Great serve, for a guinea-pig ...

As tennis players have got bigger and stronger, the balls have been flying ever faster. Now they are set to slow down, says Charles Arthur

The professional tennis players gracing the courts at Wimbledon may be rich and famous. But they are also unwitting guinea-pigs in a worldwide experiment, in which the laws of physics - and especially aerodynamics - are being harnessed to make the game more entertaining.

The key to the experiment lies in the tennis balls. What tournament organisers really want are balls that look and feel just like the present ones, but fly through the air more slowly.

The past 10 years have seen the emergence of the "power game". Tennis rackets have become up to 20 per cent more powerful as manufacturers use stiff graphite fibres to make larger-headed models. The players themselves have become taller and stronger: most of the top 10 men players are over 6ft, compared to only a handful 20 years ago. But the tennis balls have stayed the same.

The result is that service speeds have risen, so that on Centre Court they typically average 100mph, leading to more points where the service is not returned. In a number of countries, especially the United States, many spectators are not coming back either: when asked why not, they say the game is just too boring.

This has put the International Tennis Federation (ITF) in a quandary. Unlike golf, where the twin powers of the Royal & Ancient Club and the US Golf Association legislate on every technical detail - from the number and shape of the grooves in the clubheads to the pattern of dimples on the golf ball - in tennis the ITF sets only a maximum size for the racket, with no limits on its "power". The replacement of wooden rackets with graphite overwhelmed the game, but it's too late to turn back. So now the only option left is to try to slow the game down (and stop spectators getting whiplash as they follow rallies) by altering the balls.

This year, Wimbledon officials and players have tried using balls which are pressurised at 2 per cent below the norm. But in last week's hot weather the balls heated up unusually quickly, reaching the same speed as normal balls within a couple of rallies. Statistics collected by IBM bear this out - there was no difference in the percentage of serves returned.

Dr Geoff Thwaites, of GT technology in Cambridge, last week ran a series of computer simulations of the effect of pressure reduction on tennis balls. He found that while a ball rebounds about 5 per cent less when bouncing from a hard surface, "the velocity of the ball off the racket is only decreased by 1 per cent for the same pressure reduction". And, of course, it is precisely the velocity of the ball off the server's racket which matters if you are trying to give his or her opponent an increased chance of returning a service. Dr Thwaites also warned that softer balls will skid further "and may cause more difficulty with line calls".

The organisers of the French Open, held at Roland Garros in Paris in June, tried a similar experiment, but this time using smaller balls. Their object was to speed up the game rather than slow it down: their floor surface, of crushed brick, is one of the slowest in the world, while Wimbledon's is the fastest.

Two factors are crucial in deciding how fast a ball will fly: the resilience of the rubber (to which the internal pressure contributes); and its effective cross-section, which affects the ball's air resistance. The Wimbledon balls use the same rubber as those in the French Open, but at a lower pressure, meaning that when a racket hits the ball, it "gives" more. This reduces the ball's initial speed. But when the rubber - and consequently the gas inside the ball - heats up, the pressure rises again.

Increasing the cross-section means that the ball slows down more quickly as it flies through the air. "To really have an effect, you would have to look for a 10 per cent reduction in the speed of the balls," says Robert Haines, a technical consultant to the ITF who has worked for ball manufacturer Dunlop-Slazenger for the past 30 years. "You would have to make the ball's cross-section about 20 per cent bigger. To get a 10 per cent reduction through lowering the pressure, you'd get a pretty squashy ball. It would be unsatisfactory to the players." Dr Thwaites reaches a similarly gloomy conclusion from his computer studies: "It seems from these results that little can be done with the ball to change the game."

Some players have been complaining that the softer balls at Wimbledon have led to more wrist strains, because the impact lasts longer. But they can probably console themselves with the thought that they are the best- paid scientific guinea-pigs in the world.

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