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The Draft

NASCAR aerodynamics

An edge is in the air for stock car racers

Roaring around a racetrack at nearly 200 mph, NASCAR drivers often find themselves speeding in a single line of tightly bunched cars. As other cars run side by side, fighting for position, the line of leaders gradually pulls away from the pack. One of the more interesting lessons of aerodynamics is that a line of two or more cars running bumper to bumper can move faster than any of them can travel alone. On the other hand, cars racing fender to fender can actually slow each other down. The two major forces created by the car's movement are the drag produced by air pressure and the downforce generated over the whole car. Drag is the car's enemy because it robs horsepower and speed. Downforce - helping hold the car to the ground - is its friend, but not always. Not surprisingly, experts on the subject, who are often associated with racing teams, are reluctant to talk specifics.

AIR PRESSURES A race car is what aerodynamicists call a "bluff body," one on which air pressure creates a drag. Another variable is what's termed "ground effect," the change in airflow around the car because it is close to the ground. The airflow around and over a car produces both high- and low-pressure areas. Designers try to produce a properly balanced net downforce on their racers. But there is no free lunch: Increased downforce yields better control, but adds drag that subtracts speed.

A competitive edge

"If you discovered something that makes you go faster, you don't want your competitor to have it," said Dr. Harry Law, head of Clemson University's Motorsports Engineering Program. Even very small improvements have an outsized effect in the highly competitive world of NASCAR's Winston Cup Series, according to Dr. Joe David of North Carolina State University. "A significant change would be one part in 300 in either aerodynamic downforce or drag," David said. What everyone in racing knows is that drag and downforce change radically when racing cars are near one another. As a car speeds along the racetrack, some air tends to pile up in front of it, and some air gets dragged along in its wake. To see how this works, watch what happens when you stir a spoon through a thick bowl of soup. The surface will actually hollow out behind the spoon as it starts to move, and the waves of soup around the spoons edge's will gradually move in and fill the depression.

A car moving by itself is pushing the air in front of it out of the way. That air is dragged in its wake, filling the area behind the car with low pressure and creating a partial vacuum. This creates drag, which eats up engine power.

Slowing down the car

Most of the horsepower generated by a racing engine is eaten up by the high-pressure air pushing the front of the car and the low-pressure air - a partial vacuum - dragging at the car from behind, according to David, who is known as "Mr. Stock Car" at N.C. State, where he teaches mechanical and aerospace engineering. Both forces work to slow down the car. So what happens when another car enters the picture, creeping up on the rear bumper of the leader? "When a car starts to move up on the lead car, he's actually raising the pressure on the back of the lead car," said Gary Eaker, the aerodynamics engineer and "Wind Wizard" for Hendrick Motorsports of Harrisburg, N.C., which fields the NASCAR teams of Jeff Gordon, Terry Labonte and Ricky Craven. "One's low-pressure area cancels the high-pressure area of the other so they can run faster," said David, a Richmond native who has designed and engineered cars for several racing teams. This technique of reducing effective drag on one car by driving closely behind another is called "drafting." Dick Trickle, who drives for Richmond's Junie Donlavey:

As another car moves in behind, its high pressure cancels the low pressure of the lead car. The second car is actually pulled in behind the first, making both move faster.

Let's say I'm going 183 miles an hour by myself at Daytona. Now we get four cars hooked up - now we've got four motors pushing and pulling that same air. We can run 189, 190.

Because race cars typically are operating at or near their maximum power - which is to say, wide open - on the straightaways, they have little excess horsepower available to buck unobstructed high-drag air and pass cars ahead of them.

A fundamental fact of aerodynamic life is that drag increases with the square of the speed. In other words, when you double the speed, you quadruple the drag. That means going a little bit faster is not just a little bit harder, it's a whole lot harder. "Today the cars are running close to terminal velocity," Eaker said. A single car may not be able to pass, but several cars working together can use their joint reduced drag to get around traffic. When two cars travel side by side, their proximity blocks the flow of air between them, forcing that extra air out and around.

Two cars traveling side by side are slower because air is blocked from flowing between them, forcing more air around and over both cars, giving them extra air to push. This is know as "interference drag."

Side-by-side racing slows cars up terribly. In a single car traveling at 185 mph as soon as another car comes up, you'll run 183 because you're trying to push a bigger column of air.

Eaker said that slowing phenomenon is known as "interference drag." "The sum of the drag of two bodies that are close to each other is higher than their individual drags," he said. "You've essentially added resistance to the flow and pressure to the nose of the car."

