In several previous articles, we have shown that paddle power and topspin capability go hand-in-hand (see for example, Can Topspin Enable a Faster Serve?) In an effort to limit paddle spin capability (and indirectly paddle power), the USAP established limits on the paddle face surface roughness and coefficient of (kinetic) friction (or CoF). We have addressed surface roughness and friction in previous articles (see for example, Does Friction Create More Spin?) and determined that neither of these factors contributes to the paddle’s spin capability. This has been independently corroborated by other researchers.
The USAP has recently adapted the test apparatus used to measure the paddle / ball coefficient of restitution (PBCoR) for use in measuring the paddle’s spin capability. We analyzed the PBCoR test in a previous article and identified some issues with the test methodology. In this article, we will examine the physics behind the USAP spin rate test to determine its suitability for predicting the spin capability of a paddle. For these analyses, we obtained background information from videos posted by JohnKew Pickleball and The Pickleball Tech Dude that they recorded while attending the recent USAP Media Day.
The USAP Spin Rate Test
The USAP Spin Rate Test is performed by using an air cannon to fire a pickleball at a speed of 40 MPH at the face of a pickleball paddle that is oriented 30 degrees to the horizontal. When the ball strikes the paddle, a surface traction is developed by friction at the interface between the ball and paddle. This surface traction has both normal (perpendicular) and tangential (parallel) components, causing the ball to rebound from the paddle and creating a torque that causes the ball to spin (Figure 1). Based on physics, if the paddle is held at an inclination angle (θ) the incidence and reflection angles must also equal the inclination angle (θ).
Real-Life Paddle Inclination Angles
Would you normally try to hit a pickleball with your paddle inclined at 30°? If so, what would such a shot look like? Figure 2 shows two examples, where (a) the paddle face is “open” (i.e. facing upward), receiving a ball in the horizontal plan at 40 MPH, and (b) the paddle face is “closed” (i.e., facing downward. If you tried to return a ball with your paddle facing upward at an inclination angle of +30°, the ball would hit your paddle and fly upward behind you. Similarly, if you tried to return a ball with your paddle facing downward at an inclination angle of -30°, the ball will land on the ground behind you.
Clearly, the ±30° inclination angle is not realistic and would not normally be used to hit a pickleball. Why then, is USAP using such a shallow inclination angle in its paddle spin rate test? Would it not be more pertinent to use a realistic inclination angle range, of say, 75° to 90°?
The answer lies in the fact that the USAP spin rate test cannot differentiate paddles using realistic inclination angles! In the JohnKew video around the 24:20 mark, the PPL test engineer admits that at inclination angles of 45° (and steeper), all paddles develop about the same amount of spin because the ball does not slip relative to the paddle surface! Paddles show differences in spin rate only when the inclination angle is less than 45° with the largest differences at angles of about 30°.
Wait a second!! The USAP spin rate test doesn’t work at realistic inclination angles?! So they run the test at inclination angles that would never be used during actual play??!! If a paddle exceeds the allowable spin rate at these unrealistically shallow inclination angles, would we expect it to have too much spin capability at realistic inclination angles? Don’t all paddles exhibit the same amount of spin capability at realistic inclination angles? If so, how could USAP possibly disqualify or de-list a paddle under conditions where it will not be used? How is this in any way fair or scientific?
Summary and Conclusions
Further interpretation of these results verifies our findings in a previous article and the conclusions drawn by other researchers that for reasonable inclination angles (>45°), all paddles will develop the same amount of spin, and therefore surface friction of the paddle face is not a factor in producing topspin of a pickleball! Consequently, the USAP spin rate test, the USAP coefficient of (kinetic) friction test, and the USAP surface roughness test are not relevant and do not provide any insight into which paddles will actually develop more spin. These tests should therefore be discontinued and not used to disqualify or de-list a paddle based on spin capability.
If face surface friction does not contribute to spin capability, what does? As we alluded to in previous articles, (see Paddle Spin Capability, Friction, and Stiffness and Does Friction Create More Spin?, we believe that the paddle/ball contact time (or dwell time) is the key element in determining the spin capability of a paddle. That is, paddles that are engineered to create longer dwell times will be capable of generating more spin than paddles with shorter dwell times. Various factors affect dwell time, including the paddle and ball dynamics, mass, inertia, and stiffness properties, and vibration damping characteristics. We will take an in-depth look at these factors in a future article.
