Why this year’s World Cup ball may not fly as far

Much is new about this month’s upcoming FIFA World Cup tournament, which will be held in the US, Canada, and Mexico. It hosts more teams than ever before. It’s the first to occur in three different host countries. And, like predecessor cups for over half a century, it will employ a soccer ball with a brand-new design. One group of researchers that has been testing the physics of World Cup balls for the past 20 years recently studied this new entry, called the Trionda. Made by Adidas, the Trionda features four red, green, and blue panels textured with deep grooves and maple leaf, green eagle, and star emblems to represent the three host countries. Through wind-tunnel experiments, the research team found that this ball improves over previous versions in some ways, but long-distance kicks might not go as far as they did in the past. “The simple picture is that Trionda may very slightly punish extreme distance, but it should reward clean technique and predictable flight,” says team member John Eric Goff, who researches sports physics and is an incoming professor of engineering practice at Purdue University. “Goalkeepers, defenders hitting long passes, and long-range shooters are where I would look first for visible differences.” Researchers used a wind tunnel to study the Trionda ball at the University of Tsukuba. TAKESHI ASAI, SUNGCHAN HONG, AND RICHONG LIU Adidas has been designing new balls for each World Cup since the 1970s. Some of the design changes in the first few decades were aesthetic: The 1986 ball featured graphics inspired by Aztec temples for the Mexico tournament, and 1994’s had space graphics in honor of the moon landing’s 25th anniversary. There were some structural differences too, such as upgraded foam cores and improved water resistance. But by and large, the balls used the same design of 32 pentagonal panels stitched together. That changed in the 2006 World Cup in Germany, when Adidas introduced the +Teamgeist ball. It featured just 14 curved panels, which were thermally bonded together rather than stitched. The design helped keep moisture out so the ball wouldn’t grow heavier throughout the game, Goff says. It was around this time that he started studying soccer balls. In the years since then, he and his colleagues have followed the transformations as Adidas has released balls with different surface textures and even fewer panels—design changes significant enough to affect game play. In-flight motion Goff discovered early on that by analyzing a ball’s trajectory data, he could derive its drag coefficient—a number that determines the air resistance it experiences midflight at a given speed. Shortly after, he began working with a team in Japan to analyze how the World Cup ball’s in-flight behavior changes with each new design. The experiments, carried out at the University of Tsukuba in Japan, have been purposely consistent over the years because “maintaining continuity is important for comparing new data with historical data sets,” says Takeshi Asai, a professor there who works on the experiments. They entail attaching the ball to a metal rod connected to an instrument called a force balance, which measures aerodynamic forces such as drag and lift as the ball is exposed to the same wind speeds it would experience in a real soccer game—seven to 35 meters per second. The team tests the ball in different orientations, “but you can only do a few because the Trionda ball is $170,” Goff says, and each new test effectively destroys it. The experiments show the team how the drag coefficient changes with speed, and Goff then writes code to simulate the ball’s overall trajectory as it flies through the air. The team’s analysis has shown how recent World Cup balls evolved since the eight-panel Jabulani ball for the 2010 event. The Jabulani faced much criticism from players—particularly goalkeepers, who said it had a deceptive trajectory that “dipped wickedly,” as one player told the Guardian . ALAMY ADOBE STOCK TAKESHI ASAI, SUNGCHAN HONG, RICHONG LIU The 2010 Jabulani ball (left) had eight panels and a smooth texture that translated into unpredictable performance. Later balls, like the 2014 Brazuca (center) and this year’s Trionda (right) have fewer panels but more roughness. The ball had one key flaw: It was too smooth. Even though its drag coefficient was relatively low at high speeds, once the ball slowed to a certain point the coefficient would ratchet up, causing it to lose speed quite fast and behave as the 2010 players complained. This sudden transition—called the drag crisis—occurs at higher speeds for smoother balls, but with added texture like seams and grooves, the transition can be avoided until a ball reaches lower speeds. This allows the ball to travel farther and generally behave in a more predictable way during typical play. “It’s the same reason why golf balls have dimples and baseball