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Inside NASCAR: The science of the tandem draft

April 20, 2011, Mark Aumann, NASCAR.com

Greg Biffle sticks perfectly to the bumper of Matt Kenseth while racing at Talladega Superspeedway. (Autostock)

Engineers rely on laws of motion to get best out of cars on superspeedways

Sir Isaac Newton died 220 years before Bill France sat down with a group of race car owners at the Streamline Hotel and founded NASCAR in 1948. But Newton's laws of motion -- three physical laws that describe the relationship between the forces acting on an object and its motion due to those forces -- are behind almost every decision made when engineers look for mechanical competitive advantages in today's NASCAR.

To explain what's happening to a Cup car at high rates of speed, particularly with the recent phenomenon of two-car bump drafting, I turned to three NASCAR engineers to help explain the physics behind the performance.

-- Steve Hallam is executive vice president of competition for Michael Waltrip Racing. A graduate of Loughborough University in England, Hallam previously worked in Formula 1, first as the track engineer for Team Lotus and then heading up McLaren's F1 efforts. After three decades and six world championships in open-wheel racing, Hallam moved to NASCAR in 2009 as director of race engineering for MWR.

-- Travis Geisler is competition director for Penske Racing. A former go kart racer who earned his degree in mechanical engineering from Vanderbilt University, Geisler drove in both the ARCA Series and the Nationwide Series. He joined Robert Yates Racing as a race engineer, and has worked as team engineer and crew chief at Penske.

-- Steve Hoegler is team engineer for Clint Bowyer's No. 33 Chevrolet as part of Richard Childress Racing, A native of Cleveland, Hoegler earned his mechanical engineering degree from Ohio University. After working for Petty Enterprises and Hall of Fame Racing, Hoegler joined RCR three seasons ago and works hand-in-hand with crew chief Shane Wilson.

"Everything can be traced back to Newton's three laws of motion, which basically define how we operate today," Hallam said. "You can reduce pretty well everything back to force equals mass times acceleration."

The new car's (left) body style allows for more drag than the old chassis, according to one engineer. (Autostock)

And it's especially true when engineers try to determine the physics behind the push drafting now prevalent at superspeedways such as Daytona and Talladega. It's all about fluid dynamics: moving air out of the way in an effort to get through it as cleanly as possible, while keeping enough air around to provide sufficient thrust while cooling the engine.

Kind of a drag

What we call "air" is a combination of molecules prevalent in the earth's atmosphere: approximately 78 percent nitrogen, 21 percent oxygen, and minute amounts of water vapor, argon, carbon dioxide, hydrogen, helium and other gases. Even though air is mainly transparent, it has substance. If you could weigh a one-inch column of the atmosphere, it would come out to about 14 pounds.

Fluid dynamics is the study of how objects move through a fluid, like the atmosphere. Because the shape of the car creates resistance against air, it creates a drag force. At 194 mph, a Cup car is traveling at 284.5 feet per second -- almost the length of a football field -- so it's having to push its way through a lot of air.

Every object has a coefficient of drag based on its shape. The lower the coefficient, the more efficiently the shape travels through a fluid. A cube's coefficient of drag is 1.05, while a sphere's coefficient of drag is .47. The coefficient of drag for a current Cup chassis is approximately .4. Amazingly, that's better than a Formula 1 car.

However, the coefficient of drag is almost useless without multiplying it by the projected frontal area of an object. That's where the F1 car is way ahead of the game. Its frontal area is miniscule compared to the front bumper of a Cup car.

It's like comparing a cocktail umbrella to a golf umbrella. Both have the same coefficient of drag, but try holding them outstretched while running into a headwind and I'll bet you can tell which one will cause you to use more energy to maintain velocity.

For Geisler, the differences between the old Cup chassis and the current configuration are more than skin deep.

"The cars have a ton more drag than a typical old-style speedway car," Geisler said. "We were really able to change the shapes on things and really make a big difference on the amount of drag, to where an intermediate car would have a lot more drag, I mean double if not a little more than would a speedway car.

"Now, you can trim things and make them a little smoother and do some little things here and there to try and trim the car out. But you can't make a big step change on the amount of drag that they have, just because you're locked in on your shape so much more."

"Drafting's been in motorsports for as long as the sport's been going. They could pull more revs in the draft and they could go faster."

--STEVE HALLAM

And there are a number of reasons for that. One, the tolerances on the new chassis are much more strictly enforced. Two, a larger cockpit makes for a windshield that stands more upright.

"You don't have half-inches of flexibility, you've got 30- to 70-thousandths [of an inch]," Geisler said. "It's much more difficult to really affect the overall drag on the cars. So I think the cars are draggier, I think they are boxier. If you kind of look at the windshield angle of an old car versus the new one, you were able to make everything just a lot smoother. It was just a lot smaller package in the air.

