Yesterday I did some measurements on the springs, and these are the results.
Both springs had a free length of 295mm (just at the service limit for SP springs according to service manual), when loading them with 10 kg they extended to 312mm, and when loading them with 20 kg they extended to 331mm.
Weight/displacement
10kg/17mm => 0.59 kgf/mm
20kg/35mm => 0.57 kgf/mm
If I would have measured smaller weights I would probably get a lower spring rate, but since I am not going to use progressive springs and comfort is not the issue on the race track I focused on the heavier weights. Also it is more difficult to measure displacement for small weights.
I round of the result and use the spring rate 0.6 kgf/mm.
Afterwards I did some measurements on the static/free sag and ride height.
No preload, sag = 33,0 mm
Medium preload setting (dont remember exactrly how much), sag = 24,1 mm
Full preload (using tyga fork caps = 25mm preload), sag = 14,3 mm
While using full preload I got a ride height of 36,9 mm, and then I wasnt wearing any equipment (adds a few kg to ride weight and mm to ride hight.)
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MC21, GP-Replica (Daijiro Kato, Telefonica Movistar Team 2001)