Last Friday in Dallas was beautiful. It started out cloudy, but cleared off and got warmer. By the time I finished a TV interview about 10:00 a.m., we had to dig out our sunglasses before we (my husband and I) started off for Texas Motor Speedway. We spent the midday working with Channel 33 on a segment that turned out wonderfully, thanks to the enthusiasm of Michael and Barry, the reporter and cameraman. After catching up with some friends in the garage and watching the No. 19 qualify (they went first), we decided to move to a better vantage point and watch the rest of qualifying sitting on the hill that runs alongside the South tunnel entrance. That spot gives you a great view of turn 1 and much of turn 2.
The No. 00 Aarons ‘Dream Machine’ with rookie Michael McDowell driving and Lucky Dog grinning from the hood, took to the track. Our dog, Darwin, looks just like Lucky Dog. In fact, I’ve been meaning to suggest to their PR person that they should have a contest because I think we’d have a good shot at winning.
“He’s not very fast,” my husband said.
“Yeah,” I replied, “But he just needs a solid lap…”
You know what happened next, because even if you don’t follow NASCAR, McDowell’s crash made just about every newscast. From our vantage point, which was only about 60 yards away, we watched him hit the SAFER barrier hard in turn 1, roll over on the roof, spin a couple times, then barrel roll down turn 2. The crowd quieted and all you could hear was the stomach-crunching sound of sheet metal hitting pavement. My heart stopped. The car finally came to a stop at the bottom of the track. We couldn’t see anything but some flames and a plume of smoke.Watching the car as it tumbled down the 24-degree banking, it was hard to reconcile everything I know about the safety designed into the cars and the track with what I was seeing. It wasn’t until we heard a cheer rise from the crowd that we knew he was OK.
A man standing nearby said, “I like watching this on TV, but I don’t like seeing it in person.” He looked down at his young son. “It makes it too real.” Even with all the emphasis NASCAR has placed on safety in recent years, everyone knows that all it takes is for a car to hit at precisely the wrong angle, or for a fifty-cent bolt to fail and the result could be catastrophic. A reporter asked McDowell what he thought the most important safety device was and McDowell gave exactly the right answer: All of the safety improvements NASCAR has made work together to protect the drivers (and the fans). The NASCAR Research and Development Center and their research partners deserve a huge amount of credit for McDowell being able to walk away from the accident and race Sunday.
The basic physics of safety is simple. (As is often the car, however, coming up with real-world implementations is more challenging.) Anything moving has kinetic energy. A car traveling normally around the track has linear kinetic energy. (I’m qualifying the kinetic energy with the world “linear” to contrast with the situation in which the car is spinning, in which case we would have to consider the rotational kinetic energy as well.)
The linear kinetic energy is proportional to the mass and the square of the speed, which means that a stock car moving at 180 mph has nine times the kinetic energy of the same car moving at 60 mph.
The amount of kinetic energy the car has is important because of the law of conservation of energy. Conservation of energy tells us that energy cannot be created or destroyed. Energy can only change forms. When a car comes to a stop, all of its kinetic energy has to be converted into other forms of energy. When a car pulls into the pits, its kinetic energy is converted into other types of energy: heat (the brake pads and the rotors heat up), light (you’ve seen glowing rotors on television), and sound (squealing). This conversion happens gradually from the time the driver starts slowing as he drops to the apron to enter pit road until he comes to a complete stop.
Accidents require the conversion of energy to happen much more quickly, which often means that energy is converted into less desirable forms, such as deformation of the car. It takes energy to change the shape of a material, because you are literally breaking the bonds that hold the atoms together. If you squish a marshmallow, for example, it takes energy to change the marshmallow from a 1-1/2″ tall cylinder to a 1/2″ cylinder. First, you compress the marshmallow by pushing all the air out from the inside. Then, you break the bonds holding together the sugars from which the marshmallow is made. The same thing applies to a car, although it takes much more energy to bend a tube in the chassis or a piece of sheet metal than a marshmallow.
Every bit of safety equipment used in NASCAR has the same purpose: To convert the kinetic energy of the car into other forms of energy, with a minimum of that energy reaching the driver. Let’s start with the car, the chassis of which is pictured below.
