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An explanation of Cricket

The physics of legspin bowling

by Mike Whitaker


I will freely admit to getting progressively more annoyed by successive websites' and TV commentators' attempts to unravel the mysteries of Messrs. Warne, Mushtaq, Strang and the like. Far too many seem to get caught up in the hype and hysteria surrounding the art, and apparently ignore basic physics in their descriptions of the various deliveries.

Let me set my credentials out: I bowl the stuff - physically, no, I don't bowl it very well, but I understand what it's MEANT to do. I passed A-level physics. I own the Ritchie Benaud MCC coaching video on the subject. I watch Warne, Strang, Mushtaq, Afridi, even Salisbury avidly every chance I get. And I play other sports involving spinning balls well enough to understand how a ball behaves when it's spun. There is no mystery to legspin, beyond that of actually reading it! If you know how the ball is spinning, you can predict its behaviour in the air and off the wicket. Just because Shane Warne has bowled it, it won't behave any differently if spun the same way by anyone else.

This article is going to be long, and at times have the tone of a mechanics lecture. Unfortunately, to achieve a complete understanding of, for example, why a top-spinner does what it does, this is necessary.

Lets start with the basios.

Speed and Velocity

People often use these two words interchangeably. They aren't, not quite. Speed is a simple measure of how fast a thing is going, expressed as so many units of distance per unit time. Velocity, on the other hand, adds one crucial but useful element: an object's velocity is its speed AND the direction it's moving in.

It's also possible to refer to an object's speed in a certain direction. This is usually called the component of the object's velocity in that direction. So, for example, a thrown ball as it starts to fall in flight will have a horizontal component, and a downward one, to its velocity.

When we consider the flight of a cricket ball, we're going to look at three components of its velocity. The vertical component (which will affect how high it bounces), the horizontal one towards the batsman (how fast it appears to move), and the horizontal one from side to side (its deviation in flight). Because these three are all at right angles to each other, we can play with each one separately without affecting the others.

The Motion of a Bouncing Ball

The simplest possible case. No spin at all, just a hard surface and a ball. Lets assume for a moment that we have a perfect bounce (of course, we don't, but we'll address that a little later on). If we drop it vertically, then it will bounce back at the same speed it had when it hit, or, put another way, the vertical component of its velocity will reverse direction.

If the ball is moving horizontally as well, then the horizontal component of its velocity won't change when it bounces, just the vertical.

Of course, the world isn't ideal, and cricket balls don't bounce perfectly off even the hardest Jamaican strip. What happens in the real world is that, when the ball bounces, the vertical component of its velocity is reversed and reduced a little, so the ball doesn't bounce quite as high as it was dropped from. And there's a little friction as the ball bounces which will reduce the horizontal component of its velocity too. Be grateful for this friction - without it, legspin would be a pointless exercise.

The Motion of a Spinning Ball

The motion of a spinning ball divides into two parts, for our purposes. Motion through the air (Warne's much-vaunted dip and drift) and motion off the wicket. We'll deal with the latter first, because it's simple.

Movement off the Wicket

What happens when a spinning ball hits a flat surface? Actually, it's simple, and obvious. What we need to consider is the direction the surface of the ball that contacts the pitch is moving in, so lets imagine for a moment that what we have is not a ball but a very flat car tyre, spinning about the same axis. If we were to drop it vertically, then when it landed, the tyre would grip and roll in what I'd hope would be the direction we expect - the opposite one to the direction the lower surface of the tyre is moving. If we inflated it enough to bounce, it would grip as it bounced, and bounce in that direction too. And the same happens with a spinning ball. It acquires velocity in the direction opposite to the one in which the point of contact of the ball with the ground is moving.

Now, obviously, the above thought experiment assumes the ball is dropped vertically. If it isn't, then the prior motion of the ball comes into play. The extra velocity imparted by the spin will combine with the existing motion of the ball, in a manner that is consistent with basic mechanics. Even when Shane Warne bowls it.

The Three Types of Spin

Ok, let's continue to play with our imaginary car tyre for a while.

