Ever had a rock kicked up by a car in front of you impact the windshield of your car? Scares the heck out of you, doesn't it?
Imagine the object impacting your wind screen weighing more than a pound and traveling at, oh, 200 mph or so.
That's what NASCAR drivers face every time out -- an unwelcome passenger could at any moment climb aboard through the windshield, and depending on the angle, could do more than that.
"I grew up racing on dirt," said Tyler Reddick, who drives the No. 19 Ford for Brad Keselowski Racing in the NASCAR Camping World Truck Series. "I'm not even used to having a windshield. On dirt, you see a lot of stuff flying into the car with you, like brake rotors and other parts, and even dirt clods big enough to break your bones."
NASCAR, with the introduction of the Generation-6 car for the NASCAR Sprint Cup Series in 2013, also upgraded the Lexan windshields in the top division along with all the other changes. This year, it expanded the use of the new, thicker material into the NASCAR Nationwide Series and Truck Series.
Lexan is a polycarbonate, which means it is layered and compressed into a single sheet with more tensile strength than regular glass. Think of it as clear plywood.
The old windshields were comprised of Lexan and about a quarter-inch thick. That's enough for good structural integrity, both from aerodynamic forces and objects striking it, but NASCAR thought it could do better.
Enter NASCAR safety guru Tom Gideon and a pneumatic cannon, full cans of soda and a bunch of machined metal slugs.
Gideon, who for years was on the cutting edge of automotive safety while at General Motors, used the cannon (which uses compressed air to accelerate projectiles) to pulverize windshields until the proper recipe was reached.
Instead of the single-piece plastic Lexan windshield, NASCAR's new laminate shields are built in two stages.
The old shields were .236 inches of Lexan. The new one takes .118-inch slabs of Lexan and separates them with a polymer film. Heat them up and press them together to form the chemical bond, and you have a much stronger piece that is just 30-thousandths of an inch thicker than the old one.
According to the website www.buildingspeed.org, the film is about as thick as eight "really good quality heavy-duty trash bag" plies.
Gideon, the technician's technician, set up the testing by first building frames for the windshields and setting them up at various angles. A single sheet of Mylar tear-off was applied to both test windshields.
Once the metrics were arranged, he unlimbered his test cannon at the NASCAR R&D Center in Concord, N.C., and started hurling full cans of soda at both the old and new windshields. None penetrated either shield, but the existing windshield did fracture.
One problem with the soda cans was that once they hit, they tended to spew their contents all over the place.
Once that test was over, Gideon brought out the heavy ammo.
Slugs made of bar steel, 2.5 inches long and weighing nearly a pound (0.86 pounds, to be exact), were fired at the old windshield with somewhat predictable results. Designed to simulate a car part traveling at up to 200 mph, they arrived with nearly 1,150 foot-pounds of kinetic energy and some of them got through.
Once they penetrated, the slugs were still traveling at 110 feet per second, according to a report by the Society of Automotive Engineers, and that translates to 161.6 foot-pounds of energy. That's enough to ruin anyone's day, even though the energy was reduced more than seven times.
The same shot at the new windshield did not penetrate, a fact that Reddick appreciates.
"I've seen a lot of guys get hit with stuff, and that can trouble you a little bit," he said. "This new windshield doesn't let anything through. At only about five-hundredths of an inch in thickness, that's amazing."
"NASCAR is always looking to make the trucks safer and they really did with this," he added. "You never want to have anything in the truck with you."
Any piece of plastic -- tough, stable plastic like a windshield -- is subjected to conditions on the race track. Grit, trash and bits of stuff that accumulate will hit it. That can result in some pitting, and if you're racing at Phoenix or Las Vegas or Sonoma, you can add sand-blasting as well.
When you thicken up a piece of polycarbonate and add a flexible film, it can cause the optics to suffer. There is a point of reduced benefit, in other words.
According to SAE's report on the tests, "acceptable optical clarity becomes increasingly more difficult to achieve as material thickness increases."
Reddick says that he has not noticed any difference in the vision from his seat, and that's the only one that matters.
"I don't see a big difference, to tell the truth," he said. "Of course, I haven't really driven anywhere there's been sand yet, but I don't think it's going to be a problem."