ratatouille wrote:Could angular inertia be the culprit? In the Python case, the pendulum is ideal, with a point mass. In Bullet, the mass is distributed over a rigid body of nonzero size (i.e. the swinging body of the pendulum). For a long enough pendulum and a low enough mass (L = 10 m, m = .1 kg) the results overlap.
I tried setting the inertia tensor and/or the angular factor to 0 to try and disable this effect. But when I do this, the spring constraint doesn't work any more and the pendulum just flies off to the right.
Well, that explains just about everything! You were not even running the same simulation so of course the bullet sim will have increased damping. Rather than approximating bullet's conditions to a point, just expand your python script to include rotational and linear inertia.