Shaft frequency is a precise method of measuring shaft flex. Cool Clubs determines frequency based on the length of the club and the units of frequency measurement known as Cycles Per Minute (CPM). A shaft with a higher frequency (more CPMs) is stiffer than one with a lower frequency (fewer CPMs). A shaft with a frequency of 7.0 is very stiff and a shaft with 2.0 frequency is very soft.
From the following chart we can see that Cycles Per Minute (CPM) and Frequency (Flex) are two sides of the same coin. As length increases CPM decreases for the same flex. For example, if we want to build a club to 6.1 FM at 39 inches length we would ensure it has a CPM of 290 while to build to the same FM at 45 inches we would build to an CPM of 240.
This is important as it means we can build a full set of clubs from Driver to wedges with exactly the same FM and thus have them feel the same when hitting them.
The advantage of this approach to shaft stiffness is that Cool Clubs can build our clubs to 0.1 FM increments. So rather than have a stiff shaft at say 6.0 FM we can build at 6.1, 6.2, 6.3 etc. This means you get a much better fit of flex to your swing dynamics.