Dirt Car Aerodynamics - Aero Importance

When it comes to aerodynamics, there is a simple rule that many of the smartest fabricators and body hangers live by: If you have to let off the gas to make it through the turns, then you need more downforce. Feel free to use this advice.

Just because you race dirt and you spend half of every lap with the car slung out sideways doesn't mean you can't use the laws of aerodynamics to help you get around the track a little bit faster. The good news is that air behaves consistently when it is moved, and even though most dirt track racers don't have a budget for wind tunnel time like the Nextel Cup guys, you can still take advantage of a few basic laws of aerodynamics when hanging your body.

In the interest of not boring you to tears, we will dispense with the physics, fluid motion theory and all the other stuff. Understanding the theory, after all, is a lot less important than knowing how to implement it on your racecar, so we'll get right to the good stuff. It's like Grandma says, "Talking about cooking don't get the bread baked."

Those big flares above the fenders aren't extra material that the team didn't bother to trim away. They are called "elephant ears" and their purpose, according to chassis builder Mark Richards of Rocket Chassis, is to direct the air away from the wheels but then direct it back to the body so that it contacts the rear spoiler. "We used to try to make them ourselves with sheet metal," he says, "but they never worked as well as the new molded ones from the manufacturers like Five Star and Performance Bodies."

Richards adds that new splitter designs on noses (at the bottom) do a good job of adding downforce at the front of the car. Notice how the skirt has been trimmed to account for body roll to the right side in the turns. "Also," Richards says, "you want the nose to have a nice, gradual angle to the hood. Nothing too sharp. A nice, long fender radius is good, too. Anything you can do to smooth the lines and limit turbulence."

"The more angle you can put on the roof, the better," says Richards. "There was once a theory that tapering the roof down in the back helps direct air onto the spoiler. But the problem with that is we have no windows, so once the air unattaches itself from the roof you cannot control where it goes. You have to use the roof in a manner like the wing on a Sprint Car. Plus, raising the roof in the rear allows more room for raising the air pressure on the rear area of the deck just in front of the spoiler."

Angling the roof raises the air pressure above the roof, but you can also lower the air pressure below the roof to gain even more downforce by angling the upper deck of the body down, like you can see here. Some prefer to drop the deck by two or three inches just behind the hood, but this can be detrimental in some cases because a sharp drop will cause air turbulence which isn't predictable. The gradual dip in the deck ensures smooth airflow all the way back to the spoiler.