cycles per minute (CPM)

Golf Terminology: Cycles Per Minute (CPM)

Golf Shaft Cycles Per Minute (CPM)

What a frequency analyzer machine does for golf

For many years a golf shaft’s stiffness was determined by using a flex board. This was a simple device. The flex board allowed the shaft butt to be hooked under a fixture at one end of the vertical board. The tip end then would have a weight hung on it, which caused the shaft to bend into a profile that could be measured against the flex board. So, you got a view of the shaft under stress to see if the butt was firm or the shaft had a weaker tip.

Most importantly, though, the board showed what flex range the shaft fell into, such as if it was a softer ladies (L), senior (A), regular (R), stiff (S) or extra stiff flex (X).

With the invention of the frequency analyzer machine, the time consuming task of using the flex board was not as important. Now club makers could get a fast, accurate reading of a golf shaft by checking the cycles per minute (CPM) the shaft would vibrate.


The frequency analyzer works by clamping the butt end of the shaft in a fixture and then twanging or plucking the tip end. The tip is bobbing up and down through a light source (usually a type of photo-electric eye) and produces CPM readings of the shaft. A cycle is created each time the shaft tip goes through the light beam. 


Here’s an illustration of this:

Take a ruler and place 1 or 2 inches of one end on a flat surface like a desktop. Hold that section down firmly. Let the remaining length hang off the desktop. Twang the ruler tip and you will see the body of the ruler bounce up and down. Note the speed it moves. Now increase the section that you are holding down where it is now about 3 to 4 inches. Go ahead and pluck the tip end again. You’ll see that the ruler now has a much faster vibration (Therefore, it is stiffer.).

The frequency analyzer also can tell you if a shaft has a slightly stiffer or softer side to it, depending on which way you orient the shaft in the machine. Due to the manner that steel shafts are manufactured, the tubular steel shafts tend to be very uniform in flex no matter which way you position the shaft in the fixture.

Composite graphite shafts, when manufactured in a professional manner, are also very uniform. However, some cheaper graphite shafts may have what is known as a “spine.” This is a slight overlap or opening in the way the composite materials were laid up during production causing either a slightly firmer or softer rib that runs down the length of the shaft.

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Some shaft companies go to the trouble of identifying the location of this spine and mark it for the club maker. What seems to be a continual bone of contention is what to do with the knowledge of where the spine is located. Some folks believe that it should be positioned down the back of the shaft at assembly, while others think it should be oriented on the left or right side of the shaft. The USGA — golf’s governing body — has a rule that says that shafts must flex uniformly in all directions. But the USGA does allow club makers to position the shafts a certain way. This tells me that the USGA doesn’t really think all this effort to identify the spine produces much of a difference in accuracy.

Here is a basic formula your average golfers can use to determine what kind of flex they should have on their shafts. The diagram below gauges flex using driver swing speed matched with what particular club one uses from 150 yards.

X-Stiff; 105-plus, pitching wedge.

S-Stiff; 90-104, 8- or 9-iron.

R-Regular; 80-94, 6- or 7-iron.

A-Senior; 70-85, 5-iron or less.

L-Lady; 70 or less, 5 iron or less.





Golf Terminology: Characteristic of Time (CT)

Measuring a Clubface’s Characteristic Time (CT) or Flexibility and Spring-like Effect

And how the precise method replaced Coefficient of Restitution (COR) in overseeing golf clubface manufacturing and performance.

In the past, golfers with wooden clubheads would gather around the bar after their round and tell tales of how much more distance they were getting from the smokin’ face inserts in their drivers or favorite fairway woods.

Inserts with metal “firing pins” milled into the face, aluminum inserts, or the mythical “gamma-fire” glass insert were all part of the lore. Whether they actually worked or not is still debated, but since very few folks still swing persimmon clubheads we’ll skip that debate today.

When metalwoods became the norm, golfers became obsessed with how far a thin face could hurtle the ball. Manufacturers began making faces thinner and thinner until they were barely able to sustain the impact of a fast-swinging player.

However, distance was improving — and this concerned the USGA.

For several years, the USGA had used Coefficient of Restitution (COR) to identify how “hot” a clubface was. There was a cumbersome method of measuring that started by detaching the clubhead from the shaft, mounting it to a fixture and then firing a golf ball at it from an air cannon to get a reading on the COR.

You may remember that COR means how much energy is maintained after the golf ball (fired from the cannon) strikes the club face. A Perfect COR would be 1.0. The max spec set by the USGA was.830 COR. Anything over that was deemed non-conforming to the Rules of Golf.

Now the USGA uses a different method of measuring this spring-like effect. It is called the Characteristic Time or “CT.” You can click HERE to see the full USGA rules on measuring CT. 


Much easier and less expensive, it measures in microseconds (with the symbol: µs) the dwell time of a steel pendulum device as it impacts the clubface. The assembled club’s shaft is clamped into a fixture and the pendulum is adjusted to allow it to strike the center of the clubface.


Right from the USGA rule book on measuring CT or clubface flexibility.

The specified limit allowable is 239 µs, with a max tolerance of 18 µs, thus equaling a maximum CT of 257 µs.

There is said to be a correlation between COR and CT, but CT is a much more precise way of measuring the spring-like effect of a clubface.

The USGA seems to have tight reigns on the speed that it will allow manufacturers to produce on clubheads. This will further limit just how hot a club face may be and perhaps begin to put a cap on the distance golfers will hit the ball.

Ah, but don’t worry too much. I’m sure that the golf ball makers will be able to squeeze out a few more yards from the clubs when max CT is reached.


Players Versus Game-Improvement Golf Clubs

Golf Terminology: Players Versus Game-Improvement Golf Clubs


Golf Terminology: Players Versus Game-Improvement Golf Clubs

How much you play and practice the game of golf will influence what type of equipment you may choose to put in your bag, whether it be players or game-improvement golf clubs. 

A combination of lots of practice and play, understanding your game and the science behind it all figure into deciding what to choose — or when to switch.


If you play only occasionally or find that you need a little help with your shot making, you may elect to use game-improvement clubs.

I’ll define these as having the following features and benefits:

The driver and fairway woods will always have graphite shafts, usually with a little extra torque to allow you to feel and flex the shaft and help launch the ball higher. The clubheads also may have some extra loft. To help limit the tendency to slice the ball, the face angles may be a touch closed.

The irons usually will have a slightly larger clubhead from heel to toe versus a players’ club. They will have a somewhat wider sole with some bounce to help the club glide through the turf. The wide sole also creates a lower center of gravity (CG) in the head that tends to help get the ball up in the air.

Game-improvement irons always will have extra perimeter weighting, usually with additional heel-toe weighting. (Although, some game-improvement clubs may be designed like a hollow iron, similar to a hybrid head.) The conspicuous deep-cavity perimeter weighting acts to increase the moment of inertia (MOI).

A club with a higher MOI will resist rotating about its axis. This is very helpful when, in those rare instances, you miss-hit the ball off the heel or toe portion of the clubface. Since the head is designed with a higher MOI, your shot will not wander as far off-line.

Finally, game-improvement irons usually are designed with additional hosel offset. This helps place the hands ahead of the leading edge of the club head to aid the golfer in hitting down on the ball — which helps get the ball up.


Let’s now take a look at what a players club has to offer. These clubs tend to be for golfers that play the game substantially more, and at a higher skill level, than the average golfer. Golf professionals and lower-handicap players are the target market.

Their clubs will have these features and benefits:

The driver and fairway woods almost always are built with graphite shafts. The shafts will be designed with stiffer tips and lower torque, so the clubs may be swung with greater speed and accuracy. The clubface angles will tend to be square to open versus the game-improvement woods. This open face helps the stronger golfer swing more freely without fear of duck hooking a shot.

The players irons also have a shorter head length from heel to toe. The soles are generally narrower and blade-like with much less weight concentrated toward the bottom of the club when compared to a game-improvement iron.