Topsin or Overspin

First of all, lets hold it so its (invisible) axle is at right angles to us, and set it spinning so its top moves away from us and its bottom towards us (Looking from the right hand side, this is clockwise). Now lets throw it, still spinning, away from us towards a suitably attired friend (bat, pads, box, gloves...).

So, what happens when it bounces? The vertical component of its velocity reverses direction, still, just like any other bouncing ball. Its underside is moving towards us, so it will acquire velocity in a direction opposite to that, namely away from us, towards our friend. This will add to the existing horizontal component of its velocity towards him.

The result of this is that the tyre will speed up when it bounces. It will get to him faster than a ordinary, non-spinning tyre, because the component of its velocity towards him has increased, and it won't have had time to bounce as high as a result.

This is topsin. The characteristic behaviour of a topspun object when it bounces, as I hope I've just shown, is that it speeds up and seems to keep low, to 'shoot'.

Backspin or Underspin

Right. Now lets reverse the spin on the tyre, so the top of it spins toward us, and the bottom of it spins away (counterclockwise if you look at it from the right side), and throw it towards our friend again.

Now what happens? The point of contact of tyre and ground is moving in the opposite direction to last time, so the tyre will acquire velocity towards us, away from our friend. This will reduce the component of the tyre's velocity towards our friend (in fact, if we spin it hard enough it will actually bounce back towards us, but this is for all practical purposes extremely unlikely to happen). If the ball is spun hard enough and travelling fast enough, it may seem to 'skid' a little on pitching.

As far as our friend is concerned, the tyre will slow down when it bounces, and will have time to bounce higher before it reaches him.

This is backspin. The characteristic behaviour of a backspun object is that it slows down on bouncing and seems to bounce higher, to 'sit up and beg'.


Ok. Lets turn our tyre sideways, and spin it clockwise as we're looking at it, so its top is moving to our right, and its bottom to our left, and toss it towards our friend again. This being a thought experiment, we can assume the tyre will stay upright, both through the air and on bouncing. Unlikely though this is, it's a valid assumption that will help here.

Applying the same rules as before, the tyre will acquire velocity sideways, to our right and our friend's left, as it bounces. This won't affect the component of its velocity towards him at all: it will simply make the tyre deviate from the straight path towards him.

This is sidespin. The characteristic behavoir of an object with sidespin is that it 'breaks', to use a cricketing term, when it bounces, to one side or the other depending on which way it is spinning.

[Obviously we could spin the tyre about a vertical axis, but this would have little practical use in this part of the discussion, althought it will be interesting when we move on to the next section.]

Movement through the Air

What follows may seem hard to grasp, and I don't intend to cover the physics behind it. What I'm about to explain can be shown by empirical observation, best with a table tennis bat and ball and a partner if you're any good at the game, by watching live tennis, or any of a number of other games with a moving ball. Note that the phenomenon I'm about to describe bears no relation to how the likes of Akram, Gough or anyone else make a ball swing - this is, to coin a phrase, a whole different ball game.

It's demonstrably possible to make a ball move in the air by putting spin on it. Ask any baseball pitcher. For a classic example, find footage of the Brazillian soccer player Romario taking a freekick, and watch him apply spin and cause a ball with no seam to swerve over four yards sideways over a distance of 25 yards.

Lets replace Romario's soccer ball with an oversized cricket ball with its seam horizontal. You will find that when he kicks it, he imparts spin about a vertical axis, such that, for the ball to swerve to the right from the viewpoint of someone facing it, the ball is spinning counterclockwise, looking down on it.

As the ball travels through the air towards the person facing it, one side of the imaginary seam (the left in this case) is also spinning towards the viewer, and the other side is moving away, and the ball demonstrably swerves as though it was being 'pulled ahead' on its left side and 'pushed back' on its right - i.e. to the right. The slower the ball is moving through the air, and the harder it is spun, the more pronounced this effect appears to be to the viewer.

For another example, watch Pete Sampras or Greg Rusedski hit a backspin volley. In this case, the underside of the ball is spinning forwards, and the top side is spinning backwards. The ball effectively 'swerves up' - which exhibits itself as a tendency for the ball to 'float', staying in the air longer before it drops. Conversely, a topspun shot will 'swerve down', and 'dip'.

