Technology

Locking retractorsedit

The purpose of locking retractors is to provide the seated occupant the convenience of some free movement of the upper torso within the compartment while providing a method of limiting this movement in the event of a crash. Starting in 1996, all passenger vehicle seatbelts must lock pre-crash meaning they have a locking mechanism in the retractor or in the latchplate. Seat belts are stowed on spring-loaded reels called "retractors" equipped with inertial locking mechanisms that stop the belt from extending off the reel during severe deceleration.

There are two main types of inertial seat belt locks. A webbing-sensitive lock is based on a centrifugal clutch activated by the rapid acceleration of the strap (webbing) from the reel. The belt can be pulled from the reel only slowly and gradually, as when the occupant extends the belt to fasten it. A sudden rapid pull of the belt—as in a sudden braking or collision event—causes the reel to lock, restraining the occupant in position. Automatic locking retractor for seat belts and shoulder harnesses in the U.S. was the Irving "Dynalock" safety device. These "Auto-lock" front lap belts were optional on AMC cars with bucket seats in 1967.

A vehicle-sensitive lock is based on a pendulum swung away from its plumb position by rapid deceleration or rollover of the vehicle. In the absence of rapid deceleration or rollover, the reel is unlocked and the belt strap may be pulled from the reel against the spring tension of the reel. The vehicle occupant can move around with relative freedom while the spring tension of the reel keeps the belt taut against the occupant. When the pendulum swings away from its normal plumb position due to sudden deceleration or rollover, a pawl is engaged, the reel locks and the strap restrains the belted occupant in position. Dual-sensing locking retractors use both vehicle G-loading and webbing payout rate to initiate the locking mechanism.

Pretensioners and webclampsedit

Seatbelts in many newer vehicles are also equipped with "pretensioners" or "web clamps", or both.

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 motion of the occupant in a violent crash. Like airbags, pretensioners are triggered by sensors in the car's body, and many pretensioners have used explosively expanding gas to drive a piston that retracts the belt. Pretensioners also lower the risk of "submarining", which occurs when a passenger slides forward under a loosely fitted seat belt.

Some systems also pre-emptively tighten the belt during fast accelerations and strong decelerations, even if no crash has happened. This has the advantage that it may help prevent the driver from sliding out of position during violent evasive maneuvers, which could cause loss of control of the vehicle. These pre-emptive safety systems may prevent some collisions from happening, as well as reducing injury in the event an actual collision occurs. Pre-emptive systems generally use electric pretensioners which can operate repeatedly and for a sustained period, rather than pyrotechnic pretensioners, which can only operate a single time.

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 an energy management loop ("rip stitching") in which a section of the webbing is looped and stitched with special stitching. The function of this is to "rip" at a predetermined load, which reduces the maximum force transmitted through the belt to the occupant during a violent collision, reducing injuries to the occupant.

A study demonstrated that standard automotive three-point restraints fitted with pyrotechnic or electric pretensioners were not able to eliminate all interior passenger compartment head strikes in rollover test conditions. Electric pretensioners are often incorporated on vehicles equipped with precrash systems; they are designed to reduce seat belt slack in a potential collision and assist in placing the occupants in a more optimal seating position. The electric pretensioners also can operate on a repeated or sustained basis, providing better protection in the event of an extended rollover or a multiple collision accident.

Inflatableedit

The inflatable seatbelt was invented by Donald Lewis and tested at the Automotive Products Division of Allied Chemical Corporation. Inflatable seatbelts have tubular inflatable bladders contained within an outer cover. When a crash occurs the bladder inflates with gas to increase the area of the restraint contacting the occupant and also shortening the length of the restraint to tighten the belt around the occupant, improving the protection. The inflatable sections may be shoulder-only or lap and shoulder. The system supports the head during the crash better than a web-only belt. It also provides side impact protection. In 2013, Ford began offering rear seat inflatable seat belts on a limited set of models, such as the Explorer and Flex.

Automaticedit

Seatbelts that automatically move into position around a vehicle occupant once the adjacent door is closed and/or the engine is started were developed as a countermeasure against low usage rates of manual seat belts, particularly in the United States. The 1972 Volkswagen ESVW1 Experimental Safety Vehicle presented passive seat belts. Volvo tried to develop a passive three point seatbelt. In 1973, Volkswagen announced they had a functional passive seat belt. The first commercial car to use automatic seat belts was the 1975 Volkswagen Golf.

