Dr. Diandra: Why aero is so important at intermediate tracks

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Kansas Speedway is only the second intermediate track on the 2022 NASCAR Cup Series schedule given that Atlanta runs a superspeedway configuration. These 1.5-mile tracks, which are often unfairly derided as ‘cookie cutter tracks’, highlight the importance of aerodynamic downforce.

Aerodynamics is the science of understanding and predicting the behavior of billions and billions of air molecules moving at high speeds around complicated objects like rockets, airplanes and race cars. I find aerodynamics to be one of the most challenging aspects of motorsports.

Dr. Eric Jacuzzi, who holds a Ph.D. in aerospace engineering from North Carolina State University, is the managing director, aerodynamics/vehicle performance at the NASCAR R&D Center. He agrees that aerodynamics can be a challenge to understand — and to explain.

“In general,” he said, “people try to be too simplistic about aerodynamics. It’s a complex subject, and I think sometimes people try to go with the soundbite.”

He laughed.

“And that usually doesn’t work.”

So let’s go beyond the soundbite to learn a little about aerodynamic downforce and why it’s so important at tracks like Kansas Speedway.

How race cars go fast

A car’s grip depends on the friction between its tires and the track. Grip is proportional to how much force presses the tires into the track. Downforce is literally force in the downward direction.

You experience the importance of downforce anytime you try to drag or slide something. Compare pushing an empty file cabinet to pushing a full file cabinet. The full file cabinet creates more friction — more opposition to motion — because it weighs more. The full file cabinet has more grip.

Grip is bad if you’re moving furniture, but good if you’re racing cars. Grip allows tires to turn without sliding too much and allowing the car to move up the track.

A car’s weight produces mechanical grip. Even the driver adds to the downforce. According to the NASCAR rule book, a Cup car and driver must weigh at least 3,665 pounds. How that 3,665 pounds of mechanical downforce is distributed among the four tires is important, but we won’t worry about that now.

Grip means speed. Teams want as much grip as they can get. A heavier car has more mechanical grip but also requires the engine to make more force to accelerate the additional weight.

So how do you get more grip without making the car heavier?

The answer is (literally) blowing in the wind.

Aerodynamics and Bernoulli’s principle

We usually express Bernoulli’s principle as a long, complicated mathematical equation. But, really, all you need to know is:

The faster air moves, the less pressure it creates.

You can understand a lot of the basics of race car aerodynamics with this one sentence.

For example: Air travels quickly over a car’s hood and roof. Without enough downforce in those areas, the car can experience lift. Lift is great for airplanes, but dangerous to race cars. That’s why roof and hood flaps deploy when the pressure in those areas gets too low.

Splitters utilize the same principle. Air moves more slowly on top of the splitter than underneath it. The force above the splitter is thus greater than the force below the splitter. The net force is down, which pushes the front tires into the track.

The other thing you need to know is that — unlike the weight of the car — aerodynamic forces change depending on how fast the car is going.

Aerodynamic forces depend on speed squared

Drag and downforce are both aerodynamic forces. Downforce pushes down on the car, while drag always acts opposite to the direction the car is moving. Both forces depend quadratically on speed. This means:

  • If you double the speed, the drag and downforce go up by a factor of four.
  • Tripling the speed increases drag and downforce by a factor of nine.

A NASCAR Cup Series car going 180 mph has nine times more downforce than the same car going 60 mph.

The faster you go, the more grip you get. This only works up to point, of course, because you eventually exceed the tires’ capabilities.

Jacuzzi cites a typical value for a non-superspeedway car of around 2,000 pounds of aerodynamic downforce at 200 mph. Superspeedway cars have about 500 additional pounds of aerodynamic downforce at 200 mph.

Below, I’ve plotted the mechanical downforce in black, the aerodynamic downforce in blue and the total downforce in red.

A line chart showing aerodynamic downforce, mechanical downforce and the total downforce on a typical NASCAR Series Cup car

Since the car and driver weigh 3,665 pounds and the aerodynamic downforce at 200 mph is 2,000 lbs, the race car experiences 5,664 pounds of downforce at 200 mph. That’s about the weight of a small elephant.

The car’s tires have to support all that downforce. Most passenger cars, whose primary source of grip is the car’s weight, require tire pressures of 30-35 psi. Goodyear’s recommended tire pressure for the right-side tires at Kansas is 50 psi (front) and 46 psi (rear). The higher tire pressure is necessary because the race cars must support the additional downforce that comes with high speeds.

Aerodynamic downforce makes up about one-third of the total downforce at 200 mph. Anything that decreases the aerodynamic downforce — like getting too close to another car — makes the driver feel as though he’s just hit a patch of ice. At 200 mph.

Below, I graph percentages of mechanical and aerodynamic downforce relative to total downforce as a function of speed. I include this plot to emphasize how much the driver depends on aerodynamic downforce at higher-speed tracks.

A graph showing the percentage of downforce at each speed broken down into mechanical and aerodynamic downforce and

Aerodynamic downforce comprises:

  • 10 percent of total downforce around 90 mph,
  • 25 percent of total downforce at 156 mph,
  • 28 percent of total downforce at 170 mph.

Aerodynamic downforce at Kansas Speedway

The clip below, of Ryan Newman at the 2021 fall Kansas Speedway race shows how his car changes speed as it travels the track.

A video showing part of a lap from Ryan Newman at the 2021 Hollywood Casino 400, showing minimum and maximum speeds

In the three seconds during which his car goes from 183 mph to 169 mph, the aerodynamic downforce goes from 1,674 pounds to 1,428 pounds, a loss of 246 pounds of downforce. That’s like losing a linebacker’s worth of weight in grip.

The driver’s job is to keep the car as close to the limits of traction as possible. Drivers must constantly adjust not only to long-term changes like track conditions and tire wear, but also to the changing grip as the car’s speed changes each lap. The Next Gen car doesn’t provide much sideforce, which means going over the traction limit has a much higher penalty than it used to. That may be why we are close to matching last year’s record for spins.

When I lived in Nebraska, I used to put a couple sacks of sand in my pickup truck’s bed in the winter. That extra rear downforce helped maintain traction when it got icy. You can think of aerodynamic downforce on a race car as a bag of sand — but a bag of sand whose weight changes depending on how fast the car is going.

And which can disappear without warning.

Anytime track speeds exceed above 150 mph, Jacuzzi says, aerodynamics will be important. That’s not to say it’s unimportant at other tracks. But the faster you go, the most important aerodynamics are.