Dutton Kit Cars and their owners
On my Melos, which had a similar arrangement to your current set up (i.e. with the damper fixed to the TCA), I worked out that the wheel moved 1.45 times as far as the damper did. I measured the spring rate as being 180lb/in. I thought that my Melos was fairly soft sprung but I never did get to drive it properly, so I can't be sure.
The Melos was 3/4 the weight of the Legerra, so I would expect it to need about 250lb/in to be equivalent in the Legerra. I would regard that as a minimum and your suggestion of 400lb/in might well be okay.
With the Legerra carrying about 400lb on each wheel then the spring would have a load of about 600lb when at rest (that is 400 x 1.45). If you assembled the spring on the damper with no preload then the spring would be deflected by 1.5" which corresponds to about 2.25" at the wheel. I don't know whether that is enough to ensure that the wheel stays in contact with the road on an uneven surface but it is probably better to allow that rather than to bottom out on bumps.
Alternatively, if you used 250lb/in springs then the static spring deflection would be 2.4" (making 3.5" at the wheel). 2.4" seems a little excessive, so I'd probably preload the spring by at least 1". You also need to consider the travel of your dampers, some only move 3" or so, so I guess you'd want to get the static position to be a little less than half the travel.
All things considered, I am rather inclined to agree with your 400lb/in spring rate and to preload the spring by 0.25" (i.e. 100lb) so that the damper has only travelled 1.25" when the car is parked.
I borrowed some 275's from a mate to try and it was still quite soft, he didn't have any 400 to try so its temporararilly sitting on 650's and not doing too badly, maybe because my lower mounting point is further inboard than other Duttons?
James's logic sounds good to me. The only other factor is if you want the spring preloaded rather than near to floating at full damper extension. This will obviously reduce the sitting deflection on a 250lb/in spring.
What spring lengths are people going for? Does a standard Triumph spitfire spring work on a Melos with a Zetec/ type 9? as the heavier engine & gearbox must nearly balance out the lighter body. I ask as that was what I was going to try,
think my springs are 7"
My springs are a bad comparison as I think they are shot along with the shockers on the front
I do have a set of 120lb springs knocking about somewhere, they came from the front of a crossflow Legerra.