The results of Blocken's CFD study confirmed the 30 percent to 35 percent reduction in air resistance for the trailing rider, but they also showed there is an effect for the leading rider -- specifically, a reduction in resistance of about 2 percent to 2.5 percent, which even goes to 3 percent if there are additional trailing riders.
The physical explanation for the findings is that a cyclist experiences air resistance caused by the overpressure of the wind on the front part of his body, pushing him backwards, and by the underpressure on the back part of his body, which sucks him backward. This underpressure is determined by the "wake" or "slipstream" behind the back of the cyclist, so when a second cyclist rides behind the first, he or she fills the wake, reducing the underpressure, and, in turn, creating less air resistance for the first rider.
While 2.5 percent seems like a minor gain, Blocken says it can actually translate into quite an advantage in cutthroat competitions. According to his calculations, there can be a 50-second gain on a distance of 50 kilometers, which happens to be the time trial distance for the Tour de France.
"Because time trial races are often won with a time difference of a few seconds, the 2.5 percent can clearly be decisive and determine whether a team wins or loses a race," according to summary documents prepared by ANSYS on the study. "The same applies to sprint races, which are sometimes won based on a few centimeters that have to be decided by photo finish."
One other note about the study's findings: The 2.5 percent applies to cyclists with identical body shapes and sizes, so if the second cyclist is larger/wider/taller than the leading rider, the reduction in air resistance for the first one will be even greater, Blocken said. This can be particularly useful for competitive riding teams developing a riding strategy, according to Thierry Marchal, industry director at ANSYS.
"This could have a major impact on preparing a time trial for the Olympics to define a strategy of where to put people," Marchal explained. "If you look at six cyclists as a system, not everyone is equally strong. This helps you think about where to put people."