Seat belt

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

A three-point seat belt.

A seat belt, sometimes called a safety belt, is a harness designed to hold in place the occupant of a car or other vehicle against harmful movement that may result from a collision or a sudden stop. As part of an overall occupant restraint system, seat belts are intended to reduce injuries by stopping the wearer from hitting hard interior elements of the vehicle or other passengers and by preventing the wearer from being thrown from the vehicle.

Types of seat belts

Three point seat belt, Citroen BX

• Lap: Adjustable strap that goes over the waist. Used frequently in older cars, now uncommon except in some rear middle seats. Passenger aircraft seats also use lap seat belts.
• Two-point: A restraint system with two attachment points. A lap belt or (less commonly) diagonal belt (rare, common prior to the 1990s).
• Automatic: Any seat belt that closes itself automatically. There is also a lap belt which should be fastened.
• Sash: Adjustable strap that goes over the shoulder. Used mainly in the 1960s, but of limited benefit because it is very easy to slip out of in a collision.
• Lap and Sash: Combination of the two above (two separate belts). Mainly used in the 1960s and 1970s, usually in the craper. Generally superseded by three-point design.
• Three-point: Similar to the lap and sash, but one single continuous length of webbing. Both three-point and lap-and-sash belts help spread out the energy of the moving body in a collision over the chest, pelvis, and shoulders. Until the 1980s three-point belts were commonly available only in the front seats of 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. By September 1, 2007, all new cars sold in the US will require a lap and shoulder belt in the center rear.[1]
• Criss Cross Belt: Experimental safety belt presented in the Volvo SCC. It forms a cross-brace across the chest [2].

Harness

• Five-point harnesses are safer but more restrictive seat belts. They are typically found in child safety seats and 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. (Strictly speaking, harnesses are never to be fastened to the seat - they should be fastened to the frame/sub-frame of the automobile.)
• Six-point harnesses is like a five-point harness but includes an extra belt between the legs. These belts are used mainly in racing. In NASCAR, the six-point harness became popular after the death of Dale Earnhardt. Earnhardt was wearing a five-point harness when he crashed and died. Because it was thought at first that his belt had broken, some teams ordered a six-point harness. The sixth point has two belts between the legs, which is seen by some to be a weaker point than the other parts.
• Inertia reel: Used almost universally today, inertia reel belts are effectively self-adjusting, which improves effectiveness. They also retract when not in use, reducing the chances of damage to the belts. A retractor reel lets out the strap or pulls it back as needed, and in the event of an accident the reel locks, preventing any more strap to come out and holding the passenger in the car. This may be augmented by pretensioners (see below). Most three-point belts are of inertia-reel construction, as are some lap-and-sash and lap belts.

History

Seat belts were invented by George Cayley in the 1800s. They were introduced in aircraft for the first time in 1913, by Adolphe Pegoud, who became the first man to fly a plane upside-down. However, seat belts did not become common on aircraft until the 1930s.

Edward J. Claghorn was granted U.S. patent 312,085 on February 10, 1885 for a safety belt providing protection for a person ascending or descending a ladder or pole.

Edward J. Hock invented the safety belt first used by the Ford Motor Company as standard equipment, while he was on active duty with the military as a flight instructor. In 1955 his idea was accepted by the naval authorities, and Hock was awarded $20.50 for his invention. The original schematic and blueprints shows that he utilized scrap parachute strapping to implement his idea. He was never awarded anything other than the $20.50 award, and a letter of recognition, a picture with the 'brass', and a newspaper article to his credit.

Nils Bohlin of Sweden invented the three point seat belt for Volvo, who introduced it in 1959 as standard equipment. Bohlin was granted U.S. patent 3,043,625 for the device.

Most US automobiles were sold with front seat belts standard in the 1964 model year. Rear seat belts were made standard in 1968.

Mechanism

Seat Belt uncovered Inertial Reel

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. This is implemented with a centrifugal clutch, which engages as the reel spins quickly. Alternatively, they may also be secured by a weighted pendulum or ball bearing: when these are deflected by deceleration or roll-over they lock into pawls on the reel.

Types of inertia reel type seatbelts:

NLR (No Locking Retractor) - Generally applies to a recoiling lapbelt

ELR V (Emergency Locking Retractor - Vehicle sensitive) - Single sensitive, is comprised of a locking mechanism activated in an emergency by deceleration or rollover of the vehicle (ie the seatbelt is vehicle sensitive).

