The mystery of just how Christiano Ronaldo and Gareth Bale manage to bamboozle some of the world’s greatest goalkeepers with their zigzag “knuckleball” free kicks has finally been unlocked.
While footballers such as David Beckham (inset) manage to make the free kicks look effortless, scientists have until now been as flummoxed by the way the ball whizzes first one way and then the other as the goalkeepers trying to stop them. However, researchers have brought state-of-the-art wind tunnels and high speed cameras – and a specially constructed mechanical kicker – to bear on the problem and they believe they have solved the conundrum.
The knuckleball has its origins in baseball as a slow pitch, or throw, that has an erratic flight. It involves releasing the ball from the knuckles of the first joints of the index and middle finger and has been attributed to a number of pitchers in the early 1900s.
While relatively common in the baseball pitcher’s repertoire there are fewer exponents in professional football, most notably Juninho, Andrea Pirlo, Gareth Bale and Christiano Ronaldo – who manage the feat with a booted foot rather than the flexibility and control of a hand.
“Unsteady lift forces”
Knuckleball shots in football and pitches in baseball, in which the ball curls first one way and then switches to the other direction, are delivered at quite slow speeds and with little or no spin. It was originally thought that the knuckleball effect was achieved by airflow turbulence caused by the seam of the ball, but footballers continued to be able to produce the same effect with seamless balls.
The unique flight of the knuckleball is due to “unsteady lift forces”, according to scientists at the Ecole Polytechnique and ESPCI ParisTech in Paris.
The research required the key development of a “kicking machine” to deliver balls at different velocities with very little spin. The motion and airflow from each delivery were captured using a high-speed camera and wind tunnel.
The strength with which the ball is kicked provides it with the force it needs to shoot forward, and as it does so the air underneath provides it with a degree of lift. The researchers established, however, that for a particular range of velocities – the slower shots – the air underneath and alongside the ball becomes comparatively unpredictable, providing “unsteady lift force”.
“No spin” is key
Key to the footballer’s skill, the team realised, is the ability to strike the ball in such a way as to ensure there is no spin. If the ball is spinning it has the effect of producing a uniform lift force, whereas when there is no spin the lift force becomes unstable.
This unsteady lift force affects the layer of air around the ball, encouraging and amplifying side-to-side movement, and results in the ball flipping from one direction to another.
The velocity range required to produce unsteady lift determines the maximum strength with which the ball can be kicked to get the full effect and explains why footballers and baseball pitchers deliver it unexpectedly slowly.
Baptiste Darbois Texier added that the reason why the knuckball effect is absent in most ball games is because the distances the ball travels are too short.