Since the better player is much more adept at striking the ball on the sweet spot of the club face, he actually prefers an iron with a lower MOI. He also wants the low-torque steel shafts in his irons. All of these features allow the skilled golfer to manipulate the clubface and hit a variety of shots, from soft high fades to boring low draws.

The players club may have a little bounce, but the sole usually has a radius camber that performs the task of digging less into the turf. Skilled players already tend to hit down on the ball, so the hosel offset is usually far less than is seen on a game-improvement iron.

In years past, players irons generally were what we called “flat back.” That meant that the back of the head had no cavity of any kind. The head may have had only a slight muscle back or small sole flange. (It is interesting to note that early Scottish irons had “mussel” backs due to the shape of the bivalve mollusks the Scots ate. Years later it was changed to “muscle” to denote a powerful club design.)

Over the years skilled golfers noticed how much easier a cavity design was to hit and they began asking for this feature on some of their clubs. Nowadays, a players club, more often than not, may come with a slight cavity to make the game somewhat easier for even the best players.

Moment of Inertia (MOI)

Golf Terminology: Moment of Inertia (MOI)

Moment of Inertia (MOI)

A ball curves due to a tilt in its axis of rotation. And the axis of rotation is the absolute center point that a ball is spinning around.

Golf Terminology: Golf Club Moment of Inertia (MOI)

The “Moment of Inertia” or MOI is a term thrown around very loosely in the golf industry. But when it comes down to it, MOI basically is how well-balanced a clubface. It can get a bit more complicated when you factor in how a golf ball reacts itself — and the clubface MOI (or twisting of the face at impact) then what will occur on an off-center hit as a result of the manufacturer’s engineering and your personal adjustments to the club, such as loft or swingweight.

The idea behind applying MOI to golf at its heart is just making sure that the face strikes the ball cleanly without without twisting too much, throwing off the ball’s motion so it doesn’t move efficiently — like straight and far. But throw in a round ball and it gets more complicated, yet the physics behind MOI essentially are the same.

Generally speaking, MOI is used in golf by distributing the weight of clubheads and balls outward to lessen twisting and other golf equipment and human variables (Hopefully, you’ll understand why as you read on and watch the video below.).

In physics, strictly speaking, MOI is a property that indicates the relative difference it takes to put an object in motion from a defined axis of rotation (Keeping up? See diagram below.). The higher the MOI of an object, the more force will have to be applied to set that object in a rotational motion. On the other hand, the lower the MOI, the less force is needed to make the object rotate about an axis. So let’s explain the whole axis and rotational stuff first.

(This is where golf club engineers get creative. But more on that at another time.)

Here’s a common example of MOI: The ice skater. When the skater starts a spin, she reaches out her arms and the speed of the spin is intentionally slow as it builds and she begins to pull her arms closer to her body. She is no longer resisting the speed of rotation, so her MOI is falls to a low point. It’s an inverse kind of formula since when she puts her arms out again, she slows and her MOI goes up higher as her resistance to the speed of rotation increases. 

Now here is the official physics definition of MOI in golf:

“Moment of inertia is the name given to rotational inertia, the rotational analog of mass for linear motion. It appears in the relationships for the dynamics of rotational motion,” according to the Georgia State University Physics Department“The moment of inertia must be specified with respect to a chosen axis of rotation. For a point massthe moment of inertia is just the mass times the square of perpendicular distance to the rotation axis, I = mr2.

“That point mass relationship becomes the basis for all other moments of inertia, since any object can be built up from a collection of point masses.” 

“There are several different moments of inertia that are factors in the performance of a golf club. Remember, MOI has to first be defined by identifying what axis the object is rotating around. There is an MOI for the whole golf club, which, when swung, is “rotated” around the golfer during the swing.”

These are the two examples of MOI in a golf club. But let’s not get too far ahead of ourselves for right now.

These are the two examples of MOI in a golf club. But let’s not get too far ahead of ourselves for right now.

Not too bad to understand, right?

As far as golf goes, here’s a nice, down-to-earth explanation of MOI from Rotary Swing golf instructor Clay Ballard: