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Tennis pros, and dedicated amateurs, can control just about everything about their rackets. Rackets are measured for weight, stiffness, balance and head size and strung for peak force, deflection and tension loss. Yet the court players run and hit on will always behave unpredictably. Even at most pro tournaments, players don’t know precisely how “fast” or “slow” a given court is playing, an expression of how much speed and height the ball retains after it bounces. Court speed is a fixture of questions at press conferences during the first few days of Grand Slam tournaments such as the French Open, now unfolding in Paris.

It wasn’t bouncing as much as it had actually the last days on the outside courts,” Milos Raonic after winning his match on Sunday.

One reason court speed is such a mystery is that it’s been so cumbersome to measure. The International Tennis Federation has helped develop and test machines that do the job, but the best ones are expensive, bulky and difficult to operate. (The equipment is so hard to transport that many court-surface manufacturers ship samples of their product, in half-meter (1.64-foot) squares of paint and sand, to the ITF’s testing laboratory in London.) For the most prominent ITF-organized events — the Davis Cup and Fed Cup — federation employees must fly with their testing devices, or ship them to sites weeks in advance, to keep hosts from giving players too big a home-court advantage.

Now, the ITF is trying to find an easier way to ensure courts are fair. On a recent Thursday at a London tennis court,1 Jamie Capel-Davies, manager of ITF’s science and technical department, was testing a new device that the federation has helped develop. It’s known as SPRite,2 and this test was of model number 007.

The device doesn’t take a vodka martini. Instead, a ball cannon powered by a bicycle pump propels a ball off the court and through the testing chamber. Seconds later, a display shows the court-speed measurement. The measurement device and ball cannon each weigh about 14 pounds and together cost $12,000, compared to the 110-pound, $45,000 behemoth that sets the standard now. The new devices can be carried by hand, and their dimensions fall within most airlines’ carry-on baggage limits. The motivation for the test, Capel-Davies said, was “democratizing court-pace rating.”3

The ITF knows it’s traded some precision for lightness and mobility. The question is, how much? Is SPRite accurate enough to replace its forebears? Capel-Davies and his colleagues were testing it alongside the Sestée, which is the current benchmark. Court-speed rating typically runs between 20 and 70 — and must run between 24 and 50 for Davis Cup matches (a higher number means a faster court).4 The ITF wants SPRite to run within two points of Sestée for the same court.

Court speed matters in tennis. During a typical match, the ball will bounce off the court hundreds of times. If the ball typically retains much of its speed, it will be harder to play. That encourages more aggressive play — players will charge the net more often to avoid skidding balls and tricky bounces. A slower court lets players camp out behind the baseline and chase down most shots.

Court-pace rating takes into account more than speed, though. The ITF tested the court speed perceptions of U.K. players who were good enough to play for their counties5 and found that the height of ball bounce mattered, too. The lower the bounce, the faster the court seemed. That’s why high-bouncing clay courts like the French Open’s seem slower than hard courts with the same coefficient of restitution, or ratio of the ball’s speed after impact to its speed before the bounce.6 And grass courts, with their lower bounce, seem faster to players than similar hard courts. The ITF formula attempts to account for all this.

Once a match is underway, players can’t do anything about the court speed. They play the bounces they get. But players can tailor their training and schedules around which courts best suit their games. For instance, clay makes Rafael Nadal’s heavily topspun forehand shots bounce even higher than on other surfaces. He has taken advantage of this throughout his career by playing more clay tournaments than some of his rivals.

For the ITF team competitions, court speed is a crucial part of home-court advantage: Host countries get to choose the surface, which is why Andy Murray had to play on his least favorite surface — clay — in Great Britain’s two away Davis Cup ties this year. (Murray and his teammates won in the U.S. but lost in Italy.)

