What are two-stage airbags

More airbags and techniques

Incidentally, the airbag is only so effective because it interacts perfectly with the three-point seat belt and handlebar in the event of an accident.

Airbag (blue), seat belt (red) and steering column (yellow) work together to protect the occupants (Image: Volvo)

On this page kfztech.de presents not only the driver airbag but also other airbags and technologies.

Will usually be on the passenger side

 Tubular inflators used.

Modules with two pot generators are not used that often.

Due to the larger airbag, the amount of fuel is approx. 1000g -1200g.


Tubular gas generator

So-called hybrid gas generators are now used more and more frequently. The term hybrid is explained by a certain hybrid position. Although the generator is still ignited pyrotechnically, the main load of the airbag filling is carried out by a compressed gas. Instead of the solid fuel, the required amount of gas is stored in a pressurized gas container. The compressed gas consists of 98% argon and 2% helium and is under a pressure of approx. 240 bar. When the airbag is triggered, a membrane that closes the pressure vessel is opened by the propellant and the gas unfolds the airbag. Since the gas cools down considerably when it leaves the pressurized gas container, a small amount of solid propellant is required to warm it up and to prevent the container opening from freezing. Hybrid gas generators are more environmentally friendly than conventional solid fuel generators due to the small amount of propellant.

The airbag is made of polyamide fabric and can be coated with a thin silicone film. The top and bottom can be connected with straps that bring the inflated air cushion into an optimal position in front of the driver and front passenger. The airbag collapses immediately after it is inflated, the gas escapes evenly through special holes on the back of the airbag and the kinetic energy of the occupants' upper body is dissipated when they are immersed. At the moment of catching the occupants, it carries the force of 2 t.

US airbag (The US bag is larger and has a shorter response time, as there is no requirement to wear seat belts in America.)

Driver airbag approx. 60 - 80 liters inflation speed approx. 240 km / h
Passenger airbag approx. 120-150 liters inflation speed about 360 km / h

Euro airbag
Driver airbag
approx. 35 liters
Passenger airbag approx. 65 liters

Since detonators and, among other things, the plastic of the airbags are subject to aging, the durability of the airbag systems is limited. The service life is 10 to 15 years, depending on the vehicle manufacturer. After that, the airbags have to be replaced, at least in the 90s that was the common opinion. Investigations in Switzerland (Dynamic Test Center and TCS) on vehicles that were over 12 years old showed in 2005 that all airbags were still deployed and protected. Even after 12 years, a vehicle with an airbag is safer in the event of a collision than the same model without an airbag. The crash series showed that airbags offer protection even at an advanced age. Most vehicle manufacturers have reacted and increased or canceled the airbag and belt tensioner running times to 15 years. At Hyundai, a change is still required after 10 years. Deactivating an airbag makes little sense.

  • Side airbag (thorax bag)

In contrast to the front and rear, there is hardly any space or mass on the side that could absorb energy in the event of a collision. The side airbags in the doors reduce the risk of injury to the occupants in the trunk area in the event of a side crash.

Although the vehicles are now equipped with stabilizing elements on the side (side impact protection), these metal structures do not replace a crumple zone. A side impact is therefore much more dangerous than a head-on collision.

If the car hits a tree from the side, in extreme cases it will penetrate the vehicle and deform it into a "banana". The inmates must expect serious injuries in the area of ​​the upper body and head. According to information from the expert organization DEKRA, a side crash often ends fatally for the occupant at an impact speed of just 25 kilometers per hour.

The driver hits the vehicle door or even the obstacle with his head on impact. In the event of a frontal collision with a tree, there would still be a chance of survival even at 50 km / h.


Thorax airbag

Therefore, in contrast to the frontal airbag (filling capacity approx. 12 l), side airbags do not have to gently intercept the occupants, but rather accelerate them gently in order to reduce the high speed difference between the passenger and the vehicle structure penetrating from the side in a fraction of a second and protect against penetrating parts.

The side airbag is also known as a thorax airbag. It mainly protects the chest, lungs and pelvis from side bruises.

Furthermore, the occupant is better held in his position in the seat.

In the event of a collision between the occupant and the intruding side of the vehicle, the airbag inflates within twelve milliseconds. The risk of severe chest injuries can thus be reduced by more than 20 percent.

The side airbag is installed in the seat back or in the door panel.

Side airbag


The side airbag is only triggered on the side facing the crash if the side impact sensor and the airbag control unit have detected an accident.

The sensors can be installed under the front seats or in the doors and their function is constantly checked by the control unit. If the sensors are installed in the doors, pressure sensors are often used.

