Wheel has no friction on mesh collider of another GameObject.

Good day!

I'm working on a mod for the Kerbal Space Program, a game that is based on Unity 4.5.5

And I have a problem with (I think) WheelColliders:

In a scene I have a terrain and a garage (GameObject with a RigidBody and several mesh colliders which are its floor, walls and ceiling). The garage stands still on that terrain. I also have a simple car with four wheels that use WheelColliders.

While the car drives on the terrain wheels work as expected and changing of FrictionCurves' parameters modifies their behaviour accordingly. But when the car rides inside the garage on top of the floor WheelCollider (which is as thick as ~1/4 of the wheel's diameter) wheels start to slip and have no grip at all, no friction. And changing their FrictionCurves have no effect.

Googling gives nothing on the problem, so I write here.

Do you have any clue as to where the problem may be, or at least some advices on debugging it?

I've spend many days on it and completely out of ideas.