For years, the trusty seat belt provided the sole form of
passive restraint in our cars. There were debates about their
safety, especially relating to children, but over time, much
of the country adopted mandatory seat-belt laws. Statistics
have shown that the use of seat belts has saved thousands of
lives that might have been lost in collisions.
Air bags have been under development for many years. The
attraction of a soft pillow to land against in a crash must be
very strong -- the first patent on
an inflatable crash-landing device for airplanes
was filed during World War II! In the 1980s, the first
commercial air bags appeared in automobiles.
Since model year 1998, all new cars have been required to
have air bags on both driver and passenger sides. (Light
trucks came under the rule in 1999.) To date, statistics show
that air bags reduce the risk of dying in a direct frontal
crash by about 30 percent. Newer than
steering-wheel-mounted or dashboard-mounted bags, but not so
widely used, are seat-mounted and door-mounted side air bags.
Some experts say that within the next few years, our cars will
go from having dual air bags to having six or even eight air
bags! Having evoked some of the same controversy that
surrounded seat-belt use in its early years, air bags are the
subject of serious government and industry research and tests.
In this edition of HowStuffWorks,
you'll learn about the science behind the air bag, how the
device works, what its problems are and where the technology
goes from here.
Before looking at specifics,
let's review our knowledge of the laws of motion.
First, we know that moving objects have momentum (the
product of the mass and the velocity of an object). Unless an
outside force acts on an object, the object will
continue to move at its present speed and direction. Cars
consist of several objects, including the vehicle itself,
loose objects in the car and, of course, passengers. If these
objects are not restrained, they will continue moving at
whatever speed the car is traveling at, even if the car is
stopped by a collision.
Stopping an object's momentum requires force acting over
a period of time. When a car crashes, the force required
to stop an object is very great because the car's momentum has
changed instantly while the passengers' has not -- there is
not much time to work with. The goal of any supplemental
restraint system is to help stop the passenger while doing as
little damage to him or her as possible.
What an air bag wants to do is to slow the passenger's
speed to zero with little or no damage. The constraints that
it has to work within are huge. The air bag has the space
between the passenger and the steering
wheel or dash board and a fraction of a second to work
with. Even that tiny amount of space and time is valuable,
however, if the system can slow the passenger evenly rather
than forcing an abrupt halt to his or her motion.
There are three parts to an air bag that help to accomplish
inflation system is not unlike a solid rocket booster (see How Rocket
Engines Work for details). The air bag system ignites a
solid propellant, which burns extremely rapidly to
create a large volume of gas to inflate the bag. The bag then
literally bursts from its storage site at up to 200 mph
(322 kph) -- faster than the blink of an eye! A second
later, the gas quickly dissipates through tiny holes in the
bag, thus deflating the bag so you can move.
- The bag itself is made of a thin, nylon fabric,
which is folded into the steering wheel or dashboard or,
more recently, the seat or door.
- The sensor is the device that tells the bag to
inflate. Inflation happens when there is a collision force
equal to running into a brick wall at 10 to 15 miles per
hour (16 to 24 km per hour). A mechanical switch is flipped
when there is a mass shift that closes an electrical
contact, telling the sensors that a crash has occurred. The
sensors receive information from an accelerometer
built into a microchip.
- The air bag's inflation system reacts sodium
azide (NaN3) with potassium nitrate
(KNO3) to produce nitrogen gas.
Hot blasts of the nitrogen inflate the air bag (click
here for the full chemical equation).
|The air bag and inflation system stored in
the steering wheel
The inflation system uses a solid propellant
Even though the whole process happens in only
one-twenty-fifth of a second, the additional time is
enough to help prevent serious injury. The powdery
substance released from the air bag, by the way, is
regular cornstarch or talcum powder, which is used by the air
bag manufacturers to keep the bags pliable and lubricated
while they're in storage.
According to Scientific
The idea of using a rapidly inflating cushion to
prevent crash injuries had a long history before the U.S.
