Driving The Wind

Wind Tunnel

Next door to A2 Wind Tunnel, where cyclists work on reducing their wind resistance like in this video, is a larger building, another wind tunnel. But this facility, called the AeroDyn Wind Tunnel, is not meant for bicycles. It is meant to test cars, either for NASCAR, land speed competitions or drag races. Several NASCAR racers, including Dale Earnhardt Jr. have brought their cars to this tunnel.

AeroDyn Wind TunnelNASCAR drivers have to grapple with the same forces as cyclists do, only with more speed comes more drag. Air is just a series of atoms and molecules and as a cyclist or a car moves forward, it collides with those atoms and molecules and has to push them out of the way. As a car pushes air out of the way, the space behind the car fills with turbulent air at lower pressure than the air in front. That pressure difference sucks the car backwards

The combined force of all those collisions and the turbulent air behind the car is drag. A cyclist makes billions of collisions every second, but a stock car travelling at many times the speed will make even more collisions and create more turbulent air, causing much more drag.

Racing drivers, therefore, must be concerned with how they can best charge forward with as few collisions and as little disturbance to the air as possible. But drivers also have to deal with a whole new set of problems as well. With the greater power of a race car comes the challenge of how to harness that power.

A car moves by transferring the power of its engine to its wheels, but a race car can put so much power into the wheels that they can overcome the static friction between the tires and the ground, making the car peel out and slide. So the drivers try to make it more difficult for the wheels to slide, instead forcing the wheels to roll. They do this by increasing the static friction between the track and the tires.

There are two ways to increase friction between track and tires, and both involve a downward force pushing the wheels into the track. Imagine sandpaper moving over wood. When you push down on the sandpaper, it is harder to move. Friction is harder to overcome.

The first way to increase that downward force and thus the friction is to add weight to the car. But simple physics tells us that a heavier car requires more power to accelerate and is more difficult to stop. Since professional drivers rely on their brakes to get them through turns and their car’s acceleration to quickly get them back up to speed again, a heavier car won’t do.

The other way to push down on the car brings us back to the wind tunnel, because even though drag and friction with the air can slow a car down, a properly designed car can harness those forces to push the car into the ground.

Formula 1 cars have very visible modifications to increase that down force. The wings at the front of the car and the spoiler on the back both slope down toward the front of the car. As air (shown in teal) passes over the car, it exerts a force on the top of these additions. Part of that force pushes the car backward and part of that force (shown in red) pushes the car down. That down force provides more friction, which keeps the wheels rolling and improves the handling of the car. In fact, at certain speeds, a Formula 1 car can corner better by going faster because higher speeds provide more down force and more friction.

Formula 1 Down Force

Even without wings and spoilers, automotive engineers can generate more down force by adjusting the car’s wheel wells and nose to allow as little air as possible under the car. The air pressure underneath the car drops while the top feels the force of all those microscopic collisions. This pushes the car into the ground, creating more friction and better handling.

That is why NASCAR and Formula 1 drivers around the world bring their cars to the AeroDyn Wind Tunnel and others like it. Inside the tunnel, drivers can get precise data on how the car’s shape affects the way it moves through the air. Using this data they can make tiny adjustments to the car to balance drag with down force.

This may seem like a lot of math and physics to make a car go around an oval, but with championships and millions of dollars on the line, each driver is looking for any edge he can get. With the help of the wind tunnel, aerodynamics can be one such edge.

-Daniel Lane

Daniel Lane covers science, engineering, medicine and the environment in North Carolina.

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