Fall 2005 Forward to a Friend





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


Part One presented a brief history of air bags and safety legislation, an overview of what air bags do and the locations of air bag and other Supplemental Restraint System (SRS) components.

SRS augments the protection offered by seatbelts, which are the primary passive-safety components. Part Two delves deeper into how air bags and other SRS components work.

How Air Bags Work

In an impact, SRS components function together to help reduce injury. In the following, assume that impacts are of magnitudes exceeding preset values for SRS to function. Remember these functions happen in fractions of a second. (The 2005 Legacy SRS is used for illustration.)



For Frontal Impacts

One or both front subsensors and sensors in the air bag electronic control module detect an impact – a sudden stop beyond normal parameters. The module uses an electronic g sensor to measure changes in inertia (1 g = the weight of the body at rest).

The electronic control module has two sensors – a safety sensor and a g sensor. In the safety sensor, mass weights act as pendulums. In an impact, the weights move far enough to close contacts and complete a connection to the control module that indicates an impact.

The g sensor has electrodes shaped like comb teeth. They detect an impact by variances in the spaces between the teeth.

Signaled by sensors, the control module determines whether or not air bags and seatbelt pretensioners should be deployed. (Electronic control module notes: First, it has a self-diagnosis function that alerts the driver through a warning light if it discovers a fault. Trouble codes stored in the module’s memory facilitate repair. Second, the control module has a backup power supply in case battery current is interrupted as a result of an accident.)

The control unit signals the frontal air bags, indicating at which of two stages they should inflate.


(1) Steering-wheel pad (4) 1st-stage
gas-generating agent
(7) 2nd-stage igniter
(2) Air bag (5) 2nd-stage gas-generating agent (8) 1st-stage ignition signal
(3) Inflator (6) 1st-stage igniter (9) 2nd-stage ignition signal


The driver’s air bag module is built into the center steering-wheel pad. The inflator behind the air bag houses igniters and gas-generating agents. These components control the speed at which the air bag inflates.

For a high-speed impact, the control module ignites both gas-generating agents simultaneously. For mid-speed and low-speed impacts, the control module ignites the second-stage agent on a slight delay to help reduce the inflation rate – tailoring inflation to the level of impact. The driver’s air bag helps prevent driver contact with the steering wheel and windshield.


(1) Inflator (4) 1st-stage igniter (7) 2nd-stage gas-generating agent
(2) Air bag (5) 2nd-stage igniter (8) 1st-stage ignition signal
(3) Lid (6) 1st-stage gas-generating agent (9) 2nd-stage ignition signal


The passenger’s air bag is built into the dashboard. Although the inflator’s shape differs from the steering-wheel-mounted driver’s air bag, it functions the same way to tailor inflation to the level of impact. The passenger’s air bag helps to prevent passenger contact with the dashboard or windshield.

For Side Impacts

Side air bag sensors are located in the B-pillars (between the front and rear doors) and curtain air bag sensors in the C-pillars (behind the rear doors). A side-impact sensor sends signals that deploy both the side air bag and the curtain air bag. A curtain air bag sensor only signals a curtain air bag to deploy.


(1) Case (3) Seat upholstery (5) Bracket
(2) Air bag (4) Seat frame (6) Inflator


Side-impact air bags are built into the backrests of the front seats, and the curtain air bags are located in the headliner above the roof pillars. They operate like the frontal air bags, but only at one stage.

Side air bags are intended to soften an impact to front-seat occupants, particularly around their chests.

The curtain air bags are designed to reduce the impact on outboard passengers, particularly around their heads.


Rear Air Bags, Part One and Three, with these and other topics:
• Air bag dos and don’ts • Occupant detection system
• Seatbelt pretensioners • How air bag inflators work