Most children who kick an inexpensive soccer ball probably don’t realize that they are playing with an object similar to a truncated icosahedron. For that is what traditional soccer balls, with their 20 white hexagonal panels and 12 black pentagonal panels most resemble, to one familiar with Archimedean solids. That mathematical pattern was also seen on the Adidas Telstar, used in the 1970 World Cup in Mexico. Moreover, the black-and-white pattern helped people without colour televisions see the ball in flight. Flash forward more than half a century to today, when people watch technologically-advanced soccer on high-definition smartphones. For all the available real-time player biometric data, synthetic pitches, clothing made of smart materials, and multi-camera virtual-reality-assisted refereeing, clubs must still get round balls into nets if they wish to advance to glory.
Evolution of World Cup soccer balls
This article’s space is devoted to the aerodynamics of the main piece of equipment used in the world’s most popular sporting event, the ball. Soccer balls are introduced for each new World Cup with hosting countries playing pivotal roles in the naming of the balls and how the balls are coloured. In this piece, limited article space necessitates that we focus on the men’s World Cup. The 32-panel ball geometry was used by Adidas up until and including the 2002 World Cup in South Korea and Japan. But computer design and manufacturing methods helped to create the Adidas Teamgeist, a 14-panel ball used in Germany for the 2006 World Cup. Besides possessing fewer panels, the Teamgeist was the first World Cup ball with thermally-bonded panels, which kept water out of the ball’s interior more effectively than stitches. The eight-panel Jabulani that Adidas introduced for the 2010 World Cup in South Africa needed textured panels to avoid being too smooth.
Surface roughness and aerodynamics
Why is ball surface smoothness important? It turns out that if a soccer ball is too smooth or too rough, the ball’s aerodynamic properties will render the ball useless for match play. The range of ball speeds for corner kicks and free kicks is close to what’s called the “drag crisis.” Airflow around a slow-moving soccer ball separates from the ball about halfway around from the front. The separation is generally quite smooth and regular, often called laminar air flow. For speeds above the drag crisis, air flow around such a fast-moving ball separates farther back on the ball, and the separation is turbulent and irregular, called turbulent air flow. For a given speed, balls with laminar air flow experience larger air drag compared to balls with turbulent air flow. Also, perhaps counterintuitively, rough balls have a drag crisis at smaller speeds compared to same-sized smooth balls. What all that means is that a hard-kicked soccer ball that is too smooth will slow down too fast due to too much air drag.
Despite the added panel texturing, Jabulani suffered from a drag crisis at too large a speed, which meant that the air drag on it was noticeably too large on some corner and free kicks. This was the cause of serious controversy, whereby feedback from various professional players was compiled by former Liverpool player Craig Johnston and sent to FIFA. The feedback outlined the perceived failings of the ball and requested it to be abandoned by FIFA.
Adidas and other ball manufacturers must ensure that the new balls they create possess aerodynamic properties very similar to the balls that the players have become accustomed to playing with. The last thing players, fans, and equipment manufacturers want is for the most important piece of equipment to behave in unfamiliar ways on the world’s biggest stage.
No more Jabulani mistakes!
When Adidas unveiled the official match ball for the 2014 World Cup in Brazil, the Brazuca, the company had corrected the mistakes found with Jabulani. Brazuca had just six panels, two fewer than Jabulani, however Brazuca’s surface texturing was rougher than Jabulani’s, and Brazuca’s total seam length was 68% longer than that of Jabulani. Brazuca’s reduced panel number was therefore compensated by rougher panels and a greater total seam length. Its surface was rough enough that its aerodynamic properties were back to what players were used to.
The Telstar 18, the Adidas ball used in the 2018 World Cup in Russia, had six panels, just like Brazuca. By 2018, one may need training in topology to understand how to create a six-panel soccer ball that is a great approximation to a sphere! One can easily tell when one feels the panels of Brazuca and Telstar 18 that the former ball’s panels are rougher than the latter’s. To make up for a possible lack of overall roughness, Telstar 18 had a total seam length that was 30% longer than that of Brazuca. Once again, aerodynamic properties for Telstar 18 were aligned with what players expected.
Soccer Ball for the upcoming Qatar 2022 World Cup
Instead of staying with six panels, or even trying for fewer panels, Adidas created a 20-panel ball, the Al Rihla, to be used in the 2022 World Cup in Qatar. High summer temperatures forced organising committees to set the World Cup dates to mid-November to mid-December. An open-roof stadium capable of keeping players and fans cool with air conditioning is just one piece of technology on display for the new World Cup. Among the many new ball features, panel texturing includes debossed square and oval shapes. Eight of the 20 panels are triangles with social messaging in various languages. The 12 bigger panels are shaped a bit like Drumstick ice cream cones. Despite having 14 more panels than Brazuca, Al Rihla’s total seam length is just 6% longer than Brazuca’s. But Al Rihla’s total seam length is nearly 19% shorter than Telstar 18’s. What keeps Al Rihla’s surface approximately as rough as Telstar 18’s surface are its seams. About 2.5 mm wider and 0.5 mm deeper as compared to Telstar 18’s, Al Rihla’s seams provide the added surface roughness.
Myself and colleagues at the University of Tsukuba in Japan have tested Al Rihla in a wind tunnel (see figure below). Wind-tunnel testing and trajectory analyses have shown that the Al Rihla’s drag crisis is right where it needs to be and its flight characteristics should be similar to the ball used in the previous World Cup.
You can work in sports science too
There is so much more physics to explore with World Cup soccer balls! Space constraints prevented a look at balls spinning and bending through the air, ball-boot interactions, headers, and so on. For those readers who are students, consider working with myself at the University of Lynchburg on exciting sports physics research.