How Your Airbags Work to Keep You Safe

Sure they’re to blame for the hideous steering wheel designs that arrived in the late 1980s and haven’t quite disappeared just yet, but they’re also responsible for saving many lives and reducing or eliminating many injuries. Airbags, aka the supplemental restraint system. They’re in every modern car, but how do they work?

The idea of putting an airbag into a vehicle dates back almost as long as the car. To 1919. And air filled bladders were considered back in the 1950s. Then, patents were awarded to bags that would fill with compressed air. Though, since you’ve never seen one in a car, the idea didn’t, well, inflate. Because the bags couldn’t inflate quickly enough to actually offer appreciable safety benefits.

Buick steering wheels, then and now.

The first modern-type airbag didn’t arrive until the 1970s. Oldsmobile was the first to offer one. A passenger-side bag called the Air Cushion Restraint System. It was available in that car in 1973, and much of GM’s full-size lineup in 1974. It wasn’t popular and the system was gone just a few years later.

In 1981, Mercedes-Benz launched the feature on the S-Class sedan. Porsche became the first to offer a driver and passenger bag as standard on the 1987 944 Turbo. Chrysler was the first US company to offer a driver bag as standard in 1988, adding it to the full car line the next year.

All cars sold in the US and built after September 1, 1998, were required to have both a driver and passenger airbag as standard. Today, cars have those bags plus knee bags, side-impact airbags, and even seatbelt airbags.

But back to how they work.

The airbag itself is a nylon bag. It’s sized and stitched to fit each model of car, then folded and packed in a very specific way. Both to make sure it fits behind the steering wheel or dashboard cover and to make sure that it inflates correctly. The bag has holes cut in it, which are usually partially covered so that once it’s fully inflated it can deflate. Because your car might still be moving, which means that you need to see out the windshield. Or if it’s come to a stop, you need to be able to get out. Not be pinned in by a giant balloon.

A steering wheel airbag is normally round, the passenger side one is normally much larger and rectangular because the dash is larger and further away than the steering wheel. Side curtain airbags can come as one bag running the length of the roofline, or as smaller bags to cover each seat. Some vehicles use a side-impact airbag that’s mounted in the seat or in the door.

The bags are inflated by a chemical reaction. Not all use the same chemical reaction, but all start with a pyrotechnic device. A small explosion that starts the reaction and causes the chemicals to generate enough gas to fill the bag. Many of these chemical compounds are proprietary and secret. Takata airbags, which you’ve probably heard of because of the massive recall, used ammonium nitrate for the propellant. The chemicals used in the airbag are generally considered to be non-toxic, though automakers suggest you wash them off as soon as possible after an inflation because they can irritate your skin or breathing temporarily.

So basically, there’s an explosion in front of you. That sends the surface of the bag flying at you at about 200 mph. It needs to be fast because you’re also moving quickly. At whatever speed your car was going before you hit another object.

At 60 miles per hour, you’re travelling about 88 feet every second. Your steering wheel is probably only about one foot from your face. Maybe less. So in just milliseconds after your bumper hits an obstacle, your face will hit the steering wheel. A car airbag’s explosion fills the bag in as little as 25 milliseconds. Quickly enough to stop you from smashing into the wheel, quickly enough to fully inflate before you hit the bag. Then it gently, well, ok, maybe not so gently, but certainly more softly then the windshield, slows you down before you hit anything solid. It slows you, instead of just abruptly stopping you, thanks to specially sized holes that let the air back out at a controlled rate once your face hits it.

So what tells the airbag to go off? Sensors and math.

There are accelerometers mounted in various places in the car, usually the front bumper. They measure how quickly the car is slowing down in a crash. The amount of force the car is experiencing. Well, they measure that all the time, but they only care when they detect it’s enough deceleration to be a crash. The faster car stops or changes direction, the more likely the airbag will deploy. Similar sensors control the side-impact bags. Another sensor can tell if the car is rolling over. And there’s another that will usually trigger the bag in case of a vehicle fire, so that they don’t randomly and uncontrollably explode in the fire. Older cars used older-style sensors with a ball held by a band that would stretch and make contact in a collision, triggering the airbags

The use of sensors makes airbags not perfect. If the impact is in just the right, or in this case the wrong spot, the sensors can slow down over a longer time than the rest of the car. So they might not go off if you hit a thin object, like a utility pole, in just the wrong spot. Modern systems can activate with different levels of force, by changing the timing and reaction. So in a slow-speed crash they might not fire at all or they might deploy with less force. They also need you to wear your seatbelt to work properly. The seatbelt makes sure you’re in the right place to hit the airbag. And the belt stretches slightly to help slow down your stop. Airbags and belts need to be used together.

The NHTSA says airbags had saved nearly 45,000 lives as of 2015. How’s that for being a super hero? It’s an impressive statistic. One worth a thought the next time you wonder why your car can’t have a nice 1960’s-style steering wheel center.

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