Motorcycle helmet

From Academic Kids

This article is part of the
hats and headgear series:
Overview of headgear
Hats; Bonnets; Caps
Hoods; Helmets; Wigs
Masks; Veils; Scarves
Tiaras; Papal tiaras
Crowns; Types of crowns
List of hats and headgear

A motorcycle helmet is a type of protective headgear used by motorcycle riders. The primary goal of a motorcycle helmet is to protect the rider's head during impact, although many helmets provide additional conveniences, such as face shields, ear protection, intercom etc.

Contents

Standards

In North America, there are two well-known sets of standards that can be used to judge the effectiveness of a motorcycle helmet in an accident. DOT is often considered the minimum for legal riding. The Snell Memorial Foundation (http://www.smf.org/) has developed stricter requirements and testing procedures for motorcycle helmets, as well as helmets for other activities (e.g. bicycling, horseback riding).

Most motorcycle helmet standards around the world use impacts at speeds between 4 and 7 m/s. At first glance, this is confusing given that motorcyclists frequently ride at speeds of 20 or 30 m/s. This confusion is relieved by understanding that the perpendicular impact speed of the helmet is usually not the same as the road speed of the motor cycle and that the severity of the impact is determined not only by the speed of the head but also by the nature of the surface it hits. For example, the surface of the road is almost parallel to the direction the motorcyclist moves in so only a small component of his velocity is directed perpendicular to the road while he is riding. Of course, other surfaces are perpendicular to the motorcylists velocity such as trees, walls and the sides of other vehicles. The other vital factor in determining the severity of an impact is the nature of the surface struck. The sheet metal wall of a car door may bend inwards to a depth of 7.5 - 10 cm (3 - 4 inches) during a helmeted head impact, meaning that it generates more stopping distance for the rider's head than the helmet itself. So a perpendicular impact against a flat steel anvil at 5 m/s might be about as severe as a 30 m/s oblique impact against a concrete surface or a 30 m/s perpendicular impact against a sheet metal car door or windscreen. Overall, there is a very wide range of severity in the impacts that could conceivably happen in a motorcycle impact. Some of these are more severe than the impacts used in the standard tests and some are less so.

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.

Basic types

There are three basic types of motorcycle helmets. From most to least protective, they are:

Missing image
BlackFullCoverageMotorcycleHelmet.jpg
Full face helmet
Full face 
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; critics dislike the increased heat, sense of isolation, lack of wind, and reduced hearing that such a helmet makes inevitable. Full face helmets intended for off-road use sometimes omit the face shield but extend the visor and chin portions.

A subset called "Convertible" or "Flip-up" is also available; in these helmets, the chin bar pivots upwards (or, in some cases, may be removed). The rider may thus eat or drink without unfastening the chinstrap and removing the helmet.

3/4 face helmet
Enlarge
3/4 face helmet
3/4 face 
This helmet's rear also covers the back of the skull, but lacks the lower chin armor of the full face helmet, as well as the face shield. Many offer visors of selectable length, some opaque, some tinted, which may be used by the rider to block out sunlight or headlights. A 3/4 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 for riders to wear wrap-around sunglasses or goggles to supplement eye protection with these helmets.
1/2 helmet 
With essentially the same front design as a 3/4 face helmet, but a raised rear, the half helmet provides the minimum coverage generally allowed by law in the US. As with the 3/4 face, it is not uncommon to augment this helmet's eye protection through other means.

All of these types of helmets are secured by a chin strap, and their protective benefits are greatly reduced if the chin strap is not fastened.

There are other helmets - often called "beanies" or "novelty helmets" - which are not certified and generally only used to provide the illusion of compliance with mandatory helmet laws. Such helmets are often smaller and lighter than DOT-approved helmets, 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.

Some motorcycle helmets have a built-in so-called MROS (Multiple Reflective Optic System): a set of reflective surfaces inside the helmet which together function as a rear-view mirror [1] (http://www.reevu.nl).

Construction

Modern helmets are constructed from plastics, often reinforced with kevlar or carbon fiber. They generally have fabric and foam interiors for both comfort and protection. Motorcycle helmets are generally designed to break in a crash (thus expending the energy otherwise destined for the wearer's skull), so they provide little or no protection after their first impact. Note that impacts may, of course, come from things other than crashing, such a dropping a helmet, and may not cause any externally visible damage. For the best protection, helmets should be replaced after any impact, and every three or so years even if no impact is known to have occurred.

Function

The conventional motorcycle helmet has two principal protective components: a thin, hard, outer shell made of acrylonitrile butadiene styrene (ABS) plastic, fiberglass or kevlar and a soft, thick, inner liner usually made of expanded polystyrene foam or expanded polypropylene foam. The purpose of the hard outer shell is

  1. to prevent penetration of the helmet by a pointed object that might otherwise puncture the skull, and
  2. to provide structure to the inner liner so it does not disintegrate upon abrasive contact with pavement. This is important because the foams used have very little resistance to penetration and abrasion.

The purpose of the foam liner is to crush during an impact, thereby increasing the distance and period of time over which the helmet stops and reducing its acceleration.

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 you from splitting your head 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. Think of how you lurch backwards and forwards while standing on a bus as it accelerates or stops. 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 bleeds.

It is clear then that it is very 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 or 2 inches 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. What happens then? Well, beyond the liner is a hard plastic shell and beyond that is whatever the helmet is hitting, which is presumably an unyielding surface. The head cannot move any further so after crushing the liner it comes suddenly to a dead stop, causing high accelerations that injure the brain.

This means that an ideal helmet liner is stiff enough to decelerate the impacting head to a dead stop in a smooth uniform manner just before it completely crushes the liner and no stiffer. So how stiff is that? The answer, significantly, is that it depends. It depends on the impact speed of the head, which is of course 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 5 and 7 m/s. This choice is described in greater detail in the standards section.

Manufacturers

Some well-known manufacturers of motorcycle helmets are:

  • AGV (http://www.agv.com/)
  • Arai (http://www.araihelmet.com)
  • Bell (http://www.bellhelmets.com/)
  • Nolan (http://www.nolan.it)
  • Schuberth (http://www.schuberth.com/)
  • Shoei (http://www.shoei.com) (pronounced show-eh)
  • Suomy (http://www.suomy.com/)

Laws

Motorcycle helmets are believed to greatly reduce casualties in motorcycle accidents, thus many states and countries have laws requiring minimum acceptable helmets to be worn by motorcycle riders. This is not the case everywhere and there are some who disagree with the enforcement of helmet use, opposing them on civil liberty or safety grounds (see Helmet law defense league).

Other

A full face motorcycle helmet is sometimes used in a robbery and other crimes, as a mask to prevent recognition. For someone participating in a riot it has a double function of protection against some harm as well as against recognition.

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