Mohammad Shami about to deliver a ball in a one-day international match against New Zealand, October 23, 2023.
| Photo Credit: Deepak K.R./The Hindu

At the 2023 ICC Men’s ODI Cricket World Cup, Mohammad Shami ended with the most wickets – 24 from only seven games. The standout feature of Mr. Shami’s bowling was his seam position, which commentators described as being “bolt upright”. When the bowled ball pitches seam-first into the pitch, the way it bounces back up becomes hard to predict – more so than if the ball is swinging, which batters can read ‘in the air’. Mr. Shami used this unpredictability to get batters into trouble.

In the ways in which it differs from spin bowling, seam bowling is a study in contradiction. In spin, the direction in which the bowled ball spins is the same as that of the arm that is spinning it. But in seam bowling, the bowler’s arm is moving counter-clockwise (when bowled towards the left) while the bowled ball spins clockwise. That is, the bowler’s fingers on the ball impart a backspin.

Finger positions

To achieve the best effect, intuitively, the two fingers can be said to be just to either side of the seam’s centre. So once the fingers impart torque (rotating force) on the ball, the angular velocity imparted by the rotating arm should vanish and the backspin velocity should increase. At this moment, the spin axis will rapidly shift hemispheres, passing through the seam.

To avoid a wobbling seam – which would reduce the chance of the ball bouncing on the seam as well as compromise the aerodynamics of the contrast swing – the spin vector should coincide with the torque vector.

This is why, when a seam bowler imparts torque to the ball, the position of the two fingers matters.

“As a very simple explanation, the spin vector coincides with the torque vector only if the middle finger imparts more force to the ball than the index finger,” Franz Konstantin Fuss, professor and Chair of Biomechanics, University of Bayreuth, told The Hindu. “This can be achieved naturally, since the middle finger is stronger than the index finger.”

Deviation from the ideal

However, a 2013 study reported a problem: at the moment the torque is imparted, if the torque vector is at the topmost part of the ball (i.e. if the torque is imparted at the ball’s ‘north pole’), the spin vector wouldn’t have time to move to coincide with the torque, leading to a wobbly delivery. The effect could be corrected by imparting more rotations on the ball. But the researchers found that there was a limit to this because the bowler has a fixed amount of time to impart those rotations: around 50 milliseconds.

As a workaround, the researchers – Dr. Fuss and Robert Masterton Smith, both then of the School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne – advised applying finger pressure around 10-15 degrees away from the topmost part of the seam, rather than right on top, and to ensure the fingers move parallel to the seam as the torque is being imparted. This way, they estimated, there was a better chance of the spin and torque vectors coinciding, reducing the wobble.

A good seam position increases the ball’s ability to take advantage of swinging conditions. Swing is the result of different air flow on either side of a ball moving through the air. This can be achieved by keeping one side of the ball rougher than the other. As a result, air flow is streamlined on the smoother side and turbulent on the rougher side. But if the seam is angled towards the smooth side, the air flow becomes turbulent on both sides and the ball no longer experiences a lateral force that produces the characteristic sideways movement.

More torque is better until it isn’t

In a 2014 study, the same researchers reported examining the bowling actions and seam deliveries bowled by four state-level cricketers using a ‘smart ball’ equipped with battery-operated gyroscopes. Based on the data recorded by this device, including the spin rate and the torque imparted, they reported some actions that could improve the accuracy of the ball bowled.

According to them, “The best way of preventing seam wobble is … a position of the middle finger closer to the seam. … [The] worst angular positions of the spin axis at release are 19 degrees undershooting, 15 degrees overshooting, and 16 degrees deviating. These angles cause a considerable side force”. ‘Undershooting’ means the angle of the spin axis with the plane of the seam is acute (from the bowler’s side); ‘overshooting’ means it is obtuse. Deviation refers to the angle between the moving spin axis and the axis at the time of the ball’s release.

The researchers also wrote in their paper that “a clear result is that spin rate and magnitude of torque influence deviation angle, and the higher spin rate and torque, the larger is this angle.” So simply imparting more and more torque isn’t a good thing either.

It isn’t easy to do what Mr. Shami did – but that’s the point.

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