When landing a vehicle on an angle the Wheel Colliders have a tendency to Immediately stick to the ground at a relative 90 degree angle, not like a real tire would or any other collider in Unity engine.

To reproduce:

1. Open project.
2. Go to "_Scenes" folder and open "main" scene.

Note: There are two vehicles on a 70 degree angle.
The "Test Vehicle 1" has the original coefficients of a sideways traction.
The "Test Vehicle 2" has sideways traction coefficients multiplied by 2.
(This is just to show that traction is not the problem here.)

3. Enter Play Mode.
4. Observe the falling vehicles.

Expected results: When vehicle wheels hit the ground the vehicle very slowly rolls on its four wheels, or even rolls on its roof because of the higher center of mass.
Actual results: As soon as one or couple of wheels touch the ground all the wheels Immediately stick to the ground at a relative 90 degree angle.
(Video: "Example1.webm")

Note: In this scenario both vehicles (even with a different sideways traction) