Directional stability

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The new model is strictly optional. You have to compile with VDRIFTN define to enable it. It is better in the sense that it is using a data set from a tire manufacturer and includes combined forces. So if anything is behaving incorrectly I can at least be sure that the tire forces are correct. My ultimate dream is to reduce parameter tweaking as much as possible, just plug in some realistic values and get somewhat realistic car behavior.

It has got more parameters, but at least the naming is more logical. The aligning torque is a bit too complex now imho, still don't fully understand what they are doing there. But I guess it is a rather complex effect, thus the complex model.

The handling will ultimately depend on the parameters. The data I've got has less grip and a bit lower ideal slip angle, but better combined handling in general I think (full combined model vs beckman).

To get a better feeling for the parameters I am trying to look at the main variables (this is better with the new model):
Example old model Fx = D * sin(C * atan(B * S - E * (B * S - atan(B * S))))
D(b1, b2): Peak factor, force maximum (scales Fx)
C(b0): Shape factor, falloff after peak
B(b3, b4, b5): Stiffness factor, slope towards peak
E(b6, b7, b8): Curvature factor, width/smoothness of the peak

I think you could get away with a simpler model, by using fixed C, B, E. But with D(b1,b2) as the force should scale with tire load.

I am not sure what is the best way to deal with gravel. I remember seeing some curves in a paper, will see if I can find it. I think it had reduced slope, smooth peak with low falloff (or almost no peak). Should result in quite slippy but smooth handling I guess.

I'll upload my tire tools soon. They allow to load a reference data set, which I found quite helpful when tweaking values. And I have got a version which allows loading old tire data as reference to compare it with the new model.