Crash Testing

There is a lot more in the modern automobile to keep you safe in a collision, than just seat belts and airbags. The vehicle itself is designed to act as a partial cushion in an impact. Areas of the vehicle are designed to fold up or crumple, partially absorbing some of the impact’s energy. These areas are called “crumple zones.” That may mean more damage to the vehicle(s), but less damage to the passengers. Some muscle car guys will tell you that their car can drive through a brick wall. That may or may not be true, but that doesn’t mean that the driver will survive. Watch any video on YouTube of a slow motion crash test, it may seem like the car takes a lot more damage than it should, but this is for the occupants protection. It takes much more engineering, testing and tuning to make a car that is safe in a collision, than it did to make the “solid” cars of the muscle car era. So why did manufactures start spending their money on this kind of research? Before a car is allowed to be sold in a specific country, it must pass the country’s crash test standards (if the country has crash test standards). It was the governments which made safety in automobiles mandatory, which is also one of the reasons North America does not get many makes/models of cars (although it is perfectly fine to drive a motorcycle). A manufacturer wants to sell as many units as it can, so limiting the number of countries it can be sold in can really limit sales. Since “safe” cars became mandatory, safety has become a major selling point on newer models.

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Frames/Chassis

Many older cars used a body-on-frame design, which as the name implies, has the body of the vehicle attached to (mostly) ladder style frames. These frames worked well for their intended use (heavy load and rigidity) but did not offer much in the form of passenger protection. Unibody frames combine the frame and the body of the vehicle into one unit. This can make the car lighter and also much safer in a collision since you have the main structure of the vehicle around you instead of just below you. One problem with the unibody design is that after a collision, it could cost much more to fully repair the damage than on a body-on-frame design. Most passenger cars today are unibody, but most pick-up trucks still use body-on-frame.

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Seatbelts

seat beltSeat belts are designed to hold the occupant in place during a collision, to prevent the occupant from hitting the dash or windshield. Many early seat belts were only a lap belt, which held the occupants lower half in place but allowed the upper body to swing forward in a collision. Some cars had this type of belt for the middle rear seat up until recently. Shoulder belts are now used along side the lap belts to control the upper and lower body in a collision. Together these two belts (or one belt) form what is called a 3-point harness. Seat belts have a retractor which tightens the seatbelt when slack in the belt is taken up quickly or suddenly, and some are also controlled by the airbag module when it detects a collision. The electrically controlled kind are a one time use device, similar to an airbag. Once the retractor is deployed, it becomes useless. On modern vehicles where seatbelts are used along side airbags, seatbelts are designed to guide the occupant into the airbag properly as well as restrain the occupant.

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Airbags

Note: I do not recommend anyone who is not a licensed technician to work anywhere near airbags or airbag systems. The following is for information purposes only. I am not responsible for any injuries or damage to property that results from unqualified personnel working on or tampering with airbag systems. Airbag systems are (usually) easily identifiable by using bright yellow wires and plugs in their circuits.

Airbags became somewhat of a hot topic a few years ago. Airbags have the power to save a life in an impact and they also have the power to further injure occupants. It all depends on if the occupant hits the airbag or if the airbag hits the occupant. If the occupant gets hit by the airbag, his/her injuries can be far more severe than if the airbag did not go off at all. Airbag systems are designed to completely deploy an airbag before the occupants even start to move forward in relation to the vehicle as a result of the impact. It is this rapid rate of deployment which makes them effective as well as potentially dangerous. Airbag systems consist of a module, crash sensors and the airbags themselves.

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Crash/Impact Sensors

Crash sensors are usually located along the outside front and sides of the vehicle. They can use a roller and a spring, a magnet and a gold-plated ball or an accelerometer. The first two are both normally open switches which complete their circuits when a crash is detected, either by compressing the spring and allowing the roller to connect two contacts, or by knocking the ball off its magnet to connect two contacts. The accelerometer type will actually generate a voltage signal when a piezoelectric element distorts during an impact. Newer systems may use pressure sensors to detect a rise in air pressure inside the doors. It is important on these systems that the door remains a sealed unit after any work is done. If air is allowed to escape during a collision, the airbags may not deploy.

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Occupant Detection

Occupant detection devices are located in the seat. If there is no occupant in that seat, the airbags intended to protect the occupant of that seat may not go off depending on the severity of the impact. Many passenger airbags come through the dash (destroying the dash), and would require much more work to fix. This is pointless when there is nobody in the seat to protect. Some occupant detection systems will also take into account the weight and seat location of the occupant to make changes to how/which airbags deploy.

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The Airbag Control Module (ACM)

airbag control moduleThe ACM is usually located in a central area of the vehicle or at least in a central location in relation to the airbags and/or sensors. The module has an internal crash sensor which is called the arming sensor. The arming sensor must be tripped before the module will listen to any of the crash sensors. (Yes, that means that video where the old lady smacks a rich guys car and the airbag goes off is fake.) So, at least two sensors must be tripped before an airbag will deploy. The module also constantly checks system components for readiness. If a fault is detected, the module will illuminate the airbag MIL in the instrument cluster. The most common fault in an airbag system is the clock-spring (explained later). The airbag module is supplied with 2 separate power sources and a case ground in case one power source is cut off in a collision. The module also stores power in case power or ground is completely lost during a collision. Most modules can store enough power to deploy an airbag for about half an hour after power is severed. The ACM also does something many people do not consider. It acts like a “black box” in an air plane. It will record things like vehicle speed before the collision, deceleration rate, brakes applied, ABS engagement as well as anything else that the manufacturer or police may want to know. This information cannot be accessed by any scan tool I have ever heard of, usually the module will need to be sent to the manufacturer to have this information extracted. (This is the module that will “rat you out” if you got into a collision doing something you are not supposed to be doing.) Once one of these modules have been through a collision where the airbags were deployed, they may have to be reset but most likely will need to be replaced. This is also not a module that you would ever want to buy used.

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Airbags

airbagAirbags contain solid fuel pellets which can be sodium azide (NaN3), potassium nitrate (KNO3), or another pyrotechnic compound. When these compounds are ignited, they burn rapidly and can fill the airbag in a very short amount of time. The airbags also have vents in them to allow the gas inside the bag to escape and allow the bag to collapse so it does not get in the way of the occupant escaping the vehicle if necessary. Some newer airbags use an argon gas canister. When this type of airbag is activated, the canister is ruptured, allowing the argon charge to inflate the bag. The suqib (Chrystler term) is the device inside the airbag which causes the deployment. It is a thin coil of wire which is sent a very low amount of current from the ACM to create an arc and ignite the charge. Many airbags have several stages of deployment. Using 2 or more squibs, the ACM can control how hard/fast the airbag deploys, depending on the severity of the impact and the weight/position of the occupant(s). The bag itself is made of nylon fabric covered in a powdered lubricant. This keeps the bag flexible in case the bag goes 20+ years without deployment. It seems that as of late, manufacturers have been cramming more and more airbags into their cars. Some newer models even have airbags for your legs.

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Clockspring

clockspringThe clockspring is used on the drivers airbag in the steering wheel. It is designed to keep the ACM in constant contact with the drivers steering wheel airbag, even when the wheel is turned. It is essentially a long strip of wires, wrapped into a coil with enough slack to allow the wheel to turn its full travel without stretching the strip. Any time the wheel is turned and the airbag MIL doesn’t come on, this piece is doing its job. If this piece is removed, it is important that it is not spun. If the clockspring is installed after it has been turned one way or the other, it may stretch or break the strip when the wheel is turned, causing an airbag MIL.

 

 

 

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