T. E. Lawrence (known as Lawrence of Arabia) had a fatal crash on a Brough Superior SS100 on a narrow road near his cottage near Wareham. The accident occurred because a dip in the road obstructed his view of two boys on bicycles. Swerving to avoid them, Lawrence lost control and was thrown over the handlebars. He was not wearing a helmet, and suffered serious head injuries which left him in a coma; he died after six days in hospital. One of the doctors attending him was the neurosurgeon Hugh Cairns. He consequently began a long study of what he saw as the unnecessary loss of life by motorcycle despatch riders through head injuries and his research led to the use of crash helmets by both military and civilian motorcyclists. As a consequence of treating Lawrence, Sir Hugh Cairns ultimately saved the lives of many motorcyclists since.
Motorcycle helmets greatly reduce injuries and fatalities in motorcycle accidents, thus many countries have laws requiring acceptable helmets to be worn by motorcycle riders. These laws vary considerably, often exempting mopeds and other small-displacement bikes. In some countries, most notably the USA, there is some opposition to compulsory helmet use (see Helmet law defense league); not all USA states have a compulsory helmet law.
Worldwide, many countries have defined their own sets of standards that are used to judge the effectiveness of a motorcycle helmet in an accident, and define the minimal acceptable standard thereof. Among them are:
The Snell Memorial Foundation has developed stricter requirements and testing procedures for motorcycle helmets with racing in mind, as well as helmets for other activities (e.g. drag racing, bicycling, horseback riding), and many riders in North America consider Snell certification a benefit when considering buying a helmet while others note that its standards allow for more force (g's) to be transferred to a rider's head than the U.S. Department of Transportation (DOT) standard. However, the DOT standard does not test the chin bar of helmets with them, while the Snell (and ECE) standards do. A motorcycle helmet with either standard will nonetheless provide vastly more protection than one with neither.
In the United Kingdom, the Auto-Cycle Union (ACU) defines a stricter standard for racing than the legal minimum ECE 22.05 specification. Only helmets with an ACU Gold sticker are allowed to be worn in competition, or at track days. Many riders in the UK choose helmets with an ACU Gold sticker for their regular on-road use.
There are five basic types of helmets intended for motorcycling, and others not intended for motorcycling but which are used by some riders. All of these types of helmets are secured by a chin strap, and their protective benefits are greatly reduced, if not eliminated, if the chin strap is not securely fastened so as to maintain a snug fit.
From most to least protective, as generally accepted by riders and manufacturers, the helmet types are:
A full face helmet covers the entire head, with a rear that covers the base of the skull, and a protective section over the front of the chin. Such helmets have an open cutout in a band across the eyes and nose, with a plastic face shield (which may be clear or tinted) that generally swivels up and down to allow access to the face. Many full face helmets include vents to increase the airflow to the rider.
The significant attraction of these helmets is their protectiveness. Some critics dislike the increased heat, sense of isolation, lack of wind, and alleged reduced hearing of such helmets. Full face helmets intended for off-road use sometimes omit the face shield but extend the visor and chin portions.
Studies have shown that full face helmets offer the most protection to motorcycle riders because 35% of all crashes showed major impact on the chin-bar area . Wearing a helmet with less coverage eliminates that protection — the less coverage the helmet offers, the less protection for the rider.
The motocross and off-road helmet has clearly elongated chin and visor portions, a chin bar, and partially open face to give the rider extra protection while wearing goggles. The visor is to keep the sun out of the eyes of the rider when he or she goes off jumps.
Originally, off-road helmets did not include a chin bar, with riders using helmets very similar to modern open face street helmets, and using a face mask to fend off dirt and debris from the nose and mouth. Modern off-road helmets include a (typically angular, rather than round) chin bar to provide some facial impact protection in addition to protection from flying dirt and debris. When properly combined with goggles, the result provides most of the same protective features of full face street helmets.
