The truth about swing bowling

The exclusive Cricbuzz series - The Name of the Game - returns to decode the art of swing bowling, in a session of myth-busting and enlightenment with NASA scientist and aerodynamics expert, Dr. Rabindra Mehta.

"He really didn't know until I told him, 24 years later."

Mike Hendrick played the last of his 30 Tests in 1981 - nearly a quarter of a century before he found out why he sometimes, inexplicably, struggled to swing the new Readers ball - a rather debilitating quandary for a fast bowler whose livelihood depended on it. By the time aerodynamics expert Dr. Rabindra Mehta decoded the problem, it was 2005 and Hendrick was a bowling coach, passing his wisdom onto the next generation of English bowling.

"I went to a coaching centre in England in 2005, where county coaches were invited to discuss swing bowling with me," recalls Dr. Mehta. "Mike Hendrick, one of the bowling coaches, came up and asked me why he couldn't, at times, swing the new ball back in his playing days.

"And I explained to him that he had been bowling at a speed where the ball hits the "zero" mark - in other words - where it crosses over from conventional to reverse swing. If he knew at the time, he could have slowed it down a little bit, or bowled a little faster."

In a nutshell, Hendrick was too quick for conventional swing, but not quick enough for reverse swing. What, however, does speed have to do with whether the ball swings conventionally or reverse? Everything.

The anatomy of a ball

Let us, however, start at the very beginning. Was the cricket ball meant to swing by design?

"No, they certainly weren't manufactured with swing in mind," says Dr. Mehta definitively. "Initially, it was much easier to manufacture four pieces of a sphere separately and stitch them together, so it's just a tradition that has continued in first-class cricket. In fact, modern two-piece balls swing more, because they don't have the roughness on the surface due to the internal stitching."

That the cricket ball was never designed to swing is pure serendipity, but it was clear that in the 20th century, bowlers had figured out how to move the ball sideways in the air. For decades, that's all it was - the ball moves in the direction of the seam. Period. In the 1980s, however, Pakistan's pacers stunned the world with what came to be known as "reverse-swing". And Dr. Rabindra Mehta of NASA delved into the "How it works" of it all, as opposed to the "How it's done" of it.

"In Test cricket your opening bowlers go and choose their preferred ball, and they tend to go by the colour of the ball - which is pretty stupid," says a dismissive Dr. Mehta, repulsed by the absence of logic in decision-making. "They think the darker red ball swings more, but the colour is one of the only factors that have nothing to do with the magnitude of swing. The width of the seam matters, but that's pretty much the same for all the international brands (Dukes, SG and Kookaburra). The height of the seam (very relevant to swing), matters a lot, and there will be differences, in addition to the way the ball is stitched.

"This," he picks up from his table a gleaming red object that, until just then, appeared to be a mere paperweight, "happens to be a Dukes ball, used for Test matches in England, and it's of a much higher quality than the Kookaburra. In the Dukes ball, all six rows of stitching go across (the external seam). In the Kookaburra, only one internal row of stitching goes across. The other ones are just cosmetic."

Despite the lack of clarity in the video call, the Dukes ball was a magnificent, polished relic, its proud seam hand-stitched to perfection. It was evident, that in terms of durability, it was made to last, far more than the mass-produced, machine-stitched Kookaburra. Consequently, the seam on a Kookaburra tears up, flattens out and gets damaged far more quickly than that on the Dukes. As a result, it doesn't swing for as long as the Dukes, or even the SG ball (used for Test matches in India).

"The condition of the seam on a new ball will determine how long it will still be usable for swing. For conventional and reverse swing, you need the seam; the better seam you have to start with, the longer the ball will continue to swing. That's the key." Conventional and reverse - that's all the kinds of swing, isn't it?

Not quite...

Conventional Swing

"The cricket ball swings in the direction the seam is pointing," says traditional cricket wisdom, that has made its way through generations via word of mouth. "When the ball gets older, it swings towards the rough side."

Why, though? At the center of swing bowling, and in fact running down the center of the cricket ball, is the protruded external seam that holds the ball together. The release of the the ball with an upright seam, stabilized by the backspin, with the seam angled towards the off-side or the leg-side creates an asymmetry in airflow that eventually causes the air to exert an equal but opposite lateral force on the ball, thereby causing the phenomenon of swing.

