Spine vs. Frequency
NOTE 13: Mother Nature always tries to take the path of least resistance, that is, water doesn't flow up hill. If a golf shaft is allowed to rotate while being bent it will always take that path of least resistance. It will rotate to the axis of the shaft which can most easily be bent, that is its weakest axis. The weakest axis will be the axis with the lowest frequency and 90 degrees away will be its stiffest axis.
I decided to test all this and built a spine tester out of a several ball bearings and a short piece of water pipe I had lying around. It helps to have a metal lathe. To make it easier I picked a pretty bad shaft, a real "wobbler". This shaft was from a very large component house with their name on it. (I thought all the really bad shafts went into over-the-counter clubs.) I noted the spine point and then attached a compass rose to the shaft with zero at the spine point. I measured the frequency as I rotated the shaft through 22.5 degree increments and plotted the results. The data resulted in a pretty nice sine wave with a peak to peak variation of 12 cpm. The minimum frequency is occurring at exactly the point determined by the spine checker and the maximum is 90 degrees away. This shaft was as bad a wobbler as I've run into yet it was very easy to measure with a Club Scout III. This of course made me very happy.
The spine tester is a nice simple device that you can make yourself for a couple of dollars. It will certainly indicate the weak axis of the shaft. Unfortunately it won't tell you much else about the shaft. I did try one interesting experiment. I checked the spine of a filament wound shaft and was able to consistently detect it. When I checked its frequency it was hard to detect any consistent variation as the shaft was rotated. More about this later. While we're on the subject here is a NEWS FLASH: as of February 1999 the USGA has approved the use of a spine tester to align shafts for consistency.