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On the rebound

The courts at the Australian Open are made of a manufactured surface called Rebound Ace.

It is an acrylic surface because irrespective of what's beneath it, what the players stand on is made of acrylic paint.

Underneath the surface is a synthetic rubber base which is mounted on concrete or asphalt.

Rebound Ace is similar to the surface used at the US Open but has more cushioning and differs in the type of sand used in the top paint.

Some players have complained that the surface becomes sticky when it gets hot but the surface has been tested in temperatures up to 70 degrees Celsius and manufacturers dispute the claims.

And it seems that injuries are more down to the timing of the Australian Open in the tennis calendar and the general cushioning and grip of the surface.

BBC tennis commentator Andrew Castle explains: "Rebound Ace is renowned for having a bit of give in it so it's easier on the body.

"But the real problem is that when it heats up, it has even more give to it.

"So you can catch the edge of your shoe on it and that can cause ankle injuries.

"Basically you've just got to pick your feet up."

BALL SPEED


There are two things which affect the speed and bounce of the ball on tennis surfaces.

The friction of the court affects the horizontal bounce of the ball. So for Rebound Ace the more sand you put in it, the slower the ball will travel after bouncing.

Then there's the vertical bounce of the ball which is affected by the stiffness of the surface. The harder the surface the higher the ball will bounce.

Officials at Melbourne Park insist they have made the courts faster in recent years by altering both factors, but this year's tournament is unchanged from 2005.

You can see in the diagram above how an acrylic surface compares to others.

If the ball is hit at 105km/h it will typically bounce off the court at 71km/h which is 68% of the initial speed.

Friction also affects the amount of spin on the ball.

When a spinning ball hits a low friction surface such as grass, it will continue to spin. The lack of grip makes the ball skid.

But on a high friction surface like clay, the ball will grip more and change the trajectory of the ball quite significantly.

That's what happens when you see the ball bounce really high at Roland Garros during a top spin rally.

Andrew Castle says: "On clay the ball is up round your ears but on grass the ball is waist high or lower.

"Rebound Ace is more versatile. Serve-volleyers can have their way with it because if you hit the ball firmly it will stay low.

"Then again if you hit top spin it will kick up high, so whatever you want to do, it will respond."

PLAYER SPEED


The surface also affects the players in terms of their speed around the court.

That's down to the shoes they're wearing and their grip on the surface.

Paradoxically while clay offers the most friction in terms of the ball bouncing off the surface, for the player's movement, it gives the least friction.

The crushed brick on the top of the clay acts like ball bearings between the player's shoes and the court so they can skid to reach the ball.

So whereas, in the diagram above, the player may only make it to position A on the Rebound Ace surface, on clay he may make it to position B by skidding.

But even though sliding helps with speed around the court, acrylic courts hold the advantage when it comes to changing direction.

That's why it's such a fine balancing act for manufacturers to develop a surface.

They have to consider how much friction they want for the ball to react, but more importantly how much grip there is for the players themselves.

Not enough grip and the players will slip. Too much grip and they will twist their ankles.




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