How NASCAR Drafting Works

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.

­Drafting's importance as one of the techniques and strategies to win a race has as much to with the driver as the car. NASCAR imposes strict rules on engine power, engine components, body design and composition so no one team can gain too much of an advantage. The end result is a game of very small numbers and percentages and those numbers play out in body design and driving skill -- which, of course, includes drafting.

Daytona and Talladega are two superspeedway racing venues where a draft-savvy driver can really shine. Both tracks offer banked curves and long straightaways where a driver can push a car to its upper limits. It's for this reason that tracks require restrictor plates and other safety measures to limit top speeds. Because of this, capable drafting is often a key to success.

Traditional restrictor plates drop the overall power of the cars by about 300 horsepower [source: Boone]. Plates were first used in the 1970s to even the playing field between larger and smaller engines, as well as a safety measure as the tracks became larger and the cars became more powerful. In 2004, NASCAR driver Rusty Wallace tested a car at Talladega without a restrictor plate and reached a reported top speed of 228 miles per hour (367 kilometers per hour) on the backstretch and had a one-lap average speed of 221 mph. Wallace described the experience as "insane" [source: NASCAR.com].

Despite the use of restrictor plates, NASCAR drivers often reach speeds of more than 180 miles per hour (290 kilometers per hour). This is in large part due to drafting where an understanding of the physics involved allow drivers to help reduce drag and gain a few more miles per hour (kilometers per hour) in the process.

Each track on the NASCAR circuit, including the superspeedways, has its own character. At smaller tracks, like Bristol Motor Speedway in Tennessee where drivers turn 16-second laps, the constant curve of the track means less opportunity to draft. Here, engines are allowed to run full power and downforce becomes critical. With more than 800 horsepower being produced in some races, downforce is maximized to stick the car to the track. At superspeedways, downforce is purposely reduced since the track layout requires higher speeds on the straightaways. The adjustments become a bit of a balancing game: Less downforce means greater straightaway speed, but less grip in the turns, and more downforce means improved grip in the turns, but slower straightaway speeds. As you can imagine, finding the perfect adjustment can prove difficult.

Until recently, the mix of track layout, engines, tires, drivers and strategies, drag and downforce required most teams to field several cars, especially for Sprint Cup series competition. This, in turn, led to the introduction of NASCAR's Car of Tomorrow -- a race car designed to be competitive on all NASCAR tracks.

NASCAR's Car of Tomorrow was introduced in 2007 and raced in just 16 events that first year. The initial plan called for the design to be featured in 26 of the 36 races on the 2008 NASCAR schedule and then bumped up to permanent use in the 2009 racing season. Instead, the new design was raced in all 36 races on the 2008 schedule and is currently the only car design NASCAR allows to race.

Dr. Jerre Hill, a professor at the University of North Carolina at Charlotte, explained how NASCAR's Car of Tomorrow standardizes the body of the vehicle, and adds a wing to the rear and an air splitter to the front.

The rear wing replaces the traditional spoiler and the front splitter is a thin horizontal piece under the front bumper that increases downforce. Teams will be allowed to adjust the angle of attack on the wing and the position of the front splitter to custom tune the car's performance on different tracks. Hill said the changes have a serious effect on the car's wake characteristics and therefore on the drafting potential of the car. The end result is a car that's harder to pass and harder to draft.

Up next, we'll learn how strategy plays a role in the NASCAR draft.

Drafting strategy involves more than simply knowing where to place your car on the track, and it often has less to do with aerodynamics and more to do with driver's knowledge of the competition's mind. Drafting strategy is where a race becomes a mental challenge as well as a battle of speed and guts. The lead driver can step on the brakes, come within inches of the following driver, and deprive them of the air needed to cool the engine. And just like that, one competitor goes down. The following driver can also pull a similar tactic, inching up on the car ahead in order to disrupt the flow of air over the lead car's body. Remember, that airflow is critical for keeping the lead car's tires stuck to the track surface. Without it, and maybe with a little help in the form of a bump, the lead car can lose traction, skid into an outside lane and quickly drop 10 places (or more) during a race. Teams, sometimes official but often an informal collaboration, use the power of drafting to rocket members past the competition, vie for the best track real estate and even steal the race lead at a moment's notice. All's fair, it seems, in love, war and drafting.

The two-car draft is the most basic draft pattern and the one most often used by a team. Pulling within a car length of a lead car benefits the trailing car by reducing drag. That same reduction also benefits the lead car as the presence of the trailing car reduces the pressure drag off the back of the lead car. The result is a speed increase for both drivers.

Brett Bodine, now the director of cost research for NASCAR's Research and Development Center, began his career as a NASCAR Truck Series driver. He said the drag created by the boxy truck designs, as well as less horsepower than Cup Series vehicles, put drafting at the top of the strategy list.

"You see two cars side-by-side, you know there's a big hole behind them and if you can get in that you can accelerate faster," Bodine said, adding that, in the truck series, the vehicles punch bigger holes in the air, allowing for even more acceleration in the draft position. "When you see that, you have to start planning your moves, take advantage of what's being offered."

