What causes airbags to deploy?
 

In another discussion ( https://www.rx8club.com/showthread.p...threadid=18053 ), I mentioned that I had survived a pretty scary crash on the highway that totaled my RX-8. An interesting question about airbags was raised there since, though the force of the accident was enough to completely crush the back of my car and send the car into a vehicle in front of me, the airbags did not deploy.

I assumed that the reason was that the car absorbed most of the impact so that the severity of the impact was under the threshold of what is required for the airbags to deploy. But it was suggested that there are sensors on the back bumper and that the airbags should have deployed when the back bumper was hit at around 35 mph--which, judging by the extent of the damage to the car, I'd guess it was.

So what, exactly, determines when an airbag goes off, in the RX-8.

Many thanks.

I doubt there are sensors on the back bumper - airbags typically do NOT trigger from a rear impact. Before the advent of side airbags and the window curtain airbags, the standard drivers and passengers dash airbags were only beneficial in a frontal impact, and would only inflate in a frontal impact of sufficient force. I imagine that there are now side impact and rollover sensors to trigger the side and curtain airbags, but it still remains true that airbags don't do any good for an impact from the rear, and therefore don't inflate.

If you hit the car in front hard enough, they should have triggered, but even then the system may identify it as a secondary impact to the primary impact from the rear and not trigger.

Regards,
Gordon

The trigger mechanism is a tube with a metal ball in it. On one end is a magnet, and the other is two electrical contacts. The deceleration Gs must be sufficient to dislodge the ball from its magnet. Not sure exactly what that Gforce number is though.

Thanks, Gordon and Chrisbert. Do you know how many triggers there are and, if more than one, what does what?

Also, I assume the magnet is permanent, not electro which would be very unsafe if the electrical current were cut off. But can't permanent magnets lose their attractivity? If so, could airbags deploy prematurely or with a smaller Gforce? Have you ever heard of this happening?

Older Mazdas had sensors mounted in the bumpers wired directly to the A/B control unit. The newer ones use an accelerometer or G-force meter control box mounted under the dash, wired directly into the OBDII ECU "black box" which can record and download impact data. Rear impacts will not deploy airbags, as initial hit is absorbed by the head restraint and seatback.

Speed...
 

With the furor lately about people being hurt by low speed airbag deployment, I think there is probably an ECU 'minimum speed' cutoff, so that they dont go off in fender benders.
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doc

This is relevant to the FC rx-7 however I should imagine the same principles apply here also.

In the case of the FC, there are "D" sensors near the bumber and a "S" sensor in the center of the car. The "S" and at least one "D" sensor must be triggered before the airbags fire..

And for the technically interested...

Of course, since the rx-8 has a bazillion airbags that go off in all kinds of directions, the control system is probably more complex than this, with sensors in different angles for side impacts and such.