Do Tour de France Cyclists Get Unfair Boosts from Support Cars?

New model suggests trailing cars could give big aerodynamic advantages.
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Marcus Woo, Contributor

(Inside Science) -- During the Tour de France, which kicked off on Saturday, the cyclists get all the attention—rightfully so. But fans might also want to keep an eye on the support cars that trail close behind.

That's because those support cars, which carry spare parts and coaches, can reduce the wind drag on a cyclist, according to new research. The effect provides an inadvertent boost—an unfair advantage that could've been especially important in Saturday's first stage, an individual time trial in which cyclists raced alone against the clock, followed by their support cars.

The idea that a vehicle can cut down drag on a cyclist in front of it certainly goes against common sense. But in 2012, Bert Blocken, an engineer at Eindhoven University of Technology in the Netherlands, found that a cyclist doesn’t even need something as big as a car to get this kind of a boost—just another cyclist close behind.

Using computer simulations, Blocken discovered that during a race, the rear cyclist can reduce the aerodynamic drag on the one in front by as much as 2.5 percent. That's a lot, considering that 90 percent of a cyclist's total resistance results from drag.

In general, a cyclist feels drag while speeding along. That’s because air gets pushed forward and squeezed into a small region of high pressure, leaving a pocket of low pressure in the wake behind the bicycle. The high-pressure region pushes back on the cyclist while the low-pressure region pulls, creating resistance.

But the simulations showed that a second cyclist close behind would sweep air forward, filling the gap that the first cyclist created. The air pressure behind the first cyclist isn't as low anymore, so the wake pulls on the first cyclist less, decreasing the aerodynamic drag.

Blocken then wondered what would happen if you replaced the rear bicycle with a larger vehicle—such as a cyclist's support car. To find out, he and graduate student Yasin Toparlar ran more computer simulations. Lo and behold, they found the same effect—only bigger, since a car is larger and can push more air.

To confirm their results, which have been accepted for publication in the Journal of Wind Engineering and Industrial Aerodynamics, the researchers conducted experiments inside a wind tunnel, measuring airflow around a plastic model cyclist and car. The data matched the computer simulations almost perfectly.

The researchers calculated that if a car is within 10 meters (about 33 feet) of the cyclist during a typical time trial that is 50 km (about 31 miles) long, and if they are traveling at a speed of 54 km/h (about 33 mph), then the car would shave 3.9 seconds off the cyclist's time. If the car were within 5 meters (about 16 feet), the cyclist would save 24.1 seconds.

These numbers only apply for individual time trials, when a car directly follows a lone cyclist. But when the total time difference between winning and losing is often mere seconds, a trailing car could offer riders a big advantage. "With this information," Blocken said, "you could influence the outcome of the race."

For safety, a support car must always be more than 10 meters away, according to the International Cycling Union (UCI). But, Blocken said, those rules are rarely enforced, and cars often do get much closer. And as the analysis showed, even 10 meters can give an advantage. Instead, he thinks cars should stay 30 meters back, a distance far enough such that they will have a negligible influence.

He has sent his analysis to members of the UCI, but hasn't received a response yet, he said.

This year, Australian Rohan Dennis won the first stage, a short, 8.6-mile circuit through the streets of Utrecht, Netherlands. But did his support car help with the victory?

Given the length of this particular trial, according to the researchers, a car within five meters could cut a cyclist's time by six seconds. Dennis won by five seconds, and the top four finishes were within eight seconds of one another.

But despite the seemingly close times, the cars probably didn't influence the results, Blocken said. The route featured many curves, so the cars wouldn't be directly behind the cyclists. The cars also didn't drive as close as they had in the past—not close enough to have a big effect. In fact, given the stage's relatively short distance, the time differences between the top riders were actually larger than he expected.

Instead, Blocken thinks the heat was the biggest factor. The starting time of individual time trials are staggered, so one rider could experience higher or lower temperatures than another.

The significant influence of a support car isn't too surprising to researchers, said Nathan Barry, an engineering graduate student at Monash University in Australia. He's done his own wind-tunnel experiments that show the same phenomenon with multiple cyclists, and has also wondered about the effect of a support car. "It's good to see that it's actually being quantified," he said.

What he'd like to do next, Barry said, is to measure the effect of camera-carrying motorcycles, which sometimes follow the cyclists even more closely than the cars to capture the dramatic footage you see on TV.

"That offers potentially a very real bias to certain athletes," he said. "In certain time trials, at least, the more high-profile athletes get a chase motorbike. Others don't. If you have a motorbike effectively giving you a push forward, that's certainly a disadvantage to those without one."

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Marcus Woo is a freelance science writer based in the San Francisco Bay Area who has written for Wired, BBC Earth, BBC Future, National Geographic, New Scientist, Slate, Discover, and other outlets.