Danger! Here it is important to pay attention to the correct installation of the door sealing foils.

Energy stores are also available for these airbags in the control unit, which may also be responsible for the front airbags.

  • Side airbag deployment

The deployment of the side airbag must be much faster due to the much smaller crumple zone.

  • 5 ms - the accident is recognized by the control unit

  • 7 ms - the side airbag is deployed

  • 12 ms - The side airbag will inflate after 15 ms is the chest area, after 19 ms
    the head area inflated

  • 20 ms - The body penetrates the vehicle interior

  • 80 ms - The impact is over.


Head airbags (filling capacity approx. 24 l) protect the front and rear passengers from head and neck injuries in the event of a side impact.

The lateral hyperextension of the head should be reduced and the strain on the cervical spine should be reduced.

It is located between the A and C pillars and extends i. d. Usually over the entire interior length on the driver and passenger side. The front and rear occupants of the vehicle are protected by this curtain-like airbag over the side windows in addition to the head and the shoulder area.

Once the head airbag has been activated, it remains inflated for a certain period of time (around 12 s) in order to provide protection for the occupants even in the event of a subsequent rollover.

Head airbag


Inflatable tubular structure

The Inflatable Tubular Structure - also known as "Weißwurst" because of its shape - was the world's first airbag for head protection in the side area. It consists of a roll of nylon fabric that is housed in the headliner above the front doors. In the event of a collision, it unfolds to a diameter of 15 cm.

Inflatable curtain

The Inflatable Curtain (ITS = Inflatable Tubular Structure at BMW) is an airbag that covers the entire upper side of the vehicle and protects the heads of the occupants. The Advanced ITS prevents the head and other extremities from swinging outwards. This leads to lower neck shear forces and head injuries.

Advantages of the system:
- Extended cover area for the front and rear side windows.
- Protection against broken glass and intruding objects.
- Optimized protection area even for very tall occupants.

Inlatable curtains go over the entire length

The curtains are located above the doors along the roof frame and unfold in less than 25 milliseconds - four times faster than the blink of an eye. They have been in use since 1998.

The head airbag consists of a polyamide fabric with a PU tube inside. The inflation speed is slower than with the front airbags. Hybrid gas generators are used for filling. The generator module is located in the A-, D-pillar or in the roof rail, depending on the vehicle manufacturer. It is activated via the airbag control unit and the side airbag sensor

Despite all the protection systems, the side of the vehicle remains extremely vulnerable. It therefore makes sense to choose a car that is equipped with additional driving aids, such as electronic stabilization programs (ESP) that regulate the driving dynamics, intervene if there is a risk of skidding and keep the vehicle stable through targeted braking interventions. However, the physics of driving sets limits here as well: If you drive too fast into a curve, ESP will not save you from getting off the lane!

Side airbag and head-shoulder airbag from Volvo

The head-shoulder airbags offer comprehensive protection for the occupants in all four seats. The airbags remain inflated even in the event of multiple collisions or a prolonged impact. They also prevent broken glass from entering.

In the event of a collision, the seat airbag is intended to prevent the dreaded “slipping” (submarining) under the belt strap.

The metal anti-submarining airbag is integrated in the seat frame under the seat and therefore does not come into direct contact with the passenger.

The airbag consists of two wafer-thin metal sheets (thickness: 0.3 mm) that are welded together. This creates a closed container with a volume of four to five liters when triggered.


Knee airbag

The Volvo V40 is equipped as standard with a knee airbag on the driver's side. This sits below the dashboard and is activated at the same time as the other airbags in the event of a collision.

In order to reduce accident repair costs, most vehicles are equipped with seat occupancy detection. If the seat is not occupied, the front passenger airbag is deactivated. Seat occupancy is either through pressure sensitive sensors Via strain gauges in a mat under the seat. , by Ultrasonic sensors or through Infrared sensorsthat can be attached to the headliner and react to body heat. A pressure measuring film in the seat cushion is often used as a sensor for the passenger seat occupancy. Newer systems also take into account the body weight of the front passenger, their weight distribution and position when the front passenger airbag is activated. If the control unit detects a failure of the sensors, the passenger airbag is triggered with the driver airbag in the event of an accident.

Child seat recognition at Bosch either via antennas in the seat mat (transponder) or monitoring of the ISOFIX child seat anchors.

Pressure sensor - Resistance sensors that are integrated into the seat are used here

The vehicle interior is monitored with several infrared or ultrasonic sensors and an "image" of the interior is made. This image is compared with the data stored in the control unit and whether or not seat occupancy is recognized accordingly. Out-of-position sensors use ultrasound to detect whether a person is sitting incorrectly (e.g. too close to the airbag).

