Air bag compatible bumpers.

[Rudolf]

What makes a bullbar/bumper airbag compatible?
Especially for a D2.

[discoveryseries1]

What makes a bullbar/bumper airbag compatible?
Especially for a D2.
ok, hear we go again, short answer, it has crush cans, or impact ubsorbors. during a colision, the srs needs time to inflate the airbag before the shock wave enters the ocupant cabin. a rigid bull bar made of rail way ties may look to do better in an impact but the truth is that it will deliver the full impact directly into the chassis and into the cabin. giving the computer no time to inflate the airbag.

srs bumpers have a rippled can somtimes looking like an air acordian like the dutch used to play, ( or still do ) (may not be dutch) this eases in the impact gradualy increasing the shock wave into the chassis, then the computer says "hey we are crashing, .......... i wonder if its going to be a big crash........ ( no minor crash) " oh i guess we dont need the air bag this time. second crash, . " oh we are crashing...... "i wonder if this is going to be a big crash"......hum 7 g's " i guess we need the air bags" at this point the crush can would be say 1/3 crumpled, (guess) then the air bags fire and inflate (via explosion) and buy the time they are inflated the crush cans would be fully squashed ( if in a serious impact) and then would comense with chassis deformation. and ripple entering cabin.

key things to rember, a bull bar with no srs will deliver full shock imedetly just like the steel marbles that hit eachother and swing on a desk top. and would inflate the air bag PREMATURELY so there is a chance that the bag will have inflated and now be deflating buy the time the customer hits the stearing wheel. from what i have understood, the airbags are sensitive to timing, as soon as they inflate they deflate, and need to be hit at the right time.

second thing to consider, the rta or road authority requires that a vhical with an airbag installed at the factory requires air bag compatibul bars be put on it for registration. now not all will check wile over the pits, but if you are in a serious crash and there is money and accountabuility it may be noted after the crash, eg a non paying insurance mob, or a passanger seuing the owner for an iligal act. ( just my two cents. )

any questions ? cheers,

jeff

[witv1]

thanks for the info, I've been wandering about this issue for a while

[davros]

One thing I saw RE ARB bars too was that their airbag bars had the rubber bumperettes to prevent shock loading, whereas the non-airbag earlier versions didn't.

Dave

[Landover]

My TJM winch bar on my SRS Disco 1 does not have crush cans. It fits to the shassy with solid plate. I questioned TJM about this and gave them the part number and they said it is SRS compatable but could not explain. It does have the two rubber pads on the front though. It may be that the whole bar is designed to crumple on impact but still not sure.

Cheers, David.

[cookiesa]

The mounting system is designed to absorb impact the same way the front of the vehicle was designed to. This prevents the airbags being triggered when they shouldn't be (eg low impact "bumps") They are designed so that when the impact is severe enough that airbag deployment is warranted, they will trigger the airbag sensors. Nothing to do with timing.

The front of a modern airbag compatable vehicle is designed to crumple and absorb the impact instead of being rigid like older vehciles which transferred the impact to the occupants. (This is why modern cars look worse off in an impact, they are meant to they absorb the impact instead of you and your passengers) There is a reinforced passenger safety cell that limits the intrusion in to the cabin area in a major accident.

Airbags are what is called an SRS (supplementary restraint system) and are there to hopefully stop your head bouncing around.

So even if you don't have airbags, having crumple zones reduces the loads placed on the occupants during an accident. Better the car folds and you walk away.

There are various ways the engineers achieve the desired results, not all have an obvious crush can style mount.

[davros]

This leads to trade offs like more modern Defenders without airbags as apparently it was felt that the knocks they were designed to take may well deploy the bags unintentionally. Although I suspect that economics had more to do with it!

Dave

[Panya]

In all the workshop manuals I have seen the D2 system has no remotely mounted crash sensors. The system uses a single DCU that "deploys based on inputs from the decelerometer and the electromechanical safing sensor" both of which are in the DCU itself. Happy to be flamed on this but I have looked at 2 different versions of the Rave Discovery II Workshop manual and I quote: "The diagnostic and control unit (DCU) controls the SRS system. The DCU is located beneath the centre console close to the handbrake area. The DCU contains both an electronic deceleration sensor as well as an electromechanical safing sensor. When the electronic deceleration sensor within the DCU detects rapid deceleration of the vehicle, it compares the deceleration rate with stored values in its' memory. If the deceleration rate exceeds the stored value and the electromechanical safing sensor triggers, the DCU deploys the airbag and the seat belt pretensioners. The DCU will not deploy the airbags and seat belt pretensioners unless both sensors trigger."

