Coefficient of Restitution (COR) is not used as a marketing tool nearly as much as it was in the early 2000’s. And the replacement of COR, Characteristic Time (CT), is talked about even less. Why does any of this matter?
First lets look at what COR really is. In physics terms, it is the ratio of velocity out to velocity in. An easier way to understand this, think of bouncing a ball against a wall. If the ball (object 1) hits a wall (object 2) at 100 mph and then returns back at 100 mph the Coefficient of Restitution would be considered perfectly elastic.
Tom Wishon explains the closest to perfectly elastic COR in the sports wold. “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. When the cue ball stops dead and 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.” It is impossible to have a perfect 1.000 in golf because the club head and golf ball are made from different materials.
The move from solid persimmon woods to metal clubs allowed engineers to create a “spring like effect” or “trampoline effect.” By thinning the metal on the clubface and utilizing bulge and roll the engineers gave the golfer the ability to hit the ball farther with less energy. In 1998 the USGA set testing max at 0.830 for metal woods. So, theoretically this means that if a ball hits a wall at 100 mph, and the wall has been limited to 0.830, the ball will return at 83 mph.
Since 2004 the R&A and the USGA have used a test where a pendulum steel ball measures the amount of time it stays in contact with the club face. The CT test is easier to measure and is portable. However, this test only works on drivers, while COR testing is still used on fairway woods, hybrids, and irons. If the steel ball stays in contact for longer then 257 microseconds the club is non-conforming.
COR AFFECTS ON DISTANCE
If two drivers are swung at 100 mph, one with COR of .820 and the other is at .830, with everything else being the same the driver at .830 will hit the ball 4.7 yds farther. Spring like effect is voided at 0.768 which should show how close a driver is from being non-conforming to dead.
Manufacturers continue to compete for distance even with COR and CT being maxed. All testing for conformity is done at the center of the club face or the “sweet spot.” Marketing has been talking about this for years as the manufacturers work on engineering the best COR values on miss hits. All the major players: Taylormade, Callaway, Titleist, Mizuno, Ping, and others have been marketing about distance on miss-hits as they all compete to engineer the longest driver, even on miss hits.
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References: liveabout.com, franklygolf.com, golfalot.com