If a bullet hits a hard surface at a slight angle -- say, 20 degrees -- it ricochets; that means the energy is channeled in another direction and lessened. But if it hits straight on, the bullet is flattened; enormous energy is concentrated right there.
Now, imagine the bullet hits a "soft wall" -- so-called because they're made of materials that dissipate energy of crashes away from drivers' bodies. But if the bullet hits at even a slight angle, it doesn't ricochet. It tends to get caught in the material, concentrate energy then and there, and cause damage to the material and the bullet.
In terms of racing, the "bullet" is the car and the "material" is the soft wall.
That's the biggest unsolved problem with soft walls. If a material -- be it rubber, polyethylene or high-density foam -- contracts enough to cushion the blow of a crash satisfactorily, then the point of impact tends to pocket, acting "like a catcher's mitt," says engineer John Pierce, chief designer of the energy-dissipation system generally considered farthest along in development.
That pocketing "snags" or "catches" the crashing car.
"That snagging is of great concern for two reasons," says Dr. John Melvin, an expert in racing safety. "One is that the snagging actually will be stopping the vehicle, and therefore adding to the crash energy. The other is it could redirect the car in a violent way so that it interacted with other cars."
In other words, "you don't want something that's going to act as a spring to shoot the car across the track" -- and possibly into onrushing traffic, Pierce says.
Concrete retaining walls have been in use since the first Indianapolis 500 in 1911. They were considered state-of-the-art replacements of other types of barriers even through the 1970s and into the '80s. Currently, they surround every major American oval track but two -- and those tracks, at Dover, Del., and Long Pond, Pa., have walls made of boilerplate steel, just as unforgiving as concrete.
Current concrete walls, though deadly in blunt impacts, usually redirect crash energy in a desirable way; that is, cars hit with glancing blows. The cars then continue to move in the direction in which they were traveling before the crash occurred.
To avoid creating new problems while solving old ones, any suitable new soft wall must allow this forward movement to continue after impact. Stopping the car completely and instantly would cause even higher concentrations of energy into the car itself -- and, far more important, into a driver's body.
In racing, "there's a lot of contact with the [concrete] wall that's at a very slight angle," says Gary Nelson, NASCAR's chief technical officer. "Just a scrape. If the wall is soft at all, I think it would literally grab hold of the car and cause it to stop suddenly.
"Ninety-nine percent of contact with the [concrete] wall at a typical race is not even reported. It's just, 'A car scrapes the wall, the side of the car was flattened, the crew hammered it out, and it went back out a few minutes later.'
"Well, each of those would be a serious accident [with soft walls] in our opinion."
NASCAR Winston Cup cars weigh 3,400 pounds, more than twice as much as Indy cars. So a stock car, crashing at the same speed as an Indy car, produces far more energy, by a formula of physics: Mass times velocity equals energy.
Thus, soft walls strong enough to absorb and withstand an Indy car hit aren't necessarily strong enough to deal effectively with a NASCAR hit.
Pierce has substituted high-density foam for the cylinders, but, says Melvin, "I liked the original PEDS because you could adjust it." Stronger cylinders and/or insertion of smaller cylinders inside larger ones could increase the system's ability to withstand crashes of heavier cars.
Despite some concerns about soft walls -- so-called because they're made of materials that dissipate energy of crashes away from drivers' bodies -- there are so many designs near fruition that track owners and sanctioning body chieftains are being forced rapidly toward a singular decision: Either they want to spend the money to develop and construct soft walls, or they don't.
Pierce is the designer of the energy-dissipation system generally considered furthest along in development. His Polyethylene Energy Dissipation System (PEDS) is being tested at the University of Nebraska's auto-crash test center, with funding from the Indy Racing League and its parent, Indianapolis Motor Speedway.
"I think 25 years from now, we'll look back and say, 'Gosh, I can't believe they raced with concrete walls,' " says former NASCAR driver Ricky Craven, whose career was ruined by one. "I really do. I think it will be a mockery."
Other systems already in use are not applicable to high-speed ovals, where soft walls are needed most.
In one sharp turn of a so-called "oval track" near Rio de Janeiro, Brazil, Championship Auto Racing Teams Inc.'s technical staff has installed what it calls an "energy attenuator," made of tightly bound tires and covered with ballistic material used in conveyor belts.
Because cars crashing into that particular corner are likely to hit almost straight on anyway, the "snagging" of cars isn't a factor. But CART technical director J. Kirk Russell says it might be deadly on high-speed ovals in that it would "catch" crashing cars.