Appendix I of the Friend at Court provides tables with the specifications for the different types of tennis balls. Specifications for the “Type 2” ball, the standard ball typically used on a 78 foot court, are provided below.
My engineer brain took one look at that table and all the “WTF” synapses simultaneously fired off. Fortunately, the ITF provides a very detailed description of the testing process.
Tennis balls are tested for compliance to the Rules of Tennis at the International Tennis Federation Technical Centre which is located in London. According to the ITF technical standards guide, that facility is the “world’s most advanced tennis-specific research facility sporting over a million dollars’ worth of equipment for testing the technical aspects of tennis.”
Tests are conducted under strict environmental conditions. Specifically, the test chamber has to be within tight ranges of temperature, humidity, and atmospheric pressure. To meet these specs, the testing room has an “air lock” type double door entry. The balls are acclimatized in that room for 24 hours prior to performance of any testing.
Before actual testing commences, balls are also pre-compressed to remove “set” in the ball, a new concept which kicked the first round of “WTF” synaptic activity in my brain. Apparently “set” is a material property that occurs when a substance is not subjected to variable forces. I found this video of a pre-compression machine in operation which helps greatly to visualize the what is done to the ball.
Now is probably a good time to mention that I am an Electrical Engineer and that the “Material Science” class that all engineering majors were required to take was not one of my better subjects. In an ironic example, I actually failed the “material failures” lab where the students select their own test subject to practice determining the cause of material failure. I selected a tennis ball that had clearly been victimized by a lawn mower, but had actually been dipped in liquid nitrogen and dropped. Brittle failures are totally different than stress/strain failures. I should have selected the sheared off bolt.
The first parameter in the ball specification table is mass, and the ITF is clear that mass is simply measured using an electronic laboratory balance scale. Technically this measures weight and not mass, but my engineer brain is going to let that one go. Really the table should simply say weight, but I am starting to suspect that the ITF likes using esoteric language in their technical specifications.
The ITF provides a lengthy description of how size is measured. A simple explanation is that there are two rings corresponding to the maximum size of the ball and one that is slightly below the minimum diameter. The ball passed this test if the ball passes through one and not the other.
Rebound is tested using a bounce test that consists of dropping a ball from a height of 100 inches and measuring the height of the resultant bounce. This is captured on high speed video against a scale and both the initial height and bounce are measured from the bottom of the ball. Chances are you have seen this type of testing on the popular TV show “Mythbusters.”
A simple explanation of the deformation testing basically a “squeeze” test. I found this useful video of tennis ball deformation testing including screen shots of the software required to interpret the results. Basically deformation testing determines the force required to bring the ball up to a certain pressure and how much of that force has to be removed to lower the pressure back to a certain level. In court-side parlance, the results would be described as “soft” or “hard” following a squeeze test.
Manufacturers who provide balls to the ITF for testing are required to submit 72 balls into the testing process. The ITF randomly selects 24 for actual testing. It is my assumption that they retain the rest of the balls to use in their own personal tennis. Balls can get expensive, doncha know.
The ITF also reserves the right to obtain additional balls from any world-wide source for retesting. So manufacturer’s can’t just submit a pristine case of balls to the ITF and then ship garbage to the rest of the world. Tennis players will notice and complain to their local federation who will most likely notify the ITF. It is possible for a previously approved ball to be removed from the approved ball list, though I found no documented instances of this occurring.
This was mentally taxing. It is definatly time to return my attention to the rules proper.
- United States Tennis Association (2020) Friend at Court. White Plains, NY
- ITF Approved Tennis Balls, Classified Surfaces & Recognised Courts: A Guide to Products & Test Methods, International Tennis Federation, 2019.
