by Jim Kerr
Nobody wants to be part of an automobile accident, but it is a fact of our environment: accidents occur. Some are small fender benders – usually nothing more than a little paint, metal, and somebody’s pride are hurt in a minor skirmish. I hope nothing worse is in all our futures.
For a variety of reasons, some driver’s will be involved in life threatening accidents. In the 1950’s and 1960’s, 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 protection for the occupants. However, today’s vehicles provide much better protection in severe accidents. Here is a scenario of what happens in a front-end collision with a modern car or truck.
First, the anti-lock brakes, traction control, and superior suspension and tires may enable you to drive around the accident. This would be the preferred choice, but sometimes there is no choice. You hit and hit hard.
As the vehicle’s front end comes into contact with the obstacle, it begins to crush. The bumper and its plastic cover move back. Moveable mounts absorb some of the force while the bumper is moving. As more force is fed into the vehicle, the body or frame starts to collapse and the hood buckles into an “A” shape when viewed from the side. The front fenders wrinkle, the engine mounts break, and the engine and transmission slide back and beneath the floorboards.
In the severest 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 this way to help absorb the impact forces so the fragile occupants inside won’t have to.
At the same time the outside of the vehicle is collapsing, the inside is also doing its part to protect the 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 20 mph (32kph) to activate the airbags on many vehicles. Hitting a moveable object like another vehicle requires a higher speed before the airbags inflate. Some 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 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 start to deflate, all within seven thousandths of a second. This is less time than a person can willingly blink 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 is much more inside the vehicle that protects you as well. The seatbelts are the main source of protection. Some seatbelts have pretensioners as part of their mechanism. At the same time an airbag is deployed, the seatbelt pretensioner ignites a charge that pulls the seatbelt tight around the passenger. This helps put them in the proper position for the airbag and counters stretch in the belts themselves.
After the belt is tight, then one of several methods is used to allow the belt to slowly loosen 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 experienced by the wearer.
Seat design has also changed. Anti-submarine bolsters are built into the front lower portion of the seat to prevent you from sliding under the seatbelt. Knee-bolster panels at the base of the steering column and on the lower passenger side of the dash prevent the passenger from sliding down under the dash.
Steering wheels bend around the hub to reduce chest impact on the driver, and steering columns collapse as the force of the impact makes the outer tube slide into an inner tube. Plastic trim on the inside pillars of the vehicle are built with honeycomb reinforcement that can crush if struck by a head or body. Windshields are glued in place to become a structural part of the body and laminated glass reduces the possibility of being ejected from the vehicle.
Walk through an insurance salvage yard and you can see how vehicles are designed to bend and crush. My latest chance to look one over was on a 2002 Dodge Ram pickup. This vehicle had been deliberately crash tested into a solid steel wall at 35 mph. This is 5 mph faster than government test standards in the USA. As expected, the hood and fenders buckled, the front end was as flat as a pancake, and the airbags had gone off.
What impressed me the most was how intact the passenger compartment was, except for the airbags being deployed. There was no visible damage to the interior structure, the doors still opened, and the glass was still all in place. The front of the truck had 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 is nice to know that there is a lot of hidden engineering in our vehicles that absorb impacts and give occupants another chance. 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 is nothing left. Wear a seat belt, drive carefully, and don’t scramble any eggs.