HOW DOES AN AIRBAG WORK?
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Adriver's airbag is normally located in the steering wheel boss and as it inflates it bursts through the surface trim breaking it open along predetermined "split fines" hidden in the foam padding. The airbag is inflated by gases which are given off when an 8.0 gram pellet of a solid chemical is ignited electrically. It burns rapidly to produce a large volume of gas which passes through a catalyst into the airbag itself. In passing through the catalyst, the gases are converted to harmless carbon dioxide, nitrogen and water. Such is the speed of the combustion that the airbag is fully inflated in 0.04sec (1/25th), literally in the blink of an eye, and it is ready to do its job even before the occupant's head moves forward as a result of the impact. Contrary to popular belief, the airbag does not stay inflated but deflates progressively under the weight of the driver's head as it hits it. If it did not deflate in this way, it would be elastic and cause the head to bounce back sharply, possibly causing secondary damage. The signal to fire the pellet is given automatically by a microprocessor if the vehicle hits a hard object at above 18km/h (12mph). There is no danger of the airbag being deployed accidentally because of the design of the microprocessor circuitry. It is programmed to recognise the vehicle's individual "crash signature" and will not respond to any other signal. The crash signature which is a characteristic trace of the vehicle's deceleration with time, is determined from as many as 50 separate crash tests. Even this advanced electronic signal is not sufficient on its own to trigger the firing; a mechanical spring/mass system also has to have sensed the crash, before firing can take place.