The angle of the seam towards a right-handed batsman's first slip causes the ball to swing conventionally away from the right-hander; and and that towards leg-slip causes it to swing conventionally into a right-hander.

"For conventional swing, in a certain speed range, the airflow splits when it hits the ball: on one side, it goes over the smooth surface, and on the other side, it's tripped by the seam," explains Dr. Mehta. "There's a thin layer in the surface that leaves the non-seam side at the apex, but on the other side (seam side), where the seam trips it (the air) into turbulence, that boundary layer is able to stay attached for much longer and separates later."

The turbulent side has air travelling faster, and according to Bernoulli's principle, an increase in speed of a fluid results in a decrease in static pressure. This would create a vacuum cleaner effect - lower pressure on the seam-side would mean the ball would be pushed into the vacuum cleaner on the seam-side.

Reverse Swing

It all began, when Imran Khan, the Imran Khan, confided in Dr. Mehta, an old school-mate, that on occasions, the ball would swing "the other way." Dr. Mehta, a firm believer in science, couldn't explain it at the time. However, the wind tunnel experiments he later conducted resulted in a startling revelation - and a plethora of misconceptions.

It turned out, that at high speeds, the ball started to swing against the direction of the seam.

And reverse-swing was born.

That, in itself, busts the first myth about reverse-swing - the ball doesn't reverse towards the shiny side. In fact, it swings against the direction of the seam.

***

"At very high speeds, or if the non-seam side is roughed up, the boundary layer becomes turbulent almost immediately, and weakens because of the high speed and separates earlier (on the seam side) compared to conventional swing. So there's a switch. In reverse-swing, the flow is turbulent on both sides - due to roughness on the non-seam side and due to high speed on the seam side."

However, due to an earlier separation on the seam-side, as opposed to conventional swing, the pressure gradient would now flip. There is more turbulence on the non-seam side, higher airspeed, and therefore lower pressure (Bernoulli's principle). This would imply that the air pushes the ball towards the non-seam side, resulting in reverse swing.

"A new ball can be reverse-swung, but at very high speeds. The reason people tamper with the ball is that not everyone can bowl at 95 miles an hour."

***

"In order to get the reverse swing, it helps if the non-seam side is rough to achieve turbulence at lower speeds. In other words, the ball is tampered or scuffed up so that reverse-swing can be achieved at lower speeds. And if they continue to bowl at such high speeds, they won't get conventional swing; they'll only get reverse-swing.

And despite what people write about reverse-swing, if you can swing a ball, you can reverse-swing a ball. There's no special arm action or grip required. If you're a swing bowler, given the correct ball, you'll be able to reverse-swing it."

Critical Speed

"The critical Reynolds number is the speed at which you start getting conventional swing - around 30mph. At the critical speed it switches over to reverse-swing. There's not an exact speed for that, since it relies on the condition of the ball."

As it turned out, exactly that was the problem with Mike Hendrick - getting far too close to the transition speed between conventional and reverse-swing for the ball to go either way.

Contrast Swing

Then again, how would one explain the ball swinging towards the shiny side? It has, after all, been observed by experts around the world that with the seam up, the old ball has been seen swinging towards the shiny side. What, then, is this sorcery?

Dr. Mehta has it ready at the tip of his tongue, as he has since the 1980s - contrast swing, a term that he admits, is "often confused with reverse swing."

"In the modern age, with high-definition slow-motion replays, it's so easy to spot the seam. If you see the ball swinging with the seam upright like Shami, with one side roughed up, that's really contrast swing and not reverse-swing."

"In India, when I was a kid, we used to tape up one side of the ball to create a contrast and it would swing - that's essentially contrast swing. You really don't need the seam," says Dr. Mehta. "One side is rough, one side is smooth, you get a laminar boundary layer on one side, turbulent on the other side, and it swings - but which way?

"The direction of swing depends on the condition of the boundary layer. In speeds you encounter in competitive cricket, it will generally swing towards the smooth/shiny side. At lower speeds, it will swing towards the rough side."

