At the 2010 World Cup in South Africa, the official Jabulani ball was the subject of complaint from many players – especially goalkeepers – for its erratic flight. For this past summer’s tournament in Brazil, Adidas designed a new ball, Brazuca, that was different not only in color and design but also surface geometry and texture. Players consistently approved the new ball. Physicist John Eric Goff explains how it works.

 

(Choo Yut Shing/Flickr)

(Choo Yut Shing/Flickr)

 

Since the 1970 World Cup, Adidas has supplied the ball that players use to battle for global supremacy on the soccer pitch. From that tournament in Mexico through the 2002 World Cup in Japan and South Korea, the ball’s design held to the traditional 32 panels that many of us know from our childhoods. The traditional ball’s 20 hexagons (usually white) and 12 pentagons (usually black) were similar in shape to what mathematicians call a truncated icosahedron. Color designs changed through the years, but the surface geometry of the balls remained the same until 2006. When Germany hosted the World Cup, the official ball was the Adidas Teamgeist, which had just 14 panels that were thermally bonded together instead of stitched.

Reducing the number of panels on a soccer ball makes the ball smoother. Though it may seem counterintuitive, making a ball smoother makes it less aerodynamic, in that it will experience more drag for a given speed through the air. Just think about all those wonderful dimples on a golf ball that cut air drag enough to allow for drives that reach a sixth of a mile (more than a quarter of a kilometer). When Adidas unveiled Jabulani, the ball used in South Africa for the 2010 World Cup, fans noticed that its surface was textured. That was because the ball had just eight thermally-bonded panels, and those panels had to be textured so that the ball would not experience too much air drag. Unfortunately for Adidas, Jabulani was not a popular ball among players and suffered from much controversy during the South African World Cup. Weird “knuckling” effects on balls with little spin baffled a few goal keepers, as, for example, with Keisuke Honda’s free kick goal against Denmark in a group-stage match (note the lack of spin on the ball in the replay).

Brazuca is the ball Adidas created for the 2014 World Cup in Brazil. That ball had only six thermally-bonded panels, and thus also had to have its surface textured. Adidas was under some pressure to create a ball that performed better than Jabulani. Brazuca was a colorful ball with texturing that was different from Jabulani, but it had one major advantage over Jabulani. Despite two fewer panels, Brazuca’s total seam length was 68% longer than Jabulani’s. Adidas was able to accomplish that by designing panels on Brazuca that looked like helicopter blades. More intricate panel geometries meant Brazuca’s total seam length could far exceed that of Jabulani, thus making Brazuca’s surface rougher than Jabulani’s.

So what’s the big deal about a rougher surface? There is a fascinating phenomenon in fluid dynamics called the “drag crisis.” Imagine someone kicking a soccer ball. Earth’s gravitational tug on the ball causes the ball to slow down on the way up to its maximum height and then speed up on its way back to the pitch. Meanwhile, air drag sucks energy from the ball throughout its flight, causing it to move slower than it would if air were absent. As a ball slows down, there is a special speed at which the air flow around the ball changes, leading to an increase in drag. That transition is called the “drag crisis.” Jabulani’s drag crisis set in as the ball slowed to about 54 mph (86 kph), which meant that intermediate-speed kicks always had Jabulani in the middle of its drag crisis.

Roughening a ball’s surface lowers the speed at which the drag crisis occurs. For Brazuca, its drag crisis appeared when its speed slowed below about 38 mph (61 kph). This meant that most intermediate-speed kicks had the ball moving above Brazuca’s drag crisis, thus keeping its aerodynamics more uniform compared to Jabulani’s. Lower drag at intermediate speeds meant that smaller players at the World Cup in Brazil were kicking a ball that had less air drag than what they had kicked in South Africa.

Surface geometry and the amount of roughness play huge roles in determining a soccer ball’s aerodynamics. The effects are imperceptible to mere mortals like us, but elite players, like those who compete in the World Cup, will notice tiny differences in ball flight when the design changes from one tournament to the next. Brazuca’s aerodynamics show it to be a better ball than Jabulani, and more in line with balls used in World Cups prior to 2010. Given that hard-hit balls experience forces from air drag that actually exceed their weight, aerodynamic properties of soccer balls are incredibly important to the world’s most popular game. Players in 2010 noticed hard-hit, non-spinning balls with lateral deflections of about 9% of the balls’ horizontal ranges. That number dropped below 6% for the 2014 World Cup. Without as much lateral deflection in 2014, goal keepers weren’t dealing with as many crazy trajectories as they confronted in 2010.

Soccer played at the highest level should not have significant changes in ball aerodynamics from one World Cup to the next. So why change the ball for each new tournament? This is one question I get asked above all others when having conversations about soccer aerodynamics. From an aesthetic point of view, each World Cup’s host country wants its own design and style for the ball. One can’t say “Teamgeist” without thinking about Germany, “Jabulani” without hearing those vuvuzelas in South Africa, or “Brazuca” without seeing a colorful ball on a wet pitch in Brazil. From an economic point of view, Adidas surely enjoys seeing new balls fly off shelves around the world each time a World Cup commences. For me, I get to study a new ball every four years, learn more about the fascinating realm of fluid dynamics, and use soccer as way to get kids interested in science.

 

John Eric Goff is professor of physics at Lynchburg College. He is the author of Gold Medal Physics: The Science of Sportsand he regular writes about sport and science on his blog.  A more detailed presentation of his findings on the Jabulani and Brazuca balls, based on wind-tunnel experiments, is available from the Journal of Sports Engineering and Technology.

space

space