Airbags

Vehicle crash strategies are primarily concerned with restraining vehicle occupants, and absorbing collision energy that would otherwise be transferred to the occupant.

The primary method of occupant restraint is the seat belt. However airbags are used to supplement the effect of the seat belt and have proven to be very effective in reducing the likelihood of severe head injuries.

How they work

A front airbag system includes a large nylon bag, which is inflated and deflated rapidly in the event of a severe frontal collision. The drivers airbag is housed in the centre pad of the steering wheel, and the passengers airbag, where fitted, in the upper left of the dash.

The airbags deployment is controlled by several sensors that detect the occurrence and severity of a frontal crash. When the airbag controller determines that the airbag should be deployed, the system triggers an inflator unit that burns chemicals very rapidly to produce large volumes of inert gas to inflate the bag.

As the bag inflates, it splits open the covers on the wheel or the dash, ballooning in front of the occupant. At the same time the occupants head and upper body is moving with significant force toward the inflated bag. As the occupants head strikes the airbag, the bag starts deflating through vent holes in its base. This controlled deflation cushions the decelerating heads forward movement. The whole process occurs within about 100 milliseconds - about the same time as the blink-of-an-eye.

In the process of deploying, considerable smoke, dust and noise will be produced and the occupant may be unaware that the bag has deployed. This is normal.

When do they deploy?

For the driver or passenger airbags to deploy, the following minimum criteria must all be met:

• A frontal collision has occurred while the vehicle was traveling at more than about 25km/h.
• The angle of impact is within a window typically around thirty degrees either side of the cars centre line (around 60 degrees in total).
• The deceleration forces produced are at least equal to those produced when the car collides head-on with an immovable barrier at approximately 25km/h.

If all these triggering conditions are not met the airbag will not deploy. Thus, a vehicle with a properly functioning airbag system could suffer significant frontal collision damage without airbag deployment.

Front airbags will not be deployed in the event of a side or rear end collision or in a rollover, as deployment would not enhance occupant protection.

Dual stage airbags

Not to be confused with dual front airbags, dual stage or adaptive airbags are a smarter generation of airbags that can optimise the level of airbag deployment to suit the severity of the crash. The airbag is of a variable volume and can be inflated in two stages depending on the severity of the crash. The level of occupant protection is improved accordingly and the risk of secondary injury reduced.

Side, curtain and knee airbags

Many vehicles offer seat mounted side airbags, and side impact curtain airbags mounted inside the sides of the roof frame. They are intended to help protect occupants against head, neck and thorax injuries during side impacts and rollovers. Knee bags protect lower limbs from injuries caused by impact with dash panels.

Airbags and bull bars

Inappropriately designed bull bars may interfere with the airbag systems ability to correctly deploy the bag. So its vital that any bull bar fitted to an air bag equipped vehicle is compatible with the system. Only bull bars that are certified as not affecting air bags systems can legally be fitted to later vehicles.

Other Airbag Issues

• Front airbags are designed to provide protection for persons of adult proportions. Children, due to their smaller size, are at risk of injury from a deploying airbag. Therefore children should not occupy the front seat when a passenger airbag is installed.

• Accessories that may restrict the deployment of the airbag or become a missile if contacted by a deploying airbag must not be fitted. Where a passenger airbag is fitted, this includes dash mats, drink or phone holders, etc. In the case of seat mounted side airbags, seat covers must be checked for compatibility ask the carmaker. 

• Airbag systems normally incorporate a dash warning lamp. If the lamp fails to extinguish soon after start-up or comes on while driving, consult your owners handbook and take the vehicle to a dealer promptly, for checking. The system has detected a fault. 

• Unless certain precautions are followed, airbags can be triggered inadvertently during certain repair procedures, especially electrical work. Serious injury can result, so leave repairs to qualified tradespersons. 

• Airbags deploy with explosive force and are far from being the big fluffy pillows some people might imagine. Minor injuries and skin abrasions from contact with the bag are common.

Related topics: Life of airbags, Windscreens