Shining Light on The Shiny Side
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 armoury. The curving ball at high speed is one of the toughest challenges for a batsman, is used ubiquitously, and 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 in 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.
I’m going to be referring to Dr. Rabindra Mehta’s work throughout this post. Here is a comprehensive Cricinfo article on the physics of swing based on his work. It explains many kinds of swing in detail, and I’d encourage you to read it to understand the general physics behind the phenomenon.
The above figure, from Dr. Mehta's own paper on swing, shows that layer of air. The ball is going to the right of 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: 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: 1) the condition of the ball, 2) the speed.
At high speeds, even the new ball can be reverse swung: it can go opposite to the seam direction.
So, if I bowl with the seam tilted towards slip for a right hander, the ball will be an inswinger if I bowl it fast enough (145 km/h or faster).
The technical explanation for this is done well in the above linked paper and article, so I’m going to skip it. The point of this post is something else.
Moving on, 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.
Note: Hereafter, I use reverse for the reverse swing defined above, and “reverse” in quotes for reverse swing as we commonly define it in cricket.
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 phenomenon is also covered well by Dr. Mehta. This is called contrast swing his work. While it is explained well in the article, I’ll point out a few relevant things.
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. This is contrast swing.
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.
Now, we know that First Class level bowlers bowl at higher speeds than 80 km/h. This is why we see the ball go towards the shiny side when it’s old. You need to release the new ball with a slanted seam to get movement. You just need to release the old ball with an upright seam.
There are multiple examples of this. Mohammed Shami bowls with an upright seam with the old ball and gets “reverse”. In this interview, Waqar Younis, one of masters of “reverse”, says that the wrist isn’t that important for reverse swing, which I took to mean that the seam is to be released straight.
This explains the swing of the ball when the seam is upright. What about when it’s not?
Newer Research
This last question has troubled me for a long time. Dr. Mehta’s research left me with two questions. One, he mentions that reverse swing can be controlled by flipping the side on which the shine is, but the ball swings towards the rough side. Secondly, it’s never specified if the ball ever swings towards the shiny side when the seam is tilted.
Something wasn’t adding up. We’ve seen countless instances in cricket of a slanted-seam ball moving towards the shiny side. The whole crux of “reverse”, as we hear it from players and commentators, is the shiny side of the ball. Where’s that?
To look at this, I looked at an excellent new paper by Dr. Sanjay Mittal’s lab at IIT Kanpur. They did a comprehensive analysis of the motion of new and old SG balls tilted at different angles, measuring the swing force, modelling the flow of air around the ball, and computing the trajectory of a ball as it goes down the pitch.
The paper presents interesting results, which I’ll quote later. Let’s come to the “reverse” swing part first. My aim here is to check if the results of this paper match up with the following notion: the old ball swings towards the shiny side.
First up, let’s define the 4 kinds of balls they used, presented in the following figure. This is important to study the key graph which I present later.
Imagine the balls are going to the left of screen, to a right hander. The seam is tilted towards leg slip. Let’s call the side of the ball towards which the seam is tilted the seam side. In this case, the seam side is the upper part of the ball in the figure.
i) New ball: no sides rough
ii) S-R: The seam side is rough. The non-seam side is shiny.
iii) NS-R: The non-seam side is rough. The seam side is shiny.
iv) C-R: Both sides rough.
Cases ii) and iii) are relevant to our questions here.
Here comes the plot:
Understanding this graph
On the x-axis is shown the speed at which the ball is released (by the bowler).
On the y-axis is the amount the ball has moved sideways (swung) by the time it reaches the end of the pitch.
They did this analysis for 5 different balls. Four of them are the ones I detailed above, with the seam angle tilted at 20 degrees (which is the optimal angle for swing). The 5th one is an NS-R ball with a different angle (30 deg).
If the line is positive, it means the ball swung towards the tilt of the seam. Which means, if I bowled it like an outswinger, it swung out.
If negative, it means the ball swung away from the tilt. Which means, if I bowled it like an outswinger, it swung in.
Now, Understanding The Two Cases of “Reverse”
I keep putting “reverse” in quotes to refer to what we conventionally call reverse swing in cricket: swinging towards the shiny side. I’ll look at the plot above and study two cases. We’ll see if we can corroborate the colloquial definition.
The physical mechanisms of these two cases are well explained in the paper, and center around the effects of speed, seam and roughness on the surface layer of air around the ball, and when it gets tripped into turbulence. I do enough physics at my day job so we’ll leave those details be, and focus on interesting results.
Revise those S-R, NS-R definitions before we move on. Gave me a lot of grief.
Case 1: S-R - Green Line
Let’s define a cricket situation with an S-R ball. Imagine:
I’m angling the seam like I’m bowling an inswinger to a right hander.
