Will your air
bag work with a bullbar? |
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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 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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