The Problematic Clamp - part 2
USAP's Questionable PBCOR Measurement
3/19/26
In the original article the groundwork was laid to question the accuracy of USAP's PBCOR measurements due to the use of a massive camp with a large inertia (swing weight=33) . It was found that the ball does not "see" the clamp in the sweet spot area, yet the clamp's inertia is [mistakenly] included in the PBCOR calculation. The result is that PBCOR values in the sweet spot area (3" to 5" from the tip of the paddle) are artificially deflated. Further experimentation shows what happens outside the sweet spot area and when it is appropriate to include the clamp's inertia in the PBCOR calculation.
Impacts in the 3 to 5 Inch Range
In the original article measurements were only taken in the sweet spot area - impacts at 3.375, 4.375 and 5.375 inches from the tip. The graph of rebound velocity and PBCOR are shown.
It would be expected the the rebound velocities of the same paddle with a massive clamp would be higher than with a lightweight clamp. However, the velocities are almost identical at these impact locations because the ball doesn't "see" the extra inertia of the clamp.
It would be expected that the PBCOR values would be identical. The PBCOR measurement should be the same regardless of what clamp is used. However, the PBCOR values at these impact locations are different because the ball doesn't "see" the extra inertia of the clamp yet the clamp's inertia is mistakenly included in the PBCOR calculation. The ball has left the paddle face (~ 2 milliseconds) before any vibratory energy from the clamp can affect the ball's rebound speed.
Impacts in the 3 to 7 Inch Range
It was hypothesized that the ball did not "see" the extra inertia of the clamp because the ball had left the paddle face before the impact wave could rebound off the clamp and affect the rebounding ball velocity. If that hypothesis is correct there should be an impact location closer to the clamp where the ball does "see" the clamp. Measurements were taken at two additional locations closer to the clamp - 6.375 and 7.375 inches.
The rebound velocities begin to diverge at the two additional impact locations. The ball is "seeing" the clamp.
The PBCOR values get closer at 6.375" and converge at 7.375". The ball is beginning to "see" the clamp at 6.375" abd fully "sees" the clamp at 7.375".
Implications
At some locations the ball "sees" the clamp; at others it doesn't. The extra inertia of the clamp should be included in the PBCOR calculations at impact locations nearer the handle. At impact locations nearer the tip the clamp's inertia should be excluded. The exact locations where it should or shouldn't be included probably vary paddle to paddle. USAP's PBCOR measurements are in error for impact locations in the 3" to 5" region. The best solution would be to use a lightweight clamp so the PBCOR determination is not affect whether the inertia is or isn't included.