The drag coefficient (C_d or C_x) is a number that describes a characteristic amount of aerodynamic drag caused by fluid flow, used in the drag equation. Two objects of the same frontal area moving at the same speed through a fluid will experience a drag force proportional to their C_d numbers. Coefficients for rough unstreamlined objects can be 1 or more, for smooth object much less.

C_d in automobiles.

The drag coefficient is a common metric in automobile 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 with the square of speed. Aerodynamics are also of increasing concern to truck designers, where a lower drag coefficient translates directly into lower fuel costs.

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, measured in m².

Drag in sports and racing cars

Reducing drag is also a factor in sports car design, where fuel efficiency is less of a factor, but where low drag helps a car achieve a high top speed. However, there are other important aspects of aerodynamics that affect cars designed for high speed, including racing cars. Notably, it is important to minimize lift, hence increasing downforce, to avoid the car ever becoming airborne. Also it is important to maximize aerodynamic stability: some racing cars have tested well at particular "attack angles", yet performed catastrophically, i.e. flipping over, when hitting a bump or experiencing turbulence from other vehicles. For best cornering and racing performance, as required in Formula 1 cars, downforce and stability are crucial and these cars have very high C_d values.

Typical values and examples

The typical modern automobile achieves a drag coefficient of between 0.30 and 0.35. SUVs, with their larger, flatter shapes, typically achieve a Cd of 0.35-0.45. Certain cars, notably can achieve figures of 0.25-0.30, although sometimes designers deliberately increase drag, in favour of reducing lift.

Some notable examples:

• 0.9 - a typical bicycle plus cyclist
• 0.7 to 1.1 - typical values for a Formula 1 car (wing settings change for each circuit)
• at least 0.6 - a typical truck
• 0.51 - Citroën 2CV
• 0.42 - Lamborghini Countach, 1974
• 0.372 - Ferrari F50, 1996
• 0.36 - Citroën DS, 1955
• 0.36 - Ferrari Testarossa, 1986
• 0.36 - Citroën CX, 1974 (the car was named after the term for drag coefficient)
• 0.34 - Ford Sierra, 1982
• 0.34 - Ferrari F40, 1987
• 0.31 - Citroën GSA, 1980
• 0.30 - Audi 100, 1983
• 0.30 - Porsche 996, 1997
• 0.29 - Lotus Elite, 1958
• 0.28 - Porsche 997, 2004
• 0.27 - Infiniti G35, 2002 (0.26 with "aero package")
• 0.26 - Toyota Prius, 2004
• 0.25 - Honda Insight, 1999
• 0.137 Ford Probe V prototype, 1985

Figures given are generally for the basic model. Faster and more luxurious models often have higher drag, thanks to wider tyres and extra spoilers.

Data for more cars:

• Mayfield Company: Coefficient of Drag for Selected Vehicles

External link

• A. Filippone's Advanced Topics in Aerodynamics: Drag
• Danish Wind Industry Association: Aerodynamics of Wind Turbines: Drag