BACK in the day when Babbage played football (with a round ball, not the pointy sort), the game was an attacking one, with five forwards, three halfbacks, and just two defenders to assist the goalkeeper. Twinkle-toed magicians like the late Sir Stanley Matthews apart, the heavy leather ball—cobbled together from 18 rectangular panels, with thick lacing across the slit for inflating the rubber bladder within—robbed players of any attempt at subtlety.
When dry, the old leather ball moved with reasonably predictability. But on a rain-soaked pitch (the rule in Babbage’s northern England), the sodden leather lump would slither through the muddy grass as if in treacle. Heading it from a corner kick could leave a player dazed for days. The beautiful game was more in the mind than on any wind-swept heath.
Football today is a far better spectacle. English teams may still insist on attacking down the wings, only to lose possession with the hit-or-miss of an airborne cross into the goal area (one reason, many suggest, for the national side’s long drought in World Cup success, despite an abundance of talent), but the modern game—when played at its highest level in places like Spain, Portugal, Italy, Argentina and Brazil—is all about maintaining possession, while probing defences and patiently building an attack. Such defensive play may produce fewer goals. And regretfully too many tournaments are settled these days by penalty shoot-outs. But when the ball is in motion, the choreography and artistry of today’s top-class players can be spell-binding beyond belief.
Depending as it does on delicacy of touch, the game demands a lot from the ball itself. It needs to be neither too heavy nor too light (15 ounces, or 425 grams, is reckoned to be about right) and should respond deftly to the tap of the foot. Yet, at the same time, the ball must be able to cannon off the boot in a shot at goal. If that were not enough, it should fly far and straight for a long pass, while being able to swerve abruptly in a free-kick or penalty. All told, the ball needs to have a reasonably high coefficient of restitution (efficiency of absorbing energy from a kick), but not so much as to bounce around the pitch like a beachball. Such conflicting requirements make designing match balls a challenging exercise, as much in aerodynamics as in mechanics and materials science.
Thanks to computer modelling and wind-tunnel testing, the Brazuca ball designed by Adidas for the 2014 World Cup, now underway in Brazil, is the best yet—and certainly a vast improvement over the disastrous Jabulani ball Adidas provided for the 2010 World Cup in South Africa. The eight-panel Jabulani was criticised by players and coaches for being too smooth and too light, and for changing direction erratically during flight. By contrast, the Brazuca uses six x-shaped panels in a slightly heavier case, with much deeper seams and a dimpled finish similar to that of a basketball.
Adidas has been the sole supplier of balls for the World Cup since 1970. The late Adolf (“Adi”) Dassler, founder of the Adidas sportsware company in Bavaria, is said to have come up with the modern football design—with its 32 patches of pentagons and hexagons—as a way of making the bulbous old leather ball more spherical. Its familiar image of black pentagons and white hexagons resulted from the need to make the ball more visible on monochrome television sets of the day. The coloured patches also helped players—especially goalkeepers—judge the swerve on the ball better.
The idea of forming a sphere from a patchwork of 12 pentagons and 20 hexagons (known in solid geometry as a “truncated icosahedron”) was borrowed from the late Buckminster Fuller, an American architect and futurist, whose geodesic domes based on the same principle were popular during the 1960s. So much so that the Adidas ball, officially called the Telstar, was often referred to as simply the Buckminster (in much the same way that nanotechnology’s hollow molecules composed of pentagon/hexagon lattices of carbon atoms became known as “fullerenes”, and their spherical versions as “bucky balls”).
Over the years, the Buckminster ball—with its outer cover of polyurethane and poly vinyl chloride, has served league football well. Absorbing little water, its weight, size and compliance barely change during a match. Apart from being more spherical than the old 18-panel leather ball, the 32 panels of pentagons and hexagons (60 vertices and 90 edges all together) provide more seams which, like the dimples on a golfball, help “trip” the air flowing over it.
Counter-intuitively, forcing the boundary layer of air passing over the surface of a ball in flight to flip from smooth, laminar flow to turbulent flow improves the ball’s aerodynamics no end. Instead of the boundary layer becoming detached from the surface as the air flows over the ball’s widest section, the more energetic turbulent air clings to the surface, delaying the onset of separation until it is much farther around the backside of the ball. The result is a smaller wake, and thus lower drag. With less energy spent on churning air in the wake behind it, the ball has more for moving forward—and thus flies farther and truer.
Like golfballs and other spherical sports balls, footballs rarely move through the air without at least some form of spin. Goalkeepers kick the ball up field with plenty of backspin, to gain height and distance. Indeed, most players pass the ball with a modicum of backspin, to help it stop dead at a colleague’s feet.
It does so because, with backspin, the ball’s upper surface is moving backwards relative to ball’s line of flight, while the lower side is moving forwards. The relative velocity of the air flowing over the top of the ball is therefore much greater than the air flowing under it. The result, known as the Magnus effect, is an upward force on the ball, like the lift of an aeroplane wing, which helps the ball scoot over the turf. On landing, the ball’s forward spinning under surface acts as a brake, bringing it quickly to a halt.
The Magnus effect—albeit about a vertical, rather than a horizontal, axis—is used most dramatically in free-kicks taken around the penalty area, where one in five attempts finish up in the net, according to World Cup statistics. Striking the ball with a glancing blow from the inner or outer side of the toe-cap can impart significant spin to the ball about a near-vertical axis, causing it to swerve left or right. David Beckham’s fame rests largely on his ability to score seemingly impossible goals from free-kicks by bending the ball unexpectedly around a wall of defenders.
The Brazuca ball is unlikely to disappoint players and spectators in Brazil. Two researchers, Sungchan Hong and Takeshi Asai, at the University of Tsukuba in Japan subjected five different ball designs to wind-tunnel tests and a kicking machine. Their studies, published in a recent issue of Scientific Reports, show why the Jabulani ball behaved so erratically. The ball’s drag varied excessively, depending on which face was pointing forward. It also suffered more variation in lift and side force than any of the other balls tested. Hence the wobbling players complained about in South Africa. Of the bunch, it was by far the worst.
Adidas clearly learned an important lesson. Some 600 professional football players have been practicing with the new ball for the past three years and sharing their experiences with the company. Meanwhile, results from Tsukuba show it behaves consistently, no matter which face is pointing forward. In wind-tunnel tests, it had the lowest variation in lift and side force of all the balls examined. Trajectories from the kicking machine hit the same spot repeatedly.
By all accounts, the Brazuca will fly far and straight when asked to do so, while allowing players to bend it like Beckham when needing to curl it round a wall of defenders. In Brazil, players will not be able to blame the ball for missed opportunities, as they could (and frequently did) in South Africa. Perhaps, then, football lovers the world over will be able to focus solely on the beauty of the game when played by the greatest practitioners around.
