Rigidbody doesn't rotate after collision

I'm new to Unity physics and I've created a test scene where I have an object (a car model) with a collider and a rigidbody, and another object (an obstacle, also with a collider) positioned below the car (the latter hovers in the air at the start of the scene). The obstacle isn't placed centrally below the car but a little to the side so that when I play the scene and the car falls down it should collide with the obstacle, bounce off of it and rotate.

In fact, it does fall, it does bounce off (or at least slide on the side of the obstacle :P) but it never rotates (it's up vector stays up). I don't know why it works that way, probably I setup something wrongly. I've tried both box collider and mesh collider for the car object. The obstacle is unity primitive capsule.