12-12-19 Airbags

A couple of times over the last month I have been asked about airbags. Questions have usually been along the lines of, “I hit a deer at 65 miles per hour and my airbag didn’t deploy, but I hit a curb at 15 mph and my airbags deployed. Why?” It has everything to do with how the airbag sensors work. This week’s article explains the sensor, what it does and why sometimes it sets off the airbags with low impact and not with high impact accidents.

The first thing you need to know is that airbag sensors are not impact sensors, but rather shock sensors. The difference is that impact sensors work like a push button switch; when an impact moves the sensor far enough, it turns on a switch. A shock sensor, on the other hand, is based on a change in momentum rather than the movement of a mechanism. In other words, your bumper can move several inches, and as long as it is moving at a steady speed, the sensor will not detect an accident, even if your bumper is completely crushed.

Momentum defines the motion of a moving body and is calculated by multiplying the object’s mass by its velocity. The faster you are moving, the more momentum you have. The heavier you are, the more momentum you have. The airbag sensor measures momentum and triggers the airbag if there is a large enough change in momentum.

The sensor does not exactly measure the momentum of the car, but rather the momentum of the sensor itself. It works by using a suspended mass and some impact sensors. The suspended mass will not touch the impact sensors unless a large change in momentum occurs, causing it to hit one of the impact sensors. You can think of it as a marble in a shallow dip in the center of a box. If the box stays level and moves steadily, the marble stays in the middle; if you bump the box, the marble will roll to one of the sides. If the marble touches the side of the box, the airbag deploys.

Now let’s look at the two scenarios. First you hit a deer at 65 mph and the airbags do not deploy. Imaging the marble in the box is setting on your dashboard. You strike the deer but the car does not slow down or swerve. The marble and the box stay firmly on the dashboard. This is because your momentum did not change. Remember, to change momentum, your speed must change. You struck the deer and maintained your speed. The car took major damage but the airbag never deployed because your momentum didn’t change.

Now for the second case, you curb the tire and the airbag deploys, causing you to lose control of the car. How did this happen and why? Think again about the marble in the box on your dashboard. You are taking the corner, and you hit the curb. The car bounces and instantly slows down from 20 miles per hour to five miles per hour. The marble rolls forward and hits the front of the box. This causes the airbags to deploy. The change in momentum because of the rapid slowing sets off the airbag.

This is both good news and bad news. The good news is that it is the change in momentum is what causes your injuries in the event of an accident and the airbag usually deploys when there is danger of your being tossed around or out of the car. The bad news is that sometimes rapid slow down due to striking a pot hole or curb, or even just hitting the sensor while the engine is running can cause the airbags to deploy, causing more damage to the car than the damage from the incident.