Week after week, month after month I am asked ”Why didn’t my airbag go off? I was hit hard enough, the frame rail is bent, the engine mount was broken but the airbag never went off.”
These situations are far too common and even more commonly misunderstood. The airbag systems are designed for specific impacts, using very specific criteria to determine the severity of the crash as well as the degree of danger to the occupants of the vehicle. The earlier models used multiple sensors placed strategically in the front end of vehicles, while many of the newer cars and trucks simply use one central Sensing and Diagnostic Module and one remote sensor to determine deployment.
With the addition of head airbags, knee bag, side curtains, and seat airbags, again multiple sensors are in play, however each sensor is designated to a specific airbag. One for frontal, one for side airbag, one fur curtain, etc... But let me get back to the question at hand. The answer is …the airbags will deploy when they need to, based on the parameters designed into the computer. The diagram below will illustrate exactly which one will deploy and which one will not.
Vehicle 2 is impacted at approximately 45 degrees from center, hitting the left front corner of the vehicle. The impact to this vehicle does not qualify for an airbag deployment because it is outside the zone designed for either a frontal or side impact deployment.
Vehicle 1 is impacted at an almost straight ahead direction and will cause the frontal airbags to deploy. All of the inertia is focused on forward movement and thus the driver and/or passenger are both subject to much more risk of personal injury form this impact that those in vehicle 2.
Airbags are basically designed to prevent the occupant from going through the windshield (or side window) as a result of an impact. If the impact is not severe enough, fast enough, or at the correct angle to risk that much personal injury, then in many cases the airbag will NOT deploy. In addition, a 'soft' impact, such as hitting a deer (or person), will also NOT transmit enough energy through the frame of the vehicle to activate a deployment. Once in a while we see virtually no damage to a vehicle and both frontal bags deployed. Upon closer inspection we discover a direct hit to the sensor or the frame rail right next to the sensor. It is with these close proximity hits to the sensing equipment that the airbags deploy when no one was in fact in danger. And on the other hand, we sometimes find a 'total loss' vehicle without a single airbag deployed. These vehicles are often hit at exactly the wrong angle for any bag to deploy.
So when airbags deploy, multiple criteria have to be met. But in general for an airbag to deploy, the speed needs to be over 30 mph, the angle of impact needs to be within 15 degrees of direct frontal or side impact, and now, with occupant sensors, someone needs to be in the seat of the occupant at risk.