Airbags: CR4 Challenge (06/17/08)

LP Ultima

This has turned into a great debate on air bags, but it seems you are the only one who has answered the question.

Nicely done!

My girlfriend worked as a paramedic and has seen the gambit.

Without an airbag, head injury is usually the prominent cause in frontal collisions for the passenger (head in windshield), while the thorax is usually the issue for the driver due to the steering column. The thorax basically gets crushed and ribs can become spears. Basil fractures of the skull as you described are another large issue. Many people have their seat adjusted so that they are too close to the steering wheel, even with seat belts and air bags.

The worst impact is the side impact. The shear stress is usually the killer. Aortic shear is the common one. The victim looks unhurt, but ends up rapidly bleeding out at the scene long before a response team gets there.

In a side impact the head will usually strike the side glass, adding to the list of injuries.

The aorta and organs are much more sensitive to side shear forces than frontal forces.

Thanks - and I agree with your additional comments. I simply ran out of steam for describing other issues.
Regarding present practice: I'm in no sense an expert, but based purely on a priori grounds I believe frontal airbags to be generally useful in frontal collisions (though they would be much improved for lighter passengers if they were segmented and maybe provided with an adaptive charge based on seat-measured body-mass). On the other hand, side airbags apperar generally incorrectly placed and therefore of limited (maybe even negative) benefit (probably too low and too far back - and what about the side of the body nearer the centre of the vehicle).

Side impacts are the big killers. There simply isn't enough crumple zone to absorb the impact.

Worse yet are the trucks and SUVs that are based on a frame rather than a unibody type of build. The frame acts as a battering ram and generally, these vehicles are sitting higher off the ground, so the frame impacts a standard car right about where the thorax of the occupant is.

The frames are not designed to absorb impact like a unibody car, instead they transfer the force into the vehicle they impact.

Maybe reactive armor on the car doors would help!

Agreed, that too is relevant. Perhaps that is another excuse for driving mid-engined cars with the air-intakes just behind the sides of the doors and necessitating significant wall thickness. Mid-engined construction also allows large unobstructed crumple zones front and rear, and coupling the engine tightly to the cockpit helps ensure that the centre of the vehicle continues moving with you without having to excessively reduce the stiffness of the car body. (Oh, and the crumple design should also lower me so I go under the frame of the worst offenders).

Nevertheless, I try to drive defensively.

I would just like to see the killer vehicles reduced in numbers so that the likelihood of such a crash is vastly reduced. Given the rise in gas prices it appears I may get my wish.

Yes, a good mid-engine coupe would be a fine ride. A Cayman comes to mind. Now, pull that wonderful engine out and replace it with an electric propulsion and it would be even more intersting.

In the last para, line 2, (as) soon as the acceleration of the vehicle exceeds some defined "safe" limit. Did you mean to say: Deceleration?

Yes and no (and if I have to choose, more 'no' than 'yes').

Acceleration is the generic term for rate-of-change-of-velocity, so acceleration is still a correct description for a frontal impact slowing the vehicle down.

So why more no than yes? Because front air-bags should also protect against a vehicle running into your front when you are stationary, and the final paragraph also covers side air-bags.

While acceleration covers all three cases, deceleration rather implies that it only covers the first.

During an accident, the vehicle's interaction with the "outside world" will be with another vehicle or a stationary object such as a tree. This results in potentially very high accelerations/decelerations of the vehicle. Deformation of the vehicle and the other object dissipates some of the energy and lessens the "rigid body" accelerations of the vehicle. However, the people inside the vehicles must undergo the same accelerations/decelerations as the vehicle or they will impact some part of the vehicle. If the people are unrestrained inside a vehicle undergoing high accelerations/decelerations, they impact things like the steering wheel, dashboard, and doors. These impacts result in very large, localized applications of force to the people. With seat belts, the forces applied to the people are mostly at the lap and shoulder belts. While this is better than impacting the inside of the vehicle, these are still more localized forces. In addition, side impacts are not as well resisted by the seat belts. Air bags do an even better job of reducing the accelerations of the people. The bags quickly inflate and provide a much larger surface area over which the forces on the body can be applied. In effect, the average surface pressure is less. Also, since the pressure is over most of the body's frontal or lateral area, all part of the body see very similar accelerations which results in less differential stress on the body. Air bags also provide much better protection from the sides than seat belts. One downside of airbags is that in minor collisions, they can deploy and cause injuries just due to the process of inflating.

I think the complete explanation has been given in the previous posts. I thought I would play around with some numbers to see what type of forces a person might be subjected to in a crash.

