by Jim Kerr

There are two types of safety systems built into modern vehicles: active and passive. Active safety is something that affects the way a driver controls a vehicle, such as ABS brakes, stability control and active steering systems. Passive safety systems include our seat belts, air bag systems and body structure design.

Many performance-oriented drivers believe that if all vehicles were built with superior handling, the passive safety systems would never be needed because drivers could easily steer their way out of danger. Driver’s skill becomes an important factor however, and then there is always the unpredictable, such as a deer jumping out in front of you.

All of these may be used during a collision to protect us from life threatening impact forces. Safety, both active and passive has improved immensely in the last couple of decades and Volvo is one of the leaders in safety design. The 2005 Volvo S40 sedan, for example, is their newest vehicle to enter the marketplace and is a fine example of current passive safety technology. The S40 is a smaller sedan but contains all the safety capabilities of their much larger vehicles. Some of this safety technology is offered on other vehicles, but Volvo continues to lead with some new and innovative systems.

2005 Volvo S40
2005 Volvo S40. Photo: Paul Williams. Click image to enlarge

Body structure plays a huge role in protecting vehicle occupants. Race fans have witnessed race-cars self-destructing against track walls at over 200 miles per hour and the driver’s have lived to tell the story. These cars protected the driver by gradually absorbing the impact forces and directing them away from the driver’s compartment. Passenger car body structure does the same thing.

For example, the Volvo S40’s front structure uses four kinds of steel to control the rate and direction of impact forces. A small car, like the S40 sedan is more difficult to design because all the deformation forces must be controlled in a shorter distance, but Volvo safety engineers have accomplished this by building the body structure in a “crash box” design. Each crash box has its own crumple rate to progressively absorb the impact forces as they get closer to the passenger compartment.

Another advantage of using this crash box design is that the crash boxes can be replaced separately if they are damaged in a collision, lowering vehicle repair costs.

Side impact forces are also absorbed by crumple zones, but this time the crumple zone is located as part of the S40’s centre floor pan tunnel. As impact forces push the side of the car in, the force is transferred into the seat, where special bars carry to forces to the centre of the floor pan tunnel. The tunnel crumples, allowing the seat to move inward and cushioning the impact forces on the passenger.

Front air bags, side impact air bags and side curtain air bags are all common on many new vehicles. While you may be familiar with air bags, many are not aware that some seatbelts also contain devices to pull the belts tight in a collision so occupants are held securely in position. Pyrotechnic charges are the most common method of deploying this type of seat belt. Some vehicles have the charge located at the base of the seat belt buckle but more and more vehicles are placing the charge by the retractor mechanism, so that the shoulder belt is pulled tight first.

The air bag’s system computer activates the seat belt charge and the belt is pulled tight. During an impact, the belt system is designed to “stretch” gradually, allowing the occupant to move forward slightly and reduce the force placed on our body. Volvo has taken this concept one step further and introduced an industry first – the retracting steering column.

Steering columns are designed to retract during an impact by shearing some plastic pins and allowing the outer column tube to slide down the inner tube. This often happens when the driver impacts the steering wheel. To further reduce the possibility of injury, Volvo’s S40 uses a pyrotechnic charge to pull the steering column away from the driver.

With 140 millimetres of travel, the steering wheel can be moved away before the driver hits it, reducing upper body injuries. Sensors in the driver’s seat monitor seat position in relationship to the steering wheel and sophisticated software activates the charge to retract the steering column only when necessary. Every collision and every driver is unique. The system is designed to react differently for each situation.

Most of the safety built into a vehicle is never visible. It is usually only truly appreciated after a serious collision. Volvo has long been known for innovative safety design and when one manufacturer leads the way, the others are usually quick to follow. Everyone is driving safer vehicles today because of the leadership shown by a few.

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