Physicist reveals tailwind has negligible effect on cycling speed

Within the cycling realm, “to Everest” involves riding up and down the same mountain until your ascents total the elevation of Mt. Everest—8,848 meters.

After a new cycling “Everesting” record was set a few years ago, a debate ensued on social media about the strong tailwind the cyclist had on climbs—5.5 meters per second (20 kilometers per hour or 12 miles per hour)—when he set the record. To what extent did the tailwind help him? Should limits be set on the allowed windspeed?

Martin Bier, a physics professor at East Carolina University in North Carolina, became intrigued by this debate and decided to explore the physics, and a little project ensued. In the American Journal of Physics, he shares his finding that ultimately, the wind turns out to be of negligible consequence.

First, a little background: From a physics perspective, cycling is easier to comprehend than running.

“In running, the motion of the legs is repeatedly accelerated and decelerated, and the runner’s center of mass moves up and down,” said Bier. “Cycling uses ‘rolling,’ which is much smoother and faster, and more efficient—all of the work is purely against gravity and friction.”

But there’s something odd about air resistance. The force of air friction you fight goes up with the square of your speed. If air resistance is the main thing limiting your speed—which is true for a cyclist on flat ground or going downhill—then to double your speed, you need four times the force. Tripling your speed requires nine times as much force. But, on the other hand, when cycling uphill, your speed is much slower, so air resistance isn’t a big factor.

“When you’re riding up a hill and fighting gravity, doubling your power input means doubling your speed. In bike races, attacks occur on climbs because it’s where your extra effort gets you a bigger gap.”

On a solo Everesting effort, calculations are straightforward. A rider isn’t getting an aerodynamic draft from another rider ahead of them. The inputs are simply watts, gravity, and resistance.

“Naively, you may think that a strong tailwind can compensate for an uphill slope,” said Bier. “You then ride up the hill as if it’s a flat road, and on the way down the headwind and downward slope balance out and again give you the feel of a flat road. But it doesn’t work—the square I mentioned earlier wreaks havoc.”

His work shows the tailwind may help a little on the climb, but most of the work on the way up is the fight against gravity. The subsequent descent is fast and lasts a much shorter time, while the headwind there actually has a huge effect. And the speed on a descent is high—about 80 kph (49.7 mph).

“Air resistance goes with the square of the speed, which leads to the headwind on the descent and causes a big reduction in speed,” Bier said. “The wind boost on the ascent is canceled out.”

The obvious implication of Bier’s work is that there’s no point in waiting for the ideal wind if you want to improve your Everesting time.

“There are no easy tricks,” he said. “If you want to be a better Everester, you need to lose weight and generate more watts (exercise). This is what matters—there’s no way around it.”