Take the scenario where a car travelling at 20m/s (approx 72km/hr) has a front on collision with a solid wall with the result that its front crumples and the driver compartment stops after travelling a further 1m towards the wall.

In this scenario the drivers compartment changes from 20m/s to 0m/s over 1m distance. So the acceleration it experiences (assuming constant acceleration during impact) is 200m/s² which it experiences for 0.1 seconds.

In the worst case the driver is unrestrained in the driver's compartment which means that there is a space between him and the inside of the drivers compartment. If we take that space as 0.5m then it will take him approx 0.07 seconds to cross that (given that the drivers compartment is accelerating towards him due to collision at 200m/s²) so he has only 0.03 seconds left to decelerate from 20m/s to 0. This results in an acceleration of 667m/s². Assuming an 80kg driver he will experience a force from the inside front of the drivers compartment of 53,333kg.m/s². This is equivalent to the driver having a 5,400 kg weight being placed on him in the shape of the front of the driver's compartment. In addition there will be sheer forces due to differential movement between the parts of the body that hit areas/protrusions of the compartment at different times.

In the best case if we assume that the driver is attached to the drivers compartment by safety devices (seatbelts, airbag, etc) he will then be accelerating at the same rate as the drivers compartment (200m/s²) and the force that he experiences from the restraining devices during the impact is 16,000kg.m/s². This is equivalent to the driver having a 1,600 kg weight being placed on him in the shape of the restraining devices. In addition there will be sheer forces due to differential movement between the parts of the body that are restrained and those that are not. Clearly in this case the areas that the restraining devices are applying the pressure to is very important. In the case of the airbag it is spreading this force over his head and torso relatively evenly and the parts of the body unrestrained (arms and legs) can take damaged without (immediately) killing the person. A seatbelt just spreads the force over the area of his torso covered by the strap of the belt which results in greater pressure on those areas than from an airbag. It also does not restrain the head leaving it to experience sheer forces at the neck (as well as potential collision with the front of the drivers compartment).

Interesting post!

-Abe

I don't exceed 80mph much anymore (I'm too old and boring) but I do agree that air bags are an oversold technology that must be a nice profit center for someone in the line. Quite frankly, if given a choice (which currently I am not) I would take a four point seat harness and roll cage instead of an airbag for protection. When I was taking glider lessons, the four point belt was simple to use, lightweight, yet seemed able to keep a passenger secure in his seat.

Unfortunately, our friends in congress (the opposite of progress!) and the federal bureaucracy think they know what is good for us and have mandated these unsafe "safety" devices.

end of rant!

A child's seat harness with the center belt is similar.

The key is to not apply loads to critical areas.

The unintended consequences of the regulations was to stop innovation that would lead to near zero deaths and little damage to the car.

Now we have kids in the back seat where a Dad can't talk to easily on one of those rides to get a "treat" which is actually one-on-one time. My young daughters are "at risk" of death just for sitting in the front seat. In fact my wife, who is a very trim and 5'8", is at risk. What a joke!

Well, safety equipment for racing versus safety equipment for street use is complex and not what it seams.

You would intrinsically think that a roll bar and a 4-point harness would trump an airbag with a standard seatbelt, but it may not.

The reason is that racing safety equipment is designed to work as a system. So, only using a portion of that equipment (i.e., roll bar and harness) may actually be less safe than what you are trying to replace. You really need the seat and a helmet (and a Hans device) to get the correct benefit. Even then, that system has some disadvantages on the street. You will also need to deactivate the airbag if you go the racing route.

Food for thought, how interested are you in wearing a helmet in your car anyway? Few people would want to be bothered with that burden.

There are some factors that reduce the effectiveness of airbags.

1) Improper use of equipment. I.e., not wearing seat belts in low speed crashes significantly increases fatalities when the airbag deploys.

2) Improper seat adjustment and position relative to the steering wheel.

3) The Offset Hypothesis where drivers believe that the safety features will protect them and they push the limits further by aggressive driving.

I had a professor in college that worked for GM previously. He worked directly with airbag development among other things. He mentioned a weakness with air bags that very few ever talk about. If you drive your car with the window rolled down and your arm resting on the opening, you will get a nasty surprise when the air bag deploys. There is a very high chance your skull will slam into the metal post that supports the roof. The air bag directs your head straight to the post as you slide off the side. He was rather impressed with the forces a crash test dummy recieved to the ol' noggin, even at lower speeds. This may explain why more modern cars are built in ways that make it uncomfortable to drive in this manner. If you are in an accident, try to square up in the middle of your seat before impact with your head against the headrest.

