They are golf clubs that breach the United States Golf Association (USGA) limit of COR. USGA holds the right to define the shapes and physical characteristics of clubs that are permissible in tournament play. COR is nothing but a value between 0 and 1 which reflects the energy lost or retained when a collision takes place between two objects. The value is zero in case of collisions where the two colliding objects stick together and do not move forward. In other words we can say that all the energy of the collision was absorbed and lost. An example for the case where COR value is 1 would be an impact (collision) taking place between two cue balls, with one ball hitting another ball resembling it exactly in size and mass at right angles. In other words all of the energy of the hitting object must be transferred to the idle object.
We cannot expect the COR value to be exactly 1 when a golf ball is hit by golf club. USGA have made it mandatory that COR value must not exceed .830 on January 2008. Why do the gold clubs not strictly adhere to the USGA limit? There are reasons. Let us see what effect this limitation has on the performance. For this first get familiar with some basic terms. Carry distance is the distance a golf ball travels in the air after being hit by the head. This distance is measured using a grid system with markers in the landing zone. Total distance is the sum of carry distance and roll distance (distance traveled from first ground contact to the point where ball travels). Swing speed is the speed with which he or she swings the club to hit the ball. Apart from swing speed the carry distance is also dependent on COR. Higher the COR value greater is the carry distance. The clubs which exceed this value to enhance the distance traveled by the ball are referred to as non conforming golf club.
