Sitemap  

 

Now that some members of the golf industry are selling drivers which do not conform to the U.S. Golf Association's limit for a club head's coefficient of restitution, two things are certain to happen. First, you will hear promises of these clubs hitting the ball 30 yards to 50yards farther; and secondly, your customers' eyes will glaze a the sound of the words, 'coefficient of restitution.'

Ko-uh-fish…er, ah, what did you say?
I may sound like a broker record, but there is NO BETTER WAY to build and cement your reputation as the maker of the best clubs in your area than to perpetuate your role as equipment expert. And the only way to become the number one source of equipment information is to learn and dispense the truth about equipment trends to every golfer.

The year's latest equipment trend are the 'hot face' drivers - the illegal drivers, or rather, those which exceed the USGA's maximum allowed limit for a clubhead's coefficient of restitution (COR). Because the USGA's COR limit only exists in the United States, Canada and Mexico, companies making non-conforming drivers can sell them in all the other countries where the Royal & Ancient has rulesmaking jurisdiction.

This May, when some of these hot drivers went into play on the European and Asian professional tours, there were initial press reports that players like Colin Montgomery and Eduardo Romero Instantly gained 30 yards off the tee. In another test conducted and circulated by a newspaper in Edinburgh, Scotland, involved a test group of six golfers with handicaps ranging form zero to 22. Comparing the hitting ability of the non-conforming driver to their own driver at a driving range, the distance increases of the non-conforming driver reportedly ranged from 25 yards up to one golfer's claim of 50 yards!

There is no better way to tantalize a golfer than with an image of greater distance simply by purchasing a new piece of equipment. The golf industry has known that for years so providing printed reports of such claims is quite literally like dropping a match on gasoline.

No doubt, serious clubmakers have fielded many questions about this topic as well as acted as a referee in debates about the truth of fiction surrounding distance claims form these non-conforming clubs.

Therefore, I do not wish to pour water on the flames and damper enthusiasm for equipment, but rather I like to set the record straight and dispense the real facts surrounding exactly what it is these clubs can and cannot deliver.

It is no myth that thin-faced drivers that are engineered properly can deliver greater ball velocity and with it, more distance. The right thin-face driver hits the ball farther, but only if it is properly engineered and selected for the specific type of golfer.

Hot-face technology only works when the golfer causes the clubface to deflect inward just short of face collapse. When this happens, the ball does not squash as much against the face during that 45 milliseconds of impact. If the ball deformation is reduced, less energy in the collision is lost and the ball velocity increases, i.e. more distance, along with a distinctly 'hotter feel' of the ball leaving the face.

The USGA has chosen to use this principle of physics (COR) to compare the energy loss at impact and limit the efficiency of the energy transfer. Simply stated, the COR is a numerical comparison of the elastic nature of an impact between two bodies. A perfectly elastic collision in which no energy is lost has a COR value of 1.0. In contrast, if all energy is lost in the impact, the COR has a designation of zero.

In the impact between the clubhead and the ball, it is impossible to achieve a perfectly elastic collision, or 1.0 COR, because the ball's weight and the clubhead's weight will never be the same. Golfing purists should not worry about drivers ever hitting the balls 20 yards to 30 yards farther simply due to a high COR. The laws of physics prevents this from happening, even with materials yet to be discovered.

Before the arrival of titanium and before modern thin-face technology surfaced, the sub-200cc to 230cc-sized stainless steel metal drivers possessed COR 0.70 to 0.78. In response to a perceived attack by 'hot drivers' on the game's integrity, the USGA ordained their absolute COR limit for clubhead to be 0.83. If the current crop of non-conforming forged titanium drivers have COR in the range of 0.85, how much more can such a driver truly generate in terms of real distance?

Answer that question requires an understanding of COR physics, which can be explained in simple mathematical terms. (Even though you are about to be presented with math formulas and examples, don't stop reading. Yours customers will think you're a genius when you explain the coefficient of restitution.)

First, obtain the COR by multiplying the ratio of the ball velocity to the clubhead speed times a value of one plus the ratio of the ball weight to the weight of the clubhead. In short…

Here's an example. A customer can hit his driver with a swing speed of 95mph. The launch monitor indicated the ball velocity off the face at 143mph. The ball weights 1.62oz and the clubhead weight is 202 grams. (Yes, after you added weight to swingweight the club. One thing about math formulas is they insist that you use common units, so let's do that.) So, 1.62oz equal 0.10125lbs., and 202 grams equals 0.445 lbs. Use miles per hour for ball and clubhead velocities.

In the above equation, the COR equal 0.848.

