Watch a car seatbelt pretensioner do its safety magic at 28,000 frames per second



The Slow-Mo guys from YouTube are at it again. This time they have rigged up their ultrahigh speed cameras to film a car’s seatbelt pretensioner in action. And the result is pretty spectacular, and just a bit terrifying.

First introduced on the 1981 Mercedes-Benz S-Class, seatbelt pretensioners are essentially passive safety devices that preemptively tighten the belt to prevent the occupant from jerking forward in a crash. If you’re not acquainted with the whole science of it, here’s how it works.

In its most basic sense, a seatbelt pretensioner sacrifices its life in an accident, in order to save yours. Using information from the vehicle’s brake and stability control sensors, the system recognizes the critical moments when a crash is likely. It then removes the seat belt slack to hold the person in the most secure position possible. It also acts as a warning to the driver that an accident could occur. If the accident is avoided, the system just resets itself automatically. The system normally works in conjunction with conventional seatbelt locking mechanisms, not in place of them. Some systems also pre-emptively tighten the belt during fast accelerations and strong decelerations, even if no crash has happened.

Some pretensioners use pyrotechnic devices that are typically controlled by the same sensor system as the airbags. Others use a mechanical spring-loaded mechanism that works just as well. In their video, the Slow-Mo Guys used both.

The idea behind the video is to capture the whole operation in video slow enough that it is visible to the normal eye. A pretensioner works so blindingly fast – it has to, else what is the benefit anyways – that its action can’t be recorded fully with a conventional camera. The Phantom Flex and Phantom Flex 4K are two of the world’s fastest cameras, used primarily in research and defense applications, but it was put the use for us pretty well here.

Watch it below.

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