Air Bags, Bull Bars and Crumple Zones
ADR 69 specifies the level of protection that must be afforded the occupants of a passenger vehicle during a 48km/h full frontal impact into a solid immovable barrier.
To achieve the levels of protection required by ADR 69, vehicle manufacturers have employed progressive crumple zones and SRS, or Supplemental Restraint Systems.
Most people will be familiar with video footage of motor vehicles as they plunge in slow motion into the crash test barrier. In more recent years these videos have shown crash test dummies pitching forward at the moment of impact, only to be saved by the inflation of the airbag.
Absorption of the impact by the crumple zones reduces the forces acting upon the vehicle occupant and as a consequence will hopefully reduce the level of injuries suffered by them.
The manufacturers of modern frontal protection devices have had to consider the effect that the fitting of their bar will have on effectiveness of these in built safety features.
Essential to the progressive crumple zone of any vehicle is the original fit bumper bar and crush cans. In order to maintain the original crumple characteristics of the vehicle, the mounting of the FPB must be able to offer similar properties. A poorly designed or incorrectly mounted bull bar could not offer the same deceleration profile, seriously compromising the safety of the vehicle occupants.
The testing required to make an 'Airbag compatible bull bar' has produced some interesting findings about accidents.
It has been found that the installation of a properly designed FPB will have much less effect on the airbag triggering than variations in vehicle weight caused by the number of passengers, load, fuel and tow weight.
Air bag triggering occurs when the rate of deceleration of the vehicle exceeds a triggering threshold. This triggering threshold may vary by as much as 30% between vehicles of the same model depending upon vehicle age and assembly variations.
A properly designed and mounted FPB has been found to have much less effect on the deceleration rate, or 'crash pulse' than the variations caused by variations in either vehicle weight or age and assembly.
It stands to reason that an FPB that has little or no effect on air bag triggering will also not compromise the level of passenger protection afforded by the SRS. In fact, in a 48 km/h barrier test, it will take about 45 milliseconds for a passenger with the seat adjusted to the position furthest from the steering wheel to reach the seating position of a passenger that has the seat adjusted to the position closest to the steering wheel.
As variations in passenger seating position, or even passenger size and weight, are not part of the calculations to determine airbag triggering, it is also reasonable to assume that the minor variations in the crash pulse will not compromise occupant protection.
As stated earlier, one of the principle reasons for the popularity of the FPB in Australia is the high incidence of animal strikes, particularly those involving kangaroos. Research by vehicle manufacturers has determined that motor vehicle accidents involving animal strikes rarely exceed the deceleration rate required to trigger the airbag.
If this is the case the FPB is fulfilling its function of minimising damage to the front of the vehicle while maintaining vehicle mobility without compromising the safety of the vehicle occupants.
"Pole" or "post" crashes are responsible for a high percentage of road fatalities . They are difficult for airbag sensors to detect because, as the impact is concentrated and therefore more intrusive, the deceleration threshold is often not exceeded. The vehicle occupant is exposed to considerable impact, without the benefit of the airbag. The FPB can assist in spreading the impact across the front of the vehicle thus providing the deceleration necessary to trigger the air bag if required.
Throughout this article the all important clause has been a "properly designed and mounted FPB". If you are fitting a FPB to a late model vehicle with SRS it is imperative that it be 'airbag compatible'. A poorly designed FPB will not necessarily prevent the air bag from deploying. In fact it is more likely to cause it to deploy prematurely, considerably increasing repair costs and defeating the purpose of the FPB.
Millions of dollars worth of ongoing research is being carried out by bull bar manufacturers to ensure that the modern FPB is able to compliment the safety features of modern vehicles. An indication of the success of this commitment is the knowledge that some locally designed and built FPB's are being marketed as genuine accessories by vehicle manufacturers.
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