How does downforce help a NASCAR race car?

­­Similar to the way that geometry and billiards are closely related, there's a lot of physics involved in NASCAR racing -- or any form of auto racing, actually. If you want an easy way to remember a few of the key factors in NASCAR, just remember the three D's -- downforce, drag, and drafting.

Downforce is created by the air moving over the top of the car and pushing it down toward the track surface. Downforce increases drag. Drag is the resisting force the vehicle experiences from air pushing against it and the additional weight that downforce creates. Drivers can lessen the amount of drag that they experience on the race track by drafting. Drafting is when Driver B tucks the nose of his car almost underneath the rear bumper of Driver A's car to improve air flow over both cars. Sometimes you will hear this maneuver called "running nose-to-tail."

When it comes to sticking the tires to the track in the turns, downforce is definitely the most important of the three D's. But downforce isn't quite as important on the long, straight stretches of pavement that immediately follow the turns. That's where drivers want slightly less downforce and as a result, slightly less drag, too. It's a delicate balance, really. An absence -- or even a significant reduction -- of downforce might cause the car to lift off of the track surface, sort of like an airplane taking off. So how does the design of a NASCAR prevent this from happening?

The front bumper of a NASCAR race car is very low and also wide. It's really more of an air dam than a bumper. It directs the moving air over the top of the car rather than underneath. This creates an area of low pressure underneath the car and an area of high pressure on top of the car. This is called negative lift, and it's exactly the opposite of how an airplane works. Where the air pushes up on an airplane wing, it pushes down on a race car.

The idea is to get most of the air to flow over the top of the car to maximize downforce. That's where the front end fascia comes in. The nose of the car is as low as possible and the front fenders are flared wide to push the air up and over the car.

The problem, as you may have already figured out, is that this low nose with flared fenders presents a lot of front surface area to push through the air. As you can imagine, this creates a lot of drag is created. If you want a first hand demonstration of what drag is, the next time you're on the highway, try sticking your hand out of the car window with your palm facing forward. That's what drag feels like. Next, tilt your hand 90 degrees so your palm faces the road. You'll feel the difference immediately. With less surface area facing the wind, the air can slip around your hand, allowing it to cut through the air much easier. You can also vary the angle of your hand to cause lift (causing your hand to rise) or downforce (causing your hand to dip). So, drag is easy enough to fine tune with your hand, but what about fine tuning an entire race car? Especially one that travels at speeds at or near 200 mph (322 km/h) on varying track surfaces and in varying weather conditions.

­Balancing downforce and drag within the confines of the NASCAR-sanctioned car body is a trick the teams simply have to deal with in the best way they can. One way teams make the most of these on-track forces is by bringing the third D into the mix -- drafting. Up next, we'll take a closer look at drafting.

­Drivers always say they want more downforce in the turns. What that really means is they want maximum stickiness in the corners and minimum drag on the straights. That's difficult to do -- especially when the adjustments teams are allowed to make the NASCAR Sprint Cup body are so minor.

There are, however, a few fine tuning adjustments the teams can make, like adjusting the angle of the rear spoiler. The steeper the angle of the rear wing, the more downforce it can add to the rear end of the car. This is what keeps the rear tires firmly planted on the pavement. At the front end, a piece called a "splitter" performs a similar role to keep the front wheels stuck to the ground. The splitter is the component that you see at the leading edge of a NASCAR race car. It runs the full width of the car, is adjustable and often appears as if it's low enough to scrape the track surface.

There are times when teams choose to go for as much downforce as possible. For instance, on road-courses with plenty of turns and very few long, straight sections of track. By making slight changes to the rear wing and the front splitter, downforce can be maximized, increasing the car's grip in the turns.

­But the majority of races in the NASCAR schedule are on high-sped oval tracks. So, we're back to our question of how to balance downforce and drag. Drafting can help. Drafting on the track allows the vehicle following the lead car to lessen its drag. The air flowing over the front car carries right over the windshield and roof of the second car, too. That's great for the second car and any car that happens to be drafting behind it, but what does the lead car get out of all of this? The lead car in a drafting pair gets something out of the maneuver, too. The second car cuts down on the pressure drag of the front car. You can think of pressure drag as a low pressure wake left behind by the car as it speeds along the track. This type of wake, however, actually pulls the vehicle backwards. By eliminating the pressure drag on the lead car, the two drafting cars can gain up to a 5 mph (8 km/h) advantage over a car turning laps solo [source: Schirber].

­Once the little tweaks for just the right amount of downforce have been made, the driver has a good feel for the race car (and the track) and maybe even practiced his drafting skills, there's really only one way to take full advantage of the aerodynamics during the race: Don't crash. That's logical advice, right? When each NASCAR Sprint Cup team has made the tiniest of adjustments to the vehicle's aerodynamics in an effort to gain an advantage of just a few hundredths of a second per lap, something as small as a crumpled front corner can end their hopes for a victory on that day.