Literature Review

Relatively little research has been conducted on non-destructive sport-balls friction testing methods. Among the few researchers to have made significant advances in this sector are Dr. J. Hubbard, Dr. M. Hubbard and Dr. H. Okubo, and Dr. Alaways. Dr. M. Hubbard was Alaways’ mentor and doctorate dissertation reviewer.

Spalding’s leather basketball was the official NBA game-ball from 1983 to 2006, when a new synthetic ball was introduced. This synthetic ball was subsequently retired in 2007 to be replaced by its predecessor because of negative surface properties (Alaways, Szczerbinski & Krumenacker). The reason for this premature retirement prompted Dr. J. Hubbard to investigate the synthetic surface properties of the 2006 composite ball. His research led to his 2006 Science News Online article, “Experiment finds that new basketball gets slick.” Also researching this topic, M. Hubbard and Okubo, and Alaways , presented two conference papers at the 7th International Sport Engineering Conference, which took place in Biarritz, France, June 2008.

Okubo and M. Hubbard’s conference paper, “Differences Between Leather and Synthetic NBA Basketballs,” used an inclined plane test (with notable alterations) to determine the static friction coefficients between balls and powder coated NBA basketball hoops. Their approach to stabilizing the basketballs was to physically attach two or more balls together (Okubo & Hubbard). Unfortunately, this method prohibits specific knowledge of a particular ball’s properties. Moreover, the experimentally determined coefficients tend to be greater than the average coefficient that one would determine were the balls tested independently. This misrepresentation of the average coefficient is due to one of the balls slipping last; this value elevates the average coefficient of all the balls.

Alaways’ paper, “Determining Friction Coefficients for Round Balls: Comparing Basketballs” with coauthors Zachary S. Szczerbinski and Nicolas Krumenacker, explored an alternative, non-destructive static friction testing method. Instead of using the traditional inclined plane test, Alaways and Szczerbinski developed a device at Temple University in 2006 illustrated in Fig. 1. This device grabs the ball axially and adds a known torque until the ball reaches impending slip. The crucial normal force is detected by a scale placed underneath the apparatus in contact with the ball. This device significantly reduced the time required for and the human error included in testing. However, this device still required a painstaking setup and testing procedure resulting in observed error while determining the point of impending slip.

In his paper, Alaways observed that, “Few studies have been presented reporting results on basketball [friction coefficients].” Alaways also reported that, in 2006, Okubo and M. Hubbard had measured the friction coefficient between basketballs and an acrylic board. Further tests reported by Weiss were performed at the University of Texas, Arlington by Horwitz and De. These tests demonstrated how the official Spalding leather ball performed better in wet conditions and the 2006 synthetic basketball performed better in dry conditions (Weiss).

Finally, Nicolas Krumenacker received the 2008 Summer Undergraduate Research Fellowship. An integral part of this research focused on improving Alaways’ device (Fig. 1). Most of the attention focused on motion detection and particularly the moment of impending slip determination. The approach was to use a torque arm (similarly to Alaways’ device) whose subsequent motion completes a simple contact circuit. This circuit warns the person running the test to stop loading the arm. This aspect of the project proved to be the most complex. How does one determine that the ball slips rather than deforms. This research project was well started and a prototype illustrated in Fig. 2 was build by September 2008. It was subsequently converted to the present project in order to better determine the moment of impending slip and in order to create a fully automated device in terms of both running tests and collecting data.

Figure 1: Alaways’ Prototype (2006)

Figure 2: Krumenacker’s Prototype (2008)