This explains the physics of the crash much better than I ever could duckv, but basically calls for the same thing. Slow the cars down, by reducing horsepower and pulling the restrictor plates off of them. Whether you watch it live, or on T.V., your eyes aren't quick enough to tell the difference in speed. Speed is relative to how fast others are travelling around you, just like on the freeway....

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Talladega Teardown: Roof Flaps and Catch Fences

“You have to understand that, like, for years, we have had wrecks like this every time we come to Talladega ever since the (restrictor) plate got here, and for years, it was celebrated. The media celebrated it, the network celebrated it, calling it the Big One, just trying to attract attention.” –Dale Earnhardt, Jr.

My least favorite part of every Talladega pre-race show used to be watching Elliott Sadler tumbling through the infield. And you know, regardless of which network is carrying the race, they’re going to show a montage of crashes. Most will also show Bobby Allison’s 1987 crash - the one that instigated restrictor plates. Did anyone else have a really sick sense of deja vu watching the 99 car of Carl Edwards go airborne and tumble along the fence?

In both crashes, the cars because airborne and, instead of hitting the wall, the cars ended up in the catchfence. In 1987, Bobby Allison’s car pulled about 100 ft of chain-link fence down and sprayed debris into the crowd. A number of fans were injured, including a woman who lost an eye.

Most of the publicity about track safety has focused on SAFER barriers; however, there are researchers improving catchfences as well and the results were evident Sunday. The 99 rolled along the fence, but if you compare the two incidents, today’s fence did a much better job. Fences have difficult tasks: They have to allow spectators to see while being strong enough to contain a flying racecar. A stock car going 190 mph into a rigid barrier would produce a peak force of about 9700 lbs. The force the catchfence experienced would be less because fences have some give, but that’s still pretty impressive for steel and wire, especially if you look at how the 1987 fence did.

Fans and pundits are calling for NASCAR to ‘fix the car’ so that it stays on the ground. If you watch closely, you’ll notice that the primary safety device that guards against cars going airborne did work. Edwards’ roof flaps deployed and, in fact, you can see the car start to lower when the flaps come all the way up. That’s when the 39 got into him and was just far enough under to send the 99 into the air.

Matt Kenseth took a nasty roll during the Nationwide series race at Talladega on Saturday. In the numerous replays of the accident, I kept looking for the roof flaps rising as the car spun sideways. As far as I can tell, the roof flaps didn’t deploy on his Nationwide car. (NOTE: As Nick points out in the comments, this wasn’t an aerodynamic issue - it was more of a mechanical issue, like a SUV rollover. Thanks to Nick for pointing this out.)

Roof flaps are the two inserts on the top of the car. One runs along a line from left to right and one is angled at 45 degrees to the first, as shown in the figure below (which comes from the original patent #5374098).

Roof flaps (the invention of which I detail in my book The Physics of NASCAR) are designed to keep cars on the ground. Faster-moving air exerts less pressure and slower-moving air exerts more pressure. A wing develops lift because the air flowing under the wing moves slower than the air going over the wing. That creates more pressure underneath the wing than over the wing, which generates a net force upward. You want that for an airplane, but you don’t want it for a race car.

A NASCAR race car is pretty stable when airflow comes from the nose to the tail. The problems start when the car turns sideways because a sideways racecar looks a little too much like a wing. Air flows over the roof of a sideways racecar very quickly. It stays attached to the car’s surface for a long time, and that creates a low pressure region on the top of the car. A little air (or another car) gets under the car and all of a sudden, the car is an airplane. This only happens when the car rotates enough, so you need a solution that only becomes active when the car is really yawed.

You want the air to detach from the car’s roof, which increases the pressure on the top of the car and decreases the lift. That’s where the roof flaps come in. As shown below, the roof flaps are flaps of metal that are normally flush with the roof. If the pressure on the roof gets low enough, the pressure differential between the underside of the flap and the top of the lap causes the roof flap to pop up. A tether keeps the flaps perpendicular to the roof surface until the car gets going the right direction.

There’s an additional bit of help as well from the cowl area - the part of the car where the windshield meets the hood which also has flaps, as shown below.

