Basics of auto aerodynamics

By Allen Penticoff

DrafCoeffI have been an “airplane nut” since first grade. I drew pictures of airplanes then, and throughout my school years I did any project possible from book reports to science fair projects on things to do with airplanes. Part of this fascination involved the study of aerodynamics. Aerodynamics is the science of air flowing over objects; not just airplanes, but cars, trucks, bicycles and even humans in motion.

Objects cause resistance to air passing over or around them, whether the object is stationary and the wind is in motion, or if the object itself is moving through the air. All objects offer some resistance to passing through air, some more so than others. When the air passing over the object becomes turbulent, this creates “drag”. Drag is a measurable resistance to airflow. Many factors affect how much drag an object creates, none more so than its shape. A smooth tapered object has less drag while a boxy shaped object has more drag – considerably more. Compare a bird to a camping trailer.

It takes energy to put an object in motion and overcome the force of drag. And the faster you go, the more drag there is. Drag increases at the square of the doubling of velocity. Fancy language, so here are simple numbers to explain. If your car’s drag at 30 mph is 100 pounds – at 60 mph drag will be 10,000 pounds! These are not actual drag numbers, but it illustrates the point that the faster you go, the more energy it takes (horsepower) and fuel/electricity that needs to be consumed to create that energy. Since rolling resistance and other efficiency factors come into play as well, a steady 45 miles per hour is the most efficient speed for most vehicles.

For automakers, to get cars to meet new fuel efficiency standards, many of the increases in mpg come in the form of improved aerodynamics. There is a number than holds constant for any shape, and that is its coefficient of drag (Cd). While rarely shown in automakers brochures, some auto magazines doing in-depth reviews will reveal a vehicle’s Cd – the lower the Cd, the less drag or resistance it has. Some examples: Tesla S and Toyota Prius (gen 4) have a Cd of .24. A boxy Jeep Wrangler has more than double the drag with a Cd of .58. A person sitting upright riding a bicycle has a Cd of 1.0, while the fastest human, Usain Bolt, has a Cd of 1.2. This is why running or riding a bike into the wind takes so much more effort – we are not very aerodynamic. Racing bicyclists do everything they can to reduce their Cd since the amount of energy they can produce for long periods is very limited.

Smoothness of the object’s surface has an effect on drag. So airplanes have smooth wings and ski jumpers have smooth tight suits. For vehicles, the air passing under the car or truck, through the radiator and around the engine all create drag too. To this end, manufacturers limit the size of the radiator and put “air dams” under the front bumper to deflect the air around – and kill aerodynamic lift, which itself causes drag. Electric cars, lacking many of the components that create aerodynamic drag are easier to push through the air – thus increasing their efficiency.

When it comes to increased efficiency, reducing drag is nearly free compared to making a more efficient drivetrain. Big and boxy have been an American standard for a long time – yes our boxy square vehicles offer more room and carrying capacity but the price is fuel consumption, imported oil, and air pollution. In the next Mr. Green Car I’ll take a look at what got me started on this topic – having seen a very streamlined camping trailer – the trailer of my aerodynamic dreams.

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