Department of Transportation called for the equipment to be
adapted for automobiles in the 1980s. The first patent on an
inflatable crash-landing device for airplanes was filed
during World War II. Early efforts to adapt the air bag
for use in cars bumped up against prohibitive prices and
technical hurdles involving the storage and release of
compressed gas. Researchers wondered:
- If there was enough room in a car for a gas canister
- Whether the gas would remain contained at high pressure
for the life of the car
- How the bag could be made to expand quickly and reliably
at a variety of operating temperatures and without emitting
an ear-splitting bang
They needed a way to set off a chemical reaction that would
produce the nitogen that would inflate the bag. Small
solid-propellant inflators came to the rescue in the
In the early days of auto air bags, experts cautioned that
the new device was to be used in tandem with the seat belt.
Seat belts were still completely necessary because air bags
worked only in front-end collisions occurring at more than 10
mph (6 kph). Only seat belts could help in side swipes and
crashes (although side-mounted air bags are becoming more
common now), rear-end collisions and secondary impacts. Even
as the technology advances, air bags still are only effective
when used with a lap/shoulder seat belt!
It didn't take long to learn that the
force of an air bag can hurt those who are too close to it.
Researchers have determined that the risk
zone for driver air bags is the first 2 to 3 inches
(5 to 8 cm) of inflation. So, placing yourself 10 inches (25
cm) from your driver air bag gives you a clear margin of
safety. Measure this distance from the center of the steering
wheel to your breastbone. If you currently sit less than 10
inches away, you can adjust your driving position in the
- Move your seat to the rear as far as possible while
still reaching the pedals comfortably.
- Slightly recline the back of your seat. Although car
designs vary, most drivers can achieve the 10-inch distance
even with the driver seat all the way forward by slightly
reclining the back of the seat. If reclining the seat makes
it hard to see the road, you can raise yourself up by using
your car's seat-raising system (not all cars have this!) or
a firm, non-slippery cushion to achieve the same effect.
- Point the air bag toward your chest, instead of your
head and neck, by tilting your steering wheel downward (this
only works if your steering wheel is adjustable).
The rules are different for children. An air bag can
seriously injure or even kill an unbuckled child who is
sitting too close it or is thrown toward the dash during
Experts agree that the following safety
points are important:
- Children 12 and under should ride buckled up in a
properly installed, age-appropriate rear car seat.
- Infants in rear-facing child seats (under one year old
and weighing less than 20 pounds / 9 kg) should never ride
in the front seat of a car that has a passenger-side air
- If a child over one year old must ride in the front seat
with a passenger-side air bag, he or she should be in a
front-facing child safety seat, a booster seat or a properly
fitting lap/shoulder belt, and the seat should be moved as
far back as possible.
In response to concerns about
children -- and others, especially smaller people -- being
killed or seriously injured by malfunctioning or overly
powerful air bags, the National
Highway Traffic Safety Administration (NHTSA) in 1997
issued a final rule to allow auto manufacturers to use
lower-powered air bags. This rule permits air bags to be
depowered by 20 to 35 percent. In addition, starting in
1998, repair shops and dealers were allowed to install
on/off switches that allow air bags to be deactivated.
Vehicle owners could now be authorized (by the NHTSA) to get
on/off switches installed for one or both air bags in their
car if they (or other users of their car) fell into one or
more of these specific risk groups:
- For both driver and passenger sides - Individuals
with medical conditions in which the risks of deploying the
air bag exceed the risk of impact in the absence of an air
- For the driver side (in addition to medical
conditions) - Those who cannot position themselves to
properly operate their cars at least 10 inches (25.4 cm)
back from the center of the driver air bag cover
- For the passenger side (in addition to medical
conditions) - Individuals who need to transport a baby in a
rear-facing child restraint in the front seat because the
car has no rear seat, the rear seat is too small to
accommodate a rear-facing child seat or because it's
necessary to constantly monitor a child's medical condition
- For the passenger side (in addition to medical
conditions) - Individuals who need to carry children between
one and 12 years old in the front seat because (a) the car
has no rear seat, (b) the vehicle owner must carry more
children than can fit into the back seat or (c) because it's
necessary to constantly monitor a child's health
If you would like to get an on-off switch installed in your
car, you need a copy of NHTSA's brochure, "Air Bags and On-Off
Switches: Information for an Informed Decision," and the
accompanying form, Request for Air Bag On-Off Switch.
