Wimbledon 2017: Is grass the most difficult surface to play on?

As the world’s best tennis players arrive at Wimbledon, it marks the culmination of the tricky transition from the clay courts to grass.

But which surface is the biggest test for players?

Brad Gibson, director of EA Milne Centre for Astrophysics at Hull University and a tennis fan, explained: “From a scientific perspective, it really depends on which part of the physics is more conspicuous to a given player.”

So we asked Gibson to break it down and explain the science behind the tennis action on these two contrasting surfaces.

The ball travels faster, but bounces slower on grass

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“If it is the speed of the ball off the court which limits a player’s effectiveness, then grass is the more difficult surface because the ball travels approximately 25% faster bouncing off grass compared with bouncing off clay,” Gibson says.

“On the other hand, if it is differences in the bounce angle of a ball – compared with the angle it first hits the surface – which limits a player’s effectiveness, then clay is the more challenging.”

Gibson also points out that on grass, the ball bounces off with the same angle that it hits with (a true bounce) whereas on clay, the angle is estimated to be 25% greater than the angle it hits with. This means mean the “balls tend to sit up and move slower” on clay.

So ultimately, it depends on how skilled the player is in interpreting the speed and bounce angle and use it to their advantage.

The ball spins more on clay, but moves more unpredictably on grass

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“If you hit the ball with a lot of topspin, that accentuates the bounce height even more on clay,” Gibson says.

Rafael Nadal, dubbed the King of Clay, is known for using his forehand power and topspin to consistently outplay his opponents.

But despite achieving true bounce on grass, Gibson adds that its rougher surface makes it tougher to anticipate the direction in which it will travel.

“Grass, despite the best efforts of the rollers, remains a rough surface,” he continues. “This means that the ball can take random bounces in directions somewhat away from a straight line – making grass incredibly difficult to predict.”

Balls are harder to hit on grass courts

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So then why does ball behave differently on different courts? Gibson says it is down to the coefficient of friction (used in physics to determine an object’s normal force) associated with these surfaces.

“The higher coefficient of friction associated with clay, relative to grass, means the ball travels approximately 25% slower,” says Gibson.

“That, coupled with the significantly higher bounce angles associated with clay makes the ball sit up and be easier to hit compared to grass.

“Outright winners on clay courts are much rarer as the returnee has a significantly easier time in reaching any given shot.”

Some players perform better on grass than others

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A player’s own technique might also play a role in how they adapt to their surroundings in Grand Slams. For example, Nadal who has two Wimbledon titles, has always dominated the clay courts, while Roger Federer – who has a record 18 Grand Slam titles to date – has triumphed on grass, despite winning the French Open in 2009.

“Michael Chang and Rafael Nadal were born for clay, but never adapted to grass in the same way,” Gibson points out.

“Conversely, Pete Sampras and Roger Federer were born for grass but were never really comfortable on clay.

“Broadly speaking, classical serve and volleyers like Sampras are more dominant on grass – while the fitter, patient and smooth-stroking players like Nadal tend to be more dominant on clay.”

Clay courts demand greater physical fitness, but damp grass poses greater injury risk

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Gibson points out that the significantly longer rallies associated with clay mean the fitness levels must be higher.

“The slower pace means more opportunities to return points, which mean longer rallies, and that means greater cardiovascular requirements,” Gibson says.

But loose clay does allow the player to slide into shots – helping them conserve energy and reduce impact damage. But grass, on the other hand, gets harder by being worn down through games, making players more susceptible to wear and tear on their joints.

“This is another reason why grass can be more challenging,” Gibson adds. “Slightly damp grass can be too slippery, which can cause serious groin injuries and zero latitude for sliding – so if you want to hurt yourself, grass would, by far, be the worst.”

Grass behaves differently when the weather changes

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Players who perform better on clay courts tend to favour Wimbledon rains because, as Gibson points out, heavy wet grass court moves closer to clay.

“Clay needs to be kept mildly damp otherwise it can cut up and the surface gets way too rough,” he says. “Grass and the soil, on the other hand, soak it up which softens the surface more and slows the ball down.”

Also, worn-down grass behaves more like a hard court as the tournament progresses.

“The exposed dirt after the grass has been worn down is more like concrete, with a higher coefficient of friction, which means the ball slows down and start to bounce higher.”

So if the clay-court specialists can survive the first week of Wimbledon, they may find conditions more to their liking as the tournament progresses.