Automatic seat belts received a boost in the United States in 1977 when Brock Adams, United States Secretary of Transportation in the Carter Administration, mandated that by 1983 every new car should have either airbags or automatic seat belts. There was strong lobbying against the passive restraint requirement by the auto industry. Adams was criticized by Ralph Nader, who said that the 1983 deadline was too late. The Volkswagen Rabbit also had automatic seat belts, and VW said that by early 1978, 90,000 cars had sold with them.

General Motors introduced a three-point non-motorized passive belt system in 1980 to comply with the passive restraint requirement. However, it was used as an active lap-shoulder belt because of unlatching the belt to exit the vehicle. Despite this common practice, field studies of belt use still showed an increase in wearing rates with this door-mounted system. General Motors began offering automatic seat belts on the Chevrolet Chevette. However, the company reported disappointing sales because of this feature.

A study released in 1978 by the United States Department of Transportation said that cars with automatic seat belts had a fatality rate of .78 per 100 million miles, compared with 2.34 for cars with regular, manual belts.

In 1981, Drew Lewis, the first Transportation Secretary of the Reagan Administration, influenced by studies done by the auto industry, dropped the mandate; the decision was overruled in a federal appeals court the following year, and then by the Supreme Court. In 1984, the Reagan Administration reversed its course, though in the meantime the original deadline had been extended; Elizabeth Dole, then Transportation Secretary, proposed that the two passive safety restraints be phased into vehicles gradually, from vehicle model year 1987 to vehicle model year 1990, when all vehicles would be required to have either automatic seat belts or driver side air bags. Though more awkward for vehicle occupants, most manufacturers opted to use less expensive automatic belts rather than airbags during this time period.

When driver side airbags became mandatory on all passenger vehicles in model year 1995, most manufacturers stopped equipping cars with automatic seat belts. Exceptions include the 1995–96 Ford Escort/Mercury Tracer and the Eagle Summit Wagon, which had automatic safety belts along with dual airbags.citation needed

Systemsedit

• Manual lap belt with automatic motorized shoulder belt—When the door is opened, the shoulder belt moves from a fixed point near the seat back on a track mounted in the door frame of the car to a point at the other end of the track near the windshield. Once the door is closed and the car is started, the belt moves rearward along the track to its original position, thus securing the passenger. The lap belt must be fastened manually.
• Manual lap belt with automatic non-motorized shoulder belt—This system was used in American-market vehicles such as the Hyundai Excel and Volkswagen Jetta. The shoulder belt is fixed to the aft upper corner of the vehicle door and is not motorized. The lap belt must be fastened manually.
• Automatic shoulder and lap belts—This system was mainly used in General Motors vehicles, though it was also used on some Honda Civic hatchbacks and Nissan Sentra coupés. When the door is opened, the belts go from a fixed point in the middle of the car by the floor to the retractors on the door. Passengers must slide into the car under the belts. When the door closes, the seat belt retracts into the door. The belts have normal release buttons that are supposed to be used only in an emergency, but in practice are routinely used in the same manner as manual seat belt clasps.citation needed This system also found use by American Specialty Cars when they created the 1991-1994 convertible special edition of the Nissan 240SX, a car that traditionally had a motorized shoulder belt.

Disadvantagesedit

Automatic belt systems generally offer inferior occupant crash protection. In systems with belts attached to the door rather than a sturdier fixed portion of the vehicle body, a crash that causes the vehicle door to open leaves the occupant without belt protection. In such a scenario, the occupant may be thrown from the vehicle and suffer greater injury or death.

Because many automatic belt system designs compliant with the US passive-restraint mandate did not meet the safety performance requirements of Canada—which were not weakened to accommodate automatic belts—vehicle models which had been eligible for easy importation in either direction across the US-Canada border when equipped with manual belts became ineligible for importation in either direction once the US variants obtained automatic belts and the Canadian versions retained manual belts. Two particular models included the Dodge Spirit and Plymouth Acclaim.verification neededverification neededoriginal research?

Automatic belt systems also present several operational disadvantages. Motorists who would normally wear seat belts must still fasten the manual lap belt, thus rendering redundant the automation of the shoulder belt. Those who do not fasten the lap belt wind up inadequately protected only by the shoulder belt; in a crash without a lap belt such a vehicle occupant is likely to "submarine" (be thrown forward under the shoulder belt) and be seriously injured.citation needed Motorized or door-affixed shoulder belts hinder access to the vehicle, making it difficult to enter and exit—particularly if the occupant is carrying items such as a box or a purse. Vehicle owners tend to disconnect the motorized or door-affixed shoulder belt to relieve the nuisance when entering and exiting the vehicle, leaving only a lap belt for crash protection.citation needed Also, many automatic seat belt systems are incompatible with child safety seats, or only compatible with special modifications.