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

On reflection, it occurred to me after my last post that the failsafe position for airbag deployment needs to be independent of indicated speed. Speed indication is from the drive line somewhere I suppose. If the abs system is down, its slippery and you lock the brakes trying to stop there may be no registration of speed even if you are travelling fast. You'd want the airbag to work in that circumstance (ie if you hit something in that case).

Alien2,
In practice you are probably right. However, deceleration is the rate at which speed changes which is independent of the speed itself (unless you are not moving at all). So, for example, if you hit an immovable object, had no crumple zone and came to an instant halt, the deceleration would be infinite! The rate of deceleration is dependent on how soft the object you hit and your car are. However, the amount of damage to either or both in a crash depends on the relative speed and is proportional to the velocity squared.
Mal

Originally Posted by Mundy

On reflection, it occurred to me after my last post that the failsafe position for airbag deployment needs to be independent of indicated speed. Speed indication is from the drive line somewhere I suppose. If the abs system is down, its slippery and you lock the brakes trying to stop there may be no registration of speed even if you are travelling fast. You'd want the airbag to work in that circumstance (ie if you hit something in that case).

Alien2,
In practice you are probably right. However, deceleration is the rate at which speed changes which is independent of the speed itself (unless you are not moving at all). So, for example, if you hit an immovable object, had no crumple zone and came to an instant halt, the deceleration would be infinite! The rate of deceleration is dependent on how soft the object you hit and your car are. However, the amount of damage to either or both in a crash depends on the relative speed and is proportional to the velocity squared.
Mal

The comment about the crush cans was about using the crumple zone.
Air bag deployment allows for this along with the other inputs,some have been discussed.
But you've summed it up well, inside knowledge?

Originally Posted by biggin

So my questions are: is there any documented evidence of the bags deploying when not required; or evidence of bags not deploying when needed, whether crush cans or bumper crumple zones are present or not?

In the past I have attended a number of car crashs with un/deployed air bags(SRS).
I have not seen them go off when not needed or not go off when needed.
Most moder cars have a crumple zone as disscused.
Plus front wheel under-run where the wheel goes under the floor pan and not through the floor(see the video from UP's post).
Plus door bars plus...that will do as hurting the brain(havn't done it for 4 years) and heading away from the post question.
To sum it up(in laymens terms)...
If your head hits the dash/steering wheel/windscreen the air bag should have deployied.
If you didn't hit the above it wasn't needed.

What airbags achive,an over veiw...
An air bag deploys and hits full volume just before you(body/head/ect.) touchs it.
At this point the seatbelt(pre-tensioner) has retracted pulling your torso into the seat.
As you continue traveling foward(In head-on senario) the car is decelerating quicker than you body can.
The air bag then absorbs you by deflating(Thats what the holes in the back of the bag are for).
This action of slowing your deseleration reduces internal injuries.

[QUOTE=alien d2;1358277]The comment about the crush cans was about using the crumple zone.
Air bag deployment allows for this along with the other inputs,some have been discussed.
But you've summed it up well, inside knowledge?

No, the first paragraph was just thinking it through logically. The second was high school physics (I'm an old fart from a time when they taught science in schools). Acceleration = time rate change of velocity = dv/dt. Damage is proportional to kinetic energy of the vehicles = 1/2*m*v2.

Originally Posted by alien d2

I would think that at a low speed of 20KM/H the deceleration would not be great enough to triger.
If decending and hitting something hard enough to triger the bag chances are you need it(provided the crush cans are fitted).
I have hit a roo in my D2 at @60KM/H(slowing from 100KM/H) and pushed the ARB bar back bending the crush cans.
Arial mounts hit the bonnet, bar twisted, headlamp poped out, slight gaurd damage, no damage to radiator support ect..
Neither myself or my pasenger were injured and the air bags didn't deploy.

My 2 cents

Just in regard of your thoughts of low speed......

I have been rammed twice whilst engaged on her majesty's business so to speak... Both times our vehicle was static and the bandit vehicle reversed into the front of our car thus deploying the front airbags, causing an instant warming to the insides of my wrists!!!! And an end to the pursuit.. the side bags/curtains didn't deploy..And disabling the vehicle. I remember attending an RTA where a car had plowed into the side of a VW golf and yet only some of the airbags deployed as 'it' sensed which ones needed to activate... so they aren't an all or nothing system either...

