Seat belt

This article is about the safety device. For the band see The Seatbelts.

A seat belt, sometimes called a safety belt, is a harness designed to hold the occupant of an car or other vehicle in place if a collision occurs. Seat belts are intended to reduce injuries by stopping the wearer from hitting hard interior elements of the vehicle or from being thrown from the vehicle. In cars seat belts also prevent rear-seat passengers from crashing into those in the front seats.

Lap belts are seat belts that go over the wearer's hips. These were an earlier style of belt and are today less common in the developed world, being found mostly in passenger aircraft. In early SAABs a different kind of two-point belt similar to a three-point belt, but without the lap part was used. Shoulder belts, or three-point belts, include a lap belt and a second belt going from one anchor point on the lap belt to a point over and behind the occupant's shoulder. Three point harnesses were first made readily available in mass-produced vehicles by Volvo. It was Swedish engineer Nils Bohlin who patented the modern three-point belt design along with Volvo. This design is crucial in aiding a person in the event of a crash. The three-point design greatly reduces the effects of secondary collisions. A secondary collision is the impact between a person and the interior of the automobile. The three-point innovation helps spread out the energy of the moving body inside of the car during a wreck. It spreads that energy out over the stronger parts of the body. This includes the chest, pelvis, and shoulders. Until recently shoulder belts were only available in the front seats of the cars, the back seats having only lap belts. Evidence of the potential for lap belts to cause separation of the lumbar vertebrae and the sometimes associated paralysis, or "seat belt syndrome", has led to a revision of safety regulations in nearly all of the developed world requiring that all seats in a vehicle be equipped with three-point belts.

Five-point harnesses are safer but more restrictive seat belts, typically found in child safety seats, and also in racing cars. The lap portion is connected to a belt between the legs and there are two shoulder belts, making a total of five points of attachment to the seat. Seat belts were first invented by George Cayley in the 1800's. Seat belts were introduced in aircraft in the 1930s. The automotive seat belt was introduced into the United States by William Myron Noe, whose patented quick release seat belt, the AutoCrat Safety Belt, was the first seat belt installed as original equipment in the US by Ford in its 1956 model year. However, they were not required by law in the US on passenger vehicles until the 1968 model year.

Mechanism

Most seat belts are equipped with locking mechanisms that tighten the belt when pulled hard (e.g. by the force of a passenger's body during a crash) but do not tighten when pulled slowly. Many are also equipped with 'pretensioners', which preemptively tighten the belt to prevent the passenger from jerking forward in a crash.

Autoliv Corporation first developed pretensioners in 1986. In the event of a crash, a pretensioner will tighten the belt almost instantaneously. This reduces the load on the occupant in a violent crash. Like airbags, pretensioners are triggered by sensors in the car's body, and most pretensioners use explosively expanding gas to drive a piston that retracts the belt. Pretensioners also lower the risk of "submarining", which is when a passenger slides forward under a loosely worn seat belt.

See the page http://auto.howstuffworks.com/seatbelt.htm for more.

Legislation

The issue of seat belt legislation has been a source of some controversy. Hospital based studies of car accident victims, experiments using both crash test dummies and actual human cadavers have indicated that wearing seat belts should provide a reduced risk of death and injury in many types of car crash. This has led many countries to adopt mandatory seat belt wearing laws. It is generally accepted that, in comparing like-for-like accidents, a vehicle occupant wearing a properly fitted seatbelt has a significantly lower chance of death or serious injury.

However, the effects of such laws are hotly disputed. The dispute centres on the effects of seat belt legislation for both car occupants and non-vehicle occupants, such as motorcyclists, cyclists and pedestrians. The effect of seat belt laws influenced the development of the risk compensation hypothesis which argues that car drivers may adjust their driving behaviour adversely in response to the increased sense of personal safety wearing a seat belt provides.de:Sicherheitsgurt es:Cinturón de seguridad fr:Ceinture de sécurité nl:Autogordel ja:シートベルト pl:Pas bezpieczeństwa zh:安全帶