"Now the windshield's just huge and flat, the greenhouse is taller. All that stuff goes to making a bigger hole in the air. So if you've got a bigger hole in the air and you've got a car that's draggier, the draft is going to work better."

Drag force is divided into two components: friction, or viscous drag, and pressure drag. Slipstream bodies, like wings, are dominated by viscous drag, while bluff bodies are dominated by pressure drag.

Have you ever stuck your hand out the window at freeway speeds? If you keep your palm so it's pointed toward the road, your hand acts like a slipstream body. If you turn it upward, more like a bluff body, it'll just about tear off your shoulder.

"In our form of racing, the body shape is more akin to a bluff body," Hallam said. "That's not a negative term. It's just a term used to describe the sort of conventional shape, a typical shape not unlike ours in aerodynamic terms."

Any object can fly, given enough thrust

As a Delta pilot explained on a trip to Atlanta not long ago, a brick can't fly on its own. But given enough energy, you can make it fly. Add potential energy by picking it up, and kinetic energy by using the muscles in your arm to throw it, and an object without independent mobility will travel through the air until the combined forces of gravity and friction will bring it to a stop.

A space shuttle is basically a "flying brick." Add enough thrust from the combination of a liquid fuel tank and two solid rocket boosters, and you can lift four-and-a-half million pounds into orbit. Thrust -- or more specifically, a 358-cubic inch, V8 engine capable of up to 850 horsepower at 9,000 revolutions per minute -- enables 3,450 pounds of iron, steel and aluminum to travel at speeds close to 200 miles an hour on a closed course, although in a test at Talladega in 2004, Rusty Wallace was able to turn a lap at 221 mph with an unrestricted carburetor.

Even with restrictor plates limiting the airflow and cutting the horsepower nearly in half, cars at Talladega last weekend qualified by themselves at nearly 180 mph and raced in two-car packs at close to 195 mph. That's close to the sustained winds created by a West Pacific super typhoon and faster than the average takeoff speed of a 747 jumbo jet.

The reason why Cup cars don't fly off the track without help is because of their aerodynamic shape. Unlike an open-wheel car, which uses wings to create downforce, the bodywork is designed in a way to keep the tires in contact with the asphalt. And at speed, it creates a partial vacuum -- a region of low pressure -- behind the car as the air is pushed out of the way.

There's a draft in here

Junior Johnson was one of the first NASCAR drivers to experience this phenomenon when Daytona International Speedway opened in 1959. He soon realized when he tucked in close behind another car, his car picked up speed and made it easier for him to pass.

"Drafting's been in motorsports for as long as the sport's been going," Hallam said. "Even if in the early days, the drivers didn't understand exactly what was going on, they would report back to their engineers or they would work it out themselves, that if they were close behind the car in front, what they would see initially [is] they could pull more revs in the draft and they could go faster.

"What would happen is they would pull up to the car in front and they would use that technique to slingshot past them and execute an overtaking maneuver. And that became quite a well-established technique in a range of motorsport disciplines."

Geisler said you can experience the draft on a smaller scale on a freeway.

Juan Montoya pushes Clint Bowyer to victory this past fall at Talladega. (Autostock)

"If you're driving down the highway and you pull up behind a tractor-trailer -- like a big box truck -- there's that area behind it where you'll feel where you have to get out of the gas a little bit, because there's no air force on the front of your car," Geiser said. "You go through that little buffer zone when the car gets buffeted because the air's not smooth and then you get to where there's just a vacuum beyond the back of it and the air can't fill that void quick enough."

A series of fortunate events

So why has tandem drafting become such a hit, and why haven't we seen this form of racing in the past? It all has to do with the configuration of the cars and the configuration of the track, a perfect storm of push drafting, so to speak.

When NASCAR designed its new chassis for introduction in 2007, it added something new: front and rear bumpers that matched up nearly perfectly. And according to Hoegler, that makes a huge difference.

"The bumpers are lined up so they hit and stay attached," Hoegler said. "They don't just bump and release. When you bump and release, you have two separate cars, basically. When they hit and stay together, two cars kind of turn into one.

"Instead of having 400 horsepower and 400 horsepower with two separate cars, when they hook up, they make 800 horsepower and are only limited by the drag of the two cars put together, which is more than one car but less than if they were separated."

"When they hit and stay together, two cars kind of turn into one. Instead of having 400 horsepower and 400 horsepower with two separate cars, when they hook up, they make 800 horsepower."

--STEVE HOEGLER

But because the power needed to push an object through a fluid increases as the cube of the velocity -- and the majority of power generated by the second car is used just to keep up -- you only see incremental increases in speed compared to the traditional multi-car drafting.