Contrast swing towards the rough side, let's call it rookie swing,is often spotted among part-time dibbly-dobblers who just run in and bowl seam-up at low pace. In this case, the turbulent layer of air on the rough side doesn't weaken enough to separate early, and therefore creates a low-pressure zone on the rough side, causing the ball to swing towards the rough side.

This is often observed when a part-time dibbly-dobbler comes into bowl, and offers seam-up pies, only to find out that they've managed to swing the ball - a manifestation of slow contrast swing, or rookie swing.

The contrast swing towards the shiny side would be due to the higher speeds encountered in international cricket, and follows the same principle as reverse-swing. For purposes of distinction, let us call this "counter-swing". On the shiny side, at high pace, there is higher turbulence for the boundary layer to weaken and separate early, and on the rough side, there is less turbulence, and therefore higher pressure, causing the air to push the ball into the shiny side.

Sound familiar? This is because it is often confused for reverse-swing, owing to the popular theory, claimed by many experts and proponents, that "the old ball starts to swing towards the heavier side." On the contrary, if one side was, in fact, heavier than the other, an upright seam would fail to sustain itself, and come out of the hand all scrambled.

Contrast swing simply relies on how well the ball has been maintained, to create a contrast in roughness between the sides that will eventually cause an asymmetry in airflow, and therefore swing. Given that the seam is not angled, it doesn't play a part. Reverse-swing, on the other hand, happens when the ball swings against the direction of the seam, often aided by the rough non-seam side that brings down the speed at which it is attainable.

There. Another myth busted.

Malinga swing: The curveball

There are certain assumptions one makes when one thinks of a fast bowler. He will definitely not run backwards in his run-up, and that he will definitely impart backspin when he releases the ball - with the exception of the knuckle-ball. An upright seam and backspin to keep it stable are prerequisites for the ball to swing.

However, there had to be someone who thought out of the box - it happened to be a maverick by the name of Lasith Malinga, who didn't even need a cricket ball to get it to swing.

"When a quick bowler releases the ball, it already has backspin on it which keeps the seam in its upright position. Malinga, with his round-arm action, makes the ball spin around the vertical axis instead of the horizontal. The seam is tilted - and that causes a Magnus force (lateral) due to the spin - just like a slice in tennis, or a banana kick in football."

The working of the Magnus effect, entails a ball rotating around a vertical axis, as shown in the picture, with the airflow assisting the downwards force on the right, and opposing the upward force on the left of the ball. Eventually, this creates a current in the wake of the ball, on its left, causing the air to exert and opposing force to the right - essentially making the ball swing into the right-handed batsman.

"The spin doesn't play a big role in conventional or reverse-swing because the actual force is upwards (not lateral). Once you tilt that axis, then he's bowling in-swingers (to the right-hander) just due to the tilted axis and the spin. It could be a fully rough or a smooth ball, or a tennis ball, it doesn't really matter. This is also the reason spin bowlers achieve what the pundits call "drift" - particularly the bowlers who impart side-spin and give the ball air.

The myths the truths about swing

Swing after bouncing:

"That's not under the bowler's control. That just happens."

"Quite a lot can happen after the bounce, depending on which part of the ball hits the pitch and the surface condition, so quite often you'll see that the ball keeps going straight, bounces, and then starts swinging, and if you noticed the seam orientation, it ends up slightly angled, one way or the other, and that's what causes the ball to swing. That's why 'keepers have a tough time.

"Another thing that can happen is the knuckling effect, such that it stops spinning due to the friction between the ground and the ball."

This would imply that on drier surfaces with more friction, like those in the subcontinent, this effect is more prevalent, as opposed to grassier wickets in England, where the ball may retain some backspin and perhaps maintain the seam position and swing after bouncing.

Colour and coating:

The color, by itself, has nothing to do with how much a ball swings. However, the amount of lacquer, which reflects in the shade of color on the ball, may have an indirect effect.

"The color doesn't really matter; but the first time they used the white ball in a World Cup (1999), they used an extra layer of lacquer so that the ball doesn't discolour easily. At the time they didn't use two balls. That was, in fact, my idea."

"I told the Australian manager, that if you want to stop or curb reverse swing, use two new balls, and sure enough, pretty soon they started doing exactly that."

