my cursed Micra

[LilGeckos]

Geez. Three hit cars. I'm glad to hear you guys are OK; especially you Pete, that's quite the hit you took there. Palourde77 pretty bent up too. Would anyone know why Pete's airbags didn't go off?

[Howie]

Thats because its an SRS system. Airbags and seatbelt pretensioners, controlled by a computer that determines the type of crash and if air bag deployment would be useful or not.

Air bag deployment is triggered by 3 sensors. A main crash zone sensor in the front bumper, and two satellite sensors, one on either side of the car near the B pillar on the bottom of the car. There is no rear crash sensor.

In an impact the car has to determine which sensor is tripped. If its one of the side impact ones, the side air bags will deploy. If its the front, the front will go off.

It also uses the vehicles speed sensor system and brake force distribution system to determine the vehicles state on the road.

It is important to note the system is designed to protect the body from moving and striking interior parts, and to reduce the speed the body travels at in a controlled manner.

If you are sitting at zero speed with your foot on the brake. If you are hit from the front, the system sees:

Front impact crash zone sensor tripped.
Speed, zero?
Front driver seat occupied?
Front passenger seat occupied?
seat belts fastened?
pretensioner state?
brake state? low pressure

From the data it sees, it will determine if there is a requirement to deploy the bags. In this case the speed is zero, the body should be restrained by the pretensioner alone and that is what the car will do.

Same accident but change a few things:

Front impact crash zone sensor tripped.
Speed, 82kph
Front driver seat occupied yes
Front passenger seat occupied no
seat belts fastened? driver yes.
pretensioner state? Engaged.
brake state: full pressure

The system will see that in this case the speed is high, and there is a driver of the vehicle. The brakes are on fully and the pretensioner is engaged on the seatbelt. In this case the chance of the driver impacting the interior of the car is extremely high. The front airbag will be engaged.

Same scenario, more speed, or an offset collision.

Front impact crash zone sensor tripped.
LH side impact crash zone sensor tripped
Speed, 120kph
Front driver seat occupied? yes
Front passenger seat occupied? yes
seat belts fastened? yes
pretensioner state? disengaged
brake state: no pressure

In this case, the driver hasnt seen the impact coming, or perhaps fell asleep. The speed is extremely high, and the impact has been great enough to distort the body to affect the side impact crash sensor. As there is no brake state and the pretensioner hasn't been set, the pretensioner lock will fire and permanently lock the seatbelt. The system knows there are two front passengers with airbags on and will deploy all airbags as it knows from the lh side impact the impact has been extreme.

Front impact crash zone sensor not tripped
RH side impact crash sensor tripped
Speed, 15kph
Front driver seat occupied? yes
Front passenger seat occupied? no
seat belts fastened? yes
pretensioner state? disengaged
brake state? no pressure

This accident would be the case of someone heading across the intersection and oncoming traffic hits the passengers side of the car. What you would see here is the pretensioner fire, and drivers side airbag deploy, as well as the passengers to catch glass.

In the case of a rear impact, there is no crash sensor to trip. The air bag module isnt alerted. The pretensioner in this case will engage. Thats about it.


So as you guys can see, the system isnt a basic one where the air bag sensor gets hit and there is a ton of airbags going off. The system takes a careful calculation based on it's programming, tries to figure out if there is a need for the airbag, then picks the right course of action. It even has its own emergency power supply internally in case the vehicles main battery is compromised, and complicated G sensors.

Generally in front impacts, the body continues to move after the car has stopped from hitting something. The pretensioner system holds as much of the upper torso as it can. As the head isn't held by it, the airbag deploys to help slow down the head. Once you hit the airbag, the headrest provides the soft backstop to hopefully prevent whiplash. If there is a frontal collision with side yaw (the car rotates after the impact) side airbags will prevent the head from striking the window, a and b pillar of the car.

In a rear collision, the impact forces are backwards. Your body is at rest and youre hit. The pretensioner locks and the headrest catches your head. The body accelerates and moves foreward, but is limited by the body being held against the seat. In a frontal colision the body stops and the head is whipped toward the dashboard. In a rear collision the seat pushes the back of the head foreward, typically with less force then in a front impact. But still worth a sore neck. But the head rarely comes in contact with the steering wheel, and if it does, it is typically less then what you would see in a frontal impact. Headache, concussion, maybe some facial/dental injury. But not the crushed skull you might see without the airbag.

This being said. Some cars DO have rear impact crash zone sensors and will deploy the airbag if the impact is severe enough. If your face isnt close enough to impact the steering wheel or dashboard, then smacking you in the face with the airbag isn't really a good idea if it isn't needed but you might still see it in some cases. It depends on the specific scenario the air bag module is faced with.

Most cars have pretensioners. You can test them by pulling rapidly and sharply on the seatbelt. It will lock. I believe the micra catches a signal from the G meter inside of the micra's air bag module and can electronically lock them and unlock them based on brake force and yaw. Ever slam on the brake and the seatbelt locks? Its supposed to work that way. Its getting ready for a crash.

Keep in mind, that most cars have 2 seperate pretensioner modes. A passive one, where the seatbelt locks and unlocks freely and a seperate one with a small pyrotechnical charge that activates to permanently lock the belt and secure you in a crash. It's also how police can tell if you were wearing the seatbelt at time of impact. If the belt is loosely flopping about after you were in the accident with slack to a point. (As if you were in the seat still) then it was worn. If the belt is locked tightly against the B pillar as if you were never in the car, then it wasnt on your body.

The nissan micra service manual section SR and SRC overview basic system functions and situations where the pretensioners and airbags will go off. There is also a basic "Flash" code self test, where it will blink the SRS light on the dash when the key is on after the self-check completes.

Flashes Fault
0 No fault
7 Air bag diagnosis sensor unit fault
6 Crash zone sensor fault
2 Driver air bag fault
ON Air bag deployment

I don't recommend you play with the system. If the light acts weird, have it serviced by a professional.

[Pete]

I got the call from the insurance adjuster today....... my micra is a total loss suprise suprise!

[Howie]

^ I wasn't terribly surprised. Sad thing, but in the end for the best (expecially since it wasnt you at fault.)

So what's on your mind for a replacement?

[palourde77]

^ I wasn't terribly surprised. Sad thing, but in the end for the best (expecially since it wasnt you at fault.)

So what's on your mind for a replacement?

ferrari!!!!!

[micrapolis]

how bout a big gas guzzling f150 or full size SUV, not cuz you need all that huge honking vehicle, but ya know, cuz you can "afford" the monthly payments?

[Howie]

Maybe a nice 16 Micra SR?

[Pete]

Took my plates off the old micra today. Here's a few more pics of the damage.

[MetroMPG]

Ouch. Performed as designed at least.

Any news about the woman who hit you?

[Pete]

No news about her but the good news of the day is that I have a clause 43 on my insurance and I am able to keep my snow tires Clause 43 means they have to pay full cost to replace no depreciation for mileage or wear and tear.