The Three Ds of NASCAR Racing
Aerodynamics plays a critical role in racing. Good design can stick a race car to the track better and allow it to move faster through the air. It also deals with one of the most basic tenets of physics in racing -- the faster a car goes, the more effect air will have on it and work to slow it down. Given that inescapable law, NASCAR teams design, redesign, refine, tweak and nudge their cars to control what happens when several thousand pounds of vehicle rip through the air at speeds often approaching 200 mph (322 kilometers per hour).
And that level of speed is critical. Drafting comes into play when drivers begin pushing the limits of their cars and the engines but are still looking for more. Two forces play key roles -- downforce and drag.
As air moves faster it creates low-pressure systems. This physical law is what allows planes to fly. A wing is designed so airflow creates a low-pressure system over the top of the wing and a relative high-pressure system underneath. That pressure differential, low on top and high on the bottom, creates lift that allows the plane to soar through the air. This pressure differential also explains why windows blow out of well-sealed homes during a hurricane. As the wind whips around the house the pressure outside of the house is lowered, and the pressure inside the house becomes comparatively higher. Soon the pressure differential is enough for the windows to shatter out from the higher interior pressure.
Race cars essentially turn the airplane principle upside down. Air flows around the car as it screams around the track and each minute flow of wind along the hood, windshield, fairings, doors, spoilers and air dams has a subtle effect of increasing or decreasing the air pressure on every surface of the car. But, overall, there is more pressure coming over the top of the car than underneath. This sucks the car downward toward the track surface, allowing it better handling on the turns and a more stable ride. This is referred to as negative lift in aerodynamic-speak and racers typically seek to increase this effect. At speed, downforce can add the equivalent of 1,650 to 1,750 pounds (748 to 794 kilograms) of downforce to the tires. On smaller tracks with fewer straightaways a car is tuned to have even more downforce to keep it stuck to the pavement and handle the turns better.
Drag is the downside of downforce. There are essentially two kinds of drag -- friction and pressure. Friction drag is the contact of air and the object moving through it, like a race car. Pressure drag has to do with the low pressure created as the air moves around the object. NASCAR has become one of racing's leaders in tweaking drag and constantly strives to reduce this effect on its cars. In fact, a drag reduction of about a half-percent is seen as a significant gain in NASCAR. Kurt Romberg, chief aerodynamicist at Hendrick Motorsports, said as little as one percent drop in drag is worth an improvement of about 10 positions on the starting grid at Daytona. "You work the numbers as much as you can to affect the end product," Romberg said.
Hendrick counts drivers Jimmie Johnson, Jeff Gordon, Dale Earnhardt Jr. and Mark Martin among the drivers fielded from their North Carolina headquarters. Romberg works on all their cars.
The draft is when downforce and drag, as well as a few other factors, come together on the track. Drag and downforce are affected by airflow coming off of cars driving close by -- usually within a car length, though effects can extend up to three car lengths away. In common drafting situations, the lead car breaks through the air in front of the line, or pack, and reduces the friction drag for trailing cars. But the trailing cars play a role as well. In addition to friction drag, there is pressure drag created by the low pressure behind the leading car. By staying close to the lead car the trailing car interrupts that low-pressure system and cuts down on its effects. The end result is about a 5-mile per hour (8-kilometer per hour) increase in speed for each car in the draft. This increase makes having a drafting partner at superspeedways critical.
Now that you know the basics of drafting, it's time to discover how NASCAR drivers use the air to their advantage.