"You get the extra horsepower but you get only 3 or 4 mph more because as the velocity increases, the drag shoots through the roof in a hurry," Hoegler said.

Hallam agrees with that assessment.

"You've got the classic situation of the same frontal area as one car but essentially -- and it doesn't quite work out like this -- twice the horsepower," Hallam said. "You've got more power. And you've got a longer single shape -- maybe slightly less optimized, which is why you don't see the benefit of twice the horsepower.

"But the overall effect is that you go faster together. And you go so much faster together, that if you break out where you have to breathe it, you do in effect pull the overall performance. If you clearly break out, you'll both go slower."

Switching from a rear wing to a spoiler may have played a role in creating a stronger low pressure area.

"I think the spoiler exaggerated it," Geisler said. "I think we did see it with the wing, absolutely. The case was there. Everybody knew it was kind of there with the wing. It does seem like it's a bigger speed differential, though, now with the spoiler. That just shows the importance of having that amount of air going over the back of the second car, or the least amount of car you can have going over the second car.

"I think that the spoiler makes the air going over the first car kick up more than the wing did. When you think about a wing, it's really supposed to reattach the air on the back side of it. The shape of the wing is to kind of seamlessly reattach air on the back side so that it's less turbulent and more efficient. The spoiler is basically just a ramp to send all the air up and create the vacuum behind it. You'd get a bigger wake behind a car with a spoiler."

Both tracks were recently repaved: Talladega in 2006 and Daytona this offseason. That made them grippier and more importantly, significantly less bumpy. Suddenly, drivers had much more control in their efforts to keep attached to the rear bumper of the cars they were following.

"We saw this start to happen at the fall Talladega race," Hoegler said. "That's how we won the race. We got together with Juan [Montoya] and [Kevin Harvick] got together with [David Reutimann]. They were starting to figure it out then but they hadn't mastered it. There were some cars that could go from the back of the pack to the front, and they were making some changes and experimenting. Then everybody went to work over the winter, because we knew this was the way it was going to go."

"The deal at Daytona really came out of the track being repaved," Geisler said. "We saw this at Talladega and it works there very well because it's smooth and it's got bigger radius corners so it's easier to stay hooked up."

Drivers know you can't dance at Talladega without a partner. (Autostock)

Nature abhors a vacuum

It was the Greek philosopher Aristotle who famously claimed "nature abhors a vacuum." And it's never truer than in superspeedway racing, where the trailing car can use the partial vacuum created in the wake of the leading car to push both cars along.

"Most of us figure the majority of the gain comes from the amount of air that gets kicked up over the first car and how long it takes for it to come back down," Geisler said. "The second car is just able to stay kind of tucked up in that jet wash or whatever you want to call it. That car sneaks underneath it, so you stay in that envelope of virtually no pressure whatsoever. A car at 200 mph will go pretty well with your air drag eliminated at that point. So if you cut out the air drag, obviously you can go a lot faster."

There is a bubble of undisturbed air trailing the lead car, according to Hallam, that creates the optimal drafting effect.

"With the nature of our cars, and again, the wake structure that exists behind our cars, you get this to a smaller effect, this degree of effect of being able to suck up behind a car," Hallam said. "But where our drafting techniques have escalated to, is if you pull out, you go slower.

"What they find now is if you can push through that bubble, there is a point at which the trailing car takes advantage of the draft. It gets close and then it has to go through a bubble where the drag on the trailing car goes up, and then it goes down again when it gets to the bumper."

That creates a silhouette that takes advantage of the wake.

"You're effectively getting a single vehicle that's twice as long and it has a more complex shape," Hallam said.

No good deed goes unpunished

But for every gain made by engineers in designing cars to take advantage of the draft, something has to be lost.

Combustion creates energy, but an unwelcome byproduct is heat. And those big V8s crank out a lot of both. The boiling point of water is 212 degrees Fahrenheit at standard pressure. However, with pressurized cooling systems, NASCAR engines can survive for short periods of time at temperatures well above that.

You need air to cool the engine. But there's almost no air coming through the radiator under drafting. That creates a quandary for engineers.

"That really becomes the limiting factor in how tightly you can stay packed up behind another car is how hot the engine gets," Geisler said. "That's really the balancing act the drivers are playing the whole time, how do they take care of that, how do they keep their temperatures under control.

"All you can do is try to maximize your cooling system, whatever you think you can do to make your cooling system work more efficiently. But once you're close to zero flow across your radiator, it doesn't really matter what you do, it's just a matter of time until you're going to overheat. You've got to have airflow. If you don't have that, you've just got the amount of time it takes to heat the water in your system to the temperature that you can't handle."

And according to Hoegler, it's just as important to keep an eye on the oil gauges.

