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High Stake Balls

Jani Macari Pallis, 
Ph.D. Photo
Jani Macari Pallis, Ph.D.

When you contemplate the speed of the game and the rewards at stake in professional tennis, I am sometimes surprised that there is not more emphasis on understanding the technology of the equipment.

Colleagues have worked with top cycling professionals like Lance Armstrong on small nuances of body position. I've seen this same group worry about how a piece of tape is positioned on an athletic suit, let alone where the water bottle is positioned on the bike. Personally, I had a great experience working on a small part of the design of an America's Cup yacht. I mentioned trying to shave a knot off of the speed and I was told, by the head designer, that I needed to worry about every fraction of a knot - just not the knots.

So I was NOT surprised when four-time semifinalist and All England Club member, Tim Henman, questioned when the cans of balls for Wmbledon are opened. Henman contended that the ball cans were opened weeks in advance, slowing the game down. For a serve-and-volley player like Henman, this is a disadvantage.

Wimbledon chief executive Chris Gorringe noted that opening the cans beforehand began ten years ago and that Slazenger (who manufactures the balls used at Wimbledon) had assured officials that this did not alter the balls' playing characteristics. Gorringe argued that the same practice had been observed at Wimbledon for a decade and that the balls were not opened two weeks in advance. The other Grand Slams tournaments open the cans on court.

But this is not the first ball controversy or probably the last. A while back, balls were refrigerated. The concern was that balls sitting in cans on the court in the sun heat up and that changes their bounce properties. (Warm balls bounce higher than cold balls. See our student tennis science experiment.) However, on the other hand, refrigerating them would make them bounce even less. Dr. Carol Otis, who was the chief medical officer for the WTA, noted to me once that placing balls in a clothes dryer was an old tour trick for "reviving" the bounce in balls for practice.

You might not know that the balls used by women at the US Open are different than the balls used for the men. The men use the Wilson US Open balls with heavy duty felt and the balls used by the women are the Wilson clay court balls which have lighter felt. On a hard court those lighter felt balls have somewhat "greater speed". I place that in quotes because I believe even this needs to be qualified: a) faster because the ball is lighter?; b) faster because the balls rebound angle is smaller and there is a greater horizontal velocity?

But the point is, why are different balls used for the women's tournament? In an interview with the Wall Street Journal last year, tournament referee Brian Early noted that it was the request of the WTA (women's tennis association). Early also noted that the other Grand Slams use the same balls, but that the 2004 Olympics used the "gender-specific" Wilson balls.

So it really is not copacetic to compare two serves of the same initial speed - one by a man and the other by a woman - at the US Open and call them "the same". The velocity after the bounce (the ball to be returned) will be different. There's probably a list of ball characteristics that could be different - contact time with strings, deformation properties, etc.

You may recall a 1999 US Open match between Andre Agassi and Justin Gimblestob. A ball went into the stands and a young boy threw it back on the court. Gimblestob threw it back to the boy as a kind gesture. The crowd cheered, but Agassi went to the official and complained because now a new can would be opened to compensate for the lost ball. New balls - different characteristics.

During some tennis ball aerodynamics tests, I recall using two different "vintages" of Slazenger balls - the type used at Wimbledon. The latter "vintage" had a new innovation - a coating placed on the ball so that the ball was more visible. The ones with the coating had different aerodynamic drag characteristics. The coating was enough to keep the felt nap down and cause less drag (less drag, faster forward speed).

Tennis players have a lot to contend with - crowds, travel, different court surfaces. Let's be honest, Wimbledon starts on a grass court and ends on a grass and soil court. I'm not surprised that Henman questioned the Wimbledon officials and I think that this is a very positive indication that understanding equipment technology and function is going to be one of the tools in a player's arsenal.

Until Next Month ... Jani

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This column is copyrighted by Jani Macari Pallis, Ph.D., all rights reserved.

Dr. Jani Macari Pallis is the founder and CEO of Cislunar Aerospace, Inc., an engineering and research firm in San Francisco. In addition to her engineering practice, she has led two collaborations between NASA and Cislunar, creating educational materials on the aerodynamics of sports for pre-college students and educators. As the head of NASA's "Aerodynamics in Sports" project, she has led a team of researchers investigating the aerodynamics, physics and biomechanics of tennis. The group has conducted high speed video data capture at the US Open and research of ball/court interaction, footwork, serve speeds, trajectories and ball aerodynamics. Pallis received a BS and MS from the Georgia Institute of Technology, an MS in mechanical engineering from the University of California, Berkeley and a Ph.D. in mechanical and aeronautical engineering from the University of California, Davis. She is a member of the Executive Committee of The International Sports Engineering Association.

Questions and comments about these columns can be directed to Jani by using this form.


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