A hybrid between full face and open face helmets for street use is the modular or "flip-up" helmet, also sometimes termed "convertible" or "flip-face". When fully assembled and closed, they resemble full face helmets by bearing a chin bar for absorbing face impacts. Its chin bar may be pivoted upwards (or, in some cases, may be removed) by a special lever to allow access to most of the face, as in an open face helmet. The rider may thus eat or drink without unfastening the chinstrap and removing the helmet.
Modular helmets are designed to be worn in the closed position for riding, as the movable chin bar is designed as a convenience feature, useful while not actively riding. The curved shape of an open chin bar and face shield section can cause increased wind drag during riding, as air will not flow around an open modular helmet in the same way as a three-quarters helmet. Since the chin bar section also protrudes further from the forehead than a three-quarters visor, riding with the helmet in the open position may pose increased risk of neck injury in a crash.
As of 2008, there have not been wide scientific studies of modular helmets to assess how protective the pivoting or removable chin bars are. Observation and unofficial testing suggest that significantly greater protection exists beyond that for an open face helmet, and may be enough to pass full-face helmet standardized tests, but the extent of protection is not fully established by all standards bodies.
The DOT standard does not require chin bar testing. The Snell Memorial Foundation has not yet certified any manufactured modular helmet using full-face standards. ECE 22.05 allows certification of modular helmets with or without chin bar tests, distinguished by -P (protective lower face cover) and -NP (non-protective) suffixes to the certification number, and additional warning text for non-certified chin bars.
The open face, or "three-quarters", helmet has a rear which also covers the back of the skull, but lacks the lower chin bar of the full face helmet, and does not necessarily include a face shield. Many offer visors of selectable length which may be used by the rider to reduce sunlight glare. An open face helmet provides the same rear protection as a full face helmet, but little protection to the face, even from non-crash events.
Bugs, dust or even wind to the face and eyes can cause rider discomfort or injury. As a result, it is not uncommon (and in some states, is required by law) for riders to wear wrap-around sunglasses or goggles to supplement eye protection with these helmets. Alternatively, many open face helmets include, or can be fitted with, a face shield extending over the upper portion of the face to protect the eyes.
The half helmet, also referred to as a "shorty", has essentially the same front design as an open face helmet but with a raised rear. The half helmet provides the minimum coverage generally allowed by law in the U.S. As with the open face, it is not uncommon to augment this helmet's eye protection through other means. Unlike open face and full face helmets, half helmets are also prone to shifting and sometimes coming off of the rider's head during an accident. Because of their inferiority compared to other helmet styles, some Motorcycle Safety Foundation courses prohibit the use of half helmets during riding exercises.
There are other types of headwear - often called "beanies," "brain buckets" or "novelty helmets" (a term which arose since they can not legally be called "motorcycle helmets") - which are not certified and generally only used to provide the illusion of compliance with mandatory helmet laws. Such items are often smaller and lighter than helmets made to DOT standards, and are unsuitable for crash protection because they lack the energy-absorbing foam that protects the brain by allowing it to come to a gradual stop during an impact. A "novelty helmet" can protect the scalp against sunburn while riding and - if it stays on during a crash - might protect the scalp against abrasion, but it has no capability to protect the skull or brain from an impact.
The purpose of the foam liner is to crush during an impact, thereby increasing the distance and period of time over which the head stops and reducing its deceleration.
To understand the action of a helmet, it is first necessary to understand the mechanism of head injury. The common perception that a helmet's purpose is to save the rider's head from splitting open is misleading. Skull fractures are usually not life threatening unless the fracture is depressed and impinges on the brain beneath and bone fractures usually heal over a relatively short period. Brain injuries are much more serious. They frequently result in death, permanent disability or personality change and, unlike bone, neurological tissue has very limited ability to recover after an injury. Therefore, the primary purpose of a helmet is to prevent traumatic brain injury while skull and face injuries are a significant secondary concern.