Am I missing something? Were crash sensors mounted on later models? Anyone got a pic of a remote crash sensor?!

[cookiesa]

I think you may be confusing crumple zones and sensors. Some vehciles have a central sensor or unit with the sensors built in (eg D2) others have multiple sensors (eg D1) located generally under the bonnet.

The crumple zones absorb the impact, the sensors detect when this wont be sufficient and the airbags will be required to reduce occupant injury.

Without a crumple zone all the force is absorbed (eg during deacleration stage) by the occupants bodies.

A crumple zone absorbs some of the crash energy (thus the folding up and what looks like heavy damage) rather than transferring it to the occupants (eg full chassis no crumple zones looks fine after a relatively heavy impact however more serious injuries are cuased to the occupants)

The sensors are completley different triggering the airbags which effectively "pin" you to the headrest preventing your head moving forwards (This is why side curtain airbags will not necessarily deploy just because the front ones have) They provide side impact protection.

Whilst the two systems work together they are not actually related. They work independantly of each other.

If you fit a non airbag bar to an airbag equipped vehicle in a minor impact that the crumple zone would have absorbed (crumple zones were introduced prioir to airbags so the airbag system has been designed around them), you will trigger the airbags simply because the crash energy is transferred straight to the sensors thus trigger them even though the impact is minor.

This of course also alters the crash "pulse" of the vehicle and will trigger the airbags prematurely, meaning they are in the deflate phase when you need them to be inflated.

There is also a big difference in airbag deployment "forces" for example in Australia we have slower inflating airbags due to it being compulsory for us to be wearing seatbelts, this reduces the injuries caused by the airbags themselves to the occupants. In the states they inflate faster due to some states still not requiring occupants to wear seat belts, they need to deploy quicker to prevent the occupants moving forward.

all this is of course condensed and simplified.

[Panya]

Thanks cookiesa. The point I was trying to make is that there are many posts and comments referring to remote crash sensors specifically for the D2 SRS system with reference to "airbag compatible" bull bars. The official documentation I have read so far says the SRS system triggers from a decelerometer in the middle of the car not from remote sensors (crash cans?) being squashed/triggered by an impact. IF that is the case then putting a big steel bullbar on the front of the car would not affect the reaction of the decelerometer since presumably your rate of deceleration with or without a bullbar is similar in any given collision (so the SRS triggers as normal) and an "airbag compatible" bullbar is a marketing ploy...

The main part of the crumple zone is in the design of the chassis and the way it deforms under impact, correct? Why should that change because you have a bullbar fitted? If you hit a stationary wall without a bullbar, your plastic bumpers absorb very little energy from the impact. Your crumple zone starts to deform as your chassis starts to absorb the force of the impact and so on. Difference in deceleration with or without bullbar at any kind of speed that would be negligible as far as the decelerometer and would therefore not make a difference in whether the criteria for airbag deployment have been met? I don't see the "airbag compatible" sticker being anything more than a marketing ploy UNLESS remote "crush cans" are used to trigger the SRS for the D2... and my question was has anyone got a picture of one - perhaps in a later D2 than the RAVE manual refers to??

[cookiesa]

I think the issue is you are confusing the two systems. Crush cans or similar mounting methods are there to "mimic" the crumple zones built in to the vehicle (The chassis is not generally part of this, a chassis by it's very nature is actually the opposite with a few exceptions).

A crumple zone is there to "crumple" and absorb impact thereby reducing the force transmitted to the occupants.

There are several ways to trigger airbags (the two systems aren't reliant on each other but do however work in conjunction with each other to ultimately reduce the force occupants are subjected too.) (I won't comment on the specific system in the D2 other than to base it on your comments) The D1 has seperate sensors they are yellow and mounted under the bonnet. The D2 from your description uses a single sensor (decelerometer) mounted centrally. (As a side note this type of sensor is also often used to alter the ABS sensitivity to allow it to work better on gravel (Eg wheel lock up on gravel does not provide the same rate of deacleration as it does on bitumen, the abs alters the pulsing to allow more effective braking on dirt/loose gravel.... but that is a whole different post! lol)

