01 March 2008

The Science Of Spin

Written by Published in Technology

'Most of today’s elite players use sidespin but a gifted few, such as Beckham and the Brazilian, Juninho Pernambucano, manage to squeeze some topspin into their deliveries, making the ball both swerve and dip.'


In my work as a sports scientist I analyse a great deal of football videotape. Sometimes this is high-quality footage shot in the lab, or occasionally during my assignments with professional clubs. Movement sensors, combined with powerful software, open a window on the game’s key moves. We can look at the biomechanics of a player’s kicking action or the detail of a goalkeeper’s dive in a penalty. We can also trace, in 3-D, the curving path of a David Beckham special and, more importantly, simulate free-kicks like his using computer models. Free-kicks are important: they are often potential match-winners when games have reached defensive stalemate, and so a little look at the history is revealing.


Spin’s the thing


Until the mid-1950s free-kicks were very much hit-and-miss. The tactic was to blast the ball at the defensive wall, hoping for a gap or a fortuitous deflection. A young Brazilian midfielder nicknamed Didi changed all this. He discovered that a ball kicked with spin would swerve in flight, and by intensive practice learned how to ‘shape’ his deliveries over or around the wall. His technique was like a scalpel replacing a bludgeon. He kicked with sidespin so the ball spun like a top about a vertical axis and deflected sideways. Most of today’s elite players use sidespin but a gifted few, such as Beckham and the Brazilian, Juninho Pernambucano, manage to squeeze some topspin into their deliveries, making the ball both swerve and dip. Spin is easily seen in practice by following the movement of the ball markings in the many slow-motion replays of televised games. Didi and his successors understood ball spin intuitively, but modern aerodynamics reveals the science behind the swerve. A non-spinning ball has no deflecting force because the airflow around it is symmetrical. Spin disrupts the flow pattern, producing a force that depends on the ball’s speed and spin rate combined. Under the right conditions this force can easily exceed the weight of the ball so it’s not surprising that it moves so much in a good free-kick.


Goalkeepers always complain


Modern ball design has accelerated development of these skills and goalkeepers are in danger of being left behind. Today’s football is a high-tech product made from synthetic materials designed to improve the energy transfer in a kick. Greater initial speeds result and slick aerodynamics ensure that the ball flies faster. A free-kick from 25 yards takes about eight-tenths of a second to reach the goal; not much time to assess the movement and shape up for a save. But that’s not the half of it. In 2006, goalkeepers’ reactions to the new World Cup ball were unusually vociferous. ‘It moves more’ and ‘It’s unpredictable’ were frequent complaints. I took these comments with a pinch of salt, but looked carefully at the television replays of the scoring free-kicks. Most were conventional sidespin efforts, but some were completely anomalous. The ball moved first in one direction, as in a conventional kick, then with a mid-air twitch, deflected another way. Close study of the video footage was very revealing. The ball markings showed that there was virtually no spin. Since 2006, the evidence for this radical free-kick has been mounting but one player, Manchester United’s Cristiano Ronaldo, stands out. Portsmouth’s goalkeeper, David James, was the latest victim of a Ronaldo sucker punch. From video evidence it’s clear that Ronaldo’s free-kicks are repeatable, not flukes. As in the World Cup footage, the ball moves unpredictably and the ball markings indicate a very low spin rate. What could possibly be going on?


Kicking a knuckleball


In baseball, pitchers use spin to throw a curveball, much as footballers do in a swerving free-kick. Aerodynamic effects ensure a similar outcome – a regularly curving trajectory. But in baseball, pitchers sometimes throw a wicked delivery called a knuckleball. Here the ball is released at speed, but with very little spin. It dodges about unpredictably in flight and the batter is often reduced to swiping at fresh air when the ball arrives. This behaviour is well understood and is caused by the ball’s seam drifting into and out of the air flow as it gently rotates. Each incursion of the seam causes aerodynamic instability and a resultant deflecting force. An instant later a different portion of the seam comes into play and the direction of the force switches, hence the chaotic movement. Is it possible to kick a knuckleball? It seems highly likely that Ronaldo has discovered the knack and is exploiting the seam pattern on the modern football. Whether you think this is a good thing or not depends on your stance on fairness. My personal view is that genuine skill should be rewarded in football. It’s actually very difficult to kick a ball with a very low spin rate, much harder than applying controlled spin. The difference, though, is that Beckham can reasonably predict where his shot will end up, but with Ronaldo, after the ball leaves his foot, it’s all down to the physics of chaos. And it makes goalkeepers look like complete idiots.


Making the perfect ball


At this very moment, Adidas’s technologists are designing the ball to be used for the 2010 World Cup in South Africa. Sales for the 2006 ball, the Teamgeist (team spirit), were estimated to exceed 10 million units, a lot of product and a lot of profit. FIFA’s rules are exacting and control the ball’s weight, its size, sphericity, recoil in a bounce and water absorption. There is no mention of aerodynamic performance, however, and no control over how much the ball should swerve for a given speed and spin. Such a ball could be designed with today’s technology and knuckleball behaviour could be completely eliminated. Would this be a good thing? My answer is an emphatic ‘Yes’ – fair play for goalkeepers! But you can watch Euro 2008 for yourself this summer, where Ronaldo will be strutting his stuff, and make your own mind up.


Ken Bray is visiting Fellow of the Sport and Exercise Science Group at the University of Bath and has acted as a consultant to Southampton Football Club. His first book, How to Score, is published by Granta

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