Yes, there's no 3rd spring in GP4, which is sometimes annoying - particulary when I'm forced to run on bump rubbers instead to setting up the 3rd spring. I haven't checked any anti-roll bars influence. As you said, they should not do anything in straight line, but I'll have a look at it.
I want to measure downforce by analysing available telemetry data just out of curiosity and to have a better understanding of the car's dynamics. It would also helpe me in some hobbies/projetcs, but nothing really serious. I chose a 50-50 weight distribution just to offset car's weight effects on the springs when stopped or when traveling at top (and constant) speed.
Anyway, I tried to sum spring compression to ride height variation and I got aprox 2202kg of downforce, exactly the figure of the game's physics engine. Although I've got the answer I wanted, I can't see why I had to sum spring compression and ride height variation (top speed RH - stopped RH). Perhaps the suspension guys here could clarify this issue