Sunday, February 14, 2010

NASCAR Basics: Window Strips & Roof Flaps

An article at explains how the new window strips are supposed to keep the Cup cars from going airborne at Daytona and Talladega...
"If you ride down the highway and you put your hand out the window and you hold your hand in the direction the car is traveling, there's not a lot of resistance," Darby said. "But turn your hand sidewise and it'll come back and slap you in the forehead. It's a very similar effect."

"If you picture the strip on the car, and you're going down the straightaway, it really doesn't have that much effect. But as the car starts to spin and the strip now sees the air pressure, it does two things. It slows the rotation of the car down, and as the air comes across it, it packs air on top of the trunk lid to help push the car back to the ground before it lifts."

And the reason why the trunk fin is on the left side of the window is also inherently logical.

"It's actually lined up with the left edge of the left roof flap," Darby said. "Because as the air hits that fin, part of what happens is that the airflow travels up to the flap. There's a misconception about the flap. A lot of people think that air pressure blows them open. It's quite the contrary.

"The flaps are contoured on the top and it works much like an airplane wing to where the airspeed that travels over the flap actually pulls it open. If we can put more air to the flap and accelerate where it goes across it, then it's going to open faster and stay up stronger. So part of the air helps deploy the flap and the rest of the air -- because the trunk lid is wide and you have more surface area to come across -- helps hold the car back down on the ground."

Even though the trunk fin seems to be the best solution for Daytona and Talladega at this point, Darby said the process is always evolving -- and more improvements may come from additional wind tunnel testing.


MyMaracas said...

It's unbelievable that the track is in such poor shape. I mean, potholes? Really?

The science and technology behind racing is fascinating. (Well, for us geekesses, anyway.) I enjoyed the explanation of how the window strips and roof flaps work.

And yeah. I secretly want a gorilla lawn ornament too.

fdtate said...

For more on the pothole, see the post above.

Chuckles8 said...

A slight clarification: It isn't the airspeed moving over the flap that opens it, but the angle of the air.

Any slightly curved surface, like a hood or roof, will generate downforce at a certain angle. As that angle is rotated up, the surface first becomes neutral to the approaching air and then begins generating lift and finally generates drag. It's the angle that generates lift that's the problem, and that helps cause rollovers.

So, as a car gets turned in a spin, and the wind angle moving over the curved surface gets more narrow (flatter) it goes from creating downforce to creating lift, and it lifts the flaps, and the now open flaps interfer with the lifting airflow, effectively killing it. That's why you'll see the flaps flutter and open and close in slo-mo video.