Thread: how does a SRS airbag system on a D2 work?

not knowing anything about this--
I would think the crush cans and SRS bar deformation is calculated in the deceleration figures.

also if you have a front end crash big enough to trigger the bags,, and the trigger mechanism was up the front in the crash area, would the bags still deploy?

Not sure of the full technical details but the first post sounds like what I have been told by my mechanic. I recently installed my ARB bar and saw no evidence of any sensors in and around its mounting points. My simple understanding is that in the event of a very sudden decelleration (impact) the sensor shifts forward (obviously) and triggers the system......I don't really understand the need for a specific bar for airbag equipped Disco's but I wanted it "legal".....

My understanding is the first D1s with airbags had crush cans at the front with sensors that set off the bags.

Later model D1s and the D2s no longer have the airbags set off by sensors but by decelerometers and shock sensors (basically if the vehicle is slowing faster than the brakes can stop it this is detected and the airbag deployed).
I think the crush area at the front of the chassis is still there but only to help with the crumple zone (not sure on this though).

Garry

It does not matter where the sensor is, could be on the roof, the point is it sense's deceleration, and that "rate" will / would change with / without crush cans on the front of the vehicle.

ARB for example take this into account and "replicate" the crush areas into the bumper design so it reflects the standard vehicle as much as possible.

With a bumper bolted directly to the chassis and no "crushable zone" the impact is sent directly into the stiffer chassis and thus the deceleration rate will be bigger as their is less "give" in the solid parts.

It's the crush zone in any vehicle construction which absorbs a lot of the impact and protects the occupants more, as less energy is passed in / onto the rest of the vehicle. This is why if you have a reasonable impact on a "New" type / designed car, you will walk away most of the time but the car will be totalled.

In a nutshell, in a frontal impact it is "more" likely you will damage the chassis AND set the airbags off at a "lesser" speed with no crush zone than you would do with say a factory spec vehicle.

As we are "on the topic", heres a vid for a D2 frontal crash test. I for one would like to see the SAME extact test with an ARB on the front not just see the "crush" effect, but to also see steel bumper V plastic.

[ame="http://www.youtube.com/watch?v=AIJFLVkCuuI"]YouTube - 1999 Land Rover Discovery Series II frontal offset test[/ame]

Thanks guys very interesting stuff, so the stores values in the DCU in which the Decelerometer compares against when a crash occurs obviously take into account the crushing of the standard bumper and the front subframe/Chassis and by having a stiffer bumper on the front of the car you are changing the rate at which the car "Crumples" therefore making the SRS system deploy the airbags and the seatbelt pretensioners later than what they should, would only need to be a few millisecond in a 100km/h crash to make a difference
does that make sense guys? thoughts, opinions?

Bluelightdisco
If you have a stiffer bumper, for a given 'bump' the deceleration will be greater and the airbag will deploy sooner not later.

I am surprised that the deployment of the SRS is independent of vehicle speed. I thought there was a threshold speed below which it didn't deploy, something around 20kph. This was to prevent deployments in low speed crashes but where there was high deceleration. So, for example, if you're going down a rock step and hit an immovable object with the front of the vehicle, a steel bullbar will give a higher deceleration than the plastic bumper and could deploy the SRS even if you're only doing a few kph.

Mal

So generally speaking whatever bar you use it should be ok provided you use appropriate crush cans or crumple zones.

from what i have read in the rave CD
The electromechanical safing sensor uses a magnet and ball bearing system to provide a mechanical confirmation of
vehicle deceleration. The ball bearing is held in place by the magnet. The strength of the magnet is calibrated such
that the force required for the ball bearing to move away from the magnet will only occur if the vehicle decelerates
suddenly, as in an accident.
The SRS deploys only if both the decelerometer reading and the safing sensor operation occur at the same time.
yes Mundy i was also of the opinion that a speed reading of some sort would factor into the equation,so regardless of speed the the airbag will trigger if the force of the sudden stop is enough to dislodge the ball bearing from the force of the magnet and then set off the SRS
i think its all quite facinating i'd love to be an automotive engineer!