Watching A Seatbelt Pre-Tensioner Work In Super Slo-Mo Is Fascinating

I’m not sure how aware you are that there’s a very good possibility that you’re driving around with explosives right by your ass cheek. It’s okay, though—those explosives are there to help you in case you wreck. More importantly, they’re fun to cram full of paint and watch in super slow-motion.

The explosives in question are one method of making a super-fast-acting seatbelt pre-tensioner. Essentially, the pre-tensioner’s job is to tighten your belt so you get hugged fast and tight to your seat at the very first sign of an accident, which can save you all kinds of unpleasantness.


In this video from The Slo-Mo Guys, they’ve got ahold of an explosive-fired seat belt pre-tensioner. They’ve crammed it full of paint, and set it off, capturing the result on 28,000 frames per second. This is something that’s designed to happen almost instantaneously, so seeing it happen in a gradual ballet of smoke and paint is remarkable:

Here’s what this mechanism is like inside, courtesy a Mazda service manual:


I also want to mention that there’s a variation of this explosive-powered pre-tensioner that uses a tiny Wankel engine instead of a linear actuator like the one in the video.


I’ve always wondered if you could rig a fuel system up to something like that and make a go-kart motor or something.

Anyway, this is just a reminder you’re possibly sitting right next to some explosives, so, you know, enjoy.

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Jason Torchinsky

Senior Editor, Jalopnik • Running: 1973 VW Beetle, 2006 Scion xB, 1990 Nissan Pao, 1991 Yugo GV Plus • Not-so-running: 1973 Reliant Scimitar, 1977 Dodge Tioga RV (also, buy my book!)