The central flap is the air intake for the engine and the two piece called out on either side are the cowl flaps. Cowl flaps work the same way as the roof flaps, opening when the pressure on top becomes much lower that the pressure underneath. The opening of the cowl flaps allows air to escape from under the car and that also decreases lift. (The patent number on that one is #5544931.)

“(Drivers) have been saying this for years: racing like this is not a whole lot of fun.” –Dale Earnhardt, Jr.

In my opinion, calls for NASCAR to improve the roof flaps or other aerodynamic components so that they can keep the cars on the ground at Dega and Daytona regardless of circumstances are wrong-headed. NASCAR needs to decrease engine horsepower at the big tracks so that restrictor plates aren’t necessary. Slowing down the cars by making the restrictor plate hole smaller isn’t going to help. Most engine programs already have a totally separate plate engine program, so they’re putting a ton of effort into motors for just those four races already. Decrease the banking a bit, or make the banking different in different corners to introduce a ‘handling aspect’ that is present at Daytona and missing at Talladega. You can’t make the drivers ‘take it easy’, or suggest that changing the yellow line rule is going to solve the problem, because all it takes is a single freak coincidence. If Newman’s car hadn’t been right there, I suspect Carl’s car would have come down and hit more of the SAFER barrier instead of the catchfence. It looks from the most recent reports that the worst injury in the crowd is a broken jaw, but it could have been much, much worse.

What makes good racing? It’s the relative speed, not the absolute speed. If two cars are going nose-to-nose for the win, it’s just as exciting at 170 mph as it is at 190 mph. Does it really matter that much to you? It does to the drivers.

“I don’t know how I’d change this racing. I know it’s a spectacle for everybody and that’s great and all – but it’s not right to ask all these guys to come out and do this. What if the car goes up in the grandstands and kills 25 people? … I don’t know if I could live with myself if I ended up in the grandstands.” –Carl Edwards

NASCAR has a long, long history of being reactive. Here’s an instance in which NASCAR must be proactive and make changes before something really serious happens.

Here’s why I’m optimistic that might actually happen. I realized only recently that NASCAR hired Tom Gideon to be Director of Safety Initiatives for the R and D Center. Tom - someone whose integrity is respected throughout motorsports - was a prime moving force in motorsports safety at GM prior to his retirement. Tom will continue the tradition Steve Peterson started and, I’m sure, bring a number of his own initiatives to the job.

NOTE ADDED 4/28/09: A couple comments on some other reports: First, Reid Spencer reports that the roof flaps popped up in the wrong order; however, that should have very minimal - if any - impact on their function. Roof flaps are designed to disturb the flow of air over the roof of the car, as described above. The faster they deploy, the faster they can do their job. Even if one of the flaps ’stuck’, it’s hard to believe that anything would have prevented the combination of the 09’s wake and the 39’s rapid approach from launching the 99.

“I would assume they just adapted their principles and the locations of the old style car to this new style car when it comes to the roof flaps and the cowl flaps and the things like that,” said Newman, who emphasized the importance of keeping the cars on the pavement.

Second, Ryan Newman has a Bachelor’s degree in vehicle dynamics. That doesn’t make him the authority on everything scientific. To Reid Spencer’s credit, he doesn’t let Newman have the last word. He cites Robin Pemberton, who notes that the roof flaps on the new car are larger to take into account the differing aerodynamics, and Bernie Marcus, Ford’s aerodynamicist, who points out that they did “at least three wind-tunnel tests” looking at differences in roof flap function on the new car. Unless Newman has some prior knowledge, it is arrogant and ignorant to suggest that NASCAR just transferred the roof flap design from the old car to the new car without testing it.

Patrick Canupp, Director of Aerodynamics at Joe Gibbs Racing, notes that he hasn’t seen the test data, but that NASCAR usually tests new car designs “at Lockheed where the yaw table can put the car at nearly any yaw angle.", which means that they were in a position to look at roof flap function over a range of orientations of the car with respect to the airflow. Patrick also points out the most important element here - something that most media coverage glosses over. Engineering is not simple. You’re looking at complex machines and interactions between multiple complex machines. You can’t predict all the possible solutions and, even if you could, “that is still just a simulation of the actual event, whose detailed surface pressure history can be quite different,” Patrick notes.