So the sensor regardless of the type used, is there to trigger the airbags if the deaceleration is within those determined as appropriate to deploy airbags. (Thus why a side impact won't generally trigger the front airbags, and why a frontal impact won't necessarily deploy side curtain style airbags)

Now if we ad a bull bar or other solid object which is mounted to the chassis (The chassis does Not form part of the crumple zone, there are on modern vehciles often a crash can built in to the front of the chassis to act as a crumple zone to prevent airbags deploying when they shouldn't/at the wrong time) and don't allow some form of crush can/crumple zone mimic effect then we have altered the crash "pulse" of the vehicle changing the scenario as to when the airbag will/wont deploy. As a side consequence more of the force of an impact will be transferred to the occupants rather than being absorbed by the crumple zones of the vehicle. So as you see whilst the two systems aren't exactly the same thing they do work in conjunction with each other.

A vehicle with airbags but no crumple zones would have very different deployment timing than a vehcile with both crumple zones and airbags.

A vehcile with no airbags but has crumple zones will transmit less of the crash force to the occupants than a vehicle with no crumple zone.

So in short, no the sticker is not a marketting ploy.

[Panya]

Thanks cookiesa that makes sense. What I really wanted to know before we got sidetracked is whether anyone out there with a D2 actually had remote sensors for their SRS system and if they could post a pic.:angel::wasntme:
I'm still a bit of sceptic about the whole "will change the airbag deployment timing" bullbar debate but you do sound like you know those particular onions (I am simply a sceptic by nature rather than at all informed!). Has the difference in deployment between standard bullbar/no bullbar/compatible bullbar been shown to be significantly different in practice?

[cookiesa]

Definately. Lots of crash testing (physical, not just simulated).

[Remy]

Definately. Lots of crash testing (physical, not just simulated).

Is there any evidence of crash testing (lots??) or is it part of the rumour??????;):wasntme:

[Remy]

How come Alloy bars with brackets a certain thickness don't require testing and are considered SRS compliant look at HRA for a P38.:p

[cookiesa]

Yes there is.

The brackets/mounts are designed on that style of bar to not effect the crash pulse by offering very little resistance, alloy will also bend and crumple with much less force than a steel bar, these factors combined, when properly engineered, don't interfere with the crash pulse of the vehicle. Not all bull bars (steel included) have obvious cruch cans built in to the mounting system, there are several ways to achieve the same outcome, but for example those that have say a D1 with an airbag compatable bar can have a look at their mounting system and will see the crush can, it looks like a concertina in the steel.

[Landover]

Yes there is.

The brackets/mounts are designed on that style of bar to not effect the crash pulse by offering very little resistance, alloy will also bend and crumple with much less force than a steel bar, these factors combined, when properly engineered, don't interfere with the crash pulse of the vehicle. Not all bull bars (steel included) have obvious cruch cans built in to the mounting system, there are several ways to achieve the same outcome, but for example those that have say a D1 with an airbag compatable bar can have a look at their mounting system and will see the crush can, it looks like a concertina in the steel.

As stated earlier my TJM SRS compatable bar on my D1 has no crush cans. It has solid flat steel and bolts straight to the shassy rails. It concerns me a little and hope it is designed to colapse on impact. Does anyone have any info on how the TJM bar has SRS approval. The standard bumper originally on the car had crush cans built into it.

Cheers, David.

[robert42]

Hi Panya and others, I had a roo bar fitted the other month after stuffing the plastic one.. I asked the panel shop would it make any differance, he just smiled and said no.. I asked what made them go off then .. answer is a tube with a steel ball in it held back by a magnet at the end of the tub is the contacts to set of the air bag.. When the impact is enough to break the strenth of the magnet it will move down the tub and bang .. He said dont worry it take a lot to make them move...????Bob

[seano87]

Yes there is.

The brackets/mounts are designed on that style of bar to not effect the crash pulse by offering very little resistance, alloy will also bend and crumple with much less force than a steel bar, these factors combined, when properly engineered, don't interfere with the crash pulse of the vehicle. Not all bull bars (steel included) have obvious cruch cans built in to the mounting system, there are several ways to achieve the same outcome, but for example those that have say a D1 with an airbag compatable bar can have a look at their mounting system and will see the crush can, it looks like a concertina in the steel.

