Automobile drag coefficient
The drag coefficient is a common metric in automotive design pertaining to aerodynamic effects. As aerodynamic drag increases as the square of speed, a low value is preferable to a high one. As about 60% of the power required to cruise at highway speeds is used to overcome aerodynamic effects, minimizing drag translates directly into improved fuel efficiency.
For the same reason aerodynamics are of increasing concern to truck designers, where greater surface area presents substantial potential savings in fuel costs.
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[edit] Reducing drag
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, such as racing cars. Notably, it is important to minimize lift, hence increasing downforce, to avoid the car becoming airborne. Increasing the downforce pushes the car down onto the race track—allowing higher cornering speed. It is also important to maximize aerodynamic stability: some racing cars have tested well at particular "attack angles", yet performed catastrophically, e.g. flipping over, when hitting a bump or experiencing turbulence from other vehicles (most notably the Mercedes-Benz CLR). For best cornering and racing performance, as required in Formula One cars, downforce and stability are crucial and these cars must attempt to maximize downforce and maintain stability while attempting to minimize the overall Cd value.
[edit] Typical drag coefficients
The average modern automobile achieves a drag coefficient of between 0.30 and 0.35. SUVs, with their typically boxy shapes and larger frontal area, typically achieve a Cd of 0.35–0.45. A very gently inclined windshield gives a lower drag coefficient but has safety disadvantages, including reduced driver visibility. Certain cars can achieve figures of 0.25–0.30, although sometimes designers deliberately increase drag to reduce lift.
Some examples of Cd follow. Figures given are generally for the basic model. Some "high performance" models may actually have higher drag, due to wider tires and extra spoilers.
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[edit] Drag area
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 - drag area - is represented as CdA (or CxA), a multiplication of the Cd value by the area.
The term drag area derives from aerodynamics, where it is the product of some reference area (such as cross-sectional area, total surface area, or similar) and the drag coefficient. In 2003, Car and Driver magazine adopted this metric as a more intuitive way to compare the aerodynamic efficiency of various automobiles.
Average full-size passenger cars have a drag area of roughly 8.50 sq ft (0.790 m2). Reported drag areas range from the 1999 Honda Insight at 5.1 sq ft (0.47 m2) to the 2003 Hummer H2 at 26.5 sq ft (2.46 m2). The drag area of a bicycle is also in the range of 6.5–7.5 sq ft (0.60–0.70 m2).[82]
| CdA | Automobile model |
|---|---|
| 2.50 sq ft (0.232 m2) | 1986 Twike[84] |
| 2.69 sq ft (0.250 m2) | 2009 Loremo |
| 3.00 sq ft (0.279 m2) | 2011 Volkswagen XL1 |
