For a fast bowler, a key weapon in their bowling arsenal is the use of swing to make it harder for batsman to follow the ball through the air. This ‘swing’ makes the ball swerve in the air either towards or away from the batsman, creating opportunities where the batsman may edge the ball to the keeper or slips, or even miss the ball entirely, possibly getting bowled out as a consequence.
The average cricket fan will often wonder what the cause of this movement could be. There is no single simple cause of this swing, with the seam, the surface texture, speed and spin of the ball all contributing to the overall effect.
Swing isn’t the only technique at a bowler’s disposal when trying to deceive batsmen. Spin bowlers can use their wrist and fingers to impart a spin on the ball as they release it. This alters both its movement through the air and what happens when it hits the ground. This unpredictability in movement is what makes good spin bowlers so hard to face as a batsman.
Do you want to master the arts of spin or swing bowling? Maybe you want to have a better knowledge for the next time you have to face one whilst batting? Well, read on to discover more about the physical and aerodynamical principles underlying these sporting phenomena.
Swing bowling is usually done by fast bowlers, namely those bowling at speeds greater than 70mph. The clip above shows England bowler Jimmy Anderson bowling New Zealand’s Aaron Redmond during a test match in 2008. You can see in the clip that the ball swerves in the air, causing Redmond to misjudge it and miss his defensive shot.
The often-unknown cause of swing bowling is the fact that the thin boundary layer of air that forms along the ball’s surface does not separate at the same point on each side of the ball. The location of this point is dependent on factors such as whether the boundary layer is in a laminar or turbulentstate, the potential asymmetry due to the presence of the seam and surface roughness of the ball.
To make a cricket ball swing conventionally, one side of the ball should be smooth and the other rough. You may have observed bowlers rubbing one side of the ball on their trousers before bowling. They do this to shine one side of the ball, making it smoother than the other and maximise the amount of swing they can achieve.
The bowler holds the ball so that the seam is at an angle from the direction of travel. As the ball travels, the boundary layer on the smooth side of the ball is in a laminar flow state, which separates from the ball quite early. On the other side, the air is disturbed by the seam which makes it transition to a turbulent state. This turbulent air sticks to the rough side of the ball and leaves the surface at a later point. This effect creates a pressure force on the ball, making the ball swing in the direction the seam is pointing. In other words, the smooth side of the ball travels faster in air and the rough side acts as a brake, arcing its path in the air.
In the 1960s, Pakistani cricketer Sarfaraz Nawaz discovered that he could swing the ball in the direction opposite to the seam. This is now known as ‘reverse swing’. It happens when the ball is travelling at more than 80 mph and is rough on both sides, for example - later in the match when the ball has been scuffed by the surface for a longer period of time.
As before, the bowler holds the ball with the seam at an angle to the direction of motion as shown in the picture below. As the ball travels, the air flow on the right side of the ball is also now turbulent because the rough surface disrupts the laminar flow of air. Meanwhile, on the other side, the incident air layer again hits the seam causing turbulence, but the layer of turbulent air on this side is thicker than on the other. Thus, the layer separates from the ball earlier, and causes a resultant force in the direction perpendicular to the seam.
Reverse swing can be confusing to a batsman who faces it, as often they tend to look for the orientation on the seam during the delivery to try to predict the ball’s movement. If reverse swing is used, the ball will swing in the opposite direction to what they would expect from conventional swing bowling.
In the case of spin bowling, the bowler uses his wrist or fingers to impart a spin on the ball as he releases it. The direction of this spin will determine the ball's behaviour during its flight and upon impact with the ground.
A cricket ball, like anything else, has three main axes (perpendicular to each other) about which it can spin. Each spin axis has a different effect on the flight of ball through the air and the way it bounces when it hits the ground. Once the ball hits the ground, it deflects in the direction of the spin.
A ball that spins about an axis along the pitch towards the batsman kicks sharply to the left or right (depending on the spin direction) when it hits the ground. The bowler grips the ball from the seam and rotates it anti-clockwise to make it spin away from a right-handed batsman, or rotates it clockwise to make it spin towards the batsman. These are known as leg spin and off spin, respectively.
To achieve topspin, the bowler imparts a spin to the ball along a horizontal axis, across the pitch, in a direction towards the batsman, The Magnus force causes the ball to dive down or dip towards the pitch and therefore hit the ground faster. This happens because the top surface of the ball travels in a direction opposite to the air flow which causes the air to slow down and causes high pressure.
On the other hand, the bottom surface of the ball travels in the same direction as the air flow, causing the air to speed up resulting in a low pressure. This pressure difference causes the ball to deviate towards the low-pressure side. Finally, as the ball hits the ground, it then kicks up forwards at a reduced angle. The reason for this is because as the ball pitches, the bottom surface of the ball (in contact with the ground) tends to slide backwards. Frictional force then acts to oppose this motion, tending the ball to slide forwards. The result is the ball moving forwards at a low angle.
For backspin bowling, the bowler imparts a spin to the ball along the horizontal axis across the pitch in a direction away from the batsman. The Magnus force in this case, causes the ball to float through the air for longer. The reason for this is because contrary to top spin, the bottom surface of the ball now travels in a direction opposite to the air flow while the top surface travels in the same direction as the flow of air. This causes a low pressure near the top surface, causing the ball to deviate towards it.
As the ball hits the ground, it kicks up, but at a lower speed than that of a non-spinning ball. This happens because as the ball pitches, the bottom surface of the ball in contact with the ground tends to move forwards, while the frictional force acts in an opposite direction to oppose this motion. The result is the ball kicking up at a greater angle but with a reduced speed.