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It can be the sweetest of sounds and feelings when a driver strikes a golf ball, but it’s also brutal: A club-ball collision can produce upwards of 3,000 pounds of force. That’s equivalent to the weight of 12 full-size refrigerators, or about 30 times the force of a hammer hitting a nail.
At that moment of impact, some of the kinetic energy in the clubhead becomes elastic potential energy in the ball – briefly. Impact lasts for all of about 500 microseconds, i.e., half a millisecond. That time of contact is 16 times faster than how long it takes a fly to flap its wings.
Much of this elastic potential energy is converted back into kinetic energy as the ball leaves the clubface and flies off on its journey, hopefully to the fairway. USGA scientists use the Coefficient of Restitution (COR) to measure the efficiency of this energy transfer. To calculate COR, you take the ball speed after impact and subtract the club speed after impact, then divide that number by club speed before impact.
That’s the math; in practice, the USGA uses a pendulum test – a metal ball swings down and impacts the face of the club, and the time the ball stays in contact with the face (aka “characteristic time,” or CT) provides the verdict on whether it conforms to the Rules of Golf or is too elastic or “spring-like.” As ever, USGA testing seeks to protect the role that skill plays in the game. The USGA’s limit for COR for a golf club is 0.822, which hasn’t changed since 1998.
Research demonstrates that COR drops as clubhead speed increases, meaning that the ball is less efficient at translating energy at higher swing speeds. Golfers with high swing speeds may generate high ball speeds and more distance, but they are less efficient at doing so. Feel free to share that nugget with any playing partner who drives it 30 yards past you.