Physicist Reveals Tailwind Has Negligible Effect on Bike Speed

In the cycling world, “to Everest” means going up and down the same mountain until your climbs total the altitude of Mount Everest, which is 8,848 meters.

After a new Everest cycling record was set a few years ago, a debate broke out on social media about the strong tailwind the cyclist was experiencing on the climbs (5.5 meters per second, or 20 kilometers per hour) when he set the record. To what extent did the tailwind help him? Should there be limits on the permitted wind speed?

Martin Bier, a physics professor at East Carolina University in North Carolina, was intrigued by this debate and decided to explore the physics, and a small project followed. American Journal of Physicshe shares his findings that, ultimately, wind turns out to be of negligible consequence.

First, some context: From a physical perspective, cycling is easier to understand than running.

“In running, the leg movement is accelerated and decelerated repeatedly, and the runner’s center of gravity moves up and down,” Bier explains. “Cycling uses ‘rolling,’ which is much smoother, faster, and more efficient: all the work is done purely against gravity and friction.”

But there’s something strange about air resistance. The frictional force of the air you’re fighting increases with the square of your speed. If air resistance is the primary limit to your speed (which is true for a cyclist on flat ground or downhill), then to double your speed, you need four times as much force. To triple your speed, you need nine times as much force. But, on the other hand, when you’re riding uphill, your speed is much slower, so air resistance is not a significant factor.

“When you’re going up a hill and fighting gravity, doubling your power means doubling your speed. In bike racing, attacks happen on the climbs because that’s where your extra effort allows you to pull away.”

In a solo effort on Everest, the math is simple. A runner does not benefit from aerodynamic pull from another runner in front of him. The data is simply watts, gravity and resistance.

“Naively, you might think that a strong tailwind can compensate for an uphill climb,” Bier says. “You then go up the hill as if it were a flat road, and as you go down, the headwind and the downward slope balance out and make it feel like a flat road again. But that doesn’t work: the square I mentioned earlier plays havoc.”

His work shows that the tailwind can help a little on the climb, but that most of the work on the climb is fighting gravity. The descent that follows is fast and lasts much less time, while the headwind has a huge effect. And the speed on the descent is high, about 80 km/h.

“Air resistance is proportional to the square of the speed, which results in a headwind on the descent and a sharp reduction in speed,” Bier explains. “The effect of the wind on the ascent is cancelled out.”

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

“There is no magic bullet,” he said. “If you want to become a better Everest climber, you have to lose weight and generate more watts (of exercise). That’s what matters, there is no other solution.”