Which brings us back to downforce. Racing teams have found a number of ways to increase the stabilizing downward pressure on the car. There are air dams or valances below the front bumper, rocker skirts below the side panels and spoilers on the rear deck. Spoilers are 57-inch-long flat panels about 6 inches tall, set at angles of as much as 60 or 70 degrees. Of all the downforce enhancers, they figure most prominently in the aerodynamic strategy of racing. While necessary to give race cars adequate traction, downforce becomes particularly critical in turns. "Going down the straightaway is not much of an issue," Eaker said. "When we get to the corner, we'll see who really wants to race, who has the best handling car, who can stay on the gas the longest." Dr. Venkataramani Sumantran of the General Motors Research and Development Center in Detroit said more downforce generally translates into faster turns. "You're trying to make sure you have sufficient tire loads so that you're able to sustain the kinds of cornering speeds you need," he said. Just as cars mutually affect each other's speed when they are drafting, they also affect each other's handling. "The airflow pattern can change around the lead and trailing cars," David said. The "streamline" of air over the lead car rolls down toward the rear deck until it hits the spoiler.

This configuration can also affect the way a car handles. The airstream is raised due to the increased pressure of the car behind. "Taking the air off the spoilers" makes the car "loose"; the rear end loses traction and some control.

The increased air pressure ahead of the trailing car fills in the wake of the lead car, and the air off the spoiler "sees that pressure before it even gets there, so the streamline over the rear of the lead car is raised," David said. drivers call this "taking the air off the spoiler," and the shift can destablize the car ahead. The lead car may get "loose" as its rear end loses traction. At the same time, the trailing car's stability also can be impaired. The same airflow that is reducing pressure on the rear of the forward car in the draft will lower the pressure on the front of the trailing car. This effect, called an "aero push," means the front of the trailing car wants to slide out rather than smoothly turning. There are two sets of engine rules in NASCAR's big-league Winston Cup series. At most of the races, teams can get about 700 horsepower from the 358-cubic-inch engines. But for four of the 32 races on the circuit - two each at Daytona and Talledega, the biggest and fastest tracks - NASCAR imposes engine restrictions that cut the horsepower by almost half. On the long straightaways of the superspeedways, racing crews struggle to reduce drag and increase top speed. At shorter tracks, such as Richmond, handling in the corners is paramount and crews emphasize downforce. NASCAR racing is so competitive that teams "who are able to get their hands around these subtleties will be able to improve their performance quite dramatically," Eaker said. The paradox is that the setup of a race car is based on compromise. Minimizing one problem or maximizing one desirable characteristic of a car can only be done at the expense of aggravating another problem or diminishing another virtue. Or, as Eaker put it, "you can never have a perfect race car."

STAYING GROUNDED A race car is designed to stay on the ground as long as it is moving forward. If it is turned sideways or backwards, air moving over and under the car creates lift, and it can become airborne. As a result, many anti-lift devices have been developed for racers to keep from losing control. SPOILERS: A metal strip that helps control airflow, downforce (the pressure of the air on a car as it races), and drag (a resisting force in a car's airstream). The front spoiler or "air dam" is underneath the car's front end near the axle; the rear spoiler is attached to the trunk lid. ROOF RAILS: Airflow over a smooth surface can create lift. These strips are designed to break up streamlined airflow across that lift. SIDE SKIRT: This skirting keeps air from getting under the car if it moves sideways.

Keeping It Down Means Keeping It Safe On The Track

Aerodynamics play a key role in keeping NASCAR racing safe.

"The biggest problem for race cars usually is simply keeping the rear end on the ground in a turn," said Dr. James F. Marchman III, a Virginia Tech aerospace engineer. "People think about drag as the big problem, but the biggest problem is the lift," he said. "The car develops lift, and you don't want it to." Veteran driver Dick Trickle described the aerodynamics of race cars in simpler terms:

They know how to fly. The problem is they don't know how to land.

Race cars are designed to stay on the ground, as long as they are traveling forward. But if they are "yawed" - turned sideways or backward - they can generate a lot of lift, according to Gary Eaker, the aerodynamics engineer for Hendrick Motorsports. Winston Cup Series cars weigh 3,400 pounds, but the aerodynamic forces as they speed at almost 200 mph are powerful enough to send them flying out of control and through the air, creating serious danger for drivers. To reduce the chances of cars getting airborne, engineers have developed several "extreme yaw anti-lift devices" for use on racers. Roof rails, side skirts and recessed edges on the right-side windows are passive aerodynamic safety features on stock cars. Rails are thin vertical strips running front to rear along the edges of the roof. In order to produce lift, air has to be moving smoothly. The sharp protuberances of the roof rails break up that streamlined airflow and reduce unwanted lift. Skirting around the lower edge of a car's side keeps high-pressure air from getting under the vehicle if it is moving sideways, lessening the tendency to spin, or fly. Because racetracks turn left, and cars tend to move to the right when they go out of control, the right windows are recessed. That gives the window an edge to interrupt streamlined air when a car yaws right, producing the same effect as roof rails.

