What Is SRS On A Car? Understanding Your Car's Supplemental Restraint Systems

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What does SRS actually mean? And why will you find it on almost all modern cars?

Your car's dashboard and instrument cluster are packed with acronyms like ABS, TPMS, ESC, and SRS; this article provides an in-depth guide to the latter. But what does SRS mean?

SRS stands for Supplemental Restraint System, also called Passive Restraint System or Secondary Restraint System. These alternative names stem from the system requiring no action from the driver or passengers to work.

The SRS includes airbags, sensors, and any other device fitted to your vehicle that operates automatically during a collision. The SRS is designed to detect high-speed impact and deploy the airbags during a crash to keep you and your passengers safe and prevent injuries.

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Various Types Of SRS

The central element of a car's SRS, airbags, is a nylon bag that rapidly inflates with gas during a high-speed collision to protect the car's occupants from impact.

There are several different types of airbags, and depending on the model and spec of your car, you may find up to 10 airbags in your vehicle. The main airbag types you may find in your car are the following:

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Frontal (Driver/Passenger) Airbags

These are the most well-known types of airbag and the first to be implemented. In a forward collision, they inflate directly towards the driver and front passenger to prevent head and upper body injuries.

However, they are unsafe for small children in rear-facing seats, which is why infant car seats should not be installed in the front seat unless absolutely necessary (for example, in a two-seater car). If you need to install a child car seat in the front, the passenger-side airbag must be disabled first.

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Side-Curtain Airbags

This airbag protects passengers in side collisions; it is mounted above the car's windows and "rolls down" like a curtain when the vehicle is struck from the side.

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Seat Mounted Airbags

These airbags are directly attached to the seat and unfurl to provide additional protection in side impacts.

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Knee Airbags

Placed in front of the passenger at knee level, these airbags have been shown to have a negligible impact on reducing injury rates and may even increase them.

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Inflatable Seat Belt Airbags

These are seatbelts with their own airbag built in to provide additional protection to the occupant's chest in a forward collision and avoid injury from the seatbelts themselves.

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Rear Seat Centre Airbags

First developed by Toyota in 2009, this type of airbag helps prevent passengers in the rear seat from colliding with one another in a side impact.

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Pedestrian Airbags

Pedestrian airbags are pretty uncommon and relatively new; they were developed to bring down fatalities for pedestrians and vulnerable road users, who are currently the majority of victims of road traffic collisions. Pedestrian airbags were only installed on a handful of cars such as the Volvo V40, Land Rover Discovery Sport, and Jaguar I-Pace; GM patented their own version in 2017.

Airbags are the main component of the SRS. Still, a host of devices form part of the SRS to ensure they function correctly when needed: other components of the SRS include the igniter, pyrotechnic inflator, warning light, passenger seat switches, electronic airbag control unit, and crash sensors.

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History Of The SRS

The first airbag was designed and patented in 1919, but it was in the 1950s that designs intended specifically for use in passenger cars started to materialize. However, since airbag tech was still in its infancy, compressed air inflated the airbag, resulting in a deployment speed far too slow to function as intended.

Things began to improve in the 1970s when fully operational airbags started to appear on commercially available vehicles by GM and Ford. These were advertised as a replacement for seatbelts rather than a complementary feature; the "supplemental restraint system" concept hadn't yet appeared. The first vehicle with something resembling a modern SRS was the Mercedes-Benz S-Class in 1981. Ten years later, in 1991, US legislation was passed stating that all US cars and light trucks built after September 1, 1998, were legally required to be fitted with airbags.

Around the time the new law came into force, airbag maker Takata was looking for a more efficient chemical to inflate their airbags quickly as a replacement for sodium azide; they settled on ammonium nitrate, which proved to be a disastrous choice. Heat and humidity made the ammonium nitrate deteriorate and turn into an uncontrolled explosive, which launched shrapnel into the vehicle when the airbag was deployed, effectively turning the car into a bomb.

What followed was one of the most extensive product recalls in automotive history, which is still ongoing. Modern airbags typically use the more stable guanidinium nitrate instead, plus a copper nitrate oxidizer to lower the exhaust gas temperature.

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What Happens When the SRS Is Deployed?

Modern cars have pre-programmed safety parameters (such as collision with a hard object above a set speed) that trigger SRS deployment. The inflation process is akin to a small controlled explosion: the airbags' electronic control unit sends an electric signal to a heated element that triggers a chemical reaction.

The resulting explosion produces large amounts of harmless gas (usually nitrogen or argon), inflating the bag and creating a cushion to soften the impact for the car's occupants. The process happens in around 1/20th of a second, less than the blink of an eye. The effect of the passenger against the bag helps it deflate relatively quickly after an accident, and the gas is dispersed into the air. SRS control units usually act as a "black box" of sorts, recording instances of airbag deployment.

The SRS is designed to prevent unnecessary deployment and will not deploy the airbags:

  • When colliding against a soft surface.
  • In a low-speed collision.
  • During sharp braking / an emergency stop.
  • In an impact from the rear.
  • In a side impact, if the vehicle isn't fitted with side airbags.
Mercedes-Benz AG Mercedes-Benz AG Mercedes-Benz AG Mercedes-Benz AG

The SRS Warning Light

The SRS light normally appears on the dashboard for about 5 seconds when the vehicle is turned on as it completes its self-test sequence. If the warning light stays on longer than that, this may indicate a fault with the SRS system. Some of the most common causes include:

  • An error in the SRS computer.
  • A previous collision where the airbag did not activate.
  • Sensor fault - one of the sensors that make up the SRS could be damaged or worn out.
  • Clock spring fault - a clock spring is a device that ensures electrical controls mounted on the steering wheel (horn, airbags, cruise control, and other buttons) keep their electrical connection while the wheel turns. This issue is prevalent in older cars.
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