Note that the correspondence between the frequency and the tension involves also the nature of the belt and the distance between the ends of the free part of the belt (same as in a guitar
).
Since the belts differ among our bikes, and probably the distance also, data from one bike are not necessarily good for another bike.
By the way, I have measured again today for my bike (Zero FXS 2016 ZF6.5 11kW, which has a drive belt with 14mm width, 8mm pitch, 54.5 cm between the axes of the sprockets).
Previous measurement, April 4th 2017, bike had 1613 kms, the Krikit tool gave 30 to 32 kgs depending on the measurements, and the frequency was 58 Hz.
Today measurement, Sept. 1st 2017, bike has 3502 kms, the Krikit tool gives 36 to 38 kgs depending on the measurements, and the frequency 69 or 70 Hz.
Note that I have not changed anything on my bike between the two measurements. So, in that case, the tension has increased with time...
Finally, I noticed that if the belt is vibrated by tapping it with a tool, the frequency is not reliable.
On the other hand, by stretching the belt by hand and releasing it, the frequency is quite reproducible.