Part of the reason for the changes is indeed to provide enough distance between the side airbag and the occupant for it actually to provide any protection at all.

My sister-in-law is a surgeon in the USA who does some trauma work. She has been involved in several cases where the front seat passengers survived a crash and the back seat passengers died. The main difference appearing to be the protection afforded by the airbags in front. I don't know what the accident statistics show.

I also wonder what impact (literally) seat back LCD screens will have. I would rather embed my face in a seatback than a glass screen in the event of an accident, of course I rather not embed my face in anything at all!

I think that last statement says it all, if you know your face is going to smash into something would you prefer a brick wall or a cushion?

The earlier comments about airbags give a fascinating insight into the 'safety' business.

There are MANY ways to "cushion" an occupant. One of the worst ways is an airbag because of the time response and decision criteria, along with the package opening debris issue. Having a "temperpedic" substructure instead of what is now there and removing any hard points is a start. Airbags are a fix to a bad instrument panel and window design. If you were to start from new, I sit down for an hour with me, I think you would see that the entire car needs a ground up remake, including where the engine goes and a host of other things needed to get to 1000 lb cars.

Your statement of a brick implies that someone is advocating such, therefore the regulation is needed. That is not a fair comment in any context. Sorry...

I don't think anyone is suggesting making cars out of brick, I was only highlighting the difference between a hard & soft surface which is pretty much agreeing with everything else in your post however, you do a much better job than me in explaining the problem.

Seaplaneguy

Very interesting information. One of my co-workers recently lost control of his car during a rainstorm. He wasn't moving very fast but slid sideways and hit the front quarter panel on his vehicle into a divider. The front airbags barely touched him but the side airbags inflated and caused his liver and spleen to rupture. Dude was in the hospital for quite a while and they did not know if he was going to make it. Fortunately he did. I question also the validity of some of the devices and statistics presented by those in authority. Statistics and percentages all can be manipulated to give whatever results you desire.

My cynical side also wonders if car manufacturers consider the value of spare parts sales. Even in a minor crash, if the air bags deploy, you might need a complete new dashboard moulding.

My cynical side also wonders if car manufacturers consider the value of spare parts sales. Even in a minor crash, if the air bags deploy, you might need a complete new dashboard moulding.

In truth many vehicles are "totalled " by insurance companies after airbag deployment. This action is often done with no other mechanical issues but the actual deployment.

In the very least, I should have a decision point at which I can choose to employ or not employ an airbag feature dependent on my driving situation. As a 4x4 enthusiast I have a recurring nightmare of bumping a tree coming down a slope and getting punched in the face by an airbag followed by an uncontrolled descent in which I wreck my truck and perhaps myself.

In a real life accident between a Tractor Trailer and a Hyundai at speed 60MPH U.S. on the freeway the tractor trailer t-boned the car, they were traveling in the same direction and converging at which time the car came over too soon and ended up being plowed forward sideways while the driver of the truck attempted to stop over the course of 150Yards what saved the driver was not her airbag that deployed bloodying her nose - but rather the reinforced door panels - two distinct bars running horizontally across the otherwise crushed door.

I was very impressed. As was the little girl in the back seat.

/endrant

Like I said in my previous post, I know the people who design them. They have been given the answers and they "have knowledge." Under the law, this industry is libel for hundred of Billions...yes Billion. The Congressmen/women who forced the regulations onto us are all bankrupt in my opinion. The lack of new entrants into the auto industry, which is for the most part government (read Keiretsu) monopoly (Oligopoly) in nature is the problem. Just try to enter the airbag market now. The entire system stops innovation. Small companies will buy from small companies, but large ones will not because of the risk of stopping a production line. It is just that simple. No conspiracy, just raw economics. There are three players in airbags, two don't have any clue, and one is run by a guy I would never hire. This is NOT capitalism. We have NO choice in the matter as consumers.

The 'dogmatic' postulation of what we all 'know' to be true is an interesting challenge.

Some writers in the automotive press have speculated that the "automatic deployment" trumps any voluntary restraint technology (regardless of actual cost/ benefit merits) due to political considerations. A dogma is born!

Thus far no one has mentioned the 'faulty deployment' problems that have plagued some airbag installations -presumably due to electrical system glitches that are not directly related to the airbag. Exactly why it happens has been the subject of speculation among service techs - but some vehicle types are commonly known to suffer from the problem.

Imagine having a deployment without any collision - and while moving at speed! It does happen. And it causes injuries as a result of both direct deployment - and contact with extra-vehicular objects resulting from control loss during deployment.

Some safety enhancement!

