According to the Factory w/shop manual, the specifications of the rear springs are as follows:----Wire diameter:---------------------------13.5mm +/- 0.05mm
Working coils:----------------------------4-1/4
Free height:------------------------------221mm
Height under load of 410 +/-10kg:-----151mm
Height under load of575 +/- 29kg:-----123mm
Solid height:------------------------------93mm
Flexibility:---------------------------------17mm per 100kg
I think that the 'flexibility' factor might be the one you want----I would read that as 'spring rate'