Automobile drag coefficient

The drag coefficient is a common metric in automotive design, where designers strive to achieve a low coefficient. Minimizing drag is done to improve fuel efficiency at highway speeds, where aerodynamic effects represent a substantial fraction of the energy needed to keep the car moving. Indeed, aerodynamic drag increases as the square of speed. Aerodynamics are also of increasing concern to truck designers, where a lower drag coefficient translates directly into lower fuel costs.

About 60% of the power required to cruise at highway speeds is taken up overcoming air drag, and this increases very quickly at high speed. Therefore, a vehicle with substantially better aerodynamics will be much more fuel efficient.

C_dA

While designers pay attention to the overall shape of the automobile, they also bear in mind that reducing the frontal area of the shape helps reduce the drag. The combination of drag coefficient and area is C_dA (or C_xA), a multiplication of the C_d value by the area.

In aerodynamics, the product of some reference area (such as cross-sectional area, total surface area, or similar) and the drag coefficient is called drag area. In 2003, "Car and Driver" adapted this metric and adopted it as a more intuitive way to compare the aerodynamic efficiency of various automobiles. Average full-size passenger cars have a drag area of roughly 8.5 ft² (.79 m²). Reported drag area ranges from the 1999 Honda Insight at 5.1 ft² (.47 m²) to the 2003 Hummer H2 at 26.3 ft² (2.44 m²). The drag area of a bicycle is also in the range of 6.5-7.5 ft². [http://www.lafn.org/~dave/trans/energy/bicycle-energy.html#aero_drag (a bicycle's lower frontal area is offset by a higher drag coefficient)]

----Automobile examples of C_dA ft² are shown below: [ [http://www.mayfco.com/tbls.htm The Mayfield Company Homepage - Coefficient of Drag Tables and Curves ] ]

Selected photographs

parachute shape
Formula 1 car
0.9_-a_typical_bicycle plus cyclist
Caterham Seven
truck
Hummer H2, 2003

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0.51_-_Citroën 2CV due to the mudguard, despite the round roof
Volkswagen Beetle due to the mudguard, despite the round roof

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0.425_-_Duple 425 coach
Lamborghini Countach, 1974
Triumph Spitfire Mk IV, 1971-1980

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0.38_-_Mazda Miata, 1989

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0.38_-_Rolls-Royce Silver Seraph, 1998 [http://www.autozine.org/Graveyard/html/RR/Seraph.html]
Ford Capri Mk III, 1978-1986
Ferrari F50, 1996 high drag due to aerodynamic aids and cooling ducts.
Citroën DS, 1955, relative high drag despite the aerodynamic headlights, due to the rough windshield-roof transition.
Ferrari Testarossa, 1986
Honda Civic, 2001

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0.36_-_Citroën CX, 1974 (the car was named after the term for drag coefficient)
NSU Ro 80, 1967 despite the edgy front
Audi TT, 1998 [http://www.autozine.org/Graveyard/html/Audi/TT_Mk1.html] much drag despite smooth shape, the similar 2007 version has 0.30 [http://www.carpassion.com/de/forum/audi/22676-neue-audi-tt-pressetext.html]
Dodge Viper GTS
Pontiac Trans Am 1982 [http://www.youtube.com/watch?v=XI06RFqt4m4]
Ford Sierra, 1982, at least one combi in this gallery! The passat 2003, has 0.32
Ferrari F40, 1987
Chevrolet Camaro, 1995
Citroën SM, 1970 low drag despite edgy front

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0.32_-_Buick Riviera, 1995
Citroën AX, 1986 low drag due to down pulled hood
Citroën GS, 1970

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0.31_-_Renault 25, 1984
0.31_-_Saab Sonett III, 1970
Audi 100, 1983 smooth nose to flow transition
BMW E90, 2006
Porsche 996, 1997
0.30_-_Saab 92, 1947 - developed using wind tunnel testing

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0.29_-_Honda CRX HF 1988
Subaru XT, 1985
Lancia Dedra, 1990-1998
Lotus Elite, 1958
Rumpler Tropfenwagen, 1921 thin tires have low aerodynamic drag
Toyota Camry and sister model Lexus ES, 2005

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0.28_-_Porsche 997, 2004, and that with the wide tires

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0.28_-_Renault 25 TS, 1984, low drag despite its overall edgy shape
Saab 9-3, 2003
Audi A2, 1999 [http://www.autozine.org/Graveyard/html/Audi/A2_2000.html] . Note the Kammback shape.
Chevrolet Corvette Z06, 2006
Infiniti G35, 2002 (0.26 with "aero package")
Toyota Camry Hybrid, 2007
Mercedes-Benz W221 S-Class, 2006, low drag despite wheelarches an a large grill and a Mercedes sign in the airflow
0.26_-_Lexus LS 430, 2001 (0.25 with air suspension)
0.26_-_Toyota Prius, 2004
Vauxhall Calibra, 1989 even the transition from the bumpers to the head light is smooth
Audi A2 1.2 TDI, 2001, low drag despite wheelarches, due to down pulled hood and fastback
Honda Insight, 1999, low drag due to down pulled hood,fastback and wheel fairings
Tatra T77 a, 1935 low drag despite an edgy windscreen-roof transition.
General Motors EV1 (electric), 1996 having wheel fairings
Aptera Motors Typ-1 (2008 planned) having a tail, wheel fairings, smooth underbody, aerodynamic suspension

ee also

* Automotive aerodynamics
* Drag (physics)
* Drag equation

References

External links

* [http://rc.opelgt.org/indexcw.php Further 500 drag coefficients]
* [http://www.edmunds.com/advice/specialreports/articles/106954/article.html Improving Aerodynamics to Boost Fuel Economy]
* [http://tauac.typepad.com/ac/2007/05/tau_drag_reduct.html Tel Aviv University reduces drag on trucks by 10%]
* [http://physics.technion.ac.il/~rutman/car/Roll-down%20test.pdf Simple roll-down test for measuring Cd and Crr for cars and bikes]