Science of Swing Bowling
Truth Behind Conventional, Reverse And Contrast Swing
Conventional swing, reverse swing, contrast swing. In the past few
years cricketers have been barraged with new and convoluted descriptions
of a ball’s deviation through the air, but only the select few
can differentiate between each, let alone unravel the science behind
them. One such man is Rabindra Mehta, a NASA scientist and former
club fast bowler based in California, who has studied cricket-ball
aerodynamics for nearly three decades. Here, in response to an article
published at Cricinfo last week, he gives the definitive answer to
the mysteries of swing.
Does it really matter what you call a particular type of swing? Well,
it matters because the science dictates what type of swing it is.
Of course, bowlers do not have to know or understand the science,
but I’m sure they would appreciate knowing when a certain type
of swing will be effective and in which direction the ball is likely
Having studied cricket ball aerodynamics for over 25 years, my mission
here is to reveal the answers to all the mysteries surrounding swing
bowling, and hopefully, to quash once and for all, all the myths and
erroneous explanations that are still floating around the cricketing
world. To assure you that these three types of swing do actually exist,
and are regularly practised on the cricket ground, I have quoted examples
from the unforgettable 2005 Ashes series between England and Australia.
In each example, the batsman ends up losing his wicket mainly because
he is deceived by the lateral movement of the ball through the air.
Let us first look at some of the fundamental flow physics that will
help to explain all three types of cricket ball swing. As the ball
is flying through the air, a thin layer of air called the “boundary
layer” forms along the ball’s surface. The boundary layer
cannot stay attached to the ball’s surface all the way around
the ball and it tends to leave or “separate” from the
surface at some point. The location of this separation point determines
the pressure, and a relatively late separation results in lower pressure
on that side. A side force or swing will only be generated if there
is a pressure difference between the two sides of the ball.
Now the boundary layer can have two states: a smooth and steady “laminar”
state, or a time-varying and chaotic “turbulent” state.
The transition from a laminar to a turbulent state occurs at a critical
speed that is determined by the surface roughness; the rougher the
surface the lower the critical speed. However, on a very smooth surface
and at nominal speeds, a laminar boundary layer can be forced to turn
turbulent by “tripping” it with a disturbance. The disturbance
can be in the form of a local protuberance or surface roughness which
adds turbulent eddies to the laminar layer and forces it to become
turbulent. It is similar to putting your finger into a smooth stream
of water from a tap: note how small chaotic turbulent motions are
generated downstream of the finger location. Now it turns out that
a turbulent boundary layer (because of its increased activity and
energy) can stay attached to the ball’s surface for a longer
distance compared to a laminar layer.
It is said that this type of swing originated around the
turn of the century, but there is evidence that it was in existence
well before that time. Bowlers from that era, including WG Grace,
had realised that a perfectly new ball favoured the “peculiar
flight,” so there is not much doubt that it was conventional
swing that these bowlers were referring to.
Fast bowlers in cricket make the ball swing by judicious use of the
primary seam. The ball is released with the seam at an angle to the
initial line of flight. For conventional swing, the ball swings in
the same direction that the seam is pointing. So a ball released with
the seam angled towards the slip fielders will swing away from the
batsman (outswinger) and one released with the seam pointed towards
fine leg will swing into the batsman (inswinger). A great example
of inswing can be found in the first innings of the fourth Test when
Marcus Trescothick was bowled by Shaun Tait, and for outswing, check
out Simon Jones to Michael Kasprowicz when Australia’s turn
came to bat.
So how and why does a cricket ball swing in the conventional mode?
Let us see what happens to the flow over a cricket ball released with
the seam angled (Fig. 1). Between about 30 and 70 mph, the laminar
boundary layer along the bottom surface separates at about the apex
of the ball. However, the boundary layer along the top surface is
tripped by the seam into a turbulent state and its separation is therefore
delayed. This asymmetry results in a pressure differential (lower
pressure over the top) and hence side force which makes the ball swing
in the same direction that the seam is pointing (upwards).
So this is the theory. Does this really occur in practice? The photograph
in Fig.2 shows a cricket ball with the seam angled and held in a wind
tunnel. The flow is from right to left and smoke is injected into
the region behind the ball. It is seen clearly how the boundary layer
over the bottom surface separates relatively early compared to that
over the top. Also, the clean, smooth edge of the smoke at the bottom
separation point indicates a laminar boundary layer while the rather
chaotic nature of the smoke near the top separation point indicates
a turbulent state.