The most common type of head injury in motorcycle accidents is closed head injury, meaning injury in which the skull is not broken as distinct from an open head injury like a bullet wound. Closed head injury results from violent acceleration of the head which causes the brain to move around inside the skull. During an impact to the front of the head, the brain lurches forwards inside the skull, squeezing the tissue near the impact site and stretching the tissue on the opposite side of the head. Then the brain rebounds in the opposite direction, stretching the tissue near the impact site and squeezing the tissue on the other side of the head. Blood vessels linking the brain to the inside of the skull may also break during this process, causing dangerous bleeding.
Another hazard, susceptibility of the brain to shearing forces, plays a role primarily in injuries which involve rapid and forceful movements of the head, such as in motor vehicle accidents. In these situations rotational forces such as might occur in whiplash-type injuries are particularly important. These forces, associated with the rapid acceleration and deceleration of the head, are smallest at the point of rotation of the brain near the lower end of the brain stem and successively increase at increasing distances from this point. The resulting shearing forces cause different levels in the brain to move relative to one another. This movement produces stretching and tearing of axons (diffuse axonal injury) and the insulating myelin sheath, injuries which are the major cause of loss of consciousness in a head trauma. Small blood vessels are also damaged causing bleeding (petechial hemorrhages) deep within the brain.
It is important that the liner in a motorcycle helmet is soft and thick so the head decelerates at a gentle rate as it sinks into it. Unfortunately, there is a limit to how thick the helmet can be for the simple reason that the helmet quickly becomes impractical if the liner is more than 1–2 inches (2.5–5 cm) thick. This implies a limit to how soft the liner can be. If the liner is too soft, the head will crush it completely upon impact without coming to a stop. Outside the liner is a hard plastic shell and beyond that is whatever the helmet is hitting, which is usually an unyielding surface, like concrete pavement. Consequently, the head cannot move any further, so after crushing the liner it comes suddenly to an abrupt stop, causing high accelerations that injure the brain.
Therefore, an ideal helmet liner is stiff enough to decelerate the impacting head to an abrupt stop in a smooth uniform manner just before it completely crushes the liner and no stiffer. The required stiffness depends on the impact speed of the head, which is unknown at the time of manufacture of the helmet. The result is that the manufacturer must choose a likely speed of impact and optimize the helmet for that impact speed. If the helmet is in a real impact that is slower than the one for which it was designed, it will still help but the head will be decelerated a little more violently than was actually necessary given the available space between the inside and outside of the helmet, although that deceleration will still be much less than what is would have been in the absence of the helmet. If the impact is faster than the one the helmet was designed for, the head will completely crush the liner and slow down but not stop in the process. When the crush space of the liner runs out, the head will stop suddenly which is not ideal. However, in the absence of the helmet, the head would have been brought to a sudden stop from a higher speed causing more injury. Still, a helmet with a stiffer foam that stopped the head before the liner crush space ran out would have done a better job. So helmets help most in impacts at the speeds they were designed for, and continue to help but not as much in impacts that are at different speeds. In practice, motorcycle helmet manufacturers choose the impact speed they will design for based on the speed used in standard helmet tests. Most standard helmet tests use speeds between 4 and 7 m/s (9 and 16 mph, or 14 and 25 km/h).
The speeds are chosen based on modern knowledge of the human tolerance for head impact, which is by no means complete. It is possible to deduce how well the 'perfect' helmet outlined in the Function section of this page would perform in an impact of a given severity. If currently available data suggest that the rider is unlikely to survive in such an impact, regardless of how well his helmet performs, then there is little point in demanding that helmets be optimized for this impact. On the other hand, if an impact is so mild that the rider is unlikely to be injured at all so long as he is wearing a helmet than that impact is not a demanding test. Modern standards setters choose the severity of the standard test impact to be somewhere between these two extremes, so that manufacturers are doing their best to protect the riders who can be helped by their helmet during a head impact.