The seam is thus tilted towards the leg side.
I’ve kept the shiny side towards off. This means the seam side is rough (S-R).
I’m bowling at a 135 km/h or higher.
This is a fairly typical situation. Now, conventional cricketing logic dictates that the ball will be an outswinger, because the shiny side is facing towards off.
Now, does the plot show that? Look at the green line (which shows the S-R ball) to the right side of the plot. It is in the negative region. Which means it swings away from the seam direction.
But “away from the seam” is the same direction as the shiny side!
This ball is moving towards the shiny side. Your best chance of getting this kind of swing is to bowl higher than around 130 km/h.
This is true reverse swing (as defined by going against the seam).
This wicket by Josh Hazlewood is a great example of this. In the replay from the batsman’s angle, you can see:
The seam is facing towards the slips.
The shiny side is facing towards the leg side.
This means this is a case of S-R (seam side is rough).
The ball swings in, i.e. towards the shine.
Notice that this is also reverse swing as it is defined in literature: the ball is released like an outswinger, but swings in.
My postulate is that this must be the dominant mode of “reverse” swing for right armers. Their default release is for an outswinger, and their default “reverse” direction goes into the right-hander. So this is what happens when a right-armer “reverses” the ball with the seam tilted. This was Waqar Younis’s MO when he bowled “reverse”. His natural ball was the outswinger, but with the old ball he used to swing it in. As one can see from the plot (and the video), the magnitude of swing is huge in this case.
Here is a left-armer’s version of this exact mechanism by Zaheer Khan.
Case 2: NS-R - The Red Line
I haven’t been able to find video examples for this one. But let’s try to break it down like the S-R case.
I’m angling the seam like I’m bowling an inswinger to a right hander.
The seam is thus tilted towards the leg side.
I’ve kept the shiny side towards leg side. This means the non-seam side is rough (NS-R).
I’m bowling at a 135 km/h or higher.
Look at the red line in the plot above in the high speed region. Its value is greater than zero.
This means the ball swings towards the seam. Which means the ball is swinging in. This is also the same direction as the shiny side.
Notice that this is conventional swing as defined in the literature, as the ball swings towards the seam direction.
By covering these two cases, we’ve confirmed “reverse” swing as it is called in cricket:
In both cases, the ball swings towards the shiny side. Check.
This means that the bowler can change the direction of swing based on where he keeps the shiny side. Most importantly, he can do this without changing the seam direction. If I know how to bowl one kind of swing, I can still achieve it in both directions with the old ball. This is what bowlers do with the old ball, often hiding the direction of the shiny side to not let the batsmen know which way it will move.
Other Interesting Stuff
With this out of the way, there are some interesting conclusions to be drawn from the above plot and the rest of their paper.
The magnitude of swing in the NS-R case is much lower than that in the S-R case. Maybe this is why bowlers use a “default” mode of reverse swing: Waqar brings it in, for instance. Getting it to go the other way is a good surprise, but maybe it doesn’t move as much.
Looking at the blue line (new ball) in the plot, we confirm Dr. Mehta’s result: the new ball can indeed swing in the opposite direction to the seam if it’s bowled fast enough. This has to do with the surface layer becoming turbulent before it reaches the seam if the ball is fast enough (or rough enough).
To get the maximum lateral movement with the new ball, bowl it at 90-100 km/h.
For the NS-R case with a 30 degree tilt (orange), “reverse” swing is achievable at much lower speeds compared to the 20 degree case.
The NS-R case with 20 degree tilt (red) shows that sometimes the old ball can go towards the rough side, if the ball is bowled at less than 130 km/h.
Most strikingly, a ball which is roughed up on both sides (black) will get appreciable reverse swing (against the seam direction) at a wide range of low speeds (80-140 km/h). If you don’t have a leadership group taking care of the ball and it gets all scuffed up, you can use your Ganguly-Chris Harris clones to try and swing it. It will go the other way, though.
Two Final Plots
This, on the right shows a plot of swing vs release speed. The seam angle is kept the same (20 deg).
Here’s more confirmation that the new ball goes against the seam direction if it’s fast enough. Also, there’s an unfortunate speed range (125ish km/h) where you might not get any swing at all.
Now, a cousin of this plot: same release speed (146 km/h), different release angles.
This shows some wild results. Changing the angle of the seam has a massive impact on whether the ball will go with the seam direction or against it.
The most bonkers result is the case when the ball is released at a 30 degree-angle. The ball starts going one way, but as it speed reduces as it moves along, the direction of swing switches. This has to do with something called the critical Reynolds number, which is the magic threshold that dictates whether the ball will go this way or that relative to the seam angle.
God help the guy who has to face that 30-degree ball.
Conclusion
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 (a la Waqar) 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.
Nice study