Passive Car Safety Systems - A Passive Attack!

So-called ‘passive safety’ systems in cars do nothing … until you crash. Then they spring into action literally faster than the blink of an eye, with the express intent of saving your neck. They’re called passive because no action or intervention by the driver brings them into play.
- shares

Perhaps we should back up a sec. Modern cars are jam-packed with safety kit, which is split into two fundamental camps – ‘active’ safety systems, like brakes, which are designed to help you avoid crashing in the first place, and ‘passive’ safety systems, like airbags, designed to lessen the impact on you when you are actually crashing. Passive systems are invoked automatically by the car.

Australia has the dubious distinction of being a developed country with one of the world’s oldest fleet of cars actually ‘out there’ on the road. Average age: about 10 years. In terms of technology this is a bad thing – it means the average Aussie driving around today is 10 years behind the eight-ball on safety technology (and emissions technology, and…) It’s easy to forget that if you’re a motoring journo, swanning around in new cars all the time.

So, although many high-end Euro luxury/performance cars will offer the lucky few who buy them something like nine airbags, the average Australian in a car today is lucky to be protected by just two – at the front.

It’s interesting to note that while airbags are passive devices, seatbelts are active – because the driver must elect to use them. Thankfully we’ve been pretty good at that in Australia since the 1970s, and it remains the main reason why our road toll, per capita, is much better than that of the USA.

Airbags are just part of a crash safety system that is fundamentally integrated into the car’s structure – which is why you can’t add them to a car in an aftermarket sense. This is because although the hardware itself (crash sensor; steering wheel with inbuilt airbag module, seatbelt pre-tensioner) could be easily boxed up and sold over the counter at Super Cheap or Repco, you could never tune it so it would work effectively.

Airbags must deploy at the ‘Goldilocks moment’ – not too early, and not too late. And here, a few milliseconds either way makes the difference between life or death. That’s why car companies do all that expensive crash testing. It’s the precise choreography of all that in-car explosive stuff – down to one-thousandth of a second accuracy – that really does the job of saving lives.

The blink of an eye takes about a fifth of a second (200 milliseconds). The bit of a car crash that can kill you is all over in about 40-80 milliseconds – two-and-a-half to five times faster than the blink of an eye. That’s from the moment the car first contacts the thing it hits, to the point the impact danger is effectively past. In that impossibly small window of opportunity the crash sensor must determine that a crash is taking place, decide if it is serious enough to deploy the airbag, send a pulse to the airbag’s detonator if it is (and a separate pulse at a separate time to the detonator in the seatbelt pre-tensioner). Then both charges in the pre-tensioner have to explode and, in the case of the airbag, generate enough nitrogen gas to deploy the bag, get it out there, ready, fractions of a second before your head hits it. Once the crash sensor says the binary equivalent of ‘go for it’, it’s all over in under 40 milliseconds.

There are variations, but most airbags use a detonator and solid propellant. The detonator fires into the solid propellant, which causes a rapid chemical reaction. An inert gas, usually nitrogen, is created at high pressure. It’s this that inflates the bag. Nitrogen is comparatively harmless, which is a good thing … since ordinary air is about 78 per cent nitrogen. (You can actually suffocate in a 100 per cent nitrogen environment, since there’s no oxygen, but that’s not much of a risk in a car crash.)

Sodium-azide was a popular propellant in older airbags. Unfortunately, it was highly toxic (sodium-azide itself, that is – not the post-deployment byproducts, which were carbon-monoxide and nitrogen oxide). It was mostly phased out in the 1990s. Nitrocellulose (think: gunpowder) has largely also been phased out because it’s not as stable as newer propellants.

If you’ve ever been in a crash in which the airbags deploy, you’ll probably notice the cabin is liberally dusted with white powder. You will be, too. Some people are moved to ponder what it is, and the health implications of breathing this stuff. Don’t worry – it’s not a combustion product. It’s just talcum powder or commercial chalk, which is used to lubricate the bag.