"Just as big a concern as water temperature is oil temperature," Hoegler said. "That's something you don't hear a lot about on TV, but you get a fairly big water to oil [temperature] split, that's how you can cook bearings and stuff like that. It's a two-sided interrelated situation, because they flow through the same cylinder heads. If you get either the water or oil cool, it helps cool the other one."

So in order to deal with two conflicting situations, drivers have learned that if the trailing car slides to the left or right -- just enough to present the opening in the grille to the flow of air from the leading car -- they can mitigate the temperature issue somewhat.

"You'll see a lot of the guys try to keep pushing but get almost three-quarters offset in order to get some air," Hoegler said. "They're staying attached and they're both sharing the horsepower but they're increasing the drag a lot because the frontal area gets bigger. They can push faster when they're tucked up but they have to get air and they peek the nose out and that slows both cars down. You can see it in lap times in practice."

What teams worked on during the offseason was to maximize efficiency, within the confines of NASCAR's rulebook. Hallam said the sanctioning body placed restrictions on header tank volumes and introduced water pressure reducing valves in an effort to keep the playing field balanced.

"The bottom line is it's pretty well the same for everybody," Hallam said. "The difference comes with the teams' cooling systems and how much effort they've been preparing to devote to optimizing their cooling systems for running at a superspeedway, nose to tail.

Denny Hamlin finds himself on the proverbial island as the third car in a two-car draft. (Getty Images)

"And NASCAR has very quickly caught that and implemented some things. They sent sufficient signs that they were on it at Daytona."

Two's company, three's a crowd

So if two cars can bump draft and run faster, why doesn't it seem to work with three or more cars? No one seems to have a definitive answer.

"I think it's a very difficult thing to get accurate models on," Geisler said. "I think the wind tunnel deal isn't set up for a two-car situation, so it's difficult to measure in a test environment other than on track. If you can put two cars together in a wind tunnel and you're able to get that done, there's only a few tunnels around where you can do that.

"And if you do, you're going to see a lot less drag or see whatever, but the key would be putting three or four behind it to find out, is there more drag? Less drag? Why is two the magic number versus three?"

Scale models can provide some decent data, but Hoegler is concerned that too much guesswork is involved.

"Computational fluid dynamics models can [give you] some fairly good information," Hoegler said. "But there again, there's a lot of assumptions that you have to put in to make it right, and do those assumptions still apply? There's a lot of questions about that. We get a general trend from the scale models."

Hoegler seems to think it has to do with the instability of the two-car draft.

"I think the third car starts to take the brunt of the air coming back down and it really adds a lot of drag to that car, compared to just the two. It's still better than one but it's not as good as two."

--TRAVIS GEISLER

"The front car on its own has to slow down to keep the second car attached," Hoegler said. "So if you put three cars worth of horsepower on there, the third car almost doesn't stand a chance, because now two cars in front can get away from the car in the back.

"They could do it at Daytona for a straightaway or maybe they can make it happen for a corner, but I think the third car just has such a hard time. They move around a lot. They're not really that stable pushing each other like that. I think it's hard to follow the path of two cars."

Geisler believes it has more to do with the air flowing over the first car's spoiler.

"I think the third car starts to take the brunt of the air coming back down and it really adds a lot of drag to that car, compared to just the two," Geisler said. "It's still better than one but it's not as good as two."

Hallam would love to spend time and energy on the problem, but it's not pertinent at the moment.

"I have my own theories but I can't prove them to you at this stage," Hallam said. "And I suspect the same exists for a number of guys out there. If we were able to study the energy in the wake of a three-car draft, we'd start to understand that.

"But that's sort of well down the list of things that we need to know now. It will elevate its way up in the fullness of time if nothing changes. Because obviously you're thinking if two is better, then logic would say three would be better again. But it's not proving to be the case at the moment."

Instead, Hallam's more concerned about tenuousness that is the status quo, because that can change with the wave of NASCAR president Mike Helton's hand.

"We tend to come to superspeedway races wondering what NASCAR is going to change on us at the last minute," Hallam said. "So it's hard to justify committing to that level of research, because you think chances are they're going to make a change so we won't be two-car drafting any more or they're going to change the cooling configuration which will minimize the amount of time you can spend in this condition so it will be rendered redundant anyway."

According to Newton's first law, an object that is at rest will stay at rest unless an unbalanced force acts upon it. And an object that is in motion will not change its velocity unless an unbalanced force acts upon it. And that pretty much sums up why tandem drafting is in vogue.

"You're better off nose to tail with one of these cars, even though there are potential downsides for your engine," Hallam said. "It's a net gain for both cars. When you consider the negative effect of being on our own, it's a no-brainer. It's that big a step."

That Newton guy would have made one hell of a NASCAR engineer.