Pretty much all the cartridge mfrs (both side of Atlantic) seem to recommend about 1kg/mm. With about 30mm preload.
Maxton have 1.1kg/mm springs but advised against it for my weight - fatties
may need them.
The US place I looked at for
@Joesmoe had 1.2kg/mm too. You'd need to get the spacers made up so Static and Dynamic Sag matched.If Maxton had said bigger springs I would have done so. But tbh the ride is perfect - it's just the excess of travel and the pneumatic bump-stop is now substantially stiffer.
Pretty much any spring mfr can make custom springs and if you decide to do so - they should be just that - custom.
I have done this in the past for bikes when adding big frame mount fairings etc. Used to use a place next door to Rickman Bros.
Simply pre-loading more often leads to springs binding before full travel is reached.
Especially tricky with dual rated or progressive rated springs.
tbh - I wanted to convert the forks to Pneumatic springing to match the Fournales rears.
Kayaba do MX forks with air springs. But Fournales did not want to get involved (fair enough) and DMX indicated that the price would be a little prohibitive.
So like all of us I rune spring/air hibrids. I did look at converting the stock forks to Air Adjusted forks - but ran out of ingenuity at a key moment.