COR
COR is the coefficient of restitution of the clubhead, a measure of how efficiently the clubface transfers energy to the ball.
COR stands for Coefficient of Restitution. The value describes the ratio of ball speed after impact to the speed before impact. The higher the number, the more swing energy ends up in the ball instead of in deformation and heat. On modern drivers, the thin, springy clubface creates a trampoline effect that noticeably raises COR. So that the effect does not get out of hand, the governing bodies USGA and the R&A have set an upper limit of 0.83. Clubs above this value are not permitted in conforming play.
In practice you notice COR mainly as a distance issue. An example: two drivers with the same loft and the same swing speed produce different ball speeds because one head works closer to the permitted limit. On the launch monitor you see this as smash factor, the ratio of ball speed to head speed. Centered strikes on the clubface deliver the highest effective COR; strikes towards the toe or heel lose energy. That is why a clean strike achieves more than exploiting any technical limit.
Typical misconception: a higher COR automatically makes every shot longer, no matter what the strike looks like. The coefficient applies only to the moment of impact itself and does not help you with poor contact. Anyone who strikes off-center loses more energy than even the best 0.83 can ever give back. Another popular assumption is that old clubs are fundamentally hopeless because of lower values. Differences in material are real, but smaller than the difference between good and poor technique. More on the effect of the strike point can be found under the term moment of impact.
FAQ
How do I make sure my driver is conforming?
Check the official list of conforming club heads kept by the USGA and the R&A. It lists all models that comply with the COR limit of 0.83. When buying second-hand or abroad, a look at the list is particularly worthwhile, because there are also non-conforming variants of well-known models.
Does a driver at the COR limit automatically give me more distance?
No, the limit says something only about the moment of impact, not about your result. Anyone who strikes the ball off-center wastes more energy than even the best permitted coefficient can ever recover. A clean strike in the middle of the clubface gains you more meters than any technical limit — see also the term moment of impact.
Do I need to pay attention to the COR value when buying clubs?
Practically not. All new, legal drivers from the major manufacturers already work close to the permitted limit, and the difference between them is minimal. Far more important for your distance are loft, shaft and how well the club matches your swing speed.