Understanding COR

What is COR? In simple terms, the COR (coefficient of restitution) limit is 0.83, meaning a maximum of 83% of the clubhead’s energy may be transferred to the ball. The USGA measures COR using a standardized test that fires a golf ball at the clubface under tightly controlled conditions. In the primary test, an air cannon propels a ball at a fixed speed toward the driver face (or a calibrated baseline plate). Sensors then record the ball’s speed before impact and again after it rebounds. The COR is calculated as the ratio of the rebound speed to the incoming speed, after discarding outlier results to ensure precision. In addition, the USGA employs a Characteristic Time (CT) test—a pendulum device that gauges how long the ball stays in contact with the clubface—to cross-check and limit the face’s spring-like effect.

Not every club is tested for COR. Since 1998, the USGA has required that clubs capable of producing a trampoline (or “spring”) effect—typically drivers, and by extension many fairway woods and hybrids (those with lofts of 35° or less)—be tested to ensure their faces don’t exceed a COR of 0.83. Irons, wedges, and other clubs with thicker, stiffer faces aren’t subject to this COR test because their design doesn’t allow for the excessive flexing seen in drivers.