Modern cars come packed with an impressive array of safety devices designed to prevent accidents in the first place and to protect the driver and passengers should one still happen.
But one of the most vital pieces of safety equipment is actually one of the oldest. It's the seat belt. The three-point seat belt was invented by Swedish engineer Nils Bohlin in 1958 and became standard equipment in Volvos the following year.
Seat belts have undergone quite a bit of change since then with inertia reels, pretensioners and webclamps now incorporated into the design. They're now easier to use and more effective while giving the user some freedom of movement.
They have five functions:
- To ensure the wearer decelerates at the same rate as the vehicle in a crash.
- To spread the force of an impact over the pelvis and chest area, rather than more vulnerable body parts.
- Preventing the driver or passengers hitting the vehicle's interior.
- Stopping the occupants being thrown from the vehicle.
- To work together with airbags so that the occupant's speed is reduced so they hit the airbag safely.
The key to seat belts working with airbags to minimise harm is the seat belt pre-tensioner. It works to tighten the seat belt around the occupant the moment a collision occurs, or by monitoring the brakes when they're applied hard. An Electronic Control Unit (ECU) creates an explosive charge in a gas generator which generates a volume of gas which acts on a mechanical linkage to pull and tighten the seat belt.
The system is similar to the operation of an airbag. Pretensioners are designed to slacken off at the same time the airbag deploys, around 30 milliseconds after the collision.
Seat belts save lives when they're used properly, but use them wrongly and they can actually harm you. Internal injuries to the spine, abdomen, shoulder, neck and sternum are all possible with badly worn seat belts, frequently as a result of that rapid firing pretensioner.