Summer 2005 Forward to a Friend






Be sure to also read parts two and three of this story!


By law, all automobiles, light-duty trucks, vans and SUVs sold in the United States today are required to have seatbelts for all occupants and frontal air bags.

Seatbelt systems are fairly basic. Air bags are another matter. Even though patented in the early 1950s and offered in vehicles since the 1970s, air bags and how they work remain a mystery for most people.

What Air Bags Do

The air bag Supplemental Restraint System (SRS) uses the word supplemental because the system is intended to supplement the protection offered by the seatbelts. Seatbelts restrain an occupant’s body in an impact. Air bags are intended to function in conjunction with seatbelts to cushion parts of the occupants’ bodies and absorb impact energy to help prevent injury.

Important: Air bags do not replace seatbelts! Seatbelts are the primary passive-safety feature in the vehicle. Although air bags were originally intended as a passive restraint for people not wearing seatbelts, they are now an integral part of passive-safety systems for which seatbelts form the foundation.

Federal law creates current Department of Transportation. Congress passes the Highway Safety Act and the National Traffic and Motor Vehicle Safety Act, authorizing federal regulation of motor-vehicle safety and requiring seatbelts to be installed in new cars at the factory.
Law requires the installation of shoulder belts in the front seats of new cars.
Final ruling requiring passive restraints to be phased in for all new cars, light-duty trucks, vans and SUVs by 1995.
Dual frontal air bags made mandatory, to be phased in for all new cars, light-duty trucks, vans and SUVs by 1997.

What today’s air bags are designed to do:
• In a frontal impact exceeding a preset threshold, the frontal air bags inflate immediately to help prevent the driver and front passenger from striking the steering wheel, instrument panel and/or windshield.
• In a side impact exceeding a preset threshold:
– Side air bags help to cushion the impact to the driver or front passenger in the area of the chest.
– Curtain air bags cushion the area of the head.


Far more sophisticated than early air bag systems, the Subaru system is a complex array of electronically controlled sensors, switches and warning lights along with the air bags. An examination of air bags involves all SRS elements.

SRS components found in the B9 Tribeca include:

Side air bag sensors detect side impacts, each with a safety sensor and g sensor (measures lateral acceleration).


Air bag modules are all deployed by the control module. (Dual-stage frontal air bags tailor the amount of deployment force to the severity of the impact.)


Passenger-seat air bag ON/OFF indicator indicates whether or not the passenger-seat air bag is activated to deploy in an appropriate impact.


Front subsensors detect a frontal impact and output deceleration signals for each side of the vehicle.


Occupant detection module determines the weight of the front passenger-seat occupant based on input from the loadcell sensor.
Air bag electronic control module uses input from sensors to judge the magnitude of an impact and determine whether air bag deployment is necessary; it performs self-diagnosis of the air bag system. (The module also serves as a supplementary power supply to maintain functionality even after battery power is cut off.)


Curtain air bag sensors detect side impacts, each with a safety sensor and g sensor.


Seatbelt pretensioners take up slack in the front seatbelts for more effective protection.


Air bag warning light indicates whether or not the system is functioning properly.


Buckle switches indicate whether or not a seatbelt is fastened.



Read Air Bags, Part Two and Three, with these and other topics:

• Air bag dos and don’ts
• The interaction of SRS components in an impact
• Anatomy of an air bag module deployment
• Seatbelt pretensioners
For part two of this story, go to Air Bags, Part Two; for part three, go to Air Bags, Part Three.