The key to successful conventional swing is to keep that leading side
(the one facing the batsman) as smooth as possible so that a laminar
separation is obtained. So the age-old practice of polishing the ball
makes a lot of scientific sense (I also liked the fact that the bright
red patch on my trousers identified me as a fast bowler). In prior
years, bowlers were accused of using “Vaseline” or “Brylcreem”
to aid the polishing process. Lately though, the buzz is all about
how some special mints or sweets can help the saliva become a more
effective polishing agent. I am not sure about this, but what I do
know is that once the lacquer is worn off the surface, the natural
oils in the leather are released and this helps the polishing process.
Of course, in reality, some backspin is always imparted when a cricket
ball is bowled. For successful swing bowling, the ball should be released
so that it spins along the seam with minimal wobble. Wind-tunnel tests
on spinning cricket balls show that the maximum side force is generated
at about 70 mph with the seam angled at 20 degrees and the ball spinning
backwards at 11 revolutions/second (Nature, Vol. 303, pages 787-788,
So what happens at speeds above 70 mph? The boundary layer on the
bottom side in Fig. 1 begins towards transition, the asymmetry is
reduced and so is the swing such that at around 80 mph there is no
swing. So if you are unfortunate enough to bowl at around this critical
speed, the ball will not swing, no matter how perfectly the ball is
released. In a recent conversation with Mike Hendrick, the former
England fast bowler, he revealed to me that he always found it very
difficult to swing the ball and after all these years, he finally
figured out why when he saw our data. Of course one solution is to
slow down a bit. But, what if the bowler puts in the extra “oomph”
and releases a much faster delivery? Can you say, “reverse swing”?
My old schoolmate, Imran Khan, is rightly regarded as one
of the first and finest exponents of reverse swing. In August, 1980,
around the time I published my first article on cricket-ball aerodynamics
in the New Scientist magazine (Vol. 187, No. 1213), he told me out
of the blue that, on occasions, the ball would swing the “wrong
way”. He was predominantly an inswing bowler at the time and
so the “wrong way” meant that the ball would swing away
from the batsman. At the time, I honestly did not believe that such
a phenomenon could occur since I could not explain it scientifically.
However, in the following year, when we conducted our wind-tunnel
experiments, the whole “mystery” was revealed.
The flow over a ball exhibiting reverse swing is shown in Fig. 3.
So now, at a high enough bowling speed (over about 85mph for a new
ball) the laminar boundary layer transitions into a turbulent state
relatively early, more importantly before reaching the seam location.
In this case, the seam actually has a detrimental effect on the turbulent
boundary layer by making it thicker and weaker and it therefore separates
earlier than the turbulent layer over the bottom surface.
Still following this at the back of the class? That means that the
asymmetry is now switched and so is the side force. Result: the ball
swings in the opposite or reversed direction. It is only true reverse
swing if the ball swings in a direction that is opposed to that of
the seam. This means that the fastest bowlers in the world who bowl
at over 90mph will only produce reverse swing, even with a brand new
Of course, not many bowlers can bowl at 90mph so how can we mere mortals
produce reverse swing? Well, that is where the surface roughness comes
into play. As the roughness on this leading side (facing the batsman)
is increased, the critical bowling speed above which reverse swing
can be obtained is reduced (experimental data showing this effect
can be found in a New Scientist magazine article, Vol. 139, No. 1887,
August 21st 1993). It also means that more effective reverse swing
will be obtained at the higher bowling speeds.
This is why reverse swing generally comes into play with older balls.
The whole beauty of reverse swing is that a bowler who could only
bowl outswingers at the onset with the new ball can bowl inswingers
with an older ball without any change in the grip or bowling action.
Similarly, an inswing bowler will suddenly be able to bowl outswingers.
For a classic exposition of reverse swing, check out last summer’s
third Test, Andrew Flintoff to Simon Katich in the second innings.
One of the reasons why reverse swing has gained such notoriety is
its constant link to accusations of ball-tampering, as we just witnessed
at The Oval. Once bowlers realised the importance of the rough surface,
they started to help the process along. After Mike Atherton’s
“dirt in the pocket” affair in 1994, I was duly summoned
to Lord’s by the TCCB. They showed me several balls that had
been confiscated after the umpires suspected the fielding side of
ball-tampering. From what I saw, the most popular forms of tampering
consisted of gouging the surface using foreign objects such as bottle
tops and attempting to open up the quarter seam using fingernails.
Perhaps what is not that well-known is the fact that positive roughness
can work just as well. So if some dirt was stuck to the ball’s
surface (using sweat or saliva as the glue), reverse swing could be
obtained at nominal bowling speeds on even a brand new ball, and best
of all, the evidence is gone by the time the ball reaches the wicketkeeper.
Is that what Atherton was attempting? I doubt it, but I wonder if
ever tried it, especially in the days when sawdust was routinely used
to dry a wet ball.