With the currently approved devices, the ITF can’t test the courts at every Davis Cup site, especially on busy weekends when dozens of ties are happening around the world. Instead it audits, choosing to test just a few courts. And at tournaments it doesn’t oversee, including the French Open, there’s simply no official court-pace measurement. (Officials at the U.S. Open and Wimbledon said they test their courts for speed, but the results are confidential.)

That makes it tough to assess whether court speeds have changed — one of the goals of Capel-Davies’s department. Tennis players and writers often say — or lament — that pro tournaments are using a narrower range of court speeds than they used to, so there’s less variation among them. Indirect indicators, such as rates of aces or service breaks adjusted for tournament fields, don’t corroborate that impression. The ITF hasn’t tested long enough or often enough to settle the debate. “We don’t have that kind of data,” Capel-Davies said. “I don’t know if anyone does.”

The International Tennis Federation uses a bulky steel crate, top left, to ship heavy measuring devices to tournament sites. The outsized Sestée, top right, is difficult to transport, so manufacturers often send samples of their courts, bottom right, to the ITF. The latest-generation device is the SPRite, operated here by Jamie Capel-Davies, manager of ITF's science and technical department. Its accuracy is still being studied.
Carl Bialik The International Tennis Federation uses a bulky steel crate, top left, to ship heavy measuring devices to tournament sites. The outsized Sestée, top right, is difficult to transport, so manufacturers often send samples of their courts, bottom right, to the ITF. The latest-generation device is the SPRite, operated here by Jamie Capel-Davies, manager of ITF’s science and technical department. Its accuracy is still being studied.

Watching Capel-Davies test SPRite in London, it was easy to see the advantages of the new device. He easily carried it onto and around the court, to different testing spots. He also let me try, and within a few minutes I got each test down to under a minute: Take a ball out of a coat that’s lying next to the unit, put the coat back in place, put the ball into the cannon, give the bicycle pump a few cranks, prime the measurement unit, put my foot7 on it for stability and press the button to fire.

Yes, a coat. Needing something to stop the balls after they had fired from the cannon, bounced off the court and gone through the SPRite, Capel-Davies offered up his jacket to science, taking care to first remove his phone. After each test, the ball nestled in the coat. As we conducted our experiment, Capel-Davies’s colleagues were testing the Sestée on another part of the court. At one point, a Sestée-measured ball got loose and whistled past us.

Each court test covered a few different spots, always including ones near the baseline, the service line and the net. The speed can vary from one part of the court to the next. It’s often highest near the baseline because players’ shoes wear down the surface and make it slicker. This court was relatively new, though, so we weren’t expecting as much variation.

The measurement process has other quirks. Fail to press the button firmly, and the cannon might not fire fast enough for a reliable measurement. This happened a couple of times to us, so we omitted those readings. Capel-Davies decided when to do that, and it wasn’t a double-blind procedure. He checked with the other group and knew what readings they’d gotten, and was following along as our parallel tests produced results. Still, the quest for an accurate reading seemed genuine. The ITF already had achieved its initial goal of agreement within five points between the devices, which is what a typical player can detect. Then it set its sights on even better agreement, of within two points.

Capel-Davies and his team have their own testing lab at ITF headquarters in the southwest London neighborhood of Roehampton, in a converted squash court with the original wood floor now scuffed by equipment. Tennis scientists test balls in a wind tunnel and rackets in a serving machine, nicknamed Goran after 2001 Wimbledon champion Goran Ivanišević, in homage to his rocket serves. The apparatus gripping the racket occasionally slips, and there are marks to prove it on the side of the machine. “We have had casualties” — of rackets, not people, Capel-Davies said.

The ITF is surrounded by tennis courts. The Bank of England Sports Centre, which hosts next month’s Wimbledon qualifying tournament, is on one side. On the other is the National Tennis Centre. Many ITF employees play tennis during their lunch breaks. But they don’t have their own dedicated full-sized court for testing. So when they get a gig testing a court, like this one, they often take the opportunity to test SPRite.

