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Old 11-29-2006, 03:06 PM   #1
tjd83
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Default Airbag system question

I want to build a car. 100%, ground up, custom design.

But I'd rather not die in it.

I had given up on the idea of airbags, until reading this post http://www.saturnfans.com/forums/showthread.php?t=89269

97coupe mentioned that the airbag system is pretty simple. I seem to recall seeing somewhere that there's a sensor inside the center console, which detects any sudden deceleration. Is that it? How hard would it be to put the entire airbag system into another vehicle? Other than the steering wheel and the above sensor, what else would be required? Would it be practical to even attempt? Would it be safe?

Just to be clear, I DO NOT use any safety system as a replacement for careful driving. Seatbelts, airbags, roll cages, etc. are important, but I believe that the driver is the #1 most important safety feature in any vehicle. I just would rather not be killed by some idiot running into me, if it can be avoided.
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Old 11-29-2006, 04:20 PM   #2
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Default Re: Airbag system question

Quote:
Originally Posted by tjd83
100%, ground up, custom design.
....
Quote:
How hard would it be to put the entire airbag system into another vehicle?
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Old 11-29-2006, 05:20 PM   #3
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Default Re: Airbag system question

Quote:
Originally Posted by madpogue
....
OK, you got me When I said that I want a custom design, I was thinking more of the chassis, bodywork, etc. I have no intention of manufacturing my own engine, transmission, and so on. There are lots of things out there that are way beyond my abilities.

The question remains... can I pull an undeployed airbag system from a junkyard, and have any expectation that it will work as intended?

And of course I'm not thinking of replacing my Saturn. But that's my daily driver, and I've promised myself I won't mess with it. I'm trying my hand at something completely different.
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Old 11-29-2006, 09:04 PM   #4
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Default Re: Airbag system question

from an engineers perspective, I think the problem you will encounter is that the airbag in any given car is designed to match the collision and crumple characteristics of that vehicle. The sensors are located and calibrated such that, when that part of the car experiences a specified threshold of deceleration, the occupant are in danger and the airbags should deploy.

You would face a challenge to get that part of your car to crumple and decelerate the same way. The result would be your airbags would either trigger when you bump a shopping cart in the parking lot, or they would not go off in a 90 mph head on crash.

Personally I would rather have a 5 point road rally style seat belt system in my car, than standard seat belts and air bags. The 5 point seat belts are what fighter pilots use in F18s. You can roll the car and stay right in your seat, without ripping up your shoulders.

something to consider. If you are going to make your own car, find ways to make it better, than the all-purpose, one version fits everyone stuff that comes from the big auto makers!
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Old 11-30-2006, 10:18 AM   #5
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Default Re: Airbag system question

THe air bag system on a 97 SC2 is pretty simple and it is completely controlled by the module between seats as far as I can tell. I have not researched whether its control module control status light on dash or wheter it sends a signal to PCM to do it but there are no external crash detection sensors on that model/design so it should not be hard to transpalnt it into another vehcile in theory anyway. I have a old dead airbag control module that I am going to take apart sometime just to see what makes it tick.
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Old 11-30-2006, 10:31 AM   #6
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Default Re: Airbag system question

In many cases airbags do almost as much harm as good. I don't feel any safer with it, and in fact I'm worried about it punching me in the face one of these days. My '02 is the first I have ever owned with one. I think side impact, or side curtain airbags are better safety devices than the standard steering wheel ones. KCW has a good point, but maybe a 4-point would suffice? The 5th point seems to get a little too "friendly" if ya know what I mean

Quote:
Originally Posted by 97coupe
but there are no external crash detection sensors on that model/design
Yes there is, how else does it know when to deploy?
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Old 11-30-2006, 11:24 AM   #7
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Default Re: Airbag system question

Quote:
Originally Posted by Uzzy

Yes there is, how else does it know when to deploy?

No there is not on a 97 style system, the first Saturns had them but not later ones. The control module has a motion sensor in it that can detect a sudden stop/impact and deploy bags.. It also records crash data too on some vehicles (impact speed, angle and rate of decceleration) which a lot of people do not realize and can be used in court if need be.
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Old 11-30-2006, 05:03 PM   #8
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Default Re: Airbag system question

Quote:
Originally Posted by 97coupe
THe air bag system on a 97 SC2 is pretty simple and it is completely controlled by the module between seats as far as I can tell. I have not researched whether its control module control status light on dash or wheter it sends a signal to PCM to do it but there are no external crash detection sensors on that model/design so it should not be hard to transpalnt it into another vehcile in theory anyway. I have a old dead airbag control module that I am going to take apart sometime just to see what makes it tick.
This is exactly what I was thinking. I understand that the various sensors around the body of most vehicles would be useless in a different vehicle, but a single sensor sounds to me like it could be used in almost anything. If all you're measuring is deceleration, then it doesn't matter what the rest of the car does, right?

If you find out about the PCM, I'd love to know. Would I need to use a PCM (or a homemade interface of some sort), or can the control module fire the airbag directly?
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Old 11-30-2006, 05:28 PM   #9
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Default Re: Airbag system question

Quote:
Originally Posted by tjd83
... If all you're measuring is deceleration, then it doesn't matter what the rest of the car does, right?...
well, yes it does matter. If the sensor is located between the front seats, then in a collision the piece of metal it is bolted to is going to decelerate based on how the rest of the car is crumpling and collapsing in a collision.

Unless you design your frame and body exactly the same way, the airbag system will act differently.
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Old 11-30-2006, 06:55 PM   #10
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Default Re: Airbag system question

Quote:
Originally Posted by KCW
well, yes it does matter. If the sensor is located between the front seats, then in a collision the piece of metal it is bolted to is going to decelerate based on how the rest of the car is crumpling and collapsing in a collision.

Unless you design your frame and body exactly the same way, the airbag system will act differently.

Somewhat true but there will be a high initail shock on impact before crumpling starts which would trigger bags. No sure where it triggers at but I tend to think somewhere between 3 and 5 g's
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Old 11-30-2006, 08:20 PM   #11
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Default Re: Airbag system question

It's not just deceleration, it's deceleration from FORWARD motion. If, for example, you get t-boned, and then the car slides sideways and slams (on the opposite side) into a fixed object, the air bags should NOT deploy (because the occupants will NOT be thrown forward). Only that side's side air bag(s), if so equipped, should deploy. The air bags should ONLY deploy to prevent an occupant from being thrown in the direction toward that particular bag. This esp. applies to the driver's front air bag, as its deployment all but relieves the driver of any chance of controlling the vehicle via the steering wheel, and that should only happen when it absolutely has to.
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Old 11-30-2006, 09:51 PM   #12
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Default Re: Airbag system question

Quote:
Originally Posted by 97coupe
Somewhat true but there will be a high initail shock on impact before crumpling starts which would trigger bags. No sure where it triggers at but I tend to think somewhere between 3 and 5 g's

Im sure its more than a simple threshold- I would expect some time element to be involved, ie: 5 gs for 10 milliseconds, or some other filter equation.

and that is going to depend on body design, bumper design.... if your bumpers take up too much of the impact the frame will never see 5 gs, or if the frame crumples too easily.

The more I think about this, unless you are a mechanical engineer with vehicle chassis design experience, and a lot of mechanical CAD/CAM SW, I really dont think you can design a crash proof vehicle on your own. It would be a huge task for one person - airbags or not.

just doing a cut and paste thing with an air bag system from another car, I think you would be kidding yourself at best, and maybe facing a false trigger at worse.

Im not saying you are not smart enough to do it, only that you are taking on a huge task, that can take a team of engineers a couple years to get right.

Last edited by KCW; 11-30-2006 at 10:05 PM.
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Old 12-01-2006, 03:06 PM   #13
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Default Re: Airbag system question

I'll grant that my ME degree (University of Houston Class of '81) is pretty rusty but, assuming the air bag doesn't deploy when a bird poops on the car, I'd say an air bag that might do you some good is better than no air bag. Sure it might be less than optimal but, that's not the same as worse than useless. A car crash is not exactly a subtle thing. I'm fairly certain that these things pop based on acceleration alone. It wouldn't surprise me if direction wasn't even part of the equation.

KCW with all respect to you, I doubt very seriously that the air bag is designed to as tight a set of specifications as you suspect. Look at the requirements:
  • The air bag must deploy dependably in conditions that might be very abnormal (no battery power, car is deforming, etc).
  • The air bag must always deploy when conditions are such that serious injury can result.
  • The air bag must deploy quickly enough that it can prevent injury.
  • The air bag must not deploy in conditions that are not likely to cause injury.

Look at the first requirement. In engineering, dependable means simple. The more variables you have to deal with, the more likely one of them will be other than what you expect. So, having the thing deploy based on nothing but acceleration makes a lot of sense.

Second requirement. Under what conditions is serious injury likely to occur? Heavy accelleration. Much heavier than the car is capable of producing normally either under heavy braking or with the gas pedal. So again, you need nothing but accelleration to determine that conditions are right for the air bag to deploy.

Third requirement. Time is of the essence. A bag that deploys after the driver has splattered his brains over the steering column doesn't do a lot of good. No need to think about it. No need to worry about crumple zones. No real need to think about direction. Dangerous accelleration has been detected. The air bag might prevent or at least lessen injury. Go ahead and deploy. Granted a processor can do quite a few calculations in the time available but, why add that point of failure? Why take the risk of a bug in the embedded code?

The fourth requirement can be met in a couple ways. One is to look at what accelleration the car is capable of producing under optimal conditions and add a safety factor and use that as the triggering acceleration. The other is to look at what accelleration the human body is capable of withstanding without serious injury and subtract a safety factor. If I'm designing air bag deployment systems, I'll use the later method. That way I'm safe no matter what car the system's in and I can sell deployment modules to multiple manufacturers.

If I were thinking about taking air bags off a Saturn and using them on another vehicle as the poster is, the bigger concern would be angle and placement of deployment. Those things do deploy with some force. You don't want an air bag hitting someone in the face if they don't need it to.
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Old 12-01-2006, 04:05 PM   #14
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Default Re: Airbag system question

I think you are making a lot of assumptions on how air bags systems work, based on how you think they should work.

Years of research and testing and developement have gone into modern airbag systems. Sure it would be nice if someone designed a simple airbag sensor and deployment system, that could be installed in any car

that would be great, but is that whats out there now?

I dont know, its not my area of experience - but from what I learned about physics in college I think simple sensors and deployment is not likely.

Do you want the airbags to deploy if you hit a deer? a dog? if you run over a rock and it hits your frame? Someone had to work all those equations out, what is the acceleration on the frame of a car when you hit a deer? It might be several gs, and only last for a few milliseconds.

One problem with airbags over the years is they would deploy after an accident. Firemen and parametics would be extracting someone from a wreck, and 3 minutes after the collision the airbags would go off, injuring the rescue workers. Firemen started taking sledge hammers and wacking the car in the right spot to trigger the air bags, before helping the passengers. They were trained on where to smack the car to set them off.

My main point is these systems have taken years to design into the vehicles, so I dont think you can just take one from car A and put it in your custom chassis/frame design, and expect it to work like it was intended.
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Old 12-02-2006, 12:18 AM   #15
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Default Re: Airbag system question

From howstuffworks.com.

Quote:
The sensor is the device that tells the bag to inflate. Inflation happens when there is a collision force equal to running into a brick wall at 10 to 15 miles per hour (16 to 24 km per hour). A mechanical switch is flipped when there is a mass shift that closes an electrical contact, telling the sensors that a crash has occurred. The sensors receive information from an accelerometer built into a microchip.
Like I said it's got to be simple.
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Old 12-02-2006, 12:29 AM   #16
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Default Re: Airbag system question

I greatly appreciate everyone's input, both pro and con. As the design exists at the moment, there is no provision for airbags. My question is, will a Saturn S-series airbag system (or any other single sensor system, for that matter) be at least as safe as, and possibly safer than, no airbags at all?

I am inclined to agree with BarnOwl, that a good safety system should follow the KISS principle. KCW, you make a lot of good points. Your arguments are the reasons that I have not designed in an airbag, in fact. But, after further reading, I decided to find out if there's anybody who could offer useful information about whether or not it's a good idea to install a single sensor airbag system.

I fully understand and accept that I am unable to design a vehicle with all the safety features of a 2007 Saturn. Crumple zones, side air bags, crash testing, etc. are all beyond my ability, both technically and financially. However, if I could install a system in a decently designed custom chassis and have some expectation that that system could contribute to my staying alive, then to me it's worth it.
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Old 12-02-2006, 12:56 AM   #17
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Default Re: Airbag system question

I dug up some more info on Alldata.

Quote:
The sensing and diagnostic module (SDM) is designed to perform the following functions in1he SIR system:

1. Frontal Crash Detection: The SDM monitors vehicle acceleration changes to detect frontal crashes which are severe enough to warrant deployment.
2. Inflator Module Deployment: When a frontal crash requiring deployment is detected the SDM provides enough current to deploy the inflator modules.
3. Energy Reserve: The SDM maintains approximately a 23 volt loop energy reserve (VLR) energy supply to provide deployment current if ignition voltage is lost in a frontal crash.
4. Malfunction Detection: The SDM performs diagnostic monitoring of SIR system electrical components and sets diagnostic trouble codes when a malfunction is detected.
5. Driver Notification: The SDM warns the driver of the SIR system malfunctions by controlling the AIR BAG telltale lamp.
6. Malfunction Diagnosis: The SDM displays SIR diagnostic fault codes and system status information through a scan tool.
7. Crash Recording: The SDM records information regarding the SIR system status when deployment has been commanded.

The SDM is connected to the SIR wiring harness by a 12-way connector. This connector has a shorting bar across two terminals which connects the AIR BAG telltale lamp to ground when the SDM connector is disconnected. This will cause the AIR BAG telltale to come "On" steady whenever the ignition switch is at the RUN or START positions with the SDM disconnected.
From what I can tell on there, it doesn't look like the PCM is doing anything with regard to the air bags. So, it should be feasible.

Remember nobody on here is qualified to tell you this is safe. I think it is but, that's at best an educated guess and at worst a wild guess. I also think an improvised air bag is better than no air bag but, the same qualifier applies. So, you're on your own on whether it's a good idea.

As for how to do it, you'd want the module in the middle of the car as it is in ours to avoid damage in an accident. You'll also want to orient the module the same as it's oriented in the car it comes from. The Saturn module does take direction into account.
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Old 12-05-2006, 11:04 AM   #18
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Default Re: Airbag system question

Quote:
Originally Posted by BarnOwl View Post
I dug up some more info on Alldata.

From what I can tell on there, it doesn't look like the PCM is doing anything with regard to the air bags. So, it should be feasible.

Remember nobody on here is qualified to tell you this is safe. I think it is but, that's at best an educated guess and at worst a wild guess. I also think an improvised air bag is better than no air bag but, the same qualifier applies. So, you're on your own on whether it's a good idea.

As for how to do it, you'd want the module in the middle of the car as it is in ours to avoid damage in an accident. You'll also want to orient the module the same as it's oriented in the car it comes from. The Saturn module does take direction into account.
Thanks, BarnOwl! With this encouragement, I will seriously consider putting in an airbag. Anyody happen to know of other vehicles with a comparable system? What about any changes to the S-series over time? Did they all use a single sensor?

Regarding safety and responsibility: I fully accept any and all responsibility for my own actions and choices in my life. That includes acting on advice given. If I install an airbag, and if it deploys, and if that turns out to be a bad thing, I will not hold anyone other than myself responsible. If I live through it, I'll do my best to let everyone else know, but that's about it. People who cannot accept responsibility for themselves are a major pet peeve of mine.

As for how to do it, I agree that it needs to be as close to the same physical setup as is in our cars. Thanks for the research, the advice, and the encouragement (and for not thinking I'm a loony!).
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