The front and rear clips aren’t shown in their entirety, but you would find that the tubing used in these sections is smaller diameter than that used in the center section of the car that surrounds the driver. The center section is the strongest part of the car. The front and rear clips are built to be weaker than the center section, so that energy is dissipated by deforming the front and the rear of the car. If you look at pictures of McDowell’s car post-crash, or view the video of the crash, you’ll see that the front and rear of the car are almost gone, but that the center section remains intact. The energy used to destroy the front and rear clips is energy that isn’t transmitted to the driver. The hood and the axles, along with the rear decklid and the wing are all tethered to prevent them from flying off; however the tires did come off the car during the tumble. That’s good: Those parts are taking energy with them and away from the driver.
When McDowell started barrel rolling, energy was transformed from linear (forward) motion to rotational motion. It takes energy to make something spin, so a car slows down as it spins because linear motion energy of the car is converted into rotational motion.There is an advantage to keeping your wheels on the track during a crash, or even spinning before you hit the wall. You may have heard drivers talking about “scrubbing off speed”, which is how they describe the friction between the track and the tires. That friction generates heat and the car slows down as energy is transformed from motion energy to heat. The same principle holds if you spin on your roof.
I’ve already posted a section about the Dow Automotive IMPAXX foam that is in both doors. This energy-absorbing foam uses kinetic energy to squish (a technical term) the foam.
The importance of the SAFER barriers cannot be overemphasized. I describe their development in my book, The Physics of NASCAR, but McDowell’s accident emphasizes some specific aspects of their construction. The SAFER barrier development was a joint project of the IRL and NASCAR, lead by Dr. Dean Sicking and his colleagues from the Midwest Roadside Safety Facility at the University of Nebraska. The car’s motion energy is used to deform the metal tubing (which is a square-cross-section, hollow metal tubing about 8″ in diameter with 3/16″ thick walls). The inner metal wall is separated from the concrete back wall by pyramidal foam sections that absorb energy. The shape of the wall and the foam also direct the car along the wall after impact instead of bouncing it back out into traffic. Energy is dissipated along the length of the wall, instead of concentrated in a single spot.
For most hits, the metal wall springs back to it’s original position; however, McDowell hit hard enough that the wall was permanently deformed and a number of the foam sections destroyed. It took a little more than an hour for them to repair the wall, which included putting a plate on the steel wall and replacing a number of sections of the closed-cell polystyrene foam sections. I understand that they actually replaced the entire 28-foot section of the SAFER barrier after track activities ended for the day.
Dean Sicking told me, when I interviewed him for the book, that data recovered from accidents is extremely important because every accident is slightly different. There is no way to test every possible scenario in controlled conditions. Dean said, though, that people who work in safety view accidents with a split attitude. On the one hand, every bit of data helps in designing the next generation of safety equipment; however, you never want to see anyone have an accident. You can bet that NASCAR will examine every bit of data that they can recover from the car, from the black box information to what’s left of the car. Robin Pemberton noted that they have pre-crash measurements of the car because of the certification process the new car goes through. They will be able to compare those measurements with the post-crash car to see how the car design stood up. They may find some parts of the chassis that didn’t behave the way they expected them to behave. Safety is an ever-evolving process: You learn, you re-design, and then you learn more about your new design. No one knows how to build a car, or a track, that will be 100% safe. It is an iterative process and Friday’s crash will provide them with important data that will improve what we already know.
Alan Sobel of West Virginia University wrote a fascinating article about whether safety improvements encourage more reckless driving. He is interested in a fundamental principle of economics, but NASCAR is a good laboratory for his study because there is 100% enforcement in the use of safety devices. His conclusion is that, yes, drivers do drive more aggressively when they are confident in the safety equipment; however, their increased aggressiveness only partially offsets the effect of the safety improvements. I only hope that McDowell’s walking away from this crash doesn’t give drivers–especially the younger ones–an unrealistic confidence in their ability to survive accidents.
I’ve changed my mind about the chances of my dog winning the contest. Michael McDowell is the ultimate “Lucky Dog”.