Is an impact required for vehicle airbags to deploy?
The sensor doesn’t set off the airbag, as such, but a control module. This matches the pulse trigger shape from the sensor against a crash pulse table – if it detects behaviour that suggests a crash that requires airbags, the airbags will deploy. It is much more complicated that a green light from a sensor. You’d have to model a crash pretty precisely to get an airbag to go off (assuming no malfunction), as the inadvertent deployment of airbags is pretty much ‘a bad thing’ from a safety and marketing point of view. So lots of crash testing has been done to ensure that the module understands what a crash pulse looks like, and doesn’t set the airbags off if the accident isn’t big enough to warrant it, or just because water got in a sensor. The best way to model a crash is to drive the car into something…
“Click and Clack” were just talking about a very similar question this past Saturday on Car Talk (for those reading this later, airdate 2008-02-09), in regards to a woman wondering whether her husband using his truck to push a dumpster would set off the airbag. See Segment 8 here [link doesn’t appear to be permanent] (Short answer: no, they use accelerometers, and slow changes in pressure on the bumpers will not set them off.) The deceleration has to be greater than anything which could be achieved by braking alone to deploy the airbags, they said, so that gives you an idea of the deceleration necessary to set them off.
Standard airbags don’t deploy in rear impacts, because they would not protect you from the typical rear-impact injury, which is whiplash as you head is forced back against the headrest. In fact, they might aggravate whiplash. Special bags that are built into the headrest for deployment in rear-impact crashes have been developed, basically they cradle your head to reduce the whiplash effect.
Grither: What matters is that there is a high-magnitude negative acceleration. That’s what I would call a sudden deceleration. In other words, what matters is a change in velocity over time (this is the definition of acceleration: dv/dt), not the change in acceleration itself. Here is an example: You drive your car into a brick wall at a constant 3m/s (11km/h). Your airbag doesn’t go off because the deceleration wasn’t great enough. Just as a simplified example, say it takes 1 second for your air bag sensor to go from 3m/s to 0m/s, so you measure an acceleration of -3m/s2. Your change in acceleration is from 0m/s2 to -3m/s2. Now you drive your car into a brick wall at 3m/s while you are stomping on the accelerator and accelerating at 3m/s2. You still have an acceleration of -3m/s2, but you have twice the change in acceleration: from 3m/s2 to -3m/s2. Should your airbag go off in this case because you have a large change in acceleration? Of course not, because what matters is the change
Now you drive your car into a brick wall at 3m/s while you are stomping on the accelerator and accelerating at 3m/s2. You still have an acceleration of -3m/s2, but you have twice the change in acceleration: from 3m/s2 to -3m/s2. Some of your assumptions are off, mind you. Simplifying it like that is also, oddly, confusing the issue. The airbag probably would have gone off in that instance (I’ve had a near exact experience, and it sure as shit hurt when the airbag hit me in the face, I can tell you). I don’t think adding numbers is helping, really. Also, there’s no way the crash impulse would be a second long in that instance, and an 11kmh impact with a wall is a significant impact, particularly if the car is accelerating. The system does measure change in acceleration (it’s all part of the crash impulse). It’s not as basic as you suggest. A crash impulse is a signature of accelerations (positive and/or negative) that have been calculated as making the deployment of an airbag beneficial
