How Seat Belt Tech Saves Lives

seat belts
Seat belts come in many different configurations. Image courtesy of Lenore Edman, via Flickr (Creative Commons 2.0)

The first precursor to the modern seat belt was invented in the late 1800s, but the first automobiles lacked any kind of safety restraints. In fact, seat belts didn’t become standard equipment in any cars or trucks at all until the middle of the 20th century. Early seat belts were offered as an option by some manufacturers as early as 1949, and Saab introduced the practice of including them as standard equipment in 1958.

Legislation has been one of the driving factors behind the adoption of car safety features like seat belts, and many governments have laws that dictate how many belts a vehicle needs to have in addition to specifications that the belts need to meet.

Types of Seat Belts

There are a few main types of seat belts that have been used in cars and trucks throughout the years, though some of them have been phased out.

  • lap
  • sash
  • two-point
  • three-point

Two-point belts have two points of contact between the belt and the seat or the body of the vehicle. Lap and sash belts are both examples of this type. Most of the early seat belts offered as optional or standard equipment in cars and trucks were lap belts, which are designed to tighten directly over the lap of a driver or passenger. Sash belts are similar, but they cross diagonally over the chest. This is a less common design since it is possible to slide under a sash belt during an accident.

Most modern seat belts use three-point designs, which mount to the seat or body of the vehicle in three different places. These designs typically combine both a lap and sash belt, which provides a more secure hold during a crash.

Retraction Technologies

The first seat belts were very simple devices. Each half of the belt was bolted to the body of the car, and they would simply hang freely when not buckled together. One side tended to be static, and the other would have a tightening mechanism. This type of seat belt is still commonly used in airplanes, though it has fallen out of use in cars and trucks.

In order for early seat belts to be effective, they had to be tightened after they were buckled. That tended to be somewhat uncomfortable, and it could also reduce a person’s range of movement. In order to account for that, locking retractors were designed. This seat belt technology typically makes use of a static receptacle and a long, retractable belt that plugs into it. During normal use, the retractor allows for a little bit of movement. However, it is capable of quickly locking in place in case of an accident.

Early seat belt retractors made use of centrifugal clutches to spool out the belt and lock in place during an accident. The clutch is activated any time the belt is pulled out very quickly, which can be observed by simply yanking on it. This effectively allows for a modicum of comfort while still offering the protection of a seat belt.

Modern vehicles use a number of different technologies to provide both comfort and safety, including pretensioners and webclamps.

Passive Restraints

Most seat belts are manual, which means each driver and passenger has the choice of whether or not to buckle up. In order to remove that element of choice, some governments have passed passive restraint legislation or mandates. In the United States, the Secretary of Transportation issued a mandate in 1977 that required all passenger vehicles to have some form of passive restraint by 1983.

Today, the most common type of passive restraint is the airbag, and legislation requires vehicles sold in the United States and elsewhere to have one or more of them. However, automatic seat belts were a popular, lower cost alternative throughout the 1980s.

Some automatic seat belts were motorized during that period, though many were simply connected to the door. This allowed the driver or passenger to slide into place under the belt, which would be effectively “fastened” when the door was closed.

While automatic seat belts were cheaper and easier to implement than airbags, they presented a few disadvantages. Vehicles that have manual lap belts and automatic shoulder belts present the same dangers as vehicles that only use sash belts, since the occupants may choose to not fasten the manual lap belts. In some cases, drivers and passengers also had the option of unbuckling the automatic shoulder belt, which was often seen as an annoyance.

When airbags became standard equipment in all new passenger cars and trucks, automatic seat belts fell out of favor entirely.