Home' Australian Automotive Aftermarket Magazine : Australian Auto Aftermarket e-Zine - Feb 13 Contents TECHNICAL COLUMN
Passive safety is the general name given
to devices that help to protect the vehicle
occupants during a crash.
These occupant protection systems are
'passive' in that they do not require any action
by the occupant when an accident occurs.
Examples of passive safety devices
include airbags, vehicle construction etc.
Strictly speaking seat belts are not passive
safety devices as they require action by
occupants to fit these, but as front seat belt
wear rates are as high as 97 percent in
Australia, these will be considered passive for
the purposes of this article.
Undoubtedly, the most important safety
feature in cars has been the introduction of
Australia was the first country in the world
to mandate the fitting and wearing of seat
belts in 1970 followed by many other
countries during the '70s and '80s.
The subsequent dramatic decline in road
deaths, equivalent to thousands of lives
saved in Australia alone, is generally
attributed to seat belt laws and subsequent
road safety campaigns.
By 2002, despite the large increases in
population and number of vehicles on the
road, fatality rates in Australia, Canada and
UK have halved.
In this article, state of the art passive
safety devices and some new and innovative
technologies that should soon appear on the
market will be reviewed.
The function of a seat belt may seem very
obvious but did you know that they protect
occupants in at least three ways.
Firstly, they restrain the occupant from
hurtling toward the windscreen or steering
wheel during a sudden stop.
For example, if a car travelling at 60kph
hits a tree and stops abruptly, then the
occupants will continue moving forward at
that speed toward the front of the vehicle.
Secondly, the seat belt spreads these
stopping forces across sturdier parts of the
body to reduce the resultant injury.
Without a seat belt more vulnerable parts
of the body, namely the head and the chest,
would be in danger of hitting hard objects
such as the windscreen or steering wheel.
Last but not least, seat belt webbing is
slightly elastic and therefore will absorb some
of the energy of impact.
Modern seat-belts also incorporate such
features as pre-tensioners and force limiters.
Pre-tensioners tighten up any slack in the
belt in the event of a collision.
Whereas the conventional retractor
mechanism stops the belt from extending
further, the pre-tensioner actually pulls the
belt in, moving occupants into the optimum
crash position in their seats.
Pre-tensioners are activated by the same
central control processor that activates the
air bags and use pyrotechnics to pull the belt
in. Force Limiters as the name implies limits
the force that the seat belt can inflict on the
occupant to minimise belt-inflicted injury.
The simplest type is a fold, sewn into the
belt, where the stitches break when a pre-
determined load is applied to the belt.
More advanced limiters use a torsion
bar within the retractor mechanism which
will twist when a pre-determined load is
applied and allows the belt to extend a
Active seat belts link active and passive
safety systems and are reversible seat belt
retractors which can be activated prior to a
The active seat belt uses an algorithm to
continuously process signals from the
vehicle's relevant system sensors, such as
ABS or electronic stability control, to evaluate
exceptional driving events such as panic
braking and skidding.
In case of a recognised critical situation,
the active seat belt system 'pre-pretensions'
the seat belt, securing the occupant in a
better position prior to the crash.
Unlike pyrotechnic pre-tensioning
devices, if the critical situation ends without
incident, the system automatically releases
the belt. It is a fully reversible feature.
Active seat belt buckle is a new innovation
for the rear seats being developed by
Mercedes-Benz for possible introduction into
the 2013 S class luxury vehicle.
An electric motor extends and retracts the
seat-belt buckle automatically.
In this way, the belt slack in the area of
the pelvis and thorax is reduced and
passengers are secured more firmly in
sideways and lengthways directions.
Fastening seat belts in the rear is also
made much simpler; the seat belt buckle
emerges from the upholstery when the rear
doors are opened and is provided with an
illuminated insertion slot.
The purpose of airbags is to slow an
occupant's motion as evenly as possible in a
fraction of a second in a crash.
Airbags are not a substitute for seat belts
and are designated as Supplemental
Restraint Systems (SRS).
The airbag consists of a folded textile
cushion which is inflated quickly by nitrogen
gas resulting from a chemical reaction using
sodium azide and other chemicals when
triggered by the airbag Electronic Control
Once the airbag deploys, deflation occurs
almost immediately as gas escapes through
vent holes in the airbag fabric ensuring the
occupant a 'soft landing' into the bag.
As most airbags are inflated
pyrotechnically they can only be deployed
The airbag electronic control unit detects
and evaluates a crash before triggering the
appropriate restraint systems according to the
type of collision and its severity.
Information is supplied to the ECU by as
many as six crash sensors.
The airbag control unit is equipped with
acceleration sensors for the vehicle's
longitudinal, lateral and vertical axes.
Some also incorporate a rotational speed
sensor which monitors the rotation of the
vehicle around its longitudinal axis.
All past articles associated with vehicle safety have concentrated on active safety
features, the general name for automotive safety systems that prevent accidents or
mitigate the severity of accidents. Active safety devices sense when the vehicle is
beginning to lose control, failing to keep a safe braking distance or takes action to
warn the driver of a potentially dangerous situation prior to its occurrence.
Examples of such devices are ABS, Electronic Stability Control (ESC), automatic
cruise control, lane departure warning plus many others.
18 AUTOMOTIVE AFTERMARKET MAGAZINE FEBRUARY 2013
Readers are invited to send technical
enquiries of a general nature to:
TRW Active Seat Buckle
Links Archive Australian Automotive Aftermarket E-zine - Annual Performance Guide Australian Auto Aftermarket e-Zine - Mar 13 Navigation Previous Page Next Page