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Behind the Scenes: The science behind the tennis ball

Whether it is the Slazenger Wimbledon ball used at The Championships, or the Dunlop Fort brand in service at this week’s Qatar ITF Junior Open, each brand will have undergone rigorous testing by the International Tennis Federation before being approved for tournament use.

In the ball-testing laboratory at the ITF’s research facility in Roehampton, thousands of balls are put through their paces to ensure that they comply with the rules of tennis before being approved for competition. ITF approval lasts for 12 months, so each year manufacturers must resubmit brands as well as presenting new ones for approval. Each ball is tested for size, mass, stiffness, bounce and durability.

“We see around 200 different brands every year, as well as another 100 brands of introductory balls over the three categories,” says James Spurr, ITF Sports Engineer. Manufacturers will submit a sample of 72 balls to the ITF, of which a third (24) will undergo testing. They are acclimatized for 24 hours in the climate-controlled laboratory, which operates at 20°C and 60 per cent humidity. Before testing takes place, the balls are put through a pre-compression machine to remove any ‘set’ in the ball.

“The idea is that by squashing the ball it activates it, a bit like a squash ball,” explains Spurr. “It’s just to give it a fair chance at the start of testing because a ball could have been sat in a can for a week, or months and they do start to harden up a bit.”

The test
The first two tests, for mass and size, are the oldest ball specifications, having been in the Rules of Tennis since 1880, although specifications have changed. First the ball is weighed on the electronic scales, which are calibrated to +/- 0.01g. The mass of a Type 2 ball must be between 56.0g and 59.4g. The size of the ball is then measured, using a simple ‘go/no-go’ test. A Type 2 ball must be between 6.54cm and 6.86cm in diameter, so the ball must fit through one ring gauge but not through the smaller one. The ball is tested on all orientations to ensure it is spherical.

Using a standard material-testing device, the deformation test determines how a ball shape changes under pressure. A load of 95.64 newton (roughly 10kg) is applied to the ball and the ball must deform by between 0.56cm and 0.74cm. The machine then applies more pressure until the ball is squashed by 2.54cm (one inch). It returns to the standard 95.64N force and the return deformation measurement is taken – this must be between 0.80cm and 1.08cm.

“The core of the ball is rubber which is an anelastic material, which means when you squash it, it heats up,” explains James. “So what you find is that the ball is softer when it comes back. Some energy is lost, so you will never get identical properties on forward and return.” 

The bounce is then calculated by dropping a ball from 254cm (100 inches) onto a granite block. In order to comply with ITF regulations, the ball must rebound between 135cm and 147cm.

“Back in the day this was all done manually, but now the camera is hooked up to measurement software so it is much more accurate,” says James. “For each ball we measure four bounces and the average of the four measurements must be within range.”

Of the 24 balls of each batch tested, one ball can fail, but if two or more fail to comply with ITF regulations then approval cannot be granted. “The result will go back to the manufacturers and they can check and it might be a quality control issue – they can resubmit and we will test again,” continues James. “We do see it every so often, but it is a bell curve of quality so hopefully the majority will be within range and inevitably you get some outside, which is why we have allowance for one ball to fail.”

If the brand passes the first four tests, six of the batch of 24 are selected for a durability test, which was introduced in 2009. “We tested over 1000 used tennis balls to find out what happens to a ball which it gets played with,” says James. “What we found is that the size didn’t really change, they got a bit lighter, as you can imagine because the felt gets worn and the fibres shear off, they get a bit softer and bounce height can go either way.

“So we developed lab-based means of artificially wearing a tennis ball to the equivalent of nine games of play. We have a ball cannon that fires the ball 20 times onto a concrete block at 40m/s, which replicates nine games of play. We then put the balls into an abrasion box, which is lined with sandpaper and this wears the ball down in two minutes. We have limits for how much they can change – we are talking about fractions of grams and millimeters.

“The idea of introducing the durability test is to remove the lower-quality products from the ITF approved list. We were finding that there were a lot of cheap supermarket balls that would pass the tests but would start to fall apart after only a few games.”

Market testing
Between September and December the team’s sole priority is approval, but at this time of year they will be market testing; taking unused balls from tournaments and buying them from shops and testing them to ensure they meet the same standard.

“We test those balls to make sure they are as good as they ones that were sent in for approval,” says James. “From this year we are being more strict about it. If a particular brand is found to continually fail over a 12 month period, we give that brand a warning and the manufacturer has 12 months to do something about it or we remove their approved status.”

“We put more energy and effort into testing tennis balls than anything else because it’s the one thing that we approve. We’re saying this product is within the rules of tennis and suitable to play tennis with. Tournaments have to use ITF-approved balls so we have to make sure what we do here in the lab is as good as it can be.”