You can find these on the NHTSA
Web site, as well as at AAA clubs, new-car dealers and
state motor vehicle departments. The NHTSA will send you a
letter of authorization that you can take to a repair shop.
(Before you bother with all this, you should check with your
auto dealer or repair shop to see if an on-off switch is
available for your car.) Some retrofit on-off switches
can be found and used if federal requirements are met --
switches must be operated by a key and equipped with warning
lights to indicate whether the bags are turned off or on.
Obviously, even you have the option of turning it off, the
air bag should be left on for drivers who can sit at least 10
inches back. For those who can't (even with the suggestions
listed above), the bag can be turned off. A group of doctors
at the National Conference on Medical Indications for Air Bag
Deactivation considered the medical conditions commonly
reported in letters to the NHTSA as possible justification for
turning off air bags. They did not, however, recommend turning
off air bags for relatively common conditions, such as
mastectomy, previous back or neck surgery, advanced age,
osteoporosis, arthritis or pregnancy.
Generally speaking, you can't deactivate your air bag
without installing a retrofit on-off switch. However, if a
retrofit on-off switch is not yet available (from the vehicle
manufacturer) for your car, the NHTSA will authorize air bag
deactivation on a case-by-case basis under appropriate
conditions. Never try to disable the bag yourself -- remember,
this is no soft cushion! It packs a wallop and can hurt you
when you don't know what you're doing.
As for factory-installed on-off switches, the NHTSA allows
car manufacturers to install passenger air bag on-off
switches in new vehicles under limited circumstances --
only if the vehicle has no rear seat or if the rear seat is
too small to accommodate a rear-facing child safety seat. And
manufacturers are not currently allowed to install on-off
switches for the driver air bag in any new vehicle. Why these
rules? The NHTSA decided against widespread factory-installed
on-off switches for fear that they would become standard
equipment in all new vehicles -- even those purchased by
people not in at-risk groups. They also saw the integration of
on-off switches into new cars (and the subsequent redesign of
instrument panels) as something that would divert resources
from the development of safer, more advanced air bag systems.
The Future of Air Bags
Activities aimed at
maintaining and improving the lifesaving benefits of air bags
are in full swing. New NHTSA-sponsored tests use improved "dummy"
injury criteria based on new knowledge and research.
Until recently, most of the strides made in auto safety
were in front and rear impacts, even though 40 percent
of all serious injuries from accidents are the result of side
impacts, and 30 percent of all accidents are side-impact
collisions. Many carmakers have responded to these statistics
(and the resulting new standards) by beefing up doors, door
frames and floor and roof sections. But cars that currently
offer side air bags represent the new wave of occupant
protection. Engineers say that designing effective side air
bags is much more difficult than designing front air bags.
This is because much of the energy from a front-impact
collision is absorbed by the bumper, hood and engine, and it
takes almost 30 to 40 milliseconds before it reaches the car's
occupant. In a side impact, only a relatively thin door and a
few inches separate the occupant from another vehicle. This
means that door-mounted side air bags must begin deploying in
a mere five or six milliseconds!
engineers experimented with different ways of mounting side
air bags and chose seat-back installation because that
protects passengers of all sizes regardless of how the seat is
positioned. This arrangement allows them to place a
triggering mechanical sensor on the sides of the seat
cushions under the driver and front passenger. This prevents
the air bag on the undamaged side of the car from inflating.
Installing the entire air bag package in the seat-back also
offers the advantage of preventing unnecessary deployments
that might be caused by collisions with pedestrians or
bicycles. It takes a collision of about 12 mph (19 kph)
to trigger side air bags.
BMW engineers have chosen door-mounted air bags. The
door has more space, allowing for a bigger bag that provides
The head air bag, or Inflatable
Tubular Structure (ITS), was featured in all of BMW's 1999
models (except convertibles).
The head bags look a little like big sausages and, unlike
other air bags, are designed to stay inflated for about five
seconds to offer protection against second or third impacts.
Working with the side air bag, the ITS is supposed to offer
better protection in some side collisions.
All of this makes it pretty clear that the science of air
bags is still new and under rapid development. You can expect
many advances in this field as designers come up with new
ideas and learn from real-world crash data.
For more information on air bags and related topics, check
out the links on the next page.
Lots More Information!
More Great Links