The science of swing

The swinging ball was a vital component of this summer’s ashes success. Matt Carré explains the engineering principles behind England’s secret weapon.

The swinging ball was a vital component of this summer’s ashes success. Matt Carré explains the engineering principles behind England’s secret weapon.

England cricket fans (including myself) are recovering from the euphoria of winning back the Ashes from Australia. The fantastic batting exploits of Kevin Pietersen, Shane Warne’s mesmerising spin bowling and the all-round heroics of Andrew ‘Freddie’ Flintoff are still fresh in everyone’s minds. But an equally important ingredient in this drama was the ‘swing’ and ‘reverse swing’ bowling unleashed by Hoggard, Jones, and Flintoff.

The phenomenon of ‘swing’ has been discussed at length in many scientific articles, perhaps most notably by an American friend of mine, Dr Rabi Mehta, to whom I owe much of this explanation.

A cricket ball differs from many sports balls due to the asymmetry caused by a proud seam and the fact that the fielding players keep one side of the ball smooth, whilst the other side is allowed to become rough. Any projectile that is non-symmetrical has the potential to experience some force, or combination of forces, perpendicular to its flight. An obvious example of this is the aircraft wing. Therefore it isn’t that surprising that a cricket ball, when bowled correctly, does ‘strange things’.

A proficient swing bowler delivers the ball towards the batsman with the seam in a vertical plane, but at a slight angle to the direction of flight. For instance, a bowler wanting to deliver an inswinger to a right-handed batsman will release the ball with the seam pointing towards the leg-side of a right-handed batsman, and the rough side on the right. Some degree of backspin is usually applied to the ball to keep the seam angle stable.

On the rough side of the ball, the boundary layer (the thin layer of air close to the surface of the ball) is tripped into turbulent mode when it hits the seam. The boundary layer on the other, smoother side remains ordered, or ‘laminar’. Turbulent boundary layers generally separate from projectiles further toward the rear, compared with laminar boundary layers, because they are able to mix with the surrounding air to gain energy.

Golf balls (with dimples to cause turbulence) have a smaller wake, compared to a smooth ball, and therefore fly further. The difference in separation points on both sides of a cricket ball leads to an asymmetric wake, an unequal pressure distribution and a resultant side force which causes the ball to swing towards leg, giving an ‘in-swinger’.

The amount of swing a ball experiences will depend on a number of factors including ball wear, speed, back spin, seam angle and stability, as well as atmospheric conditions — bowlers often say that the ball swings more when the atmosphere is ‘heavy’, if it is cloudy and/or when playing near the sea.

A further complication is ‘reverse swing’, when the ball swerves in the opposite direction to that which would be expected. This phenomenon, deployed to devastating effect this summer by Simon Jones, occurs when bowling at speeds of around 85 mph and above.

When a ball is bowled fast enough, the boundary layers on both sides of the ball become turbulent. The seam now acts like a ramp to thicken the layer on the rough side so that it separates earlier. The pressure difference on both sides of the ball is now reversed and the ball will swing in the opposite direction. This effect can also happen if both sides of the ball have become rough throughout the course of play. Now, instead of a smooth and a rough side, there is a rough and a rougher side.

Ultimately though, as with many sports-related phenomena, it’s easier to understand the physics behind what is going on, than it is to carry it out in practice. The real art of swing and reverse swing bowling is the ability to adapt to conditions as they change throughout a day’s play and cause the ball to do what you want it to — consistently. At the very top, as in any sport, it’s this level of skill that sets apart the best from the rest.

Dr Matt Carré is a lecturer in sports engineering at the University of Sheffield’s department of mechanical engineering.