The science behind the swerve

Adidas's Teamgeist World Cup ball has already been criticised by Germany and Arsenal keeper Jens Lehmann for its unpredictability, but is there any scientific basis for perennial concerns over swerving balls?

In the early 1950s a young Brazilian midfielder nicknamed Didi invented the swerving free kick. He realised that a ball kicked with spin would deflect significantly in flight.

It is no accident that the technique emerged first in the South American game. The leather ball of that era was very prone to water absorption and the weight increase made it much less responsive to the aerodynamic forces caused by the spin. This was seldom a problem in the warm, dry conditions of Brazil but a serious drawback in Europe's winter game.

Not until a ball with a synthetic, impermeable surface was introduced in the 1960s could the technique catch on. European players then became as adept as their Brazilian masters and a long line of expert free-kickers stretches from Didi to the present day.

Few can forget David Beckham's marvellous last-gasp equaliser against Greece in 2001. This wickedly swerving and dipping shot took England to the 2002 World Cup in Japan and Korea and England's recent warm-up games against Hungary and Jamaica show that Beckham is back to top form.

It took the modern science of fluid dynamics to understand exactly what happens in a swerving free kick. When a football moves through the air at low speed the air flow separates from its surface at characteristic points.

A sphere is not a very efficient aerodynamic shape and when this early separation occurs turbulence is created behind the ball. Turbulence always causes drag and if things remained like this a football would be a very sluggish object when kicked.

Above a certain speed - about 12 mph for a football - a miraculous thing happens. Turbulence begins to move backwards, producing a boundary layer - a layer of very thin flow very close to the ball's surface - and this has the effect of causing the air flow to cling more closely to the ball's surface.

Turbulence is reduced and the drag plummets. When this happens we say that the boundary layer is "tripped" and since most of footballs' actions such as kicking or throwing take place above the critical speed the ball can be moved around in a pacey manner.

When the ball rotates the boundary layer remains tripped but the air flow separation around the ball is distorted. Separation occurs earlier on the side rotating against the flow and later on the side rotating in the same sense as the flow. This causes a pressure differential and a deflecting force which is responsible for moving the ball in the air in a free kick.

It was not realised for many years that the boundary layer was tripped by surface "imperfections", in fact by the slight indentations where the ball's panels are joined together.

More panels means more seams and greater aerodynamic stability but panel designs have varied enormously over the years. For example, the familiar hexagon-pattern ball has 32, the classic English model 26.

The new World Cup ball, Adidas' Teamgeist, has only fourteen panels, however. Might this be the factor behind the disquiet expressed by Jens Lehmann? To see why this might be so we can take a look at a very simple object, the baseball which has only two panels separated by a continuous seam.

Baseball pitchers conventionally swerve the ball by throwing it with spin. This "curveball" is very similar to a swerving free kick and the rotating seam trips the boundary layer in much the same way as a football does.

Occasionally though, pitchers serve up a wicked delivery known as the "knuckleball". This bobs about randomly in flight and is very disconcerting for batters. It happens because pitchers throw the ball with very little spin. Then, as the limited seam rotates lazily in the air flow, it trips the boundary layer at certain points on the surface.

This causes an unpredictable deflection which may be completely reversed when another portion of the seam rotates into the critical position. The key, of course, is to ensure a very low spin rate, easier to achieve in a baseball throw than a kick. But this does occasionally happen in football and then the panel pattern can have a big influence on the trajectory.

The Teamgeist ball with its 14 panels is aerodynamically closer to the baseball than either English ball with 26 panels or the 32 panel hexagon pattern. So no problem when the Teamgeist spins but watch out if it is kicked with a very slow rotation rate.

Goalkeepers are often criticised for punching or palming away balls which are moving unpredictably, rather than catching them cleanly, but they can be forgiven if they are facing football's equivalent of a knuckleball.

There will be plenty of spectacular scoring free kicks at the forthcoming World Cup and elite performers like David Beckham, Thierry Henry and Ronaldinho must be itching to get the ball at their feet. Watch the slow motion replays for the tell-tale movement of the markings on the ball, the best indicator for revealing the kind of spin applied in the shot: sidespin for players like Henry and Ronaldinho but a vital component of topspin when Beckham unleashes his special delivery.

Most of all look for the rare occasions when the shot produces little or no rotation and the frantic efforts of the poor goalkeeper struggling to come to terms with the ball's chaotic, meandering flight path. We are in for an enthralling four weeks of football.

http://news.bbc.co.uk/1/hi/magazine/5048238.stm