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When cars crash, drivers absorb energy of impact

Posted: Thursday, March 15, 2001

DARLINGTON, S.C. -- With all the recent talk of neck collars, malfunctioning seat belts and hanging throttles on the NASCAR Winston Cup Series, it might be easy to miss what might be the sport's greatest safety threat: rigidity.

As cars have evolved from backyard projects to four-wheeled spaceships in the past 25 years, the thirst for speed has veered too far away from basic scientific principles of safety, according to Ken Cunefare, a professor who specializes in acoustics and vibration at Georgia Tech's College of Mechanical Engineering.

The use of stronger alloys has reduced the amount of flex in the car's framework, and that means the energy expended in a crash often is transferred to the weakest link the driver.

''There have been certain design compromises made for certain purposes,'' Cunefare. ''Driver safety has not been one of those purposes.''

A system of metal tubing throughout the engine compartment is designed to keep the front end from sagging, so it won't affect the suspension. Most upper-echelon teams construct their own framework with something called chrome moly box tubing. Chrome moly is treated steel that is so hard it will break before it bends.

In addition to chrome moly box tubing, many teams are using a maze of triangle-shaped braces around the engine compartment to make the front half of the car more rigid. Cars driven by Dale Earnhardt, Adam Petty and Kenny Irwin all had triangular motor mounts to reduce the amount of flex in the car as it travels at high speed through turns.

When a car flexes, it changes the geometry of the front wheels. If the framework bows, the camber of the front tires the angle at which they meet the pavement changes.

''If there's no energy dissipation designed in the chassis, if it doesn't bend, if it doesn't flex, if it doesn't buckle, yes you end up with a more severe impact,'' Cunefare said. ''It's called shock pulse. What counts is how fast you stop. There's a correlation to injury and the shock pulse.''

Cunefare said the way the energy is spent during a crash is best described by dropping an egg from a second-story window. ''You wouldn't put it in a metal box,'' he said. ''You'd put it in bubble wrap to cushion the blow.''

Car owner Robert Yates, who's turned wrenches on the series for more than 30 years, said the steel bumpers, steel framework and four-inch-thick radiators used 20 years ago have been replaced with fiberglass bumpers and two-inch-thick radiators. That makes the significant point of impact a foot behind the bumper, which is a foot closer to the driver.

Yates would like to see the space between the bumper and the engine fitted with more shock-absorbing materials that would help dissipate some of the violent energy before it's transferred to the driver's compartment.

Cunefare is the faculty adviser for Georgia Tech's GT Motorosports team. It designs and constructs open-wheeled racers that compete on the collegiate Formula SAE circuit. Speed, he said, is secondary to safety concerns with his team's cars. The driver's seat is surrounded by energy-management devices.

''The Dale Earnhardt incident made me think about what energy-management program do they have built into these cars?'' Cunefare said. ''In an open-wheeled car, they have very well-designed energy-dissipation devices built around the driver's pod. That's not so in a stock car.''

Cunefare said the frame rails that extend all the way to the front bumper of stock cars also magnify the force of an impact. And putting them there contradicts everything U.S. manufacturers have learned about safety in the past 50 years.

Passenger cars now have collapsible front bumpers with frame rails that stop at the engine compartment.

''The same compromises made in a race car haven't been made in passenger cars,'' Cunefare said. ''They couldn't do what they're doing for a race car in today's passenger cars.''



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