What safety system automatically tightens a seatbelt during a crash?

A seatbelt pretensioner automatically tightens the belt during a crash to remove slack and hold the occupant in a safer position.

Crash sensors trigger the device almost instantaneously. In many vehicles, a small pyrotechnic charge produces expanding gas that drives a piston or retracts part of the belt webbing. By pulling the belt snug before the occupant moves far forward, the pretensioner helps reduce contact with the steering wheel, dashboard or door and lowers the risk of submarining beneath the lap belt.

Pretensioners work alongside, rather than replace, other restraint components. A locking retractor stops the belt from paying out during sudden deceleration, while a load limiter allows controlled belt movement after the initial restraint and helps reduce chest forces. The pretensioner is therefore not the same thing as either a retractor or a load limiter.

Mercedes-Benz first introduced seatbelt pretensioners on the 1981 S-Class. Newer vehicles may also use electrically operated pretensioners in pre-crash systems; these can tighten repeatedly during severe braking or evasive manoeuvres, unlike many one-shot pyrotechnic units.