Companies, organizations and industries love to use acronyms because these abbreviations of longer terms can so easily roll off the tongue. One that has been a buzz-acronym in the golf equipment industry since 1998 is COR – short for Coefficient of Restitution.
Experienced golfers know the COR is a number which represents how “hot” the face of their clubhead(s) is made – or rather how much distance they can get out of the shot for their swing speed. COR made its way into the golf industry’s vernacular back in 1998 when the United States Golf Association got freaked out at the distance the pros were hitting the ball. Acting before ever doing any testing, the USGA blamed the pros’ distance increase on the use of the relatively new (at that time) titanium drivers and enacted a rule that placed a limit on the COR of all driver faces.
COR is actually a measurement of the energy transfer in a collision of two objects. It can be expressed in a number between 0 and 1. For example, when the USGA put a COR limit of 0.830 on driver faces, that meant no driver would be deemed to be conforming to the rules if more than 83% of the energy in the collision of the driver head with a golf ball were transferred from the head to the ball.
The COR rule also became known as the “spring face rule.” This was a little unfortunate because in fact, a higher COR clubface does not really act like a spring. When you think of spring face, it is easy to think that the ball causes the clubface to flex inward, and upon flexing back out the ball is propelled as in the manner of a trampoline sling shotting a gymnast up.
Actually, higher COR faces work like this. In the collision of the clubface and the ball, there is always some energy lost. This is because the face flexes inward and the ball is compressed against the face. Both actions result in a loss of energy. Of the two, the ball loses by far the most energy when a shot is hit because it can squash as much as 30% of its diameter against the face of the driver. In a normal shot hit with an old thick face stainless steel metal wood, scientists estimate that 80% of the energy loss in such an impact came from the ball while the balance of 20% came from the clubhead.
The idea of a higher COR face design, whether done for a driver or any other clubhead, is to allow the face to flex inward a little more so that the ball is not compressed as much against the face. When that happens, the face loses a tiny bit more energy because of its increase in face flexing. But the ball then loses a lot less energy than before because it is compressed so much less because of the slight increase in face flexing.
The net result? The ball takes off at a higher velocity and flies farther for the same clubhead speed and same loft angle on the clubface. Hence high COR means more distance regardless of your clubhead speed.
And that’s how that acronym really works in the design and performance of a golf clubhead.