Air Bag System Trivia

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Thanks to Doug Lewis.

In my Supplemental Restraint Systems class, they told us that it takes a head-on collision with an immobile object at a speed of 18mph or a head-on collision with an object of the same mass at a speed of 28mph to deploy the air bags. Any angle of deflection makes the trigger speed increase due to the energy vector.

Our cars have a pressure relief system. There is a dump valve in the body that releases pressure within the cabin when the air bags deploy. This keeps the windows and the windshield from landing on the pavement. This valve is located inside the rear bumper on the passenger side. It vents the pressure through the trunk space.

There are several failsafe features designed into the system. There are firing sensors, which are on the positive side of the circuit, and safing sensors, which are on the ground side of the circuit. You must trip one safing sensor and one firing sensor to complete the circuit and deploy the air bags.

There are three firing sensors. One on each corner, behind the foglights, and a center sensor located above the radiator. This is a combination sensor that has a firing sensor and a safing sensor. There is also another safing sensor located in the center of the firewall, just below the cowl.

The sensors consist of a cup shaped magnet that holds a gold-plated ball. In front of the ball are gold-plated contacts. In an impact, inertia pulls the ball out of the cup and into the contacts. This closes the circuit to either the ground side, or the hot side of the air bag firing circuit. This is why you must trip two sensors, one safing and one firing, at the same time.