ELR VW (Emergency Locking Retractor - Vehicle and Webbing sensitive) - Dual sensitive means a seatbelt retractor that, during normal driving conditions, allows freedom of movement by the wearer of the seatbelt by means of length- adjusting components that automatically adjust the strap to the wearer, and that is activated by two or more of the following: a)deceleration or rollover of the vehicle,(V)or b)acceleration of the strap from the retractor,(W)or c)other means of activation.

Pretensioners and webclamps

Seatbelts in many newer vehicles are also equipped with 'pretensioners' and/or 'Webclamps'.

• Pretensioners preemptively tighten the belt to prevent the occupant from jerking forward in a crash. Mercedes-Benz first introduced pretensioners on the 1981 S-Class. 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.

• Webclamps clamp the webbing in the event of an accident and limit the distance the webbing can spool out (caused by the unused webbing tightening on the central drum of the mechanism)these belts also often incorporate "Rip stitching" which is when the lower part of the webbing is looped and stitched with a special stitching. The function of this is to 'rip' at a predetermined pressure rather than risk internal injuries to the occupants.

There is some research and tort liability case findings which suggest that some seatbelts are subject to inertial release. Although the NTHSA has been urged to deal with this defect it has yet not been properly addressed. "In 1992, the Institute for Injury Reduction (IIR) petitioned NHTSA to investigate such defects, but after several months, NHTSA denied the petition. Its own laboratory results indicated that inertial forces could cause a buckle to open, but that it didn’t happen in real world accidents." Source: Seat Belt Defects

Reminder chime and light

In North America and some other parts of the world, cars sold since the early 1970s have included a seat belt light on the dashboard, reminding the driver and passengers to buckle up. These systems also included a warning buzzer which sounded for several seconds before turning off (with the warning light), regardless of whether the car was started. New cars sold in the United States in 1974 and the first part of the 1975 model year were sold with a special "ignition interlock," whereby the driver could not start the car until the seat belt was fastened; however, this system was short-lived.

By the early 1980s, many car makers selling in the US market had replaced the buzzer (along with all other buzzers for functions such as headlights-on) with a seatbelt warning chime. Today, many of these carmakers use a red figure with its seatbelt on to serve as its seatbelt warning light, and it may stay on for several minutes after the car is started and the driver's seat belt is not fastened.

In Europe most modern cars include a seat-belt reminder light for the driver and some also include a reminder for the passenger, when present, activated by a pressure sensor under the passenger seat.

Some newer cars from Ford, Honda, Hyundai, and Toyota will intermittently flash the reminder light and sound the chime until the driver (and sometimes the front passenger, if present) fasten their seatbelt.

Legislation and risk compensation

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 seat belt has a significantly lower chance of death or serious injury.

The effects of such laws are disputed, stemming from the observed fact that no country is able to demonstrate a reduction in road fatalities due to passage of a seat belt law, though deaths have in some cases been migrated from drivers to other road users. This has influenced the development of risk compensation theory, which says that drivers adjust their behaviour in response to the increased sense of personal safety wearing a seat belt provides. In one trial subjects were asked to drive go-karts around a track under various conditions. It was found that that subjects who started driving unbelted drove consistently faster when subsequently belted. Similarly, a study of habitual non-seatbelt wearers driving in freeway conditions found evidence that they had adapted to seatbelt use by adopting higher driving speeds and closer following distances (similar responses have been shown in respect of anti-lock braking system and, more recently, airbags). It is also possible that the types of injury modelled in the trials were only a subset of potential serious injuries — for example, oblique impacts may produce twisting forces on the head leading to diffuse axonal injury, a particularly serious type of brain injury.

Put simply, then: if one is involved in a crash, one is almost always better off wearing a seat belt. However, the probability of being in a crash in the first place may be affected by the fact that the person feels safer, so the overall safety benefit may be offset to some unspecified degree.

See also

• Automobile safety
• Seat belt legislation
• Seat belt use rates in the USA

External links

• Seatbelt from HowStuffWorks
• Seatbelt Testing and Sled Slow Motion Video

Discussion of Forces involved in car crashes

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