Too Smooth: Football and the “KnuckleBall” Problem

Picture a football (soccer ball) in your head and you probably see the cartoon ideal—a roughly spherical shape made with polygonal patches that are sewn together, usually in a familiar pattern of black and white. A great many balls were made along these lines for a great many decades.

Eventually, though, technology moved on. Footballs got rounder, smoother, and more colorful. This was seen as a good thing, with each new international competition bringing shiny new designs with ever-greater performance. That was, until things went too far, and the new balls changed the game. Thus was borne the “knuckleball” phenomenon.

Smoother Is Better, Right?

An Adidas Telstar from the 1974 World Cup. The original Telstar design actually predates 1974, and the combination of 12 pentagonal and 20 hexagonal panels and black and white patterns had been used before, too. Regardless, the Adidas design was soon globally known, and eventually became the “default” design for a football in media. Credit: shine2010, CC BY 2.0

From the late industrial era onwards, footballs were traditionally made with leather panels wrapped around some form of rubber bladder. As it’s not easy to produce a seamless leather sphere, balls were instead sewn together from individual leather panels in order to create a vaguely spherical whole. Early designs had few panels, and weren’t particularly good at approximating the shape of a sphere. They often had large, wide seams that stretched far across the surface of the ball. Larger seams were by and large undesirable, as they made the ball harder to control. Ridges where the panels met could catch on the foot and lead to unpredictable behavior.

The 32-panel design persisted at the World Cup level until 2002, with the Fevernova ball. Credit: Nicola, CC BY-SA 4.0

Over time, there was a desire to create smoother, rounder balls for professional play. By the 1970s, football designs began to coalesce around a common format. The standard became 12 pentagonal and 20 hexagonal panels, which could be sewn together into a relatively good approximation of a sphere. This  also allowed the construction of a ball with very fine seams, creating a more predictable ball which enabled far finer control. The format was perhaps best popularized by the Adidas Telstar as used in the 1970 and 1974 World Cups. Even though it wasn’t the first to use the 12-and-20 design, the layout and the black-and-white pattern has been firmly etched in footballing consciousness ever since. Indeed, starting at the 1970 World Cup, Adidas has made every following World Cup ball since.

For a time, it seemed as if the design of the football was settled science. Adidas stuck with the 32-panel design up until the 2006 World Cup, when it revealed the +Teamgeist design. It used 14 curved panels that were bonded instead of stitched, creating a smoother ball for yet more predictable handling. It was also intended that the design would be more waterproof, to avoid gaining weight in wet matches. The ball was criticised by some for its erratic flight patterns, but by and large was seen as fit for purpose.

The Jabulani proved controversial, with its ultra-smooth design criticised for creating its unpredictability in flight. Credit: Adidas, editorial use

With another successful World Cup under its belt, Adidas innovated further for the 2010 World Cup. It created the Jabulani, which consisted of just eight spherically-moulded panels bonded together into an ultra-smooth and  cohesive sphere. If the 2006 World Cup ball was slightly controversial, the Jabulani was outrageous. The ball was referred to as “supernatural” for its tendency to suddenly change direction in flight, which pleased strikers to a degree, but frustrated goalkeepers to no end. Ultimately, though, this tendency wasn’t good for getting balls on target. Just 147 goals were scored in the 2010 World Cup, the fewest since the competition changed its tournament format in 1998.

The problem came down to a phenomenon known as “knuckling.” This happens when the ball is travelling through the air with little to no spin. At a certain speed, the seams on the ball tend to interact with the airflow, channeling it such that it creates sudden and unpredictable movements. The term first developed in baseball, but became relevant to football with the development of the 2006 and 2010 World Cup balls, which suffered this phenomenon more often in play.

The phenomenon became so well known that NASA scientists took the opportunity to throw World Cup balls in a test chamber to demonstrate the effects at play. Knuckling behavior tends to peak at a certain critical speed, with the effect lessened either side of the peak. The problem was that the smoother designs were “knuckling” at higher speeds than balls from previous generations. NASA researchers found that the Jabulani would undergo unpredictable flight due to knuckling at speeds of 50 to 55 mph—right around the speed at which professional strikers can deliver a ball to the net. Meanwhile, more traditional 32-panel balls tend to see a peak in knuckling around 30 mph. Since strikers were typically kicking beyond this speed, knuckling—and thus unpredictable flight—wasn’t such a problem with the older designs.

The Brazuca demonstrated a critical knuckling speed closer to a traditional 32-panel ball in NASA’s tests. “What we are looking for in the smoke patterns is at what speed the smoke patterns suddenly change,” notes Rabi Mehta, NASA’s chief of the Experimental Aero-Physics Branch. “This is when the knuckling effect is greatest.” Credit: NASA

Being well aware of the problem after the 2010 World Cup, Adidas went back to the drawing board and developed the Brazuca for the 2014 competition. The number of panels was reduced yet further to just six. However, Adidas wasn’t intending to just go smoother yet again. Instead, the Brazuca had longer, deeper seams than the Jabulani, and panels covered in textured bumps. Through the company’s careful design efforts, this brought the critical knuckling speed back down to around 30 mph, much more akin to a traditional 32-panel ball.

By and large, the Brazuca proved far less controversial than its two predecessors. Search for articles on the 2014 ball, and you’ll find a little speculation from before the World Cup, before the story died completely once competition began. No more were unpredictable balls confounding the world’s finest footballers. Meanwhile, democratically speaking, where the +Teamgeist and Jabulani each have hundreds of words spilled on Wikipedia over controversy and criticism, the Brazuca has none.

Adidas gave the Brazuca deeper seams and a bumpy finish to improve its stability in flight. Credit: Nicola, CC BY-SA 4.0

The story of the Jabulani is one of unintended consequences. Adidas had intended to improve its product in a predictable and routine manner, only to find an unexpected effect at play which threw a spanner in the works. Once the effect was understood, it could be controlled and refined out with careful design. Football hasn’t suffered a “supernatural” ball since, even as the technology marches ever further into the Smart Ball era. Still, who knows what comes next at the 2026 World Cup?

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