There has been much writing above of a very relevant, logical, and illuminating nature, and I am glad it has been contributed herein. But there is one issue that has not been addressed as yet. As somebody who has been in several automobile accidents (since age twelve as a rider, not a driver...), it would seem appropriate to point out one additional issue. If a car is struck hard enough from behind, say, for any reason, to cause the impacted vehicles' airbag(s) to (deploy), the driver can be incapacitated from seeing where to go, and even steering the vehicle at all. As such, said impacted vehicle then becomes, essentially, an unguidable missile, careening further down the roadway. Other vehicles nearby may be forced to (share the experience?) in subsequent collisions. If such a driver cannot control such a vehicle, then who is accountable for such consequences?...

"Several automobile accidents" of relevance to this discussion?? May I suggest you are more careful in selecting (or rather refusing) your driver in the future.

Theoretically, if you are struck centrally from behind, the direction of acceleration should be such that neither front nor side airbags should deploy. However, in practice the effective strike is often offset so there may be sufficient rotational energy to legitimately set off side airbags, with consequences similar to those you describe.

In the UK, that would make the entire chain of events the responsibility of whoever is deemed responsible for the first collision in the chain of events - and I imagine that the same would apply in most western jurisdictions. If the initiator wished to apportion some of the blame for subsequent events to the airbag manufacturer, he would need to demonstrate to the court's satisfaction that the effect of the initial accident would have been insufficient for the driver of the struck vehicle to lose control in the absence of airbags - which would be extraordinarily difficult to prove to the satisfaction of the law.

First, to clarify, I now add that the incident in which I was twelve years old actually happened back in 1958... (Thanks for thinking to ask, first...) Later on, in regards to my own driving, the capacity to continue to "see" the best response to be made helped to mitigate further accident consequences, before airbags became legally required...

Second, "theory" and "reality" tend to be more like immediate "cousins" rather than "identical twins", on average, so the primary point is not that it should be unlikely that a rearend collision would trigger an airbag deployment, but that a rearend collision is only one of several plausible ways an airbag deployment could be triggered...

Third, irrespective of how an improperly designed airbag deployment is triggered, it should be relatively easy to demonstrate, with a sufficiently complete re-enactment in a courtroom, that a deployed airbag temporarily blocked a driver's vision long enough to prevent said driver from seeing what best to do next, and further, to then inhibit said driver from recovering control of said vehicle within sufficient time in order to avoid further accidents, as direct consequences of inadequately designed airbag configuation(s)...

In short, misconfigured airbags can add (windows of vulnerability) that would not exist otherwise...

I rather assumed the accident was a few years back - your writing style suggested a certain maturity. It was the implication of "multiple accidents since the age of twelve" that prompted my comment - that you would appear to have been at best unlucky with your chauffeurs.

The court's assumption is that, if the sideways acceleration component corresponds to levels that have been specified to set off airbags, the level is sufficiently injurious that the victim would in any case be incapacitated from driving. So it is necessary to prove that these levels have not been generated, and the only party who has any real interest in this is the perpetrator of the initial collision. Actually, it is necessary to prove rather more than this: either that the airbag provides no overall benefit on average, or that the failure to prevent unwanted deployment was both predictable and that the reason for not preventing the deployment was either simple negligence or improper cost cutting.

Even the false deployment becomes rather difficult to prove when the cars have been further damaged subsequent to the initial collision - indeed, not only would you need to show that the impact was sufficiently symmetrical, but it would be necessary to demonstrate that dangerous acceleration levels that would be ameliorate by the deployment of the airbag. Possibly the only way to demonstrate this would be using instrumented tests on cars of similar design. So that gets through stage one. Next, to prove that the reason the airbag was not designed to withstand the actual shock was legally unacceptable.

All in all, not a viable course of action unless the lacks of such proof is personally catastrophic, which would be most unusual assuming the legal minimum insurance. However, if the evidence can be found it could receive legitimate support from concerned safety campaigners.

I don't think air bags would have helped in this crash test.

Your explanation is not completely correct. In an collision, as the car is decelerating, an unrestrained passenger continues moving forward at the cars original speed. By the time the passenger impacts the dash, the car is moving considerably slower than the passenger. This results in very rapid deceleration of the passenger, which results in severe trauma.

The purpose of the airbag and the seatbelt is to decelerate the passenger at the same rate as the vehicle by starting the passenger deceleration at the same time as the vehicle deceleration.

Also, I don't think I would describe an inflated airbag as soft. They often break arms as they inflate, and can decapitate small people (children).

The time to achieve zero momemtum is the only important variable here. If the time is long, you are safe; otherwise ...

Abe