For every car crash, there are actually three big hits – and it’s the third one that kills you. Hit one is between the car and whatever it hits (think: 100-year-old gum tree). In slo-mo, the metal starts to deform. The tree pushes back on the car, which accelerates backwards (decelerates, if you like). You get thrust forward, relative to the car, which leads to…

Hit two, which is between you and the car. If you’re dumb and not wearing a seatbelt, much of that impact will take place between your head and the windscreen; fade to black. If you’re one of the 99-point-something percent of Australians who does wear a seatbelt, the impact will mainly occur between your hips and thorax, and the belt itself (your head and legs might still hit the dash, however, causing life-threatening injuries). If there’s a lot of slack between you and the belt, hit two will be bigger than if the belt is snug – because if it is loose, your body will be travelling at a high speed relative to the decelerating car when you slam into the belt. But relax; you’re not dead yet.

Hit three is the one that really counts. It’s the arbiter of life and death. After your body hits the seatbelt, your internal organs slosh forward, and hit the front of your rib cage (on the inside). If that hit is big enough, it can sever your aorta, the major blood vessel in your chest, or rip a hole in your heart. That’s bad. If your head hits the dash (or the windscreen) your brain will hit the inside of your skull. Blood vessels will tear, and blood will leak into your skull cavity, which is, essentially a rigid box. The pressure of the blood inside your head – there’s nowhere for it to go – can kill you. (Also bad…)

This is what all that crash-mitigation technology seeks to avert – in simplistic terms the airbag springs into action between collisions one and two, which has a flow-on benefit to collision three.

Look at it like this: A crash sensor, which is just a box with an accelerometer in it (or accelerometers, if the car has side airbags), and a chip inside which says ‘go bang if the crash gets worse than this XXX’. If the crash severity exceeds this predetermined line in the sand, the seatbelt pre-tensioner fires off, sucking in all the slack in the belt (reducing collision two). It also sucks you square-on into the seat so you present a better target to the airbag. Within instants the airbag is full. It actually starts deflating before your heads hits it – when that happens, it’s the automotive equivalent of jumping off a four-storey building onto a stack of Sealy Posturepedic mattresses, as opposed to, say, the bare concrete footpath.

There’s one more thing happening: the car’s structure itself is protecting you. All that controlled deformation in engineered ‘crumple zones’ up the pointy end of the crash is absorbing energy – before it gets to you. This is why old crashed cars never looked that damaged, and yet the people inside died. These days, the cars often appear royally screwed over by similar collisions, and yet the people walk away.

So, if you’re in a car with airbags, there are a few things you need to do: You need to realize they’re there, all the time. An airbag is the kind of thing that will wait patiently for 20 years or more for it’s big moment, then deploy in under a heartbeat. It’s easy to forget the damn things are there. But you really need to remember – because getting in its way at exactly the wrong time is bad.

The driver’s front airbag is packed into the hub of the steering wheel. So, if you drive one-handed, with your arm across the steering wheel (say, with your right arm at the 10 o’clock position or your left at 2 o’clock) and have a crash while you’re driving that way, your arm will be between the airbag and your head. And while it’s easy to joke about having ‘Seiko’ embossed permanently into your forehead afterwards, if there is an afterwards, the real problem is that your arm will spoil the deployment, and the airbag won’t be able to protect your head. This is just another reason why driving two-handed with hands at 9 and 3 o’clock isn’t optional.

Passengers have responsibilities, too. The passenger’s front airbag (which is bigger than the driver’s because it has to fill a bigger space, and must therefore deploy even faster) comes out of the dashboard. How often have you seen a front-seat passenger driving with a leg crossed over the knee or, worse, a foot (or feet) on the dash? Imagine what happens there – knees blown back into chests at 300km/h (the approximate airbag deployment speed), and no subsequent head protection…