I have a D1 with a SmartBar, which is SRS airbag compatible, and it doesn't have crush cans... I have asked the question to the manufacturers as to whether it should have crush cans, the answer was no, which they also said was true for other manufacturers for the D1 as well. :wacko:

[Panya]

Cookiesa is going to come along and flame me but thats part of the fun!:D. I was thinking about this last night. The decelerometer sits somewhere under the handbrake according to the manual. I am a crash test dummy in my D2; with and without "airbag compatible" bullbars and hitting my usual brick wall. Presumably the decelerometer is programmed so that if the rate of deceleration in a particular plane is greater than X then airbags are deployed. This rate is calculated (again presumably) so that deployment means my crash test head doesn't make interesting patterns on the steering wheel. Since my body mass is the same in each test my head and body move towards the wheel at the same speed only dependent upon the rate of deceleration. So the question is whether my two tonne vehicles hitting the wall at the same speed will decelerate with enough difference because my bullbar has been manufactured with a 3" concertina design at the front to affect the deployment of that bag? That's the bit I have a problem with because I can't see how it will affect the deceleration rate given all other parameters are equal. I am therefore still a sceptic...:p However I will concede that having a crumple zone does reduce the force transmitted into the cab and that the mechanics of the crumple zone may indeed be affected by a non-crumply (?!) bumper. I still object to the description "airbag compatible" rather than "crumple zone compatible":p:p:p
And I still want to know if anyone has ever seen a remote SRS sensor in their D2...!:angel:

[Tombie]

Its called crash pulse...

And its down to milliseconds...

Bag goes too early, its deflating before you hit it..

Too late, its smashing you in the chest with the force of a Anti Personnel Mine. (Breaking sternum, ribs, jaw etc..)

Its another reason not to let a short arse wife / daughter / loved one sit to close to the steering wheel... In the event of SRS deployment they can be killed by the Airbag.

And NO - There are NO sensors in the factory crush cans on a D2 (Which are a specially profiled ALLOY mount...)

[clubagreenie]

What if one removed the plastic facade from the steel backing on the D2 and made a "bar" to hang off the original cans/steel backing? Still has all original mounts just not the plastic crap.

[cookiesa]

Again comes to the crash pulse of the vehicle and keeping it the same/within tolerance.

A comment often heard about new cars in what seems like a relatively minor accident is the amount of damage, particularly if it has been involved with an accident with an old vehicle.

All that crushed steel/alloy etc is what has "folded" and absorbed the impact... Meantime our old vehicle showing minimal or no damage has transferred roughly the same amount of energy (for the purposes of keeping it simple) to the occupants of the vehicle instead.

Crush cans and imapct absorbers form part of the overall picture, but there are many ways to achieve the same thing, which basically is absorb and dissapate as much of the impact energy as possible instead of passing it on to the occupants. Then, if the impact is too great deploy airbags to hold the driver/passenger etc in position and prevent them impacting on the vehicle. (very simplified)

There is not just one component, these all work together to minimise the injuries, and severity of injuries in an accident. As Tombie ellided to, they are not perfect and can do more harm than good, but these situations tend to be in the minority, overall they make the vehicle safer. Ultimately would you prefer a wreck that did it's job amd saved the lives of your loved ones, or a "stronger" one showing minimal damage and leaving them dead or badly injured?

Some of the other features found commonly on vehicles are hinges that lock together preventing the bonnet coming through the windscreen, there is also generally a weak point causing the bonnet to fold up during an impact, it isn't a coincedence that the bonnet folds so obviously. There are reinforcements/mounts designed to force the engine/gearbox down under the car instead of in to the passenger cell, a whole topic in itself!

Whilst we curse the Volvo drivers, a lot of the pioneering work in this area was done by Volvo and Mercedes.

To get it back on topic the question is do you want to play with this? Or leave it to those who know the physics, systems and design work to sort it out?

[cookiesa]

I have a D1 with a SmartBar, which is SRS airbag compatible, and it doesn't have crush cans... I have asked the question to the manufacturers as to whether it should have crush cans, the answer was no, which they also said was true for other manufacturers for the D1 as well. :wacko:

They are simply wrong. They might not use a crush can, at a guess as the plastic is doing the absorbing, taking the place of the crush cans, does that mean any other manufacturer should not use them? A crush can ads expense, if you could not have one and not effect the cruch pulse then they would, simply because you can manufacture your product cheaper.

If they don't beleive you need them for a D1 that is a scary statement as it would suggest they are unaware that some d1's are fitted with airbags.

I would investigate that statement further, I suspect there is a misunderstanding of what you have asked, or what they meant.