As you may have guessed, the more cars involved in a draft the less drag each vehicle will experience. Bodine has seen and experienced this phenomenon, firsthand. "Three cars will run faster than two, and five cars will run faster than three," he said. This is why cars often run in drafting packs and lines, each gaining a few more miles per hour from the car in front and behind as pressure drag is reduced. This can have a profound effect on the overall running of a race.

Bodine also said lead drivers often shift from one drafting lane to another as need dictates. Whatever lane they lead will go just a little faster than the one they just left. "This is especially true at tracks like Talladega," Bodine said. "You'll see a guy at Talladega moving around and you know that whatever line he gets in front of moves faster."

And if a driver makes a mistake and falls out of the drafting line it could spell doom for their chances in the race. "If you fall out of that line, if you have to go to the pits for a problem, you're probably going to get lapped," Bodine said.

The basic physics of the draft -- from the two-car draft to draft lines -- can be used by teams to accelerate their drivers or stall the competition by depriving them of reduced pressure drag. But it's not so simple streaking down the track at nosebleed speeds. Bodine said the days of practice leading up to a race allow each driver to get to know the other cars. "You want to know exactly what your car can do on the track every second of the race," he said. "You want to know who you can draft behind, what your car will do in the curves and stretches, and how it will react. There should be no surprises."

This means finding the sweet spots on a potential lead car's bumper for the draft, and in more advanced driving situations where to place your car in front of or behind another car to deprive them of downforce, increase their drag, or even rob them of an opportunity to pass.

­If you think you've heard it all, well then you'd better get ready for the next page. That's where we take a look at the more "sophisticated" side of drafting.

The last laps of a race are often the best. When the checkered flag is waved partnerships are dissolved, heated competition ratchets up to the nuclear level and drafting becomes less of a strategy and more of a weapon.

The Archer brothers, Tommy and Bobby, made their mark in the Sports Car Club of America (SCCA) when they used a technique called bump drafting, also referred to as impulse momentum in engineering circles, to steal the lead from the competition. The technique involves the trailing car driving up and actually hitting the lead car's bumper to shoot it ahead while pulling the trailing car behind it.

Dr. Jerre Hill said he was skeptical about the process, and that the math and physics didn't quite mesh with the reality. But whether it works or not, the technique is impressive -- and dangerous. The trailing driver needs to hit the lead car in precisely the right spot and at precisely the right angle. Failure to do so can lead to disaster. Hill said as the trailing car comes closer to the lead car the air stream under the car, the downforce, is disrupted. As this happens the nose of the lead car begins to raise a little adding an upforce to the equation. The lead car's tires have less contact with the track and can often slip entirely even before the bump is made. Drivers will often take advantage of this in the last laps by getting close to a lead competitor in exactly the wrong spot and unsticking their tires by manipulating their downforce stream. They may also add a small bump and just that fast, a potential first place finisher moves to the back of the field.

While the bump draft often affects the lead car, the trailing car is taking some risks as well. NASCAR race cars, in some ways, are sensitive machines. The same draft that pulls them along can also rob them of the air they need to cool their superheated engines. Several races have been won by a savvy lead driver that allowed a trailing car to creep up from behind, stay there just a little too long, and leave the field with a heat-blown engine.

The slingshot is a classic move -- a crowd-pleaser, the gold standard of drafting strategy. Simply put, the last laps of a race are coming and a trailing driver needs to steal the lead. The trailing car drafts the lead car, uses the pressure reduction to ease its movement through the air, waits for the right curve, mashes the gas and uses the extra power to slingshot ahead for the race victory.

If you're interested in a few drafting success stories, take a look at the next page.

­While the cars, technology and strategy are constantly changing, drafting will always play a role in racing. Below are a few examples of NASCAR wins where drafting attack and defense were critical to success.

During the Firecracker 400 at Daytona in 1974, David Pearson was in a two-car draft with Richard "The King" Petty. Pearson was reportedly worried about a slingshot maneuver from Petty and at the last minute allowed Petty to surge ahead. Pearson used the greater horsepower in his car to make up the distance, then drafted Petty and used a slingshot maneuver to get around him to take the win.

At the Daytona 500 in 1999, Jeff Gordon fended off a nose-to-tail challenge from racing legend Dale Earnhardt. Earnhardt attempted to "fan" Gordon's tail -- essentially disrupt the downforce off Gordon's car and unstick his wheels by sweeping the nose of his car into Gordon's slipstream. The tactic failed and Gordon went on for the victory.

At the 50th running of the Daytona 500 in 2008, driver Ryan Newman received the "push from heaven" and drafting aid from teammate Kurt Busch (both of Penske Racing) to pass leader Tony Stewart on the backstretch and take the win.

­If you enjoyed this article, then you may want to take a look at the next page, too. There, you'll find several articles focused on NASCAR and NASCAR-related topics. It's definitely worth a look.