To better understand what makes the COR go up and down, look at the math formula. If the ball velocity rises while the clubhead speed stays the same in relation, the COR increases. This is because the higher the ball velocity for the same swing speed, the greater the shot distance. Changing the ball velocity just 2mph can change the COR by 0.026!

Consider this; prior to thin-face drivers, few oversize titanium drivers had a COR higher than 0.800. The USGA's maximum clubhead COR limit is 0.830, thus, to make a design change which resulted in the same golfer gaining only 2mph in ball velocity with a 0.800 COR driver would put the head on the threshold of non-conformance!

Take a look at the role of headweight and ball weight in the following ball velocity formula. If headweight becomes lighter while the swing speed and ball velocity ratio stays at same, the COR increases. If the ball was heavier, the COR would increase as well. This is one of the reasons why the USGA and R&A have a maximum weight limit for balls. Remember, the higher the head's COR, the greater potential for distance increase…but in truth, how much is that distance increase in real terms?

Because the ball's velocity off the face is enhanced by spring-face designs, and because ball velocity had an important effect on shot distance, the best way to relate to the COR;s effect on distance is to learn how the COR affects the ball velocity off the face. Just shift the COR formula around to find the ball velocity for any club

For example, the COR and the golfer's swing speed affect ball velocity off the face. Look at a golfer with a 100mph swing speed with conventional driver with a 0.800 COR. The ball is constant at 1.62oz (0.10125ibs) and the headweight is 202 grams (0.445lbs)

To put in simple terms -speaking only about COR and not taking into effect differences in spin rate and launch angle - if a golfer with a 100mph driver swing speed hits a conforming ball with a driver with a 202 grams headweight and with a 0.800, the ball velocity off the face equals 146.64 mph. What happens when this golfer switched to a non-conforming hot face driver with a 0.850 COR?

With all other things being equal, if the same golfer with the 100mph swing speed uses an illegal driver, he generates a ball velocity that is 4.07mpn faster than with normal driver. Given an approximation of 1.6 yards or each mph of ball velocity, the illegal driver yields 6.51 yards more in distance.

But not very many customers can swing the driver at 100mph. So, what happens of a golfer with a swing speed of 90mph, 80mph or 70mph uses the same normal and hot drivers with the right shaft and other fitting specifications to meet their swing needs? Using same ball velocity formula above, change the swing speed from 100mph to the three other examples. And just for fun, take a look at a tour player swing of 120mph as well.

Clearly the physics of the coefficient of restitution shows a number of points that most golfers, including the members of the USGA and R&A rules-making committees may not understand:

One, the higher a golfer's swing speed, the more distance increase can be realized from an illegal driver; Two, the real carry distance increase of an illegal driver with a 0.850 COR - which is in the range of what the current non-conforming drivers display - is greater for the higher swing speed golfers than it is for the slower swing speed golfer;

Three, figures indicated that the distance increase gained from non-conforming driver on the USGA's current list is less than 10 yards for a long-hitting tour player, and less than five yards for an average golfer, only if the rest of the club's specification are proper fit.

Customers will still ask about claims of increased distance so clubmakers must have answer that carries more weight than, "believe me, the math is right."

Overall distance differences between any clubs can only be fairly evaluated when only one variable of the clubs is compared. When a tour player changes from an existing driver to a non-conforming driver, there are a number of specifications that may change in addition to the COR, including center of gravity location, spin rate, launch angle, loft, roll and shaft specifications.

Golf equipment scientists also know that roll is so subjective in any hit that it can never be included in the comparison. Scientists are only beginning to understand the relationship of enter of gravity, loft face thickness, point of face impact to the launch angle and spin rate.

A change of just a couple of degrees in the launch angle and 500rpms of backspin can mean a huge change in a driver's carry distance, factors which golfers are unable to account for in any COR comparison.

In real-life, Golf Digest (May 2000) asked Nick Price to compare the carry distance of a non-conforming Callaway ERC driver to his current driver. While the magazine did not isolate all variables, their testing showed a carry distance increase with the ERC of about 6 yards.

The distance increase possible from a clubhead with a higher COR is real. Compared to other distance-promising golf club developments in the last 20 years, this one is more definite than any other, including graphite shafts.

But there still remains one major developments than can over-shadow even the higher COR clubhead in terms of real game improvement - proper fitting of every golf specification to a golfer's playing characteristics.

And, if clubmakers spend time studying the principle of custom fitting, it's nice to know that they could have a 'sure thing' in their back pocket in the form of a higher COR driver. It will add to their fitting expertise and deliver more distance and more golf enjoyment to their customers.

Philip Ang, 1999-2000 Winner International Clubmaker of the Year
conferred by Golf Clubmakers Association (GCA) - USA compiles this article from Golfsmith Tech Report.