A ball spun about an axis in the same direction as its motion (as per our sidespun tyre above) doesn't swerve markedly. Why? Because the axis of spin is along the direction of travel - its the spinning surface moving in the direction of travel that causes the swerve.

<>The Legspinner's Armoury

At last, we've covered movement through the air and off the pitch, so now we can describe each of the legspinner's four classic deliveries, and how they behave. We'll replace our car tyre with a cricket ball, its seam where the tyre's tread was. I'm not going to take these in the obvious order, for reasons which will become apparent.

The Topspinner

The seam is vertical, the ball spinning clockwise if viewed from the bowler's right. Through the air, it will dip at the end of its flight, pitching shorter than an identical delivery with no spin, and then will 'shoot' forward, keeping low, as it pitches.

Perhaps one of the best examples of this is Shane Warne's dismissal of Alex Stewart in the 1994/5 Ashes tour. Stewart was drawn forward to what he thought was a ball of good length, but found it to be a topsinner, pitching short of where he read it, that kept low, 'shot' off the pitch and trapped him lbw. Classic dismissal with a topspinner.

The Legbreak

The stock ball. Essentially it is sidespun about a horizontal axis, anticlockwise as the bowler sees it, so it will break from leg to off to a right hand batsman. 'But', I hear you say, 'Warne gets dip and drift from his legspinner, and you said that didn't happen with a ball spinning that way.'

No, it doesn't. But very few legitimate legspinners bowl 'pure' legbreaks with the seam at right angles to the direction of flight. Watch 'SpinVision' film of a Warne legbreak, and more than likely you'll notice two things. One is that the axis of rotation isn't directly towards the batsman, but pointing down the leg side, and the other is he has given it one heck of a rip. In short, it's part topsinner, and very likely the seam isn't quite vertical either. The result of this is that, as perceived, the ball displays a tendency to dip and drift in a little before it pitches, then break sharply to the off.

The legbreak to end all legbreaks has to be the Gatting dismissal. So much so that I can refer to it, in the context of legspin, as 'that ball', and everyone knows exactly which ball I mean. As good an example, perhaps with more pronounced drift, is Warne's dismissal of Hollioake in the 1997 Ashes series. Bowled from wide of the crease, this drifted in and pitched on middle stump, turning to take middle and off as Hollioake obligingly shouldered arms to allow us to better view the flight of the ball.

The Googly

Simply the reverse of the legbreak, spun clockwise as the bowler sees it. Most of the comments that apply to the legbreak apply equally here, if not more so. The nature of the delivery action makes it even harder to bowl a 'pure' googly, and the ball is even more likely to be part topspinner.

A fine example is Surrey's Ian Salisbury's dismissal of Paul Strang in the Kent vs Surrey 1997 Benson and Hedges Cup final, the more so for it being one leggie dismissing another. Salisbury doesn't bowl many googlies, and this one, I suspect to his great satisfaction, beat Strang, playing for the legspin, between bat and pad and clean bowled him.

The Flipper

There are times when I have been convinced that the flipper is the commentator's 'excuse' ball, the word used to describe a ball the commentator couldn't read that bamboozled the batsman. Interestingly, Richie Benaud and Terry Jenner (Warne's friend and coach) describe it completely differently.

What Benaud describes is spun with the seam horizontal, with the same grip as a legbreak. He asserts that the ball 'skids' on pitching. What Jenner describes, and most people seem to have agreed on, is a ball which as far as I can tell is spun with backspin and a very different action to the legspinner's other weapons.

The behaviour of the ball Jenner describes would be to 'float' a little, pitching longer than expected, and then to 'sit up' when it pitches. The desired effect, one would assume is to catch a batsman playing back for what he thinks is a shorter ball, and trap him lbw playing over the top or outside the line.

I'd love to supply an example of the flipper to discuss, but it's somewhat complicated by the cries of 'flipper' from every side whenever Warne pulls out a ball that does something special. Both the legbreak and topspinner examples I quote above were called flippers by the TV commentators at the time. I'm reasonably convinced the ball is not just an exercise in mind-games, but I've yet to be convinced I've seen one.

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