Originally Posted by biggin

So my questions are: is there any documented evidence of the bags deploying when not required; or evidence of bags not deploying when needed, whether crush cans or bumper crumple zones are present or not?

Yes... been to numerous crashes where we would have expected to have seen airbags deploy.. eg heavy front end impacts with heavy braking leading up to impact... but no bags came out...

Yes... following crashes we have been subject to a deployment well after the event.... Fire now place stickers all over vehicle if airbags have'nt deployed to give warning to anyone wanting to stick their head in front of the steering wheel.... NOT that I would want to see an insurance assessor get a wollop .... well at least the one who went back on his agreed value of my old series 2a V8 hybrid upon her untimely death!!!!

Originally Posted by twodoors

Yes... been to numerous crashes where we would have expected to have seen airbags deploy.. eg heavy front end impacts with heavy braking leading up to impact... but no bags came out...

Yes... following crashes we have been subject to a deployment well after the event.... Fire now place stickers all over vehicle if airbags have'nt deployed to give warning to anyone wanting to stick their head in front of the steering wheel.... NOT that I would want to see an insurance assessor get a wollop .... well at least the one who went back on his agreed value of my old series 2a V8 hybrid upon her untimely death!!!!

If the car crumples it's absorbing the impact so my questions is...
What were the ocupants injuries?

I've have heard of SRS deploying post impact.
Have you seen the footage from the USA of the rescuer being throwen by the air bag?
Time goes slow waiting for the SRS to power down after disconecting the battery with people to help.

Originally Posted by alien d2

If the car crumples it's absorbing the impact so my questions is...
What were the ocupants injuries?

I've have heard of SRS deploying post impact.
Have you seen the footage from the USA of the rescuer being throwen by the air bag?
Time goes slow waiting for the SRS to power down after disconecting the battery with people to help.

Yes as the car crumples it absorbs some of the force...
If we take out any injuries caused by the occupants body striking any of the distorted vehicle frame/ body work/crew area intrusion etc.. as i'm sure you will know this contact will hurt!!!

However the car still undergoes a decleration force... the occupants undergo some of the same decelaration forces whilst restrained by the seatbelts and protection offered from airbags... However the contents of the body are 'floating' for want of a better description and with any violent stop these organs etc may strike the internal solid human body frame/chest cavatity/skull and do some very serious damage....

Concusion, ripped/separated aorta, burns associated with airbag striking the wrists/face, burns from the seatbelt and/or heavy bruising and potential compression injuries to the body from incorrectly placed/adjusted seatbelt.... think of the people who use the seatbelt hold off devices as they don't like the seatbelt on their shoulder... they then travel forward and place an accelerated body force over a rigid seatbelt width as opposed to an increasingly exerteted pressure across the chest...

Lap belts that cause stomach injuries.... crushed spleens etc...

Added to the unseen internal bruising there can be the obvious external aches and pains which the all encompassing 'whiplash' covers....

It was always good to do a check on occupants and see their seatbelt injuries when the driver was in doubt as the bruising runs across opposite chest angles for driver/passenger positions!...

Add that to the people who brace their feet for impact and break ankles or cause the leg to go so rigid that the shin bone breaks or the femur breaks and ends up sticking out of their thigh, or the top of their hip....

As well as those that death grip the steering wheel and their hand/wrists rotate over the bend in the wrist and pop the bone out... then the side impacts where the side by side passengers hit heads together or the b pillar....

The non seatbelted rear passengers who hit the front seat collapsing it and crush the front seat occupants...

Sorry bit doom and gloom but just remember when in doubt don't crash!!!!

That is why cargo barriers are good as they stop any rear cargo becoming a forward moving projectile... including those idiots who think it is okay to have a dog/cat on the rear parcel shelf.... (unrestrained)...Or basically a 5 - 10 kg missile in the car usually stowed at head height to the rear of someones head....

So remember 'if you have the misfortune to be spoken to by an officer of the law' their attitude may be tainted by constantly having to deal with the above scenarios on a regular basis.... most of which are avoidable or the subsequent injuries could be negated or reduced...

James

The ramblings of a man currently on night 6 of a 7 night shift roster!!

twodoors,
Good summary of the posibilities(of which I think we have both seen).

I was looking to help answer biggin's post and clarify if the SRS not deploying incresed actualy injuries to the occupants(very subjective though).

My understanding is no SRS unless warented(predetermined by factory).

Cheers.