Everything You Need to Know About COR

Golf Digest, May, 2002 by Frank Thomas

I am relatively new to golf and keep hearing references to a driver's COR. I've read that the letters stand for "coefficient of restitution," which refers to the "springlike effect" of modern clubfaces, but that's about the extent of my knowledge. Can you clue me in?

Timothy Greene, Phoenix

Simply put, COR is a measure of how efficiently energy, in the form of velocity, is transferred between two colliding bodies. Even simpler: Imagine that a ball is fired at a rigid steel plate at 100 miles per hour, and it bounces back at 75 mph. The COR of that impact is 75/100 = .75.

The reason the term comes up a lot is that the latest drivers on the market are not made with rigid clubfaces but instead exhibit a springlike effect. This phenomenon is viewed as a violation of an equipment rule that reads, "The clubface must not be designed and manufactured to have the effect at impact of a spring, which would unduly influence the movement of the ball."

I helped write that rule in 1984 when, as technical director of the U.S. Golf Association, the USGA realized the potential for clubfaces to have a trampoline effect. (In essence, the COR of the golf ball has been regulated since 1942, when the Initial Velocity Standard was adopted.) Sure enough, in time we began seeing enhanced ball velocity off then-experimental titanium drivers. The increased ball velocity translated into about 10 extra yards. Why? A clubface can deform and then recover more efficiently than the ball can.

The USGA and the Royal and Ancient Golf Club of St. Andrews--the game's governing bodies--are at odds over limiting a clubface's COR. The USGA drew the line at .83 in 1998, while the R&A has not established a limit. The COR of a club that does not spring--such as one made of persimmon or most forms of steel--and a golf ball is about .78; experts believe the practical COR limit to be near .93. It is impossible for the COR to be 1.0 because this would mean no loss of energy during impact. (For more on COR, go to www.franklygolf.com.)

I've noticed that when I add weight to my clubhead, it is easier to control. However, does increasing clubhead weight improve accuracy at the expense of distance? And how does the added weight affect shaft flex?

Gary Peck, Staunton, Va.

Adding weight to the clubhead will make the shaft feel more flexible. Although a heavier head tends to improve accuracy, it also can reduce clubhead speed and thus, distance. But not always: Sometimes the better the feel of the club, the better the timing, which may even increase clubhead speed.

A more important effect of adding significant head weight is that you'll lower the shaft frequency, which is directly related to feel. If you want to keep the same club frequency, and you have added weight to the head, then you must shorten the shaft or get a stiffer shaft. If you want to re-shaft, try to find the shaft frequency you're most comfortable with, and this can be done only through trial and error.

Got a question for Frank Thomas? E-mail him at equipment@golfdigest.com. Visit www.golfdigest.com/ equipment for answers to other technical questions.