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5 Facts: Vehicle Aerodynamics
1. Aerodynamic Forces
Drag and lift are two aerodynamic forces acting on any body traveling through the air whether it's an airplane, car, or truck. Unlike aircraft where both lift and drag are pretty much equally important, drag is far more important for vehicles, unless it's a race car or high performance sports car traveling at very high speeds. For the latter, lift is to be avoided and thus aerodynamic devices like air dams, spoilers, and wings are used to provide downward forces so the vehicle hugs the road.
2. Factors Affecting Drag
Overcoming drag, the force that opposes forward motion, represents probably the largest load on the engine and thus the largest contributor to fuel usage and fuel economy. Overcoming drag, especially at higher speeds, requires much more power than other power consumers like tire rolling resistance, engine and powertrain friction, and powering accessories. Drag force is a function of the vehicle's drag coefficient (Cd), frontal area (A), air density, and velocity (V) squared. While nothing can be done about reducing air density, the other factors can be reduced to improve fuel economy.
3. Aiming for a Low Cd
The coefficient of drag is a measure of how easily a vehicle slides through the air. A brick has Cd of 1, about the same as a box van or boxy motorhome. Since the early 1930s and the trend-setting Chrysler Airflows, manufacturers have reduced drag coefficients with much more streamlined designs. Many modern cars have a Cd of around 0.30, or about one-third less than a brick or a Ford Model T. The Toyota Prius has a Cd of 0.26. Besides having larger frontal areas, pickups and SUVs have a larger Cd - typically around 0.40 to 0.43 for trucks and 0.30 to 0.41 for SUVs. Both styling and cooling can affect the Cd. For example, requiring more airflow through the radiator and inefficient airflow paths can result in a Cd penalty. Reducing the Cd of a car by 0.01 can result in a 0.2 mpg improvement in fuel economy.
4. Speed Increases Drag
Since drag depends on velocity squared, driving at 70 mph results in about four times as much drag as driving at 35 mph. While drag isn't that important in low speed city traffic, increasing speed on the highway can have a dramatic negative impact on fuel economy. Merely increasing speed from 55 mph to 65 mph increases drag by about 40 percent, resulting in a 10 to 15 percent increase in fuel consumption.
5. Improving Your Vehicle's Aerodynamics
While you can't improve the basic aerodynamics of your vehicle without considerable expense, there are things you can do, or at the least not do to degrade the aerodynamics. For example, use air conditioning rather than rolling down windows when traveling at highway speeds. Today's vehicles are designed to be most aerodynamically efficient with their windows rolled up. Adding a hard or soft tonneau cover over a pickup bed can improve the aerodynamics of a pickup and bring a 0.1 to 0.3 mpg improvement. In most cases, dropping the tailgate or using a mesh tailgate has little effect. Adding items with a poor Cd and large frontal areas can reduce fuel economy. A blunt bug shield on the hood will increase drag. Likewise, using wider wheels will also increase drag. Since roof racks have poor drag coefficients, be sure to remove these when they're not needed. If you need to carry items on the roof then try to use a streamlined cargo carrier. Finally, increased wind noise is often a sign of increased drag that's reducing your vehicle's aerodynamics, so find the culprit if this occurs.
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