TECH TALK: EXCLUSIVE BY THE PROFESSOR
Three- Letter Acronyms Explained

If you go through all the ads in this magazine or log on to a manufacturer’s website, you will probably notice several three-letter acronyms relating to golf technology.

While they can sound confusing, it is important to understand just what is meant by COG (Centre of Gravity), COR (Coeffi cient of Restitution) or MOI (Moment of Inertia), and,therefore, what impact they have on selecting the right equipment for your game.

Manufacturuers can use differant head and face plate materials and shapes to increase COR.

COR – COEFFICIENT OF RESTITUTION
COR is a measurement of how much energy is absorbed in a collision or the ratio of speed of separation to the speed of approach.

When applied to golf, COR simply refers to how fast a ball will rebound off the face of a club. While many golfers will know about COR, or will have read about the ‘trampoline effect’ or ‘rebound’ or ‘springlike effect’ of a club, few know just how it is measured or what it really means to your game.

COR can be determined using a number of scientific tests. The testing process for COR basically looks to measure how long the ball is on the clubface and how fast it will rebound.

A simple explanation is as follows: A golf club is mounted in a vice and a golf ball is fired out of a cannon directly at the clubface. If this was filmed using a high speed camera, the impact speed and rebound speed of the ball could be measured to determine the clubs COR. For example, a ball hits the clubface at 100mph and rebounds at 83mph. We know COR equals rebound speed divided by impact speed, or in this case 83mph divided by 100mph, so the COR is 0.83. If there was less energy absorbed at the collision and the ball rebounded faster at, for example, 86mph, then the COR would increase to 0.86.

Manufacturers can get higher COR by making the clubface out of thinner, more elastic material but the payoff is that thinner faces are more susceptible to cracking and caving, especially for stronger players with faster swings.

So what is legal? Under USGA rules, and for all professional events, a club cannot exceed 0.83 COR. Under R & A rules, (which covers amateur golf in Australia), there is no COR limit but a 0.83 limit will apply from January 1, 2008.

As we get closer to this date you will see less manufacturers releasing clubs with a COR above 0.83.

As 0.83 is the maximum COR now allowed, manufacturers are now focused on getting as much of the clubface as possible to have high COR, or to be above 0.80. This is called increasing the size of the sweetspot and it can be achieved by using different materials and shapes in the head or face plates. Most of this technology is inside the head and invisible to the consumer.

The best way to test the effectiveness of this technology is to hit a few balls using a launch monitor or go to a demo day and then see how well your good shots go compared to your not so good ones. Having more of the clubface above 0.80 should mean there is still good ball speed on slightly off-centre hits. This won’t help professionals but it will certainly help average golfers who tend to use more of the clubface.

So how much difference does COR make? At a swing speed of 100mph, for every 0.01 of additional COR the ball will travel two to four metres further.

Faster swings get more benefi t for each 0.01 of COR but slower swings, which would mean most amateur golfers, get less added distance.

In the overall scheme of things COR, while relevant, is not the most important key to hitting the ball further. Other factors, especially launch angle and spin rate, make more of an impact on distance for the average player than COR alone.

For a list of non-conforming clubs beyond 2008 check out the official R & A website www.randa.org and go to the rules
section and look under equipment.

NEXT MONTH:
Centre of Gravity and Moment of Inertia explained.

From the October 2006 issue of Golf Australia magazine