Basically, every customer is advised not to deactivate the passenger airbag, as children should be in the back seat. If you previously switched off the passenger airbag using the software, newer vehicles have a key switch for activating / deactivating the passenger airbag. The decision and responsibility to deactivate it rests solely with the customer.


Overview of the airbag sensors and actuators

New concepts have multi-stage airbag deployment with reduced airbag deployment energy. The airbags are deployed in two stages, depending on the situation. The first stage with 70% and the second stage with 30% of the inflation capacity. This adaptive airbag control offers advantages, especially in collisions at speeds between 20 and 35 km / h, because the occupants are caught more gently thanks to the first stage.

more about Pre-Safe: click here

This innovative protection system is an airbag from Renault with a volume of around 60 liters, which is integrated into the seat belt.

Headrests are very important because they are supposed to protect the sensitive cervical area in rear-end collisions.

The distance to the head should be as small as possible in order to achieve an optimal protective effect. In the event of a crash, active headrests are intended to reduce the distance between the headrest and the head and thus reduce cervical vertebrae injuries.

Some manufacturers like AUDI, VW, Ford, Opel, BMW, Subaru, Saab or Volvo use it active headrests in the front seats.

These work purely mechanically and cause them to move forward in the event of a rear-end collision.

This is intended to reduce injuries to the spine and possible whiplash injuries (cervical spine syndrome).

Whiplash Protection System (WHIPS)

Volvo's patented Whiplash Protection System (WHIPS) has been one of the best whiplash protection devices on the market since it was launched in 1998. With the protection system, in the event of a severe rear-end collision, both the backrest and headrest follow the movement of the body, brake it and thus reduce the centrifugal forces that act on the cervical spine.



The world's first pedestrian airbag was used as standard in the Volvo V40. If a collision with a passer-by can no longer be prevented, the safety system helps to significantly reduce the consequences of the accident. If there is contact between the vehicle and the pedestrian, a total of seven sensors in the front bumper ensure that the bonnet is raised by around ten centimeters and then the airbag is triggered within 50 milliseconds and then remains inflated for 300 milliseconds. The space created between the bonnet and the engine block cushions the impact. The inflated, U-shaped airbag covers the lower third of the windshield, the wiper arms and a large part of the two A-pillars. The open airbag and the raised bonnet in combination help to significantly reduce the severity of pedestrian injuries.

Volvo pedestrian airbag


A Rollover protection (Roll Over Protective Structure, - ROPS) is a protective device for the vehicle occupants of convertibles with extendable "headrests" and reinforced window frames.

The Roll Over Protection System (ROPS) with Roll Stability Control (RSC) from Volvo is a rollover protection system that works together with the Vehicle dynamics control DSTC acts. The sensors of the RSC take on the task of recognizing driving situations that harbor the risk of a rollover in good time. To this end, one monitorsGyro sensor permanent both the angle of inclination and the inclination speed of the vehicle. As soon as the values ​​determined indicate the imminent risk of a rollover, the DSTC vehicle dynamics control is activated. With the controlled braking of one or more wheels as well as the reduction of the engine power, the stability of the vehicle is restored within a few fractions of a second. This ensures optimal control even in difficult situations.

Instead of a gyro sensor, in other vehicles tooRollover sensors used. These record the vehicle position and the accelerations that occur. For this purpose, a micromechanical yaw rate sensor and several acceleration sensors are housed in the control unit. Movements of the vehicle around the longitudinal axis are recorded.


Rollover protection (VW)

Serious or even fatal injuries are not uncommon in rear-end collisions with trucks and trailers - despite the mandatory underride protection. Even the further developed requirements of the EU for underrun protection systems do not change anything, as the ADAC was able to prove in a crash test. The ADAC used crash tests to determine what a really effective system should look like. The EU Commission is now dealing with these results.


  • Safety battery clamp

Pyrotechnic safety battery clamps have been in use at BMW since September 1997. These also contain a micro gas generator, which essentially corresponds to the usual gas generators for belt tensioner modules.In order to avoid fires caused by a short circuit in an accident, the starter / generator cable is completely disconnected from the battery within 3 ms in the event of a crash. In the event of a front or side crash, the airbags are deployed together.

Airbag 1 deployment

Airbag 2 components

Airbag 3 - Other airbags

Airbag 4 - safety

Airbag 5 scenario


Sources: VW, TRW, Volvo, Autoliv, Mercedes, Ford,

Author: Johannes Wiesinger

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