Aerodynamic drag is the force of air along the length of the traveling car, opposing the car's force. As the car cuts a path through the air, some air molecules collide with the front bumper, producing resistance.
Other molecules flow along the hood, only to come up against the windshield -- another source of drag. The air that glides smoothly over the roof grows turbulent above the rear window and behind the car, exerting a backward force on the vehicle.
Speed, air density, and car size, shape and design all determine the magnitude of a car's drag force.
"A faster car experiences more drag because it has to push air molecules out of the way faster," Diandra Leslie-Pelecky explains in her book, "The Physics of NASCAR." "Dense air increases drag because there are more air molecules hitting each area on the car. A larger cross-sectional area increases drag because more air molecules have to be moved out of the way" [source: Leslie-Pelecky].
Drag is the major obstacle to acceleration and racing speed. A passenger car driving on the highway spends an estimated 60 percent of its energy overcoming air drag, a far greater percentage than tire friction and the energy needs of the drive train itself [source: Beauchamp].
Defeating drag was the first major focus of automotive aerodynamics, beginning in the 1960s. It is still the most important variable in racing conditions that place a smaller premium on downforce, such as longer tracks with more straightaways.
The sleek lines, tilted windshields and rounded corners of modern race cars -- and passenger cars for that matter -- are designed to minimize drag. But the quest to engineer racecars with high net downforce sometimes leads to additional drag. The rear spoiler found on NASCAR vehicles is a case in point: It increases drag by distributing weight from the front to the back of the car [source: Circle Track]. Aerodynamics remains a vibrant and young field of engineering, with many innovations still to come down the road.
To keep up with the latest innovations in aerodynamics, visit the links below.
Related HowStuffWorks Articles
- Associated Press. "Going With the Flow." (Accessed 12/15/08) http://hosted.ap.org/specials/interactives/nascar2005/aerodynamics/aerodynamics.swf
- Beauchamp, Warren. "Passenger Car Aerodynamics." (Accessed 12/15/08) http://www.recumbents.com/car_aerodynamics/
- BMW. "BMW Side Skirts." (Accessed 12/17/08) http://www.bmwsideskirts.com/history.php
- Boone, Jerry F. "Bad Air? The Physics Behind Auto Racing." Stock Car Racing. (Accessed 12/14/08) http://www.stockcarracing.com/techarticles/scrp_0301_the_physics_behind_auto_racing/index.html
- Boone, Jerry F. "Race Car Aerodynamics - Short Track Auto." Stock Car Racing. (Accessed 12/14/08) http://www.stockcarracing.com/techarticles/scrp_0702_race_car_aerodynamics/index.html
- Circle Track. "Aerodynamics - Stock Car Aero Defined." (Accessed 12/14/08) http://www.circletrack.com/techarticles/0304_aerodynamics_tech_definitions/index.html
- Cislunar Aerospace. "Aerodynamics and Race Cars." K-8 Aeronautics Internet Textbook. (Accessed 12/15/08) http://wings.avkids.com/Book/Sports/advanced/racecar-01.html
- ESPN. "Aero Push." (Accessed 12/14/08) http://sports.espn.go.com/rpm/nascar/icons/news/story?id=3426389
- Hiestand, Michael. "ESPN Will Show the Unseen of NASCAR." USA Today, July 24, 2007. (Accessed 12/15/08) http://www.usatoday.com/sports/columnist/hiestand-tv/2007-07-24-ESPN-NASCAR_N.htm
- Leslie-Pelecky, Diandra. The Physics of NASCAR: How to Make Steel + Gas + Rubber = Speed. Dutton: 2008. (Accessed 12/17/08 via Google Books) http://books.google.com/books?id=OAK3yFlHoTAC
- Tierney, John. "NASCAR's Screech and Slam? It's All Aerodynamics." New York Times, February 12, 2008. (Accessed 12/15/08) http://www.nytimes.com/2008/02/12/science/12tier.html?pagewanted=all
- Turner, Charlie. "Drafting and Aero." Bench Racing with Steve and Charlie. (Accessed 12/16/08) http://benchracing.typepad.com/bench_racing_with_steve_a/drafting_and_aero.html