Making the Mysteries of Clay Into a Science

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As the best in tennis slide and rally their way through a cold and rainy opening week at the French Open, we take a break from the action to look at the science of clay, the sport’s most peculiar surface.

The courts of Roland Garros are made not from clay per se, but from crushed bricks that were clay in a former life. Usually described as the game’s slowest surface, clay is actually faster than other surfaces in two ways: vertically (when the ball bounces up off the court), and beneath one’s feet, where loose particles prove useful for sliding. Clay is known as “slow” because its top coating reduces the horizontal speed of the ball as it bounces toward a player. The result is fractions of a second more to react to an opponent’s incoming shot.

Why does a ball bounce higher on clay? Rod Cross, a physics professor at the University of Sydney in Australia and a prolific author on the science of tennis, explains.

“The ball kicks up steeply when it encounters the wall of clay in front of it,” Professor Cross said in an-email interview. “It gets deflected upward by the mound, hence the ball rises up at relatively high speed.”

Think of the mound as a ramp, and it becomes easier to understand why horizontal speed is reduced and vertical speed increased.

Rallies last longer on clay, and the ball moves more slowly during them. The clay itself, however, does not reduce the speed all on its own. Rather, it creates an environment in which players usually find it advantageous to reduce pace.

Consider a ball flying through the air at 90 mph. No matter what is below it – clay, grass, cement, water – it travels at 90 mph. When it collides with clay, it is launched upward at a steeper angle than it would be on other surfaces. The reduced horizontal speed (to and from the player rather than up and down) means a player has to provide more power – that is, swing harder – to send the ball back across the net at 90 mph.

The higher bounce also encourages players to brush up on the ball with their swing more than usual, a motion that produces more topspin. Topspin strokes travel at slower speeds than flat ones, as energy used to graze the ball would otherwise be used to strike it head on, sending it speeding, rather than spinning, along. As a result, shots on clay are generally less forceful, dip more drastically, bounce much higher (a combination of topspin and clay’s natural properties), and can cross the net at more extreme angles yet still land inside the lines.

These same principles apply to serves.Yet one will notice that average serve speed, which is measured while the ball is in flight, is slower on clay. Players, in effect, compensate for the inherent properties of clay by hitting more spin serves, especially kick (topspin) serves. The force of a swing remains the same, but the racket brushes against the ball, reducing its forward speed and causing it to tumble forward and dive into the service box. Once the ball hits the clay, it kicks up even higher, making it difficult to return, or at least difficult to return well. This perhaps sets up a winning shot a few strokes later.

Statistics on aces at Grand Slam events, compiled by the International Tennis Federation (ITF), help to illustrate the great serving divide between the French Open and the rest of the majors. Between 2002 and 2005 at the Australian Open, Wimbledon, and the U.S. Open, men averaged 2,227 aces per tournament.At the French Open during those same years, they averaged 1,373 – a difference of 38.4%. For the women, the average at the other three majors from 2002 to 2005 was 564 aces per tournament, compared to 407 at the French Open – a difference of 27.8% (the smaller numbers among the women also illustrate that their matches are less dominated by serving).

Professor Cross offered a few other insights about clay.

— “Clay gets into strings and causes them to break.”One does not see broken strings in tennis too often these days, as a top player will more likely pull a new racket from his or her bag before the strings are sufficiently worn. In Paris, you should notice more snaps.

— “Rallies last longer on clay, but sets don’t last longer,” Cross said. “There are fewer games per set on average, and fewer [tiebreakers].”

ITF data backs this analysis. From 2002 to 2005, 10.6% of sets ended in tiebreakers at the French Open, compared to 11.9% in Australia, 14.2% at the U.S. Open, and 15.5% at Wimbledon.For the women,the change was not as drastic: 6.1% at the French, 6.7% in Australia, 8.2% at Wimbledon, and 8.7% at the U.S. Open.

— “Despite the easier nature of [clay] on the body (more sliding and less gripping of the feet),injuries are just as common,” Cross said. “About 3% of the men’s matches will be incomplete when one player withdraws with an injury.”

Including yesterday’s results, the men have completed 77 matches. Three ended in injuries, or 3.9%.

If Cross’s prediction holds up, one of the remaining 50 matches at Roland Garros will end in injury. Let’s hope the unlucky man is not Roger Federer or Rafael Nadal.

For more information about Professor Cross and his work, visit him on the Web at www.physics.usyd.edu.au/~cross/.

***

In yesterday’s rain-delayed action, top-seeded Roger Federer won easily, defeating Alejandro Falla 6-1, 6-4, 6-3. David Nalbandian swept aside Frenchman Richard Gasquet 6-2, 3-6, 7-5, 6-0. Gasquet won just one point against Nalbandian’s serve in the final set.

Venus Williams played sloppy tennis early, but beat the darkness and Emma Laine, 7-6(2), 6-2.

Paul Capdeville, who lost to Mario Ancic 7-5, 6-3, 7-6(3), shoved Ancic after their match. Capdeville was critical of the chair umpire and thought Ancic had lunged at him. It went no further (with help from AP wire reports).

tperrotta@nysun.com


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