Be sure to also read parts one and two of this story!
In the Summer and Fall 2005 issues of Drive, “What’s Inside” discussed air bags and other components in the Supplemental Restraint System (SRS).
In Part Three, Drive delves deeper into air bags and other SRS components – in particular, air bag inflators, seatbelt pretensioners and the Occupant Detection System and how they work. (Read Air Bags, Parts One and Two at www.drivesubaru.com.)
In order to activate a frontal air bag, a frontal force equivalent to that experienced in a collision with a stationary barrier at a speed of approximately 12.5 miles per hour is required. Once such a force is encountered, coordinated SRS activation takes place in fractions of a second.
The required frontal force overcomes the inertia and the tension of roller springs for rollers in the front sensors. The rollers then make contact with the circuit terminals. “Safeing” sensors provide redundant signals that confirm those received by the front sensors, ensuring that their signals are accurate (safe).
One collision signal from the front sensors and one collision signal from safeing sensors are required to activate the air bag inflators. The front sensors provide a ground negative circuit while the safeing sensors provide a positive circuit. Both must function for the air bags to deploy.
In the driver’s air bag, signals from the control unit cause the igniter in the air bag inflator to heat to more than 300° F, which in turn ignites a squib that then ignites nitrogen pellets. The pellets generate nitrogen gas during combustion. The rapidly expanding gas inflates the air bag. The air bag deploys to absorb the forward movement of the driver.
Other air bags operate in a similar manner.
Pretensioners for the front seatbelts tighten these belts to help restrain occupants during an impact. The pretensioners are mounted in the pillars between the front and rear doors, and they react in fractions of a second along with other SRS components.
When activated, a gas generator pushes against a piston in the pretensioner assembly. The piston thrusts a rack-type gear against a geared wheel, which rotates and winds up the seatbelt webbing to tighten it. That improves how well the seatbelt holds the occupant in place so the restraint system can be more effective.
Seatbelt pretensioners are standard in all current Subaru models sold in the United States.
If the force of the occupant’s body against the shoulder belt exceeds a predetermined level, the take-up reel mount will twist to unwind and relieve some pressure just as the air bag deploys. In this way, this force limiter helps to reduce injury.
Some Subaru models have an Occupant Detection System. It determines whether or not someone is seated in the front passenger seat and how much that person weighs. Its purpose is to deactivate the front passenger air bag if no one is seated in the seat or if the person is too small for the force of the air bag. (The system does not deactivate the side-curtain or side-impact air bags.)
A loadcell sensor is installed in each of the four corners of the seat. They measure the load on the seat, which a control module underneath the seat uses to determine whether or not to turn off the air bag. The air bag indicator light in the instrument panel signals if the system is on or off.
In the end, a vehicle’s SRS is intended to support the most important safety restraint of them all – the seatbelts. Air bags, pretensioners, bolsters and the rest all play roles in keeping occupants safe, but seatbelts are the heart of the interior’s passive-safety components.
So buckle up!