Code:
btVector3 friction_force = tire[i].GetForce( normal_force, friction_coeff, camber_rads, angvel, lonvel, latvel );
GetForce 3rd argument is in radians while the function expects degrees.
Code:
btVector3 CARTIRE::GetForce (
btScalar normal_force, // Fz, normal_force in N
btScalar friction_coeff, // track surface friction scaling coefficient
btScalar inclination, // camber, degrees
btScalar ang_velocity, // rotational speed, rad/sec
btScalar lon_velocity, // Vx
btScalar lat_velocity ) // Vy
{
...
}