Some fast/swing bowlers rely heavily on swinging the ball to trouble batsmen. Naturally, there are known techniques and methods that such bowlers use to swing the ball. Sometimes the cricket ball swings in an unconventional manner, without a change in grip, seam position or bowling action. That unconventional swing is known as reverse swing.
There are many myths and theories surrounding reverse swing. Talk of reverse swing also arouses suspicion of sharp practice and brings controversy. Reverse swing is often confused with conventional swing and cricket players have different explanations or hypotheses about what causes it. The idea of reverse swing or unconventional swing can be tricky to grasp. However, you can understand it in the context of swing in general.
What is swing?
Swing is caused by pressure differentials caused by aerodynamic differences between two sides of a cricket ball. Asymmetrical points of separation of air on different sides of the ball creates the pressure differential. When the separation is delayed on one side of the ball, the resultant pressure difference in forward motion generates a side force that is called 'swing'.
The seam position, velocity of the ball and difference in the surface of the ball affect the strength of the side force that pushes the ball either into the batsman (in-swing) or away from the batsman (out-swing). With conventional swing the ball swings toward the rough side because of increased drag and inferior aerodynamics.
A fast medium bowler can generate swing with a new ball by virtue of the seam position. The ball is delivered with a seam that is angled against the line of flight. The seam should spin with minimum wobble for maximum swing. With the new ball, the bowler's action generates the swing. One of the master's of swing bowling, Wasim Akram stated that conventional and unconventional swing is dependent on both the condition of the ball and the bowler's action.
What causes reverse swing?
Reverse swing is so-called because the ball swings in a manner that is contrary to the way the seam position and asymmetrical surface of the ball suggests it should. Reverse swings can be defined by:
a) The ball swinging toward the shiny side instead of the rough side
b) The ball swinging in a manner opposite to the seam position
With conventional swing, the ball swings toward the rough side because that side moves slower through the air. With fast bowlers and the new ball, the angled, upright seam produces swing other than it would at lower speeds. This seems to defy logic but it relies on a combination of factors:
i) Bowler's action and speed
ii) Age of the ball
iii) Atmospheric conditions- humidity
According to Scientist Rabindra Mehta, true reverse swing is defined by the ball swinging in a position opposed to that of the seam. Conventional swing can only occur up to a certain speed threshold since increased velocity in the forward motion reduces the effect of pressure differentials on the ball. At higher speeds, however, the velocity magnifies the role of the seam position.
The seam's influence causes the boundary layer of air to separate earlier at the top of the ball than the bottom. This suggests that fast bowlers bowling upward of 90 miles an hour can only get reverse swing, not conventional swing. What makes reverse swing so special is that it is achieved without a change of grip or seam position.
Reverse swing is caused by the seam position disturbing the normal flow of air around the ball. AT the right speed, this can dominate the effect of the pressure differentials from the polished and rough surfaces of the ball.
In his autobiography, Marcus Trescothick suggested that he sucked mints to add shine to the ball to produce reverse swing. Many former cricket players from past eras- like Ian Chappell- ridiculed the idea. There is not yet any evidence to suggest that the swing that England's bowlers got in the 2005 Ashes series was because of mint conditioning of the ball.
The problem with reverse swing
Cricket is riddled with terms used to explain the unconventional or unexpected. Googly and doosra are used to identify balls that go the 'wrong way'. When spinners bowled off breaks with a leg break action, cricket had to come to terms with the anomaly. With "reverse swing" it is similar. Reverse swing is either in-swing or out-swing that defies normal expectation. In scientific terms, there are just different ways of producing in-swing and out-swing or it is a case of one force over-ruling another.
The controversy with reverse swing stems from the association of reverse swing with ball tampering- particularly with Pakistan fast bowlers- and that it is not fully understood in all cricket quarters. It is true that ball tampering can aid "reverse swing." If the fielding team interferes with the seam position on the shiny side of the ball, that would create extra drag on the shinier side. Technically, any object on the shiny side of the ball (dirt for instance) can interefere with its properties and behaviour.
Just as the googly and doosra is the spinner's "wrong one", "reverse swing" is the swing bowler's alternative. "Reverse swing" will remain part of cricket terminology and will be a lethal weapon for its exponents.