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How Does An Airbag Work?

Airbags – they’re in every single new car coming out and if your car doesn’t have at least one airbag somewhere, it’s probably old enough to just about qualify as a classic.  It’s probably just about getting to the point that Millennials (or the children of Millennials – whatever trendy label you slap on that generation) will probably take airbags for granted much the same way that Gen X takes seatbelts for granted. (Boomers and Busters probably remember a time before seatbelts were compulsory back and front – I’m Gen X and I’ve got vague memories of cars with no rear seatbelts.  They weren’t common.)

The airbag concept has been around since the early 1950s, with patents being granted just about simultaneously in the US and in Germany to two different inventors, Walter Linderer and John Hetrick (which makes me suspect a little idea swapping was going on during the post-war Allied occupation of Berlin).  However, airbags didn’t really become popular until the 1970s, which was when Ford decided to give them a try.  This was about the time when legislating bodies around the world were taking a long, hard look at what was happening on the roads and were really getting serious about road safety, although it wasn’t until 1984 that the US made seatbelt use compulsory.  It was also in the 1980s that saw vehicles of all marques installing airbags in an attempt to pick up accreditation from the newly established EuroNCAP.

Now, of course, airbags are everywhere: front airbags for the driver, front airbags for the front passenger, side airbags, knee airbags, curtain airbags – even “pedestrian airbags” that deploy on the outside of the vehicle in some Volvos.

Most of the time, if we’re driving properly and everyone around us is driving properly, we won’t see diddly-squat of the airbags.  So it should be.  This means that how they work can be something of a mystery.

Any airbag, no matter where it’s located, has three parts to it: the bag itself, the sensor that tells the airbag to deploy (you don’t want the airbags firing at the wrong moment) and the inflation system.  The airbag itself is somewhat uninteresting: it’s a bag of nylon fabric that can pack up nice and tight, withstand the forces of sudden inflation, is airtight enough to actually inflate but has enough holes so it can deflate afterwards.  They also come lubricated with ordinary talcum powder to help them move easily and stay supple.  It’s the sensors and the inflation system that are a bit more fun.

The sensor picks up any force that’s equal or greater to the car going head-first into a brick wall at about 16 km/h, meaning that if you nudge the back of the garage at a crawl, if you ram the front bumper with a shopping trolley or if the cat jumps onto the car to enjoy the warmth of the bonnet, the airbag won’t deploy.  The sensors usually sit in the crumple zones and the typical modern vehicle will have about three of them.  The sensors are simple affairs, consisting of a ball in a tube as the main trigger.  If the ball is jolted out of the tube, this sets the inflation system off.

The inflation system is, quite literally, rocket science.  In other words, it uses the same technology as solid fuel rocket booster systems.  One of the early teething troubles they had with airbags was finding a method of inflating the bags that didn’t take up too much room or create an additional hazard (canisters of compressed gas had storage problems), deployed quickly enough and didn’t save your life but leave you deaf thanks to a thunderous explosion.  Nasty toxic gases were also to be avoided.  No point saving your life from the crash if it’s going to asphyxiate you as the gas dissipates.

The solution came in the form of two very reactive chemical compounds: sodium azide (NaN3) and potassium nitrate (KNO3).  Don’t play with these in the chemistry lab at school.  When the trigger goes off, these two are ignited so the reaction begins.  And does it begin or what!  These compounds burn fast and release a heck of a lot of nitrogen gas in a very short space of time.  This gas inflates the airbag super-fast so the bag deploys out of wherever it’s stored at over 300 km/h.  The nitrogen gas isn’t going to harm anyone as it goes back into the atmosphere – which is about 90% oxygen anyway.  Some mechanisms use slightly different chemicals but still work on the same basic principle and produce an equally harmless gas, such as argon.

In summary, the system works like this:

1 The force of a collision knocks the ball out of the tube (the most basic version of the accelerometer that detects a massive slowdown – there are other types out there).

2 The trigger flicks an electric switch that heats up an element.

3 The heat sets off a very, very fast chemical reaction.

4 The reaction produces heaps of gas very quickly, which fills the bag.

Can you get an airbag to deploy if you brake hard enough?  No.  In spite of urban legends and speculations tossed around by younger drivers, emergency braking in itself won’t set off the airbags.  Brakes just don’t generate the sharp, short force needed to set them off – even emergency braking is more gradual than that.  The airbags also won’t deploy if you are rear-ended (you’re being shoved forwards, not brought to a sudden stop) or if you use the bull bars on the front of a 4×4 as a DIY bulldozer (unless you try to get a running start into whatever you’re trying to push).  They will probably deploy if an enraged bull or ram takes a dislike to your car and charges it head-on, although I doubt this has been tested.  The popular amusing video of an elderly pedestrian setting off the airbag on the car belonging to an impatient driver is probably a fake, meaning it’s been carefully staged with a vehicle that has had the sensor system adjusted, then hit in exactly the right place.  But it’s still funny.

Airbags are not without hazards of their own.  Yes, they have saved thousands of lives.  However, anything that is moving at over 300 km/h is going to pack a lot of punch, even if it is only air and cloth.  Anyone who’s been on the receiving end of a well-flicked tea towel while drying dishes with siblings knows that “just” cloth can draw blood if it goes fast enough.  Not that an airbag will draw blood.  It can break your glasses as your head flies forward to meet it and it’s still going to hurt.  It will hurt even more and probably will draw blood and worse if you have anything on your lap that gets between you and the airbag – another reason not to try hiding your cellphone on your lap!

The force involved in a deploying airbag is too much for small children, which is why it’s best to put little kids of an age to sit in a booster seat in the back.  However, we know that sometimes, you just have to put a small person in the front (although this isn’t likely to happen unless you have three other littlies already in the back).  This is why some vehicles have occupant seat detection for the front passenger seat that has a weight sensor, so the airbag won’t go off if whatever’s in that seat is below a certain weight.

Airbags should never, ever, ever be used as a substitute for a seatbelt.  It’s a case of “both–and” rather than “either–or” and why would you want to skimp on your personal safety anyway?  It’s not as if an undeployed airbag is going to get in your way, limit your fun or restrict your freedom of movement or ability to drive well.  Don’t be a silly muggins – wear your seatbelt!