FLAPS - Other devices designed to disrupt lift include two flaps in the rear of the roof. As a car begins to spin past 90 degrees to its path of motion, the flaps deploy. Flaps are held down by normal air pressure. When a car spins out, they pop up, breaking the lift.

Such passive devices work best when the cars have turned at a right angle to the direction in which they are hurtling. Roof flaps come into play when a car's rear spins past 90 degrees to the path of motion. Each car has two flaps, 20 inches wide and 8 inches tall. Located near the rear of the roof, the left flap is set perpendicular to the car's length, with the right flap at about a 45-degree angle to the long axis - again because cars tend to spin their rears to the right. Hinged at the forward edges, the flaps are held flat against the roof under normal conditions by the car's airflow. But as the car starts to spin, the fast-rushing air opens the flaps, which break up the lift over the roof. "These are spoilers in the true sense of the word," said Eaker, who worked on the flaps' development. Or as Trickle put it:

It'll still fly with the roof flaps, but it won't go over the fence and get to our race fans.

Winds Of Change

Two cars are faster than one on a superspeedway - at least until they get to the corner.
Here's why

If a racecar has enough power to be competitive on the straights, then the only thing keeping it from victory lane is its balance between tight (understeer) and loose (oversteer) in the corners. A car is considered tight when it is difficult to turn despite steering wheel input and loose when it turns too easily. Either case slows the driver down. A perfectly balanced car is driver nirvana, but realistically is unobtainable. The thing that stands in the way is centrifugal force, a function of vehicle mass and speed that tries to throw the car out of the turn (either frontward when tight or backwards when loose). The only thing resisting this force is tire grip. The tires with the least amount of grip (front or rear) will slide out first. If the fronts go first, the car is tight. If the rears go first, it's loose.

Teams use many tools in their quest for perfect balance. Springs, anti-roll bars, weight distribution, roll center heights, shocks, tire pressure - all are dials that can be turned as part of the process. And on faster tracks, there is one more tuning tool: aero downforce. The impact of aerodynamics is proportionate to track size. It's a valuable handling tool at Martinsville, and an invaluable one at Daytona and Talladega. Aerodynamic downforce is a special handling tool because it is a mass-less force that can be applied to the tires for more grip. Being mass-less, it adds nothing to the centrifugal forte trying to throw the car off the track. It also does not move around during cornering, acceleration, or braking like a mass force (which causes weight transfer).

Fig 1. One car by itself with drivers aero balance

Fig 2. Car pulling behind and just starting to change streamlines

Fig 3. Car pulling behind and significantly changing streamlines

Fig 4. Both cars in tight draft with streamlines going over both

Downforce can also be distributed to help balance the car's handling. Increasing aero downforce at the front gives more grip to the front tires and loosens the car. (Imagine widening the hood three feet.) If downforce to the rear is increased (imagine a three-foot-high spoiler) rear tire grip is increased and that tightens the car up. It is even possible to bias the aero downforce from left to right in search of that perfect balance. Aerodynamic downforce - measured in pounds - is a square function of speed. Its impact increases at higher speeds. If a car has 100 pounds of downforce at 80 m.p.h., it has 400 pounds at 160 m.p.h. This is especially beneficial because more downforce is needed on the tires as the car goes faster in the corners.

The problem is, aerodynamic downforce can change in the span of a single turn, which can quickly turn a pole-sifter into a back-marker. What makes this even more frustrating is that no matter how drivers a single car's aerodynamic balance is, every car is faster with another car directly in front or behind it. Two cars experience lower aerodynamic drag and run faster together than either car by itself. Sometimes that only lasts until they get to the corner, where both cars can lose their downforce balance. The front car will most likely get loose because it has less air available at the rear spoiler to push the rear end down (aero loose). The rear car can get tight because it has less air available to push the front end down (aero tight).

This extends to a multi-car situation: With three cars, the front and rear cars experience the same phenomena - front car gets loose, rear car gets tight - while the middle car will most likely maintain drivers downforce balance but lose total downforce. This will reduce tire grip at both axles and the driver may get a sense of floating - not to mention an increased heartbeat. As long as aerodynamics are used to help balance a racecar, the driver will have to deal with the consequences of aero unbalance sometime during the race. Teams anticipate this, however, and try to find the best compromise. If they believe their guy can stay out front, they'll probably adjust the car for more rear aero downforce. If it's more realistic that their guy will run in traffic until his end-of-the-race charge, they may run with more front downforce.

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