A seat belt is a vehicle safety device designed to secure the driver or a passenger of a vehicle against harmful movement that may result during a collision or a sudden stop. A seat belt reduces the likelihood of death or serious injury in a traffic collision by reducing the force of secondary impacts with interior strike hazards, by keeping occupants positioned correctly for maximum effectiveness of the airbag (if equipped) and by preventing occupants being ejected from the vehicle in a crash or if the vehicle rolls over.
American car manufacturers Nash (in 1949) and Ford (in 1955) were the first to offer seat belts as options. The real breakthrough with modern seat belts came in 1958 when engineer Nils Bohlin invented the three-point seat belt to help secure both the upper and lower body. Up until this point, seat belts in cars were two-point lap belts, which strapped across the body, with the buckle placed over the abdomen.
Since the creation of the National Safety and Motor Vehicle Traffic Act of 1966, American vehicles were required to have seat belts in their cars.
Types of Seat Belts
A lap belt is a strap that goes over the waist. This was the most common type of belt prior to legislation requiring three-point belts, and is found in older cars. Evidence of the potential of lap belts to cause separation of the lumbar vertebrae and the sometimes associated paralysis, or"seat belt syndrome," led to progressive revision of passenger safety regulations in nearly all developed countries to require three-point belts. Since September 1, 2007, all new cars sold in the U.S. require a lap and shoulder belt in the center rear seat.
A "sash" or shoulder harness is a strap that goes diagonally over the vehicle occupant's outboard shoulder and is buckled inboard of his or her lap. The shoulder harness may attach to the lap belt tongue, or it may have a tongue and buckle completely separate from those of the lap belt. Shoulder harnesses of this separate or semi-separate type were installed in conjunction with lap belts in the outboard front seating positions of many vehicles in the North American market starting in 1968. However, if the shoulder strap is used without the lap belt, the vehicle occupant is likely to "submarine", or slide forward in the seat and out from under the belt, in a frontal collision.
A Seat Belt and Buckle
Three-Point Challenger Seat Belt
In the mid-1970s, Chrysler designed the safer three-point Uni-Belt” to supplant the separate lap and shoulder belts in American-made cars. A three-point belt is a Y-shaped arrangement, similar to the separate lap and sash belts, but unitized. Like the separate lap-and-sash belt, in a collision the three-point belt spreads out the energy of the moving body over the chest, pelvis, and shoulders.
The Belt-in-Seat (BIS) is a three-point harness with the shoulder belt attached to the seat itself, rather than to the vehicle structure. BIS type belts have been used by automakers in convertibles and pillarless hardtops, where there is no "B" pillar to affix the upper mount of the belt. Chrysler and Cadillac are well known for using this design.
4, 5 and 6-point Belts
Dodge Demon Racing Harness
Five-point harnesses 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. A 4-point harness is similar, but without the strap between the legs, while a 6-point harness has two belts between the legs. In NASCAR, the 6-point harness became popular after the death of Dale Earnhardt, who was wearing a five-point harness when he suffered his fatal crash; as it was first thought that his belt had broken, and broke his neck at impact, some teams ordered a six-point harness in response.
Seat Belt Airbags
Seat belt airbags are available in some models of Ford and Mercedes (see "Inflatable" discussion, below).
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. Most modern 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 lock. A webbing-sensitive lock is based on a centrifugal clutch, activated by 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.
The second type, 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.
Pre-tensioners and Webclamps
Seatbelts in many newer vehicles are also equipped with "pre-tensioners" or "web clamps", or both. Pre-tensioners preemptively tighten the belt to prevent the occupant from jerking forward in a crash. Mercedes-Benz first introduced pre-tensioners on the 1981 S-Class. In the event of a crash, a pre-tensioner will tighten the belt almost instantaneously. This reduces the motion of the occupant in a violent crash. Like airbags, pre-tensioners are triggered by sensors in the car's body, and many pre-tensioners have used explosively expanding gas to drive a piston that retracts the belt. Pre-tensioners 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.
Pyrotechnic Pre-tensioner Diagram
Web clamps 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 a 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.
Inflatable seat belts have tubular inflatable bladders contained within an outer cover. When a crash occurs the bladder inflates with a 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.
Inflatable Seat Belt
Automatic Seat Belts
Seat belts 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 U.S.
General Motors introduced a three-point non-motorized passive belt system in 1980 to comply with the passive restraint requirement. However, 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.
Automatic seat belt in a Honda Civic
For more related information on this topic, see the following Challengerforum articles:
Crash Test Dummies
Development of Airbags