Hi Richard,
The simplest set of sums I could find were in the Haynes 'Race and Rally Car Source Book'. Without being able to weigh each corner of the car I had to use some guesstimation - the engine is behind the axle which throwa some of the weight to the rear - plus the operative moments about the wishbone pivot to the bottom damper mount and the bottom of the king pin are needed, also the combined weight of wheel and tyre will affect the best spring rate.
I was looking for a result that left the fillings in my teeth and didn't shake the car apart while driving over the rather rough roads in my area.
One really subjective test I used was 'If I push down carefully on the top of the radiator shroud does the car go down in the manner of a typical car?'.
At 375lbin the front suspension behaved as though it was almost rigid.
Tyres will be a factor in the choice - I have 185/70 15" which has some give in the sidewalls, currently at 23psi which feels about right (no science here!).


In the end my sums were some way out - the first springs would be good for a track - the set up was very solid, but not what I wanted. I am now using 250lb in x 7" springs. This means that the spring seat has to be wound some way up to get the right ride height, a higher rate spring would not compress so far.
There is a fair amount of experimentation involved in the combination of spring rates and damper settings (which I think I have at the third from lowest setting on mine).
I got the springs from Rally Design in Faversham who do a good mail order service, but there are quite a few suppliers.
One result of the settings I have is that I take speed bumps with care - the sump is quite low and my exhaust runs underneath the car.
Martin

All that happens when you wind the collar up on spring is to preload it.
wind it up an inch (250lbs per inch)wheel to spring movement), the force needed to move the spring needs to be at least 251lbs.
If you really need to work it all out put goulds maths into exel. It works...........

Yes indeed, I don't know what the linearity of any change of spring rate will be as the spring is compressed - until the coils bind of course!The spring rate is set when it is manufactured, I had assumed the rate would remain around 250lbs for all practical purposes throughout the range of operating lengths of the spring.

The point of winding the spring seat up is to adjust the ride height on the car to make it level fore-and-aft, having no adjustment at the rear on my car.
This compensates for the spring compression due to the weight of the car. When I used higher rate springs the seat needed less adjustment because the spring was compressed to a lesser extent. Sorry I didn't explain that this was about ride height, not rate.