Thank you all for your detailed comments, which I am still re-reading.
A few replies:
I wish I could better relate the existing spring rates to the coilover rate, as Vibrac suggests.
I spent a long time trying to accurately calculate the spring force to axle force ratio, as I think that might have told us a lot about Girdraulics behaviour and would be very useful now.
I did however succeed in accurately calculating and also measuring the movement ration of the springs vs the axle. It’s around 1.8: 1 with the forks fully extended, dropping to around 1.2: 1 with the forks fully compressed. It’s actually quite easy to see why this occurs and is a consequence of the axle following a 5.6” radius circular path. As the forks start to move from fully extended, although the lower spring box mounting is moving upwards towards the upper spring box mounting, it is also moving forward, away from the top mounting. So a lot of axle movement only produces a little spring movement. Approaching full compression, it’s the opposite, the lower mount is moving both upwards and backwards, so the movement ratio gets nearer to 1:1. As the JE forks don’t have this initial forward motion, it seems very likely they largely avoid this effect so need less preload. I also briefly looked at the damper vs axle movement ratio. There is some variation, but it is much less than for the springs.
I initially thought if I knew these movement ratios, I would know the spring/axle force ratio. But I realised it’s a lot more complex than that. The angle of the spring boxes varies quite significantly, changing the vertical force component. In addition, as the springs push the blades forward, the resultant tension & compression forces in the links produce vertical components. When I tried to add these calculations to my already large geometry calculating spreadsheet the complexity became too confusing. I couldn’t face starting from scratch with more modern methods.
A point about the amount of spring travel required. The original Vincent damper had a full 3” of travel because the hydraulic bump stops were very short and stiff. The dampers that came later all have much less useable travel, because they all have large rubber bump stops. I am hoping it is possible to come up with a damper that has a closed length of, say, 3/8” less than the Vincent damper but the same open length. If we then fit a suitable bump stop, the forks will be brought to a halt at the same point as the Vincent damper. This should be the best of both worlds, the greater travel of the Vincent damper but also the much gentler bump stop of more modern dampers.
But this increases the spring travel requirement.
Regarding potential sales, I was concerned about this and I hope I have made sure they don’t have unrealistic expectations. Also, IF this works, there might be a few extra sales in a version for the JE mod and also a replacement for the Monroe shock used for the Brampton coilover conversion.