What Is COR (or 'Coefficient of Restitution') in Golf Clubs?

Corey Pavin hits an iron shot during the Buick Championship tournament.
COR measures how well energy is transferred from the golf club to the golf ball at impact. Michael Cohen/Getty Images

"COR" is an acronym for "Coefficient of Restitution." It is a technical term describing the energy transference between two objects. The coefficient of restitution of Object A is a measurement of Object A's ability to transfer energy to Object B when A and B collide.

So, in a golf context, the golf club is Object A and the golf ball is Object B. Does a fairway wood or iron have a very high COR? Then there is less energy loss at impact with the golf ball compared to a fairway wood or iron with a lower COR.

Tom Wishon, golf club designer and founder of Tom Wishon Golf Technology, gives a more technical definition of COR this way:

"Coefficient of restitution is a measurement of the energy loss or retention when two objects collide. The COR measurement is always expressed as a number between 0.000 (meaning all energy is lost in the collision) and 1.000 (which means a perfect, elastic collision in which all energy is transferred from one object to the other)."

Some examples of a zero energy transfer and a perfect energy transfer will help us grasp the concept. Here's Wishon:

"An example of a COR of 0.000 would be one piece of very sticky chewing gum colliding with another similar piece. In such a collision, the two pieces of gum would stick together and not move forward, thus indicating that all of the energy of the impact was absorbed and lost. The closest example in the sports world to a COR of 1.000 would be in pool or billiards, when the cue ball collides squarely with a target ball of the same size and weight (mass). When the cue ball hits the target ball, the cue ball stops dead and the target ball takes off at almost the same, exact speed that the cue ball had when it made contact with the target ball. This indicates that virtually all of the energy of the cue ball was transferred to the target ball to propel it onward."

A "perfectly elastic collision" - a COR of 1.000 - is impossible in a golf club-golf ball collision. Therefore, no golf club can ever have a 1.000 COR. Why?

Wishon explains:

1. The clubface and the ball are made from completely different materials;
2. The clubhead and the ball are of two totally different weights, or masses.

The USGA and R&A regulate COR in golf clubs, with the current limit being 0.830. Any club with a COR higher than .830 is ruled non-conforming.

The terms "coefficient of restitution" and "COR" came into the mainstream golf lexicon as ultra-thin-faced drivers began to proliferate in the early 2000s. An effect of the thin faces is known as the "spring-like effect" or "trampoline effect": The face of the driver depresses as the ball is struck, then rebounds - providing a little extra oomph to the shot. A driver that exhibits this property will have a very high COR.

However, the governing bodies no longer use COR to regulate drivers - they instead use something called "charateristic time" or "CT." COR and CT measurements do track one another, however.

And fairway woods, hybrids and irons are still regulated using COR measurements.

What kind of differences in distance performance will two clubs of differing CORs exhibit? We turn once again to Wishon for the answer:

"To give a frame of reference for performance, with a driver the difference in carry distance between a head with a COR of 0.820 and another head with a COR of 0.830 would be 4.2 yards for a swing speed of 100 mph. It is true that as swing speed increases, the distance difference is greater. And likewise, as swing speed decreases the distance difference for each increment of the COR measurement is less. This is one of the reasons why the USGA rule which limits the COR of a clubhead has the effect of penalizing the slower swing speed golfer much more than the high swing speed player."