Reverse swing has existed in some form or another for at least the last four decades. Pakistan, through Sarfraz Nawaz and later Imran Khan, made it into art form in the late 1980s and in the 90s Wasim Akram and Waqar Younis carried the tradition. Sunday Magazine, with the help of former NASA scientist Rabindra D Mehta who has studied cricket ball aerodynamics for close to four decades, takes a deep dive into one of cricket's darker arts.
What is reverse swing?
As the ball gets older, the ball loses its shine thanks to wear and tear. But to make the old ball swing, fielders try to keep one half of the ball as shiny as possible while they let the other half deteriorate. When one side gets scuffed up and other retains its shine, bowlers can get it to swing towards the shiny side. It is popularly known as reverse swing since the ball goes in a direction opposite (or reversed) to that expected based on the conventional cricketing wisdom and previously accepted fluid mechanics principles.
How did Mehta learn about reverse swing?
From Imran Khan, who was his school mate, in 1980. When Khan, who was a natural in-swing bowler, mentioned to Mehta that he got the ball ‘to go the other way’, Mehta was puzzled. So he wanted to learn more about the phenomenon.
What did he do?
In 1981, after Mehta got some funding to conduct some tests, he found out for himself what Khan was talking about. He put the ball through a wind tunnel to find out the side force, swing force, drag and so on. While conducting these wind tunnel experiments, he found out that the ball ‘actually reversed’.
What are wind tunnels and how did he ascertain that the ball reversed?
Assume you have an airplane model that you want to test. Rather than flying the model through the air, you have this tunnel, basically a chamber through which the wind blows. So instead of an airplane model, we put a cricket ball in there. During these experiments, Mehta noted that the ball actually swung in a direction that is opposed to that of the seam.
What happened?
This is the technical bit. During reverse seam, the really interesting flow events start to occur when the Reynolds Number (whether the flow around a solid body will be turbulent or steady) is increased beyond that for zero side force. The transition point will continue to move upstream. These points on two sides are symmetrically located, but the turbulent boundary layer on the seam side still has to encounter the seam. In this case, the seam has a detrimental effect whereby the boundary layer is thickened and weakened (lower skin fraction coefficient), making it more susceptible to separation compared to the thinner turbulent boundary layer on the non-seam side. The turbulent boundary layer on the seam side separates relatively early and an asymmetric flow is set up once again, only now the orientation of the asymmetry is reversed such that the side force, ad hence swing, occurs towards the non-seam side. This essentially is reverse swing.
Can it be bowled only with the old ball?
Not really, according to Mehta. He says bowlers who are capable of touching more 92-93 mph can bowl it even with a new ball. The speed at which you get reverse swing is dependent on the condition of the ball. So if you take a new ball, and if your bowling speed is say, 95 mph, then the ball will only reverse. You won’t be able to get conventional swing.
Is there a third type of swing apart from reverse and conventional?
Yes, says Mehta. Most players and commentators confuse reverse with what he calls contrast swing. When you end up with a ball with one side very smooth because you kept polishing it and the other side you let nature take its course, you have a ball with which you produce reverse swing. You can also produce contrast swing. The thing with that is you don’t need to angle the seam. The seam can be straight. The thing is you don’t even need a seam to get contrast swing. When you are playing in places like India and Pakistan, the seam basically gets bashed in so it doesn’t have an effective role. Even then, when you have one side smooth and one side rough, you are able to swing it.
Can reverse swing be achieved without ball tampering?
The contention that bowlers cannot bowl reverse swing without ball tampering is absolute rubbish. Does the process of reverse swing help if the ball is tampered? Yes. Do you have to tamper with the ball to make it reverse? Rubbish. No.
Can an old ball stop reversing for whatever reason?
If the ball is well looked after and the bowler knows what to do, it is his belief, reverse swing can be achieved on any condition or surface.
Rabindra D Mehta is a sports aerodynamics consultant. He lives in California.