The one misconception about reverse swing that is commonly heard (even
it occurs due to a weight imbalance created by wetting one side of
the ball. This is based on comments made by some of the early exponents
of reverse swing, but it has NO scientific basis to it whatsoever.
Wetting the ball may indeed help in the gouging process, but the importance
of a dry, rough surface is now well understood by the current players
who are often seen avoiding hand contact with the rough surface. Another
misconception is that reverse swing is more lethal because the ball
swings more and late. It turns out the side-force magnitude and direction
for reverse swing are comparable to those for conventional swing and
for both types of swing, the ball follows a parabolic flight path
so that most of the movement occurs in the latter part of the flight.
Bottom line: late swing is “built-in.”
My personal attempts at ball-tampering never really materialised.
By the time I figured it out, I found myself playing in California
and our team captain – who was also the opening bowler –
refused to let me tamper with the ball; he was convinced that my actions
would “screw up” his beautiful outswing. The fact is that
the condition of the “back side” of the ball (the upper
surface behind the seam in Figs. 1 and 2) is not very critical. So
the perfect ball for conventional and reverse swing is one with one
side very smooth and the other rough. There is also another advantage
in creating a ball with a sharp contrast in surface roughness.
On a visit to the ECB National Cricket Academy last December,
where I was invited to present my research on cricket ball aerodynamics,
I realised that there was still some confusion regarding the definition
of reverse swing. They had a practice session devoted to reverse swing
with specially-prepared balls (one side deliberately roughened). Some
of the bowlers swung the ball quite well and they thought in the reverse
However, the ball generally had the seam straight up (not angled)
and swung towards the smooth side. This was obviously not reverse
swing and it was somewhat difficult for me to explain to the bowlers
and coaches what was going on. That is when I developed the new term:
“contrast swing” (The Wisden Cricketer, Vol. 3, No. 7,
April 2006). So how is contrast swing different from conventional
and reverse swing? For one thing, the swing direction is determined
by the bowling speed, as opposed to seam and smooth/rough surface
In Fig. 4(a), a ball with a contrasting surface roughness is flying
through the air at a relatively low speed with the seam straight up.
In this case, the boundary layer over the upper surface separates
relatively early in a laminar state while that on the bottom rough
side becomes turbulent and separates later. This asymmetry results
in a side force which makes the ball swing towards the rough side.
If the ball is released at a much higher speed, the flow field is
different as shown in Fig. 4(b). In this case, transition occurs on
both sides of the ball, but the turbulent boundary layer along the
rough bottom surface is thickened and weakened (in the same way that
the seam weakens the turbulent boundary layer in reverse swing). As
a result the boundary layer on the rough side separates relatively
early and the ball now swings towards the smooth side.
Note that the actual critical bowling speed that determines which
way the ball will swing is totally determined by the condition of
the ball. Superb examples of true contrast swing can be found in the
second innings of the third Test, Flintoff to Matthew Hayden and Flintoff
to Adam Gilchrist.
The most exciting feature about contrast swing is that just about
any bowler (regardless of bowling speed) can implement it in practice.
As most cricketers are aware, it is much easier to release the ball
with the seam straight up, rather than angled towards the slips or
fine leg. Thus, even mere mortals should be able to swing such a ball,
and in either direction, since the bowling action is the same for
both types of swing, the only difference being the orientation of
In fact, the medium-pace “seam” or “stock”
bowlers usually bowl with the seam in this orientation in an attempt
to make the ball bounce on its seam so that it may gain sideways movement
off the ground. With a contrast in surface roughness, these bowlers
could suddenly turn into effective swing bowlers, without any additional
effort, thus confusing not only the batsman, but perhaps themselves
as well. Another advantage of contrast swing is that it can be obtained
even if the seam is completely “bashed-in” (note that
a prominent seam is critical for conventional and reverse swing).
If you doubt that a ball with a bashed-in seam can contrast swing,
then tape over one half of a tennis ball and bowl it with the junction
line straight up.
So the ideal ball for swing bowling is one with a prominent seam,
one side smooth and the other rough. The fielding side should examine
the new ball and choose the side with the shallower or less rough
embossment and religiously polish that side throughout the match.
The other side should be allowed to roughen during the course of play,
but it should be kept as dry as possible.
Even if the seam gets completely bashed-in, a ball with a contrast
in surface roughness can be swung. So how do you tell what type of
swing a particular bowler is producing?
Make note of the seam orientation and swing direction. If they are
coincident, it is conventional swing; if opposed, it is reverse swing
and if the seam is pointing straight down the pitch, you have just
witnessed contrast swing.
Rabindra Mehta is a Sports Aerodynamics Consultant and NASA
Scientist based in California (firstname.lastname@example.org)