***

As a result, the Dukes ball, which swung around hoops in the 1999 World Cup, was disbanded, as was the use of a single ball in an ODI, which yielded reverse-swing until as recently as the 2011 World Cup, in an attempt to make the game more batsman-friendly and marketable.

Late Swing

"Again, this is total BS," says Mr. Mehta dismissively. "People often think that especially reverse swing tends to happen a lot later. That's simply not true. The sideways force is the same for reverse swing as for conventional swing.

"However, since the ball reverses at higher speeds, it might cause the illusion that the ball swings late. Most of the movement always occurs in the latter part of the flight."

Essentially, late-swing is an illusion, and perhaps one that results from a missing piece of information - that reverse swing, in fact, occurs at a much higher pace than conventional swing and therefore, only appears to happen late.

Outfields and cross-winds

Contrary to popular belief, it is perhaps the more tangible, macro-level ground conditions that have more of an effect on the swinging ball than, for instance, the air molecules, or, even more ludicrously, the floodlights. For instance, a strong cross-breeze in an open stadium, and non-abrasive outfields certainly play a subtle but significant role on the magnitude of swing and ball condition respectively - the former results in more pronounced movement of the ball, and the latter keeps the shine on for longer, therefore causing the ball to swing for longer.

"A lot of the time, even the Kookaburra ball swings for longer in places like New Zealand, where the outfields are lush and the pitches are not concrete like India. The lush outfields are another reason the red ball swings for longer in England, because there aren't many boundaries and hence less danger of the ball hitting the boards and getting scuffed up.

"Furthermore, open grounds such as those in New Zealand can affect swing. Let's say for someone like Shami, who bowls with the seam up, the wind can create some asymmetry. These things matter, but not as much as people make it out to be."

"Cloud cover"

"Look upwards. If it's dark, bowl first," says a confident commentator in his pitch report, hardly mentioning the pitch.

One of cricket's longest-standing myths has been that cloud cover aids swing bowling; that humid air is denser and heavier, and thereby offers more resistance to make the ball swing more.

This theory has two gaping holes: firstly, the resistance that air molecules can offer to a ball travelling at 80mph is negligible. More importantly, humid air or "wet" air, is in fact, counter-intuitively, less dense than dry air! "

The density of dry air is 1.225kg/m3 whereas the density of moist air is 1.200kg/m3. So if anything, dry air is perhaps negligibly more conducive to swing bowling than moist air!

***

"All this talk about 'swinging grounds' is basically BS. It's the same thing with cloud cover. They'll never talk about it when it's hot and sunny and it's still swinging. I even had an interview with Flintoff a few years ago, and I thought I had convinced him against it, but when I watched the production, it was clear that he wasn't."

The disbanding of the cloud cover theory is perhaps proof of the fact that some facets of the game are simply counter-intuitive. They're simply not what one might imagine.

The Bottomline

With all the theories and parameters hogging the domain of swing bowling, the psychology of the bowler perhaps slips under the radar. As opposed to Mike Hendrick's critical speed fiasco, the method of swinging a ball is paramount and needs to be just right - and that would mean that the bowler should neither be trying 'too hard', nor 'not hard enough'.

"In 2007 World Cup, in a league game against Australia, Chaminda Vaas was unable to swing the ball. I found out that it was because he was trying to hold it too hard and so the seam came out scrambled. The Australians, particularly Nathan Bracken, however, managed to swing the ball the very same day.

"It's all about releasing the ball the right way. A good swing bowler will be able to swing any new ball, on any ground, in any weather conditions," is Dr. Mehta's final word on a domain he has specialized in for the best part of four decades.

Jimmy Anderson mentioned in an ECB documentary, that sometimes it swings when it's cold, and sometimes it swings on a hot, clear day. With 600 Test wickets at the time of writing, his exact words about the weather and its effect on swing were 'I don't know when the ball is going to swing.'

And perhaps with the number of parameters that seem to affect swing bowling, it may be wise for bowlers to focus on the "How it works" of it more than the "How it's done". The Dukes may be more forgiving, but the poor construction of the Kookaburra won't be, and by the time the bowlers figure out a way to swing the ball in the given conditions by trial and error, they may have lost their window to win a Test match.