Seat Belt

The Science Behind Seat Belt Functionality and Repair

Seat belts are designed to manage forces on your body in the event of a crash. They also prevent ejection from the vehicle, which can be very dangerous and sometimes fatal.

The belt stretches across your pelvis and chest and fastens to three points on the car’s frame. It is designed to distribute force to these stronger, bonier areas to minimize injury and death.

Spool and Spring

When it comes to car safety, seat belts are a must. In fact, according to the National Highway Traffic Safety Administration, they save 15,000 lives each year. However, while seat belts may seem to be a relatively simple device (a strap that prevents you from flying forward), the truth is that a lot is going on behind the scenes.

For example, seat belts are designed to distribute crash forces across the body’s more vital bony parts, such as the shoulders and pelvis, to provide maximum protection. This is particularly important in the case of rollover crashes, where occupants ejected from vehicles are at much higher risk for severe injury or death.

The spool and spring are key components that enable seat belts to perform these functions. They consist of tightly woven fabric wrapped around the torso and fastened to anchor points in the vehicle.

When the car stops suddenly, a switch activates the spool and spring, which pulls the webbing tight against the occupant’s chest and lap. This locks the belt, reducing the chance of the occupant being thrown from the vehicle in a crash.

The spool and spring are made from nylon, a material known for its strength and durability. However, like any mechanical device, the spool and spring can be subject to wear and tear, so they must be inspected periodically.


A seat belt retractor holds the excess webbing in place so that it is not spooling out constantly, keeping the belt taut. It consists of a spool and a spring that apply rotation force. Pulling your seat belt, the spool spins counter-clockwise to untwist the spring, allowing the belt to tighten. In a crash, however, it is essential that the retractor locks quickly to prevent the strap from loosening.

The retractor is operated by a weighted pivoting lever attached to the spinning spool. When something yanks the belt, centrifugal force causes the weighted end of the lever to extend outward, which pushes a cam piece attached to the spool’s rotating ratchet gear. The cam moves the pin along a groove in the pawl and locks it into the gear’s teeth, preventing the spool from spinning counter-clockwise.

While it is rare for a seat belt to malfunction, like when fixing a seatbelt that won’t retract, there are cases in which the retractor does not work as it should. When this occurs, you must have the retractor replaced immediately. If you delay, you could end up with dangerous excess slack or webbing that would put you at risk of severe injury in a crash.

Another way the retractor can fail is for a piece of debris to become lodged in the device, which keeps the locking slider from sliding into its locked position when the release button is pressed. This is a common problem and the most likely cause of a seat belt that does not retract properly.

Locking Mechanism

Modern seat belts are regulated in their dimensions and tensile properties and must meet national and international performance standards (see Fung & Hardcastle, 1991). They also have to comply with abrasion and UV resistance specifications.

When a seat belt is worn correctly, it spreads crash forces across the solid and bony parts of the body – the collar bone (clavicle), chest bone (sternum), and hip bones (pelvis). This reduces the force that would otherwise be exerted on the head, preventing it from being thrown forward in an impact and causing injuries such as fractures, bruising, whiplash, or even brain damage.

It also prevents the occupant from being ejected from the vehicle – an event that is associated with a high risk of death and severe injury, especially in rollover crashes. Seat belts are also a vital line of defense against internal injuries from fire or submersion in water.

Even though they do not protect against every type of crash, seat belts have saved lives in many cases and are among the most effective passive safety systems available.

Nevertheless, some people are still hesitant to use them, and studies show that more persistent reminders can improve belt use rates. Ignition interlocks, which limit a driver’s speed until they buckle up, have also been shown to increase seat belt use by 33 percent (Kidd & Singer, 2019).


As its name implies, this device tightens the belt for you. The mechanism works by pushing the belt mechanically into channels, and then, when activated, a piston is driven upward, quickly winding up the spool. This swift retraction removes most of the slack in the belt, dramatically reducing the distance your body is thrown forward during an accident.

There are two types of tensioners – mechanical and electrical. Most cars built in the last decade have electrical pre-tensioners using sensors and pyrotechnic gas generators to actuate as your airbags do. The generator creates a small explosive charge that drives a piston in a concealed tube, rapidly winding up the spool and tightening your seatbelt.

Most seatbelts also include load limiters, which reduce the risk of chest injury by releasing a little bit of the excess belt webbing once it spools up too much force in an accident. You can see a load limiter by looking for an extra fabric fold on your seatbelt’s webbing.

It is important to remember that seat belts are a safety feature and should only be used as intended. It’s also essential to follow the owner’s manual for your vehicle and ensure all your seat belts are correctly adjusted and in good condition. If you have questions about your seat belts, Polestar’s Customer Support team is always happy to help.


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