| 3.95 sq ft (0.367 m2) | 1996 GM EV1 |
| 5.10 sq ft (0.474 m2) | 1999 Honda Insight |
| 5.40 sq ft (0.502 m2) | 1989 Opel Calibra |
| 5.71 sq ft (0.530 m2) | 1990 Honda CR-X Si |
| 5.74 sq ft (0.533 m2) | 2002 Acura NSX |
| 5.76 sq ft (0.535 m2) | 1968 Toyota 2000GT |
| 5.80 sq ft (0.539 m2) | 1986 Toyota MR2 |
| 5.81 sq ft (0.540 m2) | 1989 Mitsubishi Eclipse GSX |
| 5.86 sq ft (0.544 m2) | 2001 Audi A2 1.2 TDI 3L |
| 5.88 sq ft (0.546 m2) | 1990 Nissan 240SX / 200SX / 180SX |
| 5.92 sq ft (0.550 m2) | 1994 Porsche 911 Speedster |
| 5.95 sq ft (0.553 m2) | 1990 Mazda RX7 |
| 6.00 sq ft (0.557 m2) | 1992 Subaru SVX |
| 6.00 sq ft (0.557 m2) | 1970 Lamborghini Miura |
| 6.08 sq ft (0.565 m2) | 2008 Nissan GTR |
| 6.13 sq ft (0.569 m2) | 1991 Acura NSX |
| 6.17 sq ft (0.573 m2) | 1995 Lamborghini Diablo |
| 6.24 sq ft (0.580 m2) | 2004 Toyota Prius |
| 6.27 sq ft (0.583 m2) | 1986 Porsche 911 Carrera |
| 6.27 sq ft (0.583 m2) | 1992 Chevrolet Corvette |
| 6.35 sq ft (0.590 m2) | 1999 Lotus Elise |
| 6.37 sq ft (0.592 m2) | 2000 Vauxhall VX220 N/A |
| 6.40 sq ft (0.595 m2) | 1990 Lotus Esprit |
| 6.41 sq ft (0.596 m2) | 2003 Smart Roadster Coupé |
| 6.54 sq ft (0.608 m2) | 1991 Saturn Sports Coupe |
| 6.57 sq ft (0.610 m2) | 1985 Chevrolet Corvette |
| 6.63 sq ft (0.616 m2) | 2001 Audi A2 |
| 6.63 sq ft (0.616 m2) | 1989 Ford Thunderbird |
| 6.66 sq ft (0.619 m2) | 1996 Citroën Saxo |
| 6.77 sq ft (0.629 m2) | 1995 BMW M3 |
| 6.79 sq ft (0.631 m2) | 1993 Toyota Corolla DX |
| 6.80 sq ft (0.632 m2) | 2007 BMW 335i Coupe |
| 6.81 sq ft (0.633 m2) | 1991 Subaru Legacy |
| 6.90 sq ft (0.641 m2) | 1993 Saturn Wagon |
| 6.93 sq ft (0.644 m2) | 1982 Delorean DMC-12 |
| 6.94 sq ft (0.645 m2) | 2003 Smart Roadster |
| 6.96 sq ft (0.647 m2) | 1988 Porsche 944 S |
| 6.96 sq ft (0.647 m2) | 1995 Chevrolet Lumina LS |
| 7.02 sq ft (0.652 m2) | 1992 BMW 325I |
| 7.04 sq ft (0.654 m2) | 1991 Honda Civic EX |
| 7.06 sq ft (0.656 m2) | 2004 Vauxhall VX220 Turbo |
| 7.10 sq ft (0.660 m2) | 1995 Saab 900 |
| 7.14 sq ft (0.663 m2) | 1995 Subaru Legacy L |
| 7.20 sq ft (0.669 m2) | 1995 Nissan Maxima GLE |
| 7.34 sq ft (0.682 m2) | 2001 Honda Civic |
| 7.39 sq ft (0.687 m2) | 1994 Honda Accord EX |
| 7.48 sq ft (0.695 m2) | 1993 Chevrolet Camaro Z28 |
| 7.57 sq ft (0.703 m2) | 1992 Toyota Camry |
| 7.69 sq ft (0.714 m2) | 1994 Chrysler LHS |
| 7.72 sq ft (0.717 m2) | 1993 Subaru Impreza |
| 8.02 sq ft (0.745 m2) | 2005 Bugatti Veyron |
| 8.70 sq ft (0.808 m2) | 1990 Volvo 740 Turbo |
| 8.70 sq ft (0.808 m2) | 1992 Ford Crown Victoria |
| 8.71 sq ft (0.809 m2) | 1991 Buick LeSabre Limited |
| 9.54 sq ft (0.886 m2) | 1992 Chevrolet Caprice Wagon |
| 10.7 sq ft (0.99 m2) | 1992 Chevrolet Blazer |
| 11.6 sq ft (1.08 m2) | 2005 Ford Escape Hybrid |
| 11.7 sq ft (1.09 m2) | 1993 Jeep Grand Cherokee |
| 16.8 sq ft (1.56 m2) | 2006 Hummer H3 |
| 17.4 sq ft (1.62 m2) | 1995 Land Rover Discovery |
| 26.5 sq ft (2.46 m2) | 2003 Hummer H2 |
[edit] See also
[edit] References
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