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AUTO TECH: Crash protection in modern cars a marvel

Nobody wants to be part of an automobile accident, but it's a fact of our environment: Accidents happen. Some are small fender-benders, with no more than a little damage to paint, metal and pride. I hope for nothing worse.

In the 1950s and '60s, cars were solid, built like tanks. Heavy frames, thick metal panels, large bumpers and very little structural plastic made these cars appear to provide good passenger protection. However, today's vehicles provide much better protection in severe accidents.

First, the anti-lock brakes, traction control, stability control, superior suspension and superior tires on newer vehicles may enable you to drive around the accident. But sometimes there's no getting away from it -- you hit, and hit hard.

As the vehicle's front end begins to crush, the bumper and its plastic cover move back. Moveable mounts absorb some of the force. Next, the body or frame starts to collapse and the hood buckles into an "A" shape when viewed from the side. The front fenders wrinkle and the engine and transmission begin to slide back beneath the floorboards.

In the most severe impacts, the force is transferred back into the rest of the body. The roof buckles upwards, the door pillars move out and the floor panel moves down. Crush zones and the use of reinforced body areas with specific weak spots makes this all happen. The vehicle is designed to collapse this way to help absorb the impact forces so the fragile occupants inside won't have to.

While the outside of the vehicle is collapsing, the inside is also doing its part to protect occupants.

Sensors for the airbag system measure the severity of the crash -- it takes force about the equivalent of driving into an immovable wall at 32 kilometres per hour to activate the airbags on many vehicles. Hitting a moveable object such as another vehicle requires a higher speed before the airbags inflate. Many vehicles now activate the airbags in two stages: low energy for less severe accidents and high energy for hard impacts. Some airbag systems also detect seatbelt use, seat position and size of the occupant to determine how fast to deploy the airbag.

If the detected impact force is great enough, the airbag will go off, inflate, cushion the occupants and then start to deflate, all within seven thousandths of a second, less than the blink of an eye. For this reason, many drivers think the airbag didn't work properly in an accident, because they just didn't see it.

There's much more inside the vehicle that protects you. The seatbelts are the main source of protection. Some have pretensioners, which ignite a charge that pulls the seatbelt tight around the passenger at the same time as the airbag is deployed. This helps put them in the proper position for the airbag and counters stretch in the belts themselves.

After the belt is tight, one of several methods is used to slowly loosen the belt and allow the passenger to decelerate at a safe rate. Some methods involve sections of the belt that are allowed to extend, or mounting points that can flex or bend. Regardless of the method, the belts help control the rate of deceleration the wearer experiences.

If you walk through an insurance salvage yard, you can see how vehicles are designed to bend and crush. What impresses me the most about these damaged vehicles is how intact the passenger compartments are, except for the deployed airbags. Often, there's no visible damage to the interior structure -- the doors still open and the glass is all in place. The front of the vehicle may be crushed, just as it was designed to do, saving the occupants from the force of the impact.

I hope you never have to test the safety features built into your vehicle, but it's nice to know there's a lot of hidden engineering in our vehicles that absorbs impacts.

Being a passenger in a car crash is a lot like being a raw egg in an egg toss: Catch it slowly and smoothly and the egg is fine. Stop it quickly, and there's nothing left. Vehicle crush zones are designed to stop occupants slowly and smoothly during a collision.

 

Jim Kerr is an experienced mechanic, instructor of automotive technology, freelance journalist and member of the Automobile Journalists' Association of Canada.

kerr.jim@sasktel.net

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