The friction coefficients in PhysX seem to be quite odd.
For an object on an inclined surface of 30 degrees, the static friction coefficient at the point that the load starts to slide is tan(30 degrees) = 0.58. Therefore, if I set the friction coefficient to 0.3, for example, I expect the object to slide down the ramp.
I set this scene up in ODE, Bullet and PhysX to compare the behaviour.
First in ODE:
Second in Bullet:
Third in PhysX:
As you can see, the object does not slide down the ramp in PhysX, despite the static and dynamic friction coefficients being set to 0.3. Note that I have eDISABLE_STRONG_FRICTION set, although this shouldn’t really make a difference. The object and ramp use plain old box geometry and the contact normals are fine.