ZONE FREQUENCY MEASUREMENTS

TECH NOTE 36: Zone frequency profiling of a shaft has become a significant subject of interest lately. Two shafts with the same butt frequency might play totally differently because if variations in tip stiffness. Originally, zone profiling was done by sliding the butt deeper and deeper into the clamp. Of course as you do this, the frequency continues to go up and up. At some point you get beyond the capability of the analyzer. Especially since a 205 gram tip weight was typically used. To overcome this at some point the shaft was simply turned around and a small weight attached to the butt. The resulting data always looked a little weird to me. You had a series of points with increasing frequencies and a final point (the tip measurement) that was a very much lower frequency. With the change in test weight and flipping the shaft around the data always appeared to be a comparison of apples and oranges. In Tech Note 31 I described an approach of computing the ratio of tip to butt frequency which at the time seemed a pretty reasonable approach to me. Unfortunately there was serious flaw in my thinking (not the first time).

UNIDIRECTIONAL APPROACH
Recently I was looking at Tom Wishon's site and I noticed he was doing zone measurements all in one direction with a very heavy tip weight to keep the frequencies form getting too high. A light bulb finally went on in my head and I realized there was a fatal flaw in tip frequency measurements. The weight of the shaft itself becomes a very significant influence on frequency. Two shafts with identical tip stiffness might have tip frequencies as much as 15 cpm different just because one is a heavy shaft and one a light shaft. In Tech Note 31 you'll note many of the steel shafts appear to be tip weak. This is really not the case. They're just heavy. In Tech Note 1 you'll see the frequency is inversely proportional to the tip weight and 0.23 times the weight of the shaft. The multiplier, 0.23, is the value for a uniform rod. The shaft of course is tapered so the number is not really correct. Computing this number is too tough for me so I measured it. If you measure the tip frequency with two different tip weights, say one of 50 grams and the other say 75 and knowing the weight of the shaft the unknown multiplier an easily be computed. It turned out to be .30 for a typical shaft. The effective weight of a steel shaft might be 50 +.30x125 or about 88 grams. A light weight graphite shaft might be 50+.30x50 or 65 grams. The ratio of these two weights is 1.33. The square root (see equation in Tech Note 1 again) is 1.15. This means that in tip frequency measurements of the graphite shaft weight becomes 15% higher just due to its light weight. Therefore table in Tech Note 31 is not truly a representation of tip stiffness unless the two shafts weigh nearly the same.

TESTING
With all this in mind I did some zone profiling the way Wishon described on his site (I also noted in Golfsmith's new catalog they are doing the same thing. The old technique of flipping the shaft around with a 50 gram weight on the butt seems to be dead…and rightly so.) I first wanted to see just how short of a beam length I could get to and still get reasonable readings with a Club Scout. Frequencies up over 900 cpm were not a problem which equated to beam lengths of under 10". (It should be mentioned that a shaft vibrating at high frequency with a heavy weight on its tip is under quite a strain. Ultra light weight shaft can break under these zone tests. I've never broken shafts down in the 60 gram range but those in the 50 gram range or less can present a problem.) The vibration amplitude is pretty small and it was hard to get a beam break with the body of the shaft itself. The shaft simply couldn't be twanged enough to get the shaft to pass above and below the light beam. I attached a little 1/8" rod to the back of the 454 gram tip weight and broke the beam with the small rod. This solved that problem.

I zone profiled a few different shafts and the results were quite interesting. See Chart 1 below.

I took a PC50 shaft at random that was pretty tip stiff and just called it my "reference shaft". I generated its zone profile and then referenced all the other shafts against it. I did this by just subtracting the PC50 frequency data from the other shaft data. This basically greatly expands the plots. See Chart 2 below.

Little idiosyncrasies of the shafts now become very apparent. It would be nice if the industry could come up with some standard profile to reference all the shafts against. Another approach if you're just comparing two shafts is to subtract one from another. I did this with an SK Fiber Lite Revolution and their new Lite Revolution II. When plotted in this fashion the differences are quite obvious. The mid section of the Lite Rev.II was very much stiffer which was their intent.

CLAMPING
At very short beam lengths there is quite a bit a force on the clamping system. This is caused of course by the very heavy weight oscillating at a very high frequency. I played around with different clamping configurations and found a pretty significant variation in short beam length readings. Using a Club Scout with the slip clutch centered over the V blocks as it normally is, produced a short beam length reading of 573cpm. With the torque screw moved forward 5/8" the reading increased to 583cpm. I then built a pneumatic clamp with two air cylinders. One positioned ¾" from the front end of the clamping base the other ¾" from the rear of the base. This allowed for more uniform pressure over the entire length of the clamping surface. The readings were 2 to 3 cpm higher than the Club Scout clamp with the forward positioned torque screw. Is 2 or 3 cpm important? I really don't know. I haven't seen or generated enough data to get a feel for what's important. In terms of tip stiffness measurements maybe 5 or 10 cpm is insignificant. I also built a Club Scout clamp with two slip clutches mounted for and aft like the pneumatic clamp. It gave the same results as the pneumatic clamp.

The variations in frequency due to clamping might be a problem in comparing data generated by different clubmakers although I don't think these differences are too significant.

UNIVERSAL ZONE FREQUENCY TIP WEIGHT
As mentioned Wishon's testing has been done with a 454 brass tip wt. of unknown dimensions. Golfsmith is using a 615 gram weight and I've been using a 454 gram steel tip weight. Because of the weight differences and the possible center of gravity difference between the brass and steel 454 gram weights the data from various source may not be easily comparable. (Corrections for the weight and cg differences could be made but it's a little messy.) To achieve transportability of test data it would be nice if the industry could (for once ) come up with a standard tip weight. The Professional Clubmakers Society is trying to do just that. A modestly priced drill chuck has been found that weighs about 590 grams. I used the chuck and must admit it's much easier to use that my 454 gram weight. It has a further advantage. The chuck provides three bearing surfaces about an inch long that hold the shaft. My tip weight is attached with a 1/4x20 bolt. Even though the end of the bolt is polished it does put a small mark in the paint on the shaft tip and a lot of pressure on a single point of contact. About a dozen of these chucks have been purchased for testing purposes. Unfortunately they vary in weight by as much as 10 or 15 grams. The back of these chucks is quite soft therefore the heavier chucks have been shaved down on the lathe to 590 grams. Because of its high weight those analyzers that don't extend to the 999cpm range would still be quite usable in zone testing.

I will still offer the 454 gram weight but will also offer the 590 gram drill chuck as well or else it may be available form the PCS. This issue is still being discussed.

It's time for a small commercial. I built a dual cylinder pneumatic clamp (photo on left) for several customers and have used one myself. It can be used with gripped or ungripped clubs. It will do all the zone measurements very nicely and will accommodate the FM Precision short, 2 7/8", clamping length for their CRC certification program. The clamp is a delight to use. A flip of the switch and the V block descends to apply a precisely known force on the shaft. The switch is flipped in the other direction and spring loaded plungers return the V block to its upper position. If you have air in your shop I'd heartily recommend a pneumatic clamp. It's $350 plus S& H.