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· Seek to understand,^Value
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Discussion Starter · #1 · (Edited)
I was reading about the new E46 front-airbag recall, where the Takata OEM setup sprays shrapnel into the cabin, when I realized I had no clue how these BMW SRS Supplemental Restraint System (includes airbag, crush zones, and explosive disconnect) systems actually worked.

I dutifully typed /srs f3 into the bestlinks, as I am wont to do, and I was surprised that no bimmerfest thread came up that was specifically all about how the safety systems work to protect us in a crash such as this recent one, which occurred on a road I travel every weekend (where a double-loaded sand truck rolled over 10 cars, crushing more than one, yet, only one person was killed).

To be sure, there was this related reference in the bestlinks, but it's not a thread where we can add information as we find it (however, it's a very nice starting-point PDF):
- Everything you need to know about the airbag on BMW cars (1)

So, I open this thread, intended to be a one-stop shopping thread, for how the SRS system works, in our bmw.

To be clear, I am aware there are many fixit threads, which have good information on how the system works, such as this nice cn90 DIY for the front belt pretensioner:
- DIY: 1998 BMW 528i, Replacing Driver Seat Belt Buckle (Pretensioner)

And, I'm aware of specific component threads, e.g.:
- Where to find the BST Battery Safety Terminal battery explosive charge emergency quick disconnect (1)

But, there wasn't, in the bestlinks anyway, a specific Bimmerfest thread, that we could add value to, which covered how the entire safety system works.

Q: How do BMW Supplemental Restraint Systems work (srs, airbag, bst, crush zones, etc.)?

This is a shrunken composite that I put together from multiple pages in the attached pdf.
A larger composite is also attached.


See also:
- How do BMW Supplemental Restraint Systems (srs, airbag, bst, crush zones, etc.) really work (1) & what is the cheapest usable BMW airbag srs reset/scan tool on the net (1) & what other scan tools reset the fault (1) (2) (3) (4) (5) & what can cause a BMW SRS airbag light (1) (2) (3) (4) (5) (6) & a cn90 DIY for replacing the driver seat belt buckle pretensioner and the passenger seat emulator (1) (2) & where to obtain an airbag occupancy sensor bypass unit (0) (1) (2) & what happens when we disconnect the airbag before the battery or we connect the battery before disconnecting the airbag in seat and window regulator repairs (1) (2) (3) (4) (5) (6) (7) (8)
 

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· Seek to understand,^Value
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25,378 Posts
Discussion Starter · #2 · (Edited)
I took the liberty of cutting and pasting together some of the better diagrams from the BMW marketing PDF, for our benefit ...

If each of these diagrams is shrunk, the original size is also attached.
 

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Q: How do BMW Supplemental Restraint Systems work (srs, airbag, bst, crush zones, etc.)?
That is a very complex question, BB. In a nutshell, the intent of the crash systems are to manage the energy of impact, reduce the likelihood of injury and increase survivability of the vehicle occupants. In a crash, the moving car typically decelerates very rapidly to a complete stop. Given Newton's First Law, a body in motion tends to stay in motion, there is the need to control the deceleration as best as possible. These systems represent the best engineers can do given the constraints of the problem. The airbag system reduces the impact forces during a crash by cushioning the occupant, which reduces the sudden deceleration the upper body and head experience during a crash. The airbags are controlled by a microprocessor that analyzes feedback from various sensors (accelerometers) around the car. If the parameters of a "crash" are detected, the appropriate airbag is instantly fired off using a pyrotechnic. They then immediately deflate to prevent smothering and allow egress. Crush zones are designed to absorb the energy of an impact by crumpling in a controlled manner. If the car body was built like a tank and the impact was with an immoveable object (wall), the deceleration forces would be significant. By crumpling, the car body sacrifices itself to reduce the impact loading. Note that the doors do not have crush zones. You need to have space (e.g. front engine compartment) to design a crush zone to absorb the impact energy. The doors are so thin, they add rigidity (& side airbags) to protect the occupant from side impacts.

I can say from personal experience that the e39 SRS and crush zones works very well indeed.
 

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