Why the Adidas soccer ball, Jabulani, promises to confound goalies with its ‘otherworldly’ behavior.
As the World Cup finals kick off, the most talked about and controversial figure in the game isn’t a player but Jabulani, the new Adidas World Cup soccer ball (the name means “to celebrate” in isiZulu). Designed by researchers from Loughborough University, United Kingdom, the ball has drawn considerable criticism from players. Goalies in particular have described the ball’s in-air behaviour as “ridiculous”, “shameful” and even “supernatural”. Not only does it travel faster than previous World Cup models but players say the ball’s curve through the air or “bend” is erratic and unpredictable.
This unexpected behavior, explained Adelaide University physics professor, Derek Leinweber, in a recent pod cast, is due to Jabulani’s combination of unique design features. First, he says, 2010 World Cup ball differs from the classic 32-panel design in that Jabulani has no stitched seams. Instead, it is composed of eight thermally bonded segments, moulded from thermoplastic polyurethane-elastomer to allow for a nearly perfect sphere.
This results in a soccer ball with significantly improved aerodynamics. As Prof. Leinweber explains, the travel of a sphere through a fluid like air creates a very thin boundary layer or ‘skin’ of airflow. At relatively slow speeds, this boundary layer moves over the ball’s surface in a smooth, even and, in aerodynamic parlance, laminar flow. More importantly, at slow speeds, the boundary layer separates from the ball’s surface at a 90-degree angle—imagine rain hitting the top of an umbrella, running down the sides and dripping off the edge—which creates a large wake behind the ball with a high drag coefficient.
However, when a soccer ball travels above a critical speed, the airflow becomes turbulent and the boundary layer “sticks” to the ball’s surface, separating at around a 120-degree angle. This time, imagine raindrops falling on a beach ball, rolling down its sides and dripping off near the bottom. The result is a much smaller wake and therefore less drag.
According to Leinweber, skilled strikers will use this phenomenon to their advantage. As the ball comes off a player’s foot, the airflow is turbulent and efficient. But, as the ball slows in flight and dips below the critical speed, the airflow becomes laminar and the sudden increase in drag acts like an air break. Therefore, a properly struck ball can fly fast enough to arc over a goalie’s head but then suddenly decelerate, drop below the crossbar and enter the net.
What makes the Jabulani ball frustrating for goalies and strikers alike is that the ball travels faster for a longer period of time, because its turbulent-to-laminar transition speed is lower than the classic 32-panel ball. While some players have put the ball’s speed down to it being lighter than previous designs, Jabulani, at 440 +/- 0.2 grams, is actually at the heavier end of FIFA’s design guidelines (420-445 grams).
If the increased speed of incoming shots weren’t enough, World Cup goalies have also complained that Jabulani curves or “bends” much more dramatically and unpredictably. According to Leinweber, this is due to what Adidas markets as its Grip ’n‘ Groove profile. Unlike the smooth surface of the 2006 World Cup “Teamgeist” ball, Jabulani has “aero grooves” and concentric circles of ridges that cover its surface.