I looked for a long time for springs with spring rates lower than 8.5 N/mm. I´ve tried a set of 8.5´s, but they were definitely too hard for me (approx. 73 kg with gear).
Since I could not find any springs that would go in without modifying spacers and such, I decided to use the springs for the Öhlins R&T forks. They are almost identical in diameters to the stockers, but beeing made from higher-grade wire (presumably ...), they are shorter with steeper pitch. One reason to use them are that they ae available in rates from 7 or 7.5 N/mm up to 11.0 N/mm, in 0.5 N steps.
I used the same rate in my Showas as I do in my Öhlins R&T fork, i.e. 8.0 N/mm which works very well (part number 4745-80 acc. to the invoice).
I had to manufacture new pre-load tubes, a coupler piece between the tube and spring, and slightly modify the plastic spacer on top of the pre-load tube.
Installed pre-load with fully backed-off adjuster is between 5 and 10 mm, should not be more than 10 mm.
I did some snapshots so I´ll do a how-to one of these days; this picture will give you an idea:
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BTW, I think some parts are on their way from K-Tech for the cartridge rebuild / modification, so that how-to will also appear one of these days