Determining the ACOR of Your Racquet
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This tool allows you to determine the Apparent Coefficient of Restitution (ACOR) of your racquet, based on some simple measurements.
You can determine the ACOR of your racquet by a simple bounce test. First, hold a racquet horizontal to the ground with your hand at the handle. Next, you, or a friend, drop a ball from a measured distance onto a pre-selected spot on the racquet (preferably the point at which you hit most of your balls). Measure the height of the bounce. Granted, it will take some ingenuity to do the measurement, but it can, nonetheless, be done.
The ACOR is the ratio of the velocity at impact divided by the velocity leaving the strings. This result can be obtained by taking the square root of the ratio of bounce height to drop height. Even simpler, just feed the results into the calculator above.
For a given drop height, the bounce height will depend on a number of factors:
- drop location (affects amount of energy siphoned off into translation, rotation, twist and vibration)
- racquet weight, swingweight and twistweight (these determine how much translation, rotation and twist occur at each impact location).
- stringbed stiffness (determines how the string and ball share the energy leftover from 1 and 2 above, which, in turn, determines how much of that energy is given back to the ball).
Typical values for ACOR range from 0.0 to 0.50. In other words, the ball will bounce from 0 to 25% of the drop height (the drop height ratio is arrived at by squaring ACOR).
Why do we want to know ACOR?
This ACOR value is magical because it is the result of everything in a racquet design that affects ball speed. You don't have to know anything about the influence of any of these design parameters. ACOR measures the combined result of all of them. If you hit at the same distance from the tip of two racquets, the one with the higher ACOR will be more powerful, given the same swing speed of the impact location. ACOR measures power.
The second reason we want to know ACOR is that it allows us to determine actual serve and groundstroke velocities. All we need to know is how the ball interacts with the racquet and strings (given by ACOR) and the speeds of the swinging racquet and ball.