Secrets of the Gen-6 Car: Super-strength Windshields

Drivers sound a common refrain:  The Car of Tomorrow sacrificed looks and raciness for safety. The Gen-6 car is a much better looking vehicle and (once the teams get a handle on the engineering) will provide a better show. 

But don’t let its looks fool you:  NASCAR did not forgot safety in the Gen-6 car.

During a recent visit to the NASCAR R&D Center, Tom Gideon — NASCAR’s Safety Czar — showed me some changes you won’t notice… unless something bad happens.  He briefed drivers on these changes during Daytona testing last month and their reactions are uniformly positive.

There is a line of Diet Coke cans sitting on the front edge of Tom’s desk.  They aren’t for drinking:  they’re there to become high-speed projectiles.

Why Diet Coke?

Because, Tom explained, if you splat regular Coke onto a car, you’ll spend the rest of the afternoon cleaning up the gooey, nasty mess produced by all that high-sucrose corn syrup.

NASCAR engineers projected these soda cans (actually, their less-fortunate cousins) at car windshields. 

Lexan: A Super Material

NASCAR windshield are, of course, not made of glass.  They are made of a plastic called polycarbonate, which is better known by the trade name Lexan.   Lexan is clear, like glass, but it gives more easily and thus can take higher impacts without breaking.  The Lexan ‘shark fins’ on the Gen-6 cars make them virtually invisible when cars are on the track.

Lexan is also more expensive than glass, which is why most passenger cars still use glass in their windshields and windows.  Windshields are made from laminated safety glass.  Laminated means stacking up a bunch of layers of something and compressing them to make a single unit.   Plywood is a laminate of wood, which you can see if you look at the end grain.

Laminated autoglass sandwiches a very thin polymer film (polyvinyl butyral, a.k.a.  PVB) between two layers of glass. The layers are heated under pressure, chemically bonding the glass and the film.  The polymer film (in contrast to glass) is elastic – that’s the layer that allows the glass to absorb energy when something hits it.  This is the same idea as padding your dashboard – the padding extends the time of your collision and thus decreases the overall force.  The polymer film insert in your car’s windshield also absorbs UV rays from the sun, protecting the fabric, leather and plastic inside the car.  Most importantly, if laminated glass breaks, the polymer film holds the pieces together.  The side windows in your car are made from  a different kind of glass that is designed to shatter into a zillion tiny — and rounded — pieces if it breaks.  This allows a person to climb out a broken side window without restriction (or cuts).

The Danger of Cans

The Diet Coke cans on Tom Gideon’s desk were representative of the types of projectiles that might land on a track.  If you think something as seemingly innocent as a soda can  can’t do any real damage, look at this blog I wrote awhile back when a Dallas TV station contacted me to verify the story of a woman who claimed a styrofoam drink cup thrown from a passing car had punched a hole through her windshield.  The hole is shown at right and, yes, a styrofoam drink cup traveling at high speed can break a windshield.

A Lexan windshield wouldn’t have broken, but the windshields in NASCAR racecars have to stand up to much more than soda cans.  They have to survive impacts of things like like wayward car parts.  Last year in Charlotte, a piece of brake rotor hit Greg Biffle’s windshield.  The brake rotor didn’t go through the windshield, but the impact did blow out the interior window braces so that they were hanging over the steering wheel and impeding Biffle’s ability to steer.

Five years ago, NASCAR was in reactive mode when it came to safety.  They weren’t anticipating problems, they were rushing to develop solutions when something happened on the track.  Things have changed:  NASCAR is sufficiently ahead of the game that they aren’t waiting for a catastrophe to happen before they start innovating.

The Pneumatic Cannon

One of the R&D Center’s new toys pieces of scientific equipment is a pneumatic cannon.  Compressed gas propels a projectile toward a windshield at about 50 mph. It’s like a potato gun, but with soda cans instead.

In addition to the soda cans, NASCAR used solid metal cylinders to test the Gen-5 windshield using the new pneumatic cannon.  The old windshield wasn’t unsafe, but the R&D Center realized they could do better.

The new window uses two pieces of Lexan, each half the thickness of the old window, with a 30 mil (that’s 30 one-thousandths of an inch) polymer film between them.  A really good-quality heavy duty trash bag is 4 mil thick, so the film in the new windshield is about seven or eight trash bags thick.  After heating and pressure treatment, you have a perfectly clear, superstrong windshield that is only thirty thousandths of an inch thicker than the old window.  Robin Pemberton told the media that the new windshield could withstand the equivalent of a connecting rod going 200 mph without breaking.

The pneumatic cannon resulted in some additional developments that I’ll detail in my next post.  Questions and comments are always welcome.