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SRS and Bull Bars


Will your air bag work with a bullbar?

4WD vehicles are currently enjoying unprecedented popularity in Australia. Not only do they claim a significant percentage of new vehicle sales, but there’s an astounding array of accessories that can be purchased to either enhance the practicality or the image of the vehicle and its driver.

More so than any overseas country, the ’bull bar’ or ’Frontal Protection Bar’ is almost mandatory as a 4WD accessory in Australia. Bull bars may provide a handy mounting point for additional ’driving lights’, CB radio aerials and for winch installations, but this is by no means their primary function.

Frontal Protection Bars (or FPB’s) are - as their name suggests - designed to protect the front of the vehicle while parked, in minor impacts with posts and trees etc, and in collisions involving animals.

With the bull bar absorbing the brunt of the impact, repair costs are kept to a minimum and the vehicle hopefully remains mobile.

The reason for the popularity of these devices in Australia lies in a uniquely Australian phenomenon - the kangaroo (in fact, from an Australian perspective, the name bull bar is a misnomer because the incidence of motor vehicle accidents involving bovines is not great in any part of the world).

FPB’s are sometimes referred to in Australia by the more appropriate name of ’Roo Bar". Kangaroos may be an internationally recognised symbol of Australia, but in this country many rural panel shops owe their very existence to the kangaroo’s near legendary lack of road sense.

The design of FPB’s has changed greatly over the years. Early examples had heaps of heavy, sharp edged iron protruding from the front of the vehicle. They were generally fitted to the truck like four wheel drives of the era. The affect that these bars had in contributing to wind noise and increases in fuel consumption due to increased weight and aerodynamic profile was generally not a consideration. Aesthetics were obviously not an issue as they often had more in common with the front of a bulldozer than a replacement for a bumper bar.

The bull bar has now evolved to meet modern demands for lightweight and more aerodynamically efficient construction, as well as having a more aesthetically pleasing appearance. Thanks to these developments, the contribution that a modern bull bar makes to increased wind noise and fuel consumption is minimal.

In recent years the need for bull bars within city areas has been questioned. The motivation behind this has been to reduce the severity of accident trauma in incidents involving pedestrians and vehicles fitted with bull bars.

However, studies have shown that the severity of injuries, and head injuries in particular, is greatly reduced when the profile of the bar matches the vehicle profile or ’envelope’.

A draft for a proposed Australian Standard will provide guidelines for the construction of bull bars, with recommendations for the softness of the bar as well as the distance that a bull bar can extend from the vehicle envelope.

A bull bar’s primary function is safety related. If its installation fails to enhance the safety of the vehicle then it has failed in its principle objective.

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 positionclosest 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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