2010-11 Ashes Betting / Andrew Hughes / 11 October 2010 /

Swing bowling is an essential part of cricket but how much do we really know about it? Why makes a cricket ball swing and what exactly is 'reverse' swing? Andrew Hughes investigates.

Australia are [1.76] to regain the Ashes and if England are to stop them, a lot will depend on how well James Anderson is able to swing the ball. He has been in great form this summer and there are few sights more exhilarating on a cricket field than a swing bowler in full command of his craft. We have been fortunate in this country to have seen some of the greatest exponents of conventional swing bowling the game has ever seen, from the legendary Sydney Barnes to Ian Botham.

Despite this, for most of cricket's history, the precise reason why a ball swings remained obscure. Generations of schoolboys were taught that if one side of the ball were polished, that side would travel more quickly through the air and therefore the ball would swing towards the unpolished side. Bowlers checked for cloud cover, polished one side of the ball, held the seam in a certain way and hoped for the best.

Since the method worked more often than not, the theory was not questioned. But logically, this explanation of swing leaves some unanswered questions. For instance, why does the shiny, unpolished new ball sometimes swing in the first over of a game? And why, if the ball swings away from the polished side, does it not keep on swinging and end up at fourth slip or gully? And why should the presence of clouds affect the movement of a ball?

And how can you explain reverse swing, in which the ball appears to swing towards rather than away from the polished side? Sarfraz Nawaz and Sikander Bahkt pioneered this mysterious art and by the 1990s, Waqar Younis and Wasim Akram were regularly bruising toes and shattering stumps. Much of the controversy about reverse swing arose because for a long time, no-one else understood how it worked. Indeed, according to conventional cricket wisdom, it shouldn't work.

The debate about reverse swing led to renewed interest in the theory of swing bowling and, thanks to various scientific studies, to some tentative answers. The full explanation is too detailed and technical to go into here, though readers who are interested might like to have a look at the relevant chapter in the late Bob Woolmer's excellent book, 'The Art and Science Of Cricket'. Essentially though, it appears that swing occurs when the bowler creates low static pressure on one side of the ball.

Angling the seam can create such an effect, which explains why a new ball can swing. Then as one side of the ball is allowed to roughen up, it develops small tears. Air is trapped in these tears and thanks to the effects of airflow, low pressure is created on the rough side of the ball, which is then sucked into the low pressure region. This is conventional swing. Reverse swing occurs, when the rough side has become too rough, whilst the polished side, having absorbed sweat and saliva through polishing, expands and develops shallow valleys, which retain air and create low static pressure.

So what part does the weather play in all this? This is one area of swing bowling theory where the scientists as yet do not have all the answers. But it seems likely that temperature is the key. It is possible that the ball swings more in cold weather because cold air is denser and exaggerates the effect of the swing. Another theory is that when the pitch heats up, it creates micro-turbulence which disrupts the airflow turbulence around the ball. Cloud cover therefore prevents the pitch heating up and facilitates swing. And this may also explain why James Anderson will have his work cut out to get that Kookaburra ball to swing in a hot Australian summer.

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