The coefficient of restitution (COR) measures the elasticity of a pair of objects in collision and measures the ratio of speeds before and after an impact. In golf terms, the COR of Object A (say a golf club) measures Object A’s ability to transfer energy to Object B (the ball) when Object A and B collide.
COR is always measured between 0.000 (all energy is lost during the collision) and 1.000 (all energy is transferred from Object A to Object B during the collision). A perfect 1.000 COR would result in a full transfer of energy from object to object. In golf, it would be absolutely impossible to have a 1.000 COR measurement, as golf clubs and golf balls are made of entirely different materials and have different weights.
As more clubface “springiness” equals a distance advantage in golf, the current USGA and R&A COR limit on drivers in the U.S., Canada and Mexico is 0.830 (83%), where 83% of energy transferred from the club is transferred to the ball. Drivers that exceed this limit are considered “hot face” drivers and are illegal. As the weight and diameter of the golf ball would also increase the COR, the max weight of a legal golf ball is 1.620 ounces, and the ball can be no smaller in diameter than 1.680 inches.
Today, although the max COR has remained the same, club design and athletic performance have improved overall, resulting in distance advantages. Swing speed also impacts COR. To get the full advantage from a high COR driver you would need a fast swing speed to compress the face of the driver.