Controlling the Rebound Accuracy of a Tennis Ball’s String-Bed Impact By Dr Paul Zarda

Dr. Paul Zarda of Orlando and Sanford Florida is advancing the state-of-art in tennis racquet design. His patent application, found here https://www.google.com/patents/US20140274494, presents a new technology racquet that will

1) increase a ball’s rebound spin rate

2) increase the accuracy of a ball’s trajectory

3) increase a ball’s rebound accuracy off the frame of the racquet

This article will concentrate on the 3rd item: Controlling the Rebound Accuracy of a Tennis Ball’s String-Bed Impact. The referenced patent application discusses, in detail, all there of the above topics.

Dr. Paul Zarda of Orlando Florida notes that an objective of his patent is to increase re-bound accuracy when a ball impacts the string bed/inner frame supported by a tuned isolation system. This rebound accuracy is measured by the angle the ball rebounds off of the string bed. 

 Figure 1 shows a typical racquet suggested in the patent, and Figure 2 depicts, schematically, an in-plane and out-of-plane spring system that supports the inner frame.

Paul Zarda, of Sanford Florida, notes that Figures 3 through 5 illustrates a pictorial for a ball rebound situation. Figure 3 is a plan view (view is from the minus XG-direction; refer to Figure 1). The head 1513 of the racquet is shown schematically as an open rectangle (the handle could be on the left side in Figure 3 in the minus YG direction). The inner frame is shown as the bold rectangle 1514. In Figure 3, there is no isolation system and the inner frame is hard mounted to the outer frame. Consider a ball impact, direction 1511 that is not centered on the racquet face. The flexible string bed will deform to position 1515 (exaggerated), and ball rebound would take path 1512 to the left. The rebound direction 1512 is complicated, but the ball will rebound to the left.

Figure 4 show a situation where the string bed is very stiff (it does not deflect), and the isolation system is made flexible with some stiffness KGz (this is the out-of-plane stiffness of the isolation system; this stiffness and its control is another attribute of the proposed invention). The same off center impact occurs in Figure 4 with direction 1511, but the rebound is direction 1512 with a rebound to the right.

Dr. Paul Zarda notes that Figure 5 shows a re-bound from a properly tuned spin control system invention. For a flexible string bed (Figure 3), and a flexible isolation system (Figure 4), the re-bound illustrated in Figure 5 is the sum of those two effects. Since the two rebounds of Figures 3 and 4 oppose each other (at least the rebound direction), it is possible to choose KGz, given the string bed stiffness, to cancel the competing rebounds and produce the rebound 1512 shown in Figure 5. The rebound 1512 is in the plus ZG direction (note the normal to the string bed at point 1517 is the ZG-direction). Hence another attribute of this invention is the increase in rebound accuracy by isolator adjustment (stiffness KGz).

A more detailed discussion of this revolutionary design can be found in the patent application:  https://www.google.com/patents/US20140274494