Pacejka tire coefficients

[orangutan]

Your traction circle combining is much better than the beckman method in my opinion, but can be off like any method that assumes a circle based on max Fx and Fy. A little bit better would be to run through the friction calculation again with a combined 'purified' concept of slip to get the effective maximums. This is from a mutated codebase, so you may have to change some names and stuff to try it out. The resulting curves are similar to your traction circle method, but track better with the anisotropy of the tire.

T Fx = getFx(m_sigma, Fz, m_friction_coefficient[0], max_Fx);
T Fy = getFy(m_alpha, Fz, gamma, m_friction_coefficient[1], max_Fy);
T x = cos(m_alpha_rad);
x = fabs(Fx) / (fabs(Fx)+fabs(Fy));
T y = 1.0-x;
T one = 1.0;
if(m_sigma < 0.0)
{
one = -1.0;
}
T pure_sigma = m_sigma / (one + m_sigma);
T pure_alpha = tan(m_alpha_rad) / (one + m_sigma);
T pure_combined = sqrt(pure_sigma*pure_sigma + pure_alpha*pure_alpha);
T kappa_combined = pure_combined / (1.0 - pure_combined);
T alpha_combined = atan((one+m_sigma)*pure_combined);
T Flimit_lng = getFx(kappa_combined, Fz, m_friction_coefficient[0], max_Fx);
T Flimit_lat = getFy(alpha_combined*180.0/fe::pi, Fz, gamma, m_friction_coefficient[0], max_Fy);


T Flimit = (fabs(x*Flimit_lng) + fabs(y*Flimit_lat));

T Fmag = sqrt(Fx*Fx + Fy*Fy);

if(Fmag > Flimit)
{
T scale = Flimit/Fmag;

Fx *= scale;
Fy *= scale;
}

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