Another "It Didn't Go Off"

[Adze39]

There was an accident today.

A Toyota Camery had its front bumper pretty much ripped off and the front end was smashed in by at least a foot...and the airbags didn't go off. The other car ended up on its side as a result of the crash (but other drivers got out of their cars and pushed the car onto its wheels before any emergency service arrived...with the lady still in it!!!)

I'm trying to see if I can get a pic or two from the PD.

Any ideas? Does the camery have to have two sensors activate? I would imagine by the result of the car that more than one sensor was hit at a good speed.

[Bones42]

How perpendicular was the hit? If the Camery hit at enough angle, that would keep the bags from inflating. It still amazes me how you can have a slow speed contact, resulting in minor vehicle damage, that sets of the airbags, but a higher speed one doesn't due to enough angle on the hit.

[Firefighter1219]

I came around a curve to see that a women was sitting in the road with her car and ended up rear-ending her.

I drive a '99 Nissan Sentra. The airbads didn't go off. The right front hit the middle of her bumper. I did over $4600 worth of damage to the front end of my car. I hit her going over 30 mph. Why no airbags?

[Dalmatian90]

Did ya get up and walk out of your car afterwards?

I believe all these sensors work more on "decelleration" then anything else -- it's how fast you slow down, not how fast you were traveling.

I could be wrong, but I suspect a car parked in the road is going to move a lot more than say a piece of concrete bolted to a parking lot. And that'll affect your decelleration.

And a car like Adze's that rolls onto the side, that's probably relatively slow too.

[ptfd]

Dalmation-Exactly. Airbag deployment depends on the decelleration of the. The airbag is activated by balls that are held in place by magnets. If the force of decelleration is greater then the force of the magnet, the ball is released, and triggers a start to the whole system. If the decelleration is not great enough, the magnet will still hold on.

[mtfd89]

That's the trick - rate of change in velocity (how sudden is the decelleration). Speed that the vehicle is travelling is only a factor, as is direction of travel.

A car travelling 45 miles an hour hitting a concrete wall head-on will experience a significantly sudden change in velocity from 45mph to 0mph and air bags will deploy. On the other hand, a car travelling 45 mph hitting a stack of 20+ mattresses will experience a slow change in velocity over a relatively long period of time and the air bags will not deploy - there is not enough force generated to "break the sensor activtion mechanism loose". Note that both cars were travelling at the same speed upon impact. This is why a low speed impact sometimes triggers air bags even though it may seem odd - question is how suddenly did the velocity of the vehicle change. (All goes back to physics fomulas where mass, distance, and time all interact to equate to force)

Direction of impact is also a consideration for frontal airbag deployment. Side or angle impacts will not (or may not) create a sufficiently sudden change in velocity in the direction required to activate the sensor. Again, rate of change in velocity together with direction of that change play a role in sensor activation.

[lutan1]

The airbag is activated by balls that are held in place by magnets. If the force of decelleration is greater then the force of the magnet, the ball is released, and triggers a start to the whole system.

Not going to dispute this, but bear in mind this is pretty old technology- many systems are fully electronic now.

[dfdex1]

Not to sound like a nerd but that change in velocity is called "Delta V".

[mtfd89]

Nerd or not; here's a web page that explains it all in basic terms:

http://www.physicsclassroom.com/Clas...tum/U4L1c.html