HOW THINGS WORK: Pretensioner [VIDEO]

In this segment, we take a more in-depth look at the need for seatbelts and the vital role they play when we wear them. We learn more about the different types of pretensioners and how they work to make sure that our seat belts keep us safely in our car seats should we suffer the impact of an accident.

Estimated reading time: 1 minute, 4 seconds.

If you can’t watch the video, you can read the video transcription below: Edited for clarity and readability

Welcome to this week's episode of How Things Work on Let's Talk Automotive. And in today's episode, we are going to be talking about seatbelts and in particular, a very special device that we find on our seatbelts, which is our pretensioner. Now let's take a few steps back and just have a little chat about seatbelts and why it is so important from a safety point of view to wear your seatbelt. So, first of all, we did some calculations a little bit earlier on, and we worked out that if you have an accident in a vehicle at a hundred kilometres an hour, and you hit it into a solid object if you're not wearing your seatbelt, you will experience up to 100Gs of impact force. And the threshold normally for survivability is 40Gs so there's no doubt that you're going to be in serious, serious danger if you're not wearing a seatbelt.

However, if you do wear a seatbelt, the G-forces are reduced down to about 20 in the same accident so you’re very much within your survivability range. So let's put that in perspective. When Romain Grosjean had his accident in the last round of the Grand Prix in Bahrain last year, his accident yielded a G-Force rating of 80Gs. So if you're not wearing a seatbelt, you're going to be in a worse off position than Romain Grosjean was in his accident and you're going a hundred kilometres an hour so it's absolutely critical.

So how does a seatbelt save our lives? Well, there are a couple of functions that a seatbelt performs, and I've got one right here with me and we're going to also talk to the pretensioner as well. So first things first: the seatbelt these days has a little bit of stretch in it. I can't really simulate it now, but I promise you in an accident, my body weight would cause this seat belt to stretch a little bit. And what that's doing is it's slowing me down a little bit. The second thing that our seatbelt has is a load force limiter, which is actually a very strong spring that's connected to the end of the seatbelt and that again allows a little bit more movement in my body when I'm really, really putting a lot of forces against the seatbelt. And what that does is, is once again, it just slows me down and by slowing me down, I'm now getting rid of a whole lot of kinetic energy.

So we have three types of pretensioners that we find on a vehicle. So the first one is a mechanical pretensioner, and I can demonstrate that with this seatbelt over here. So you can see that when I pull on the seatbelt, the pretensioner will tighten up the seatbelt a little bit. Then we have an electronic pretensioner, which uses an electric motor to wind up the seatbelt, but the most sophisticated version that we have is a pyrotechnic pretensioner. And we find pyrotechnic pretensioners as standard equipment even on the Suzuki S-Presso that we’re standing next to over here.

So this is in fact, the pretensioner from this vehicle. Now, what does a pretensioner do for us when it's got pyrotechnics in it? So one of the things that I want to avoid is an impulse force that I'll experience if my seatbelt is a little bit loose.  What the pretensioner does is that when the vehicle's in an accident, the same sensors that trigger my airbags will also trigger this pretensioner and it'll immediately tighten up the seatbelt and push me back into my car seat. And that will stop me from having any forward movement relative to the seatbelt. And that's going to dramatically reduce the forces that I experience. And that's why with the pretensioner on our seatbelt in a 100km an hour accident, we can dramatically reduce the G-forces.

So how does the pretension work? Well, we have a pinion that has gears on it, we have a piston that has the same matching gears, and then we have our pyrotechnic gases that are released when the system is activated. And these pyrotechnic gases push through this pipe over here and cause the piston to move up in this motion and as it's moving up, it winds up the pinion, which then pulls me back into my seat. So a remarkable device that is certainly saving lives in the event of a heavy impact and is a must-have on vehicles these days. 

So I hope you found that useful and that you now understand what a pretensioner does and we look forward to seeing you on future episodes of Let's Talk Automotive.

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