I'm very happy that JDMjunkies has invested the time and work needed to put together his really helpful article on S30 suspensions. It certainly helped me a lot. I've had my 240Z prepared for long distance rallying and it has an FIA HTP, with the work that lies behind that done. The suspension uses adjustable base spring platforms, and is modified with Bilstein inserts. Both the struts and the dampers were modified by Gartrac (https://www.gartrac.com/) The springs are 2.25 inch OD, front spring rate is 185lbs/in and rear 225lbs/inch. Lengths are 12 in (305mm) front and 14 inch (356 mm) rear. The car has done about 10,000 regularity rally miles with this setup, with one damper rebuild. But now the whole system including the insulator mounts, top bearings and bottom mounts are showing wear.
Recently I weighed the car on Proform 7000 lb wireless scales. This is the result for the car with driver but no passenger aboard (I know, I know but full roll cage, extra electrics and no weight reduction because the marketing department thinks it looks good) all in kg:
I must admit that the 80kg heaver front left than right and the 46kg heaver rear right than left was difficult to understand. The car is left hand drive so some of the front difference must come from the driver, the steering console/column, pedal box, plus the fact the carbs and manifold are on that side. As to the rear there is a built in fire extinguisher behind the pax seat. and the fuel tank is 'taller' on the pax side and the fuel pumps are also there. Still.....
However taking these figures to be correct then with symmetrical spring lengths the car will not sit level. I assume there was always a weight difference left to right which is why the service manual shows different, as commented by JDMJunkies, front spring lengths each side (on page FA-20 on my copy) at 373.5mm LH and 386 mm RH uncompressed (note opposite heavier side to mine) to give an identical 201mm 'installed height'. The rears are shown identical. I guess that even if the lengths - or adjustable bottom mounts are set to bring the car level, the behaviour will be different on each side if the springs and dampers have to deal with significantly different weights on them, although with different lengths at least the spring travel will remain the same on the heavier side.
So I'm trying to find out what to do. I'm looking at short term fix of fitting Eibach springs which have rates similar to the existing, but an inside diameter of the springs of 76mm, which I've been told 'are better'. But what I'd really like to do is redo the suspension from first principles. That is deriving the unsprung weights by weighing, work out the suspension frequency with different spring rates, and chose an appropriate damping rear in compression and rebound. Of course all this could be short circuited if I could get the information for the works cars in period, which must be closer to correct than anything I can calculate.
So if anyone just happens to be sitting on this info and would share it, there is at a good meal with drinks waiting here in Switzerland