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What is COR?

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Question: What is COR?
Answer: "COR" is an acronym for "coefficient of restitution." Coefficient of restitution is a measurement of the energy loss or retention when two objects collide. The COR measurement is always expressed as a number between 0.000 (meaning all energy is lost in the collision) and 1.000 (which means a perfect, elastic collision in which all energy is transferred from one object to the other).

An example of a COR of 0.000 would be one piece of very sticky chewing gum colliding with another similar piece. In such a collision, the two pieces of gum would stick together and not move forward, thus indicating that all of the energy of the impact was absorbed and lost. The closest example in the sports world to a COR of 1.000 would be in pool or billiards, when the cue ball collides squarely with a target ball of the same size and weight (mass). When the cue ball hits the target ball, the cue ball stops dead and the target ball takes off at almost the same, exact speed that the cue ball had when it made contact with the target ball. This indicates that virtually all of the energy of the cue ball was transferred to the target ball to propel it onward.

It is impossible for the collision of the golf club and golf ball to produce a perfectly elastic collision (COR of 1.000) in which all energy is transferred, for two reasons:

1. The clubface and the ball are made from completely different materials;
2. The clubhead and the ball are of two totally different weights, or masses.

The current USGA rule limiting the coefficient of restitution of a clubhead states that the COR cannot be higher than a measurement of 0.830. This means that when the clubhead impacts the ball, there cannot be more than an 83-percent transfer of the energy of the head to the ball.

To give a frame of reference for performance, with a driver the difference in carry distance between a head with a COR of 0.820 and another head with a COR of 0.830 would be 4.2 yards for a swing speed of 100 mph. It is true that as swing speed increases, the distance difference is greater. And likewise, as swing speed decreases the distance difference for each increment of the COR measurement is less. This is one of the reasons why the USGA rule which limits the COR of a clubhead has the effect of penalizing the slower swing speed golfer much more than the high swing speed player.

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