Tag: Academic research

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.” 

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. 

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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 baseballs have those nice 108 double stitches. If those rough features of those balls were not there, you would not get anywhere near the kind of distance when those balls are thrown or hit that you see now,” Goff says. “There has to be some kind of a roughness on the ball to move this transition to a smaller speed.”

New grooves

Subsequent designs have been able to push the drag crisis to lower speeds, according to the analysis by Goff and his colleagues. The Brazuca ball used in 2014, for instance, has only six panels, but their total seam length is much longer, adding to the surface’s roughness. And this year’s Trionda ball contains just four panels, but each panel also has three deep grooves for more texture. 

There’s a trade-off to this roughness, though. While Goff and his colleagues found that the Trionda ball experiences the drag crisis at the slowest speed since 2010, its drag coefficient is also higher than that of the other balls at high speeds. That means that even though the most dramatic change doesn’t happen until the ball is moving quite slowly, the ball will still slow down faster than its recent predecessors during the faster portion of its flight. So the trajectories of long kicks may be a few meters shorter, Goff says. Adidas did not respond to a request for comment.

Fortunately, players in the upcoming World Cup should already be familiar with these added nuances, as they’ve had access to the new ball for at least a few months. The ball, Goff notes, is quite similar to Nike’s Flight ball in design, so players who’ve spent more time with that ball may have an added advantage. 

Meanwhile, Goff continues sending the group’s papers to his colleagues FIFA and Adidas in hope of providing some new insights, and he’s been sent balls by Adidas in the past. Adidas does perform its own unpublished tests of each new ball. The New York Times reported last year that the Trionda’s 3.5-year testing process included robotics designed to kick the ball at specific speeds as well as testing in seven of the 16 host locations. 

But as Goff sees it, soccer is “the world’s most popular sport, [this is] its most important tournament, and the most important piece of equipment in that tournament is this ball right here,” indicating the the Trionda ball that he had on camera with him during our Zoom call. “I think they’re interested in what some external testing looks like.”