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.
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.