities such as unicycling, skateboarding and inline skating, where falling over backwards is relatively common. Other helmets will be more suitable for these activities.

Some believe that helmet shells help reduce friction and reduce the tendency of the helmet to dig in to the surface on impact; there is evidence which contradicts this. The evidence regarding this effect mostly predates microshell helmets.

Bicycle helmet - Standards

In the United States the Snell Memorial Foundation, an organization initially established to create standards for motorcycle and auto-racing helmets, implemented one of the first standards. The American National Standards Institute (ANSI) created a standard called ANSI Z80.4 in 1984. Later, the United States Consumer Product Safety Commission (CPSC) created its own mandatory standard for all bicycle helmets sold in the United States, which took effect in March 1999.

In the UK the currently applicable standard is EN 1078:1997, which replaces BS 6863:1989.

The CPSC and EN1078 standards are lower than the Snell B95 (and B90) standard; Snell helmet standards are externally verified, with each helmet traceable by unique serial number. EN 1078 is also externally validated, but lacks Snell's traceability. The most common standard in the US, CPSC, is self-certified by the manufacturers. It is generally true to say that Snell standards are more exacting than other standards, and most helmets on sale these days will not meet them (no current Bell brand helmet is Snell certified, some Specialized ones are - the Snell Memorial Foundation website includes a list of certified helmets).

In 1990 the Consumers' Association (UK) market survey showed that around 90% of helmets on sale were Snell B90 certified. By their 1998 survey the number of Snell certified helmets was around zero. Hard shells declined rapidly among the general cyclist population over this period, almost disappearing by the end of the decade, but remained more popular with BMX riders as well as inline skaters and skateboarders.

Standards are generally getting weaker, driven by the market's desire for lighter and more ventilated helmets, but efficacy against minor injuries, which is the design purpose of most helmets, is not dissimilar. Nonetheless, all other things being equal, a Snell certified helmet is probably objectively safer than a non-Snell one.

All helmets sold today must meet basic safety standards. The difference between inexpensive and expensive helmets will more likely reflect ventilation, comfort and convenience issues rather than safety.

Bicycle helmet - Proper fit

It is important that a helmet should fit the cyclist properly - according to research up to 96% of helmets have been found to be incorrectly fitted, and an incorrectly fitted helmet puts you at up to three times more risk.

First, the correct size must be purchased. Most manufacturers provide a range of sizes ranging from children's to adult with additional variations from small to medium to large.

Helmets are held on the head with nylon straps, which must be adjusted to fit the individual. The ease with which adjustments can be made can be one of the major differences between a cheap helmet and a better quality one.

A common mistake is to fit the helmet so that it sits high on the forehead. The helmet should sit level on the cyclists head with only a couple of finger-widths between eyebrow and the helmet brim. It should not be possible to insert more than one finger between the strap and the throat, or to move the helmet more than a centimetre or so in any direction. The strap should be well back under the chin, close to the throat.

Adapted from the Wikipedia article "About helmets", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki