Why does it reverse towards the shiny side? What’s the deal?
The cricket ball was not originally designed or intended to swing in the air, but the raised seam and roughness that it accumulates over time makes the phenomenon possible. Fast bowlers have harnessed this and made it an essential part of their armory. The curving ball at high speed is one of the toughest challenges for a batsman, is used ubiquitously, and is admired widely for the skill it takes to bowl.
Reverse swing is a commonly used but poorly understood term. It is said that the old ball swings towards the shiny side. But is that true reverse swing, or is it contrast swing?
Conventional Swing
The physics of conventional swing is fairly well understood. Some cricketers and enthusiasts speak about swing being influenced by the “wetness” or “heaviness” of one side of the ball compared to the other, but this is not true. The layer of air around a moving ball, much like the wind that hits you on a bike, is key to generating swing.
The above figure shows that layer of air. The ball is going to the right of the screen, the seam is pointing towards the leg slip for a right hander. The air that hits the ball in front is smooth (called laminar). The seam disturbs this smoothness and makes the air flow turbulent (as shown on the bottom side of the ball in the figure). Turbulent air clings to the ball more, which means it separates from the ball later than the smooth air on the top side. This difference in when these two kinds of flow separate from the ball leads to a sideways force on it. Which we call swing.
In the figure above, the turbulent layer separates later. And that is the direction in which the ball swings. You can see that with the seam pointing into the right hander, this ball will be an inswinger. So, conventional swing moves towards the seam.
Reverse Swing: What Is It?
Reverse swing is a mysterious and somewhat maligned term even today in cricket. But what is it? How is it different from conventional swing?
Let’s get one thing clear: the scientific literature on this defines reverse swing clearly: if the ball swings away from the seam direction, it’s called reverse. Notice that this definition is in contrast with “conventional” swing, where the ball swings towards the seam. The key to achieving this reversal of direction is twofold — the condition of the ball and the speed.
At high speeds, even the new ball can be reverse swung: it can go opposite to the seam direction. So, if you bowl with the seam tilted towards slip for a right-hander, the ball will be an inswinger if you bowl it fast enough (145 km/h or faster).
Can you achieve this reverse swing at lower speeds? Yes, by roughening up one side of the ball. A rough side can aid this process, and make the ball go against the seam direction at lower speeds.
What about the Shiny Side?
Now, in common cricketing discourse, “reverse” swing usually means something else. It is said that the old ball with one shiny and one rough side will always go towards the shiny side. Bowlers can usually bowl only one direction of swing, depending on their hand. Using a properly conditioned old ball, any bowler can now swing the ball both ways, just by flipping the shiny side. This is called as contrast swing.
Contrast swing can be achieved with a straight seam. It is a hard skill to release the ball with a tilted seam, which swing bowlers perfect. But we see part-timers and other bowlers also get swing with the old ball sometimes, even though they release the ball with seam straight.
The direction of contrast swing depends on the speed. At speeds lower than 80 km/h, the ball moves towards the rough side. At higher values, it moves towards the shiny side. If you see the ball going towards the shiny side, and the seam is upright, it is contrast swing.
If you see it go towards the shiny side with the seam tilted, it is one of the two cases outlined above. The more commonly seen case is true reverse swing, as it goes away from the seam direction. The other case is actually conventional swing, as it swings in the same direction as the seam.
