I’ve been sizing up new car models and was impressed to find many have multiple-stage air bags as well as side-impact air bags and seat belt pre-tensioners. Can you please explain how these work? These seem like a great idea, especially in minor crashes or a rollover.

Larry Taylor, Dublin, Calif.

Let’s start with a generic overview of supplemental restraint systems. I’d like to use this term at least once as a reminder that air bags are designed to supplement your seat belts, not replace them. Besides holding your body in the proper position when an air bag inflates, seat belts protect you during crash situations when the air bags do not deploy, and minimize the chance of occupant ejection.

The air bag module consists of the folded nylon air bag and an inflator module containing a squib, which is similar to a blasting cap. (Early inflator modules use an igniter containing sodium azide and cupric oxide. When detonated, it produces a large quantity of expanding nitrogen gas to fill the air bag.)

How air bags work

Newer air bag modules use a squib or similar device to burst an argon or helium gas-pressure chamber, which inflates the air bag. Crash-to-fill time is amazingly fast, approximately one-twenty-fifth of a second, and splits tear-seams in the steering wheel pad or instrument panel, allowing the bag to escape its concealed home. Deflation begins in one-tenth of a second through vents in the rear of the bag, creating a small cloud of cornstarch (a packing preservative) and combustion byproducts.

Air bag deployment depends on input from one to as many as five crash sensors and a command from the SRS diagnostic module, which monitors and controls all air bag functions.

System readiness is constantly monitored, and any faults will result in an illuminated SRS warning lamp and shutdown of the system. The use of multiple sensors minimizes the chance of accidental deployment and helps insure the impact is of a frontal nature, justifying air bag use.

Multiple-stage air bags are typically of the gas-inflator type, and use two squibs instead of one. During initial deployment the first squib is ignited, which ruptures the gas chamber, filling the bag.

Depending on crash severity, the weight of the person in the seat and how he or she is positioned in the seat, the second squib also might be ignited, causing additional heat, which fills the bag faster.

Side-impact air bags and side-curtain air bags are usually of the pressurized-gas type and might be contained in the seat back, door panel, door pillar or roof. Sophisticated sensors detect occupant size and position, insuring the safest deployment strategy.

Seat belts

Like an air bag, seat-belt pre-tensioners also contain a small explosive charge. Deployment causes a cable to be pulled to snug the buckle side of the belt, or balls are shot against a fan wheel to wind up the belt retractor. Pre-tensioner operation coincides with the air bags, helping to ensure occupants are in the best position for the front or side air bags to protect them.

Brad Bergholdt teaches automotive technology at Evergreen Valley College in San Jose, Calif.