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How does that work? Airbags

How does that work? Airbags


What's behind the wheel of every car? Not just a driver or a horn but also a little bag of wind that might save your life. Airbags employ some high-tech science, but just how do they work?

Seat belts have saved thousands of lives since their mandatory introduction, and air bags are the natural progression in safety. The first commercial airbags appeared in cars in the 1980s, and have been fitted as standard in driver and passenger sides since 1998. Government statistics show that they reduce the risk of dying in a head-on crash by about 30% for belted drivers, and with side and seat-mounted airbags this percentage increases. In a few years time, it is thought that all cars will have six or eight airbags - and research into their use is serious business.

If your car stops suddenly, you’ll continue to move forward unless you are restrained. The force that propels you forward is very strong, because the car instantly stops while you don’t. Seatbelts work well by stopping you moving quickly but the force is still high and the force of the belt on your body can be as lethal as the crash itself. An airbag slows your crash speed to zero, but has very little time to work from the point of impact. Luckily, they are very high-tech.

Several questions appeared in the early development of airbags, which included; is the airbag dangerous if you are too close and how can a bag be made to inflate really quickly without causing an ear-splitting bang? The critical period for the inflation of the bag is the first 5-8 cm, during which time the exploding bag can cause fatal injury itself. Researchers have since stated that the safe distance from the steering wheel to your chest should be about 25cm - plenty of time for the bag to inflate fully before you are pushed forward into it. The steering wheel airbag should be pointed towards your chest not your head - if it’s not, make sure you adjust the steering column. For children, the rules are different so make sure you read up before any journey. Some airbags can now be deactivated, but only if the driver or passenger has a particular medical condition that requires it.

Airbags still have to be used with seatbelts, because they only help when there is a head-on crash, and not a side or rear-end impact. Although side impact airbags are increasingly more common now, they still only work when used in conjunction with a seatbelt. Approximately 30% of vehicle accidents are from side impact, but the problem faced by crash research companies is that, on front impacts, the bumper takes most of the force, so the bag can inflate in 25 milliseconds. In a side impact, because of the short distance between the door and the driver or passenger, the inflation time has to be a lot quicker - as little as 5 milliseconds! For those cars that do have them, they are triggered by a side impact of about 12 mph. Current research is looking at door mounted airbags, as there is more room in the door so the bags can be larger.

Finally, the ITS (Inflatable Tubular Structure) is the world’s first inflatable head airbag - a protection system to reduce head injuries during side impacts and in roll-overs. It is a long nylon tube about 15cm in diameter that inflates above the doors at head-level. It was introduced in 1997 on BMW cars and is becoming more common now.

With so much attention on safety, there are bound to be more innovations in the near future, all of which should make travelling safer.


For more information, visit the Royal Society for Prevention of Accidents website on http://www.rospa.com


Three parts to an airbag

1. The nylon fabric that is enclosed behind the wheel, in the door or on the dashboard.

2. A sensor that inflates the bag. This is usually set to be equivalent to a car hitting a wall at between 10 and 15 mph. A smaller bump will not set it off - but two electrical contacts touch when the impact speed reaches the critical amount.

3. The inflation system throws together sodium azide (NaN3) with potassium nitrate (KNO3) to make 35 litres of nitrogen gas. The bag inflates in a fraction of a second; to be precise it’s just 25 milliseconds (1/25 of a second) and the bag shoots out of the wheel at about 200mph. As you hit the bag, the gas escapes quickly through tiny holes thus slowing you to a complete stop.


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08 Jun 2008
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