If SPRite passes the test, the ITF, as its seller, will benefit through increased sales, though that’s not the primary motivation, Capel-Davies said. “We’ve had interest, but everyone is waiting” to see what the results are “before putting their hands in their wallets,” he said.

Mainly, the organization would like to enable more facilities, including tennis clubs, to test court speed. Having a device on site would allow clubs to measure the effect of temperature and also to track changes in the courts, to know when it’s time to resurface. A manufacturer’s rating, based on a test of a half-meter-long square patch, isn’t good enough because it doesn’t take into account what’s under the surface, how it was attached and how the court has weathered and worn.

At tournaments, organizers could test how the speed of a court changes as players play on it — for instance, at grass-court tournaments, as players’ shoes and shots turn the turf near the baseline into dirt. And they could compare speed across different courts; players often say some courts at the same venue play faster than others, something that affects them if they play consecutive matches on different courts.

These possibilities haven’t yet arrived. Our test found the SPRite and the Sestée agreed to within 2.7 points — good but not within the desired range of 2.0 points.8 One location, near the service line, was the source of the troubles: The two devices agreed to within 3.2 points, whereas everywhere else was within the limit.

“It suggests something odd happened at that location,” Capel-Davies said. “Hopefully, additional testing will determine whether it was an anomaly.” He hopes that the SPRite model we tested, which was first produced last fall, will meet the target by the end of this year.

Even if players at every level knew the speed of the courts they were playing on, they’d still have to deal with unpredictable bounces. Court speed can depend on how long the ball your opponent just served was sitting in his pocket: The warmer the ball, the slower the court appears to be. And standard measurements don’t apply when the ball collides with a hill or valley, common on clay courts.

Still, broadening access to the court-speed numbers would help further the mission of Capel-Davies’s department, which is, as he articulated it, “to balance technology and tradition in tennis,” and to “make sure the player won because of ability” — and not faulty equipment or a finicky court.

CORRECTION (June 2, 4:00 p.m.): An earlier version of this article said the average agreement of the SPRite and the Sestée in a recent test was 2.7 points in court-pace rating, or CPR, and that the two devices’ average agreement was 3.2 points in one location. Those figures were the agreement between the two devices at 95 percent confidence. The average of the absolute value of the differences between the two devices’ measurements was 1 point of CPR.

Footnotes

  1. I’m not disclosing the tennis court’s location at the ITF’s request, to protect client confidentiality. ^
  2. Said like the soft drink; the SPR is short for Surface Pace Rating. The official name for the device measures is court-pace rating, though, as Capel-Davies noted, “‘CPRite’ didn’t have the same ring to it.” ^
  3. In tennis, democracy is relative: At this price the average hacker won’t be carrying the SPRite and cannon to her local public court. ^
  4. The court-pace rating, or CPR, is calculated based on the coefficient of restitution (COR), which is the ratio of a ball’s vertical velocity after bounce to its pre-bounce vertical velocity; the coefficient of friction (COF), which is the ratio of horizontal velocity lost after the bounce to pre-bounce vertical velocity multiplied by the sum of 1 and COR; and a temperature-adjusted COR, which is the sum of the COR and 0.003 multiplied by 23 minus the mean ball temperature, in Celsius. The formula is: CPR = 100(1-COF)+150(0.81-adjusted COR). The ITF calls courts with CPR of 29 and under slow, courts with CPR between 35 and 39 medium and courts with CPR of 45 and up fast. ^
  5. It’s also tried surveying pros, and gotten similar results, but the response rate was poor. ^
  6. One theory for clay’s higher bounce: The ball pushes granules ahead of it as it collides with the surface, creating an incline to bounce off, a “ramp effect” that leads to a higher bounce. ^
  7. We’d taken off our shoes to avoid scuffing the court. ^
  8. The ITF is aiming for agreement at 95 percent confidence of within 2.0 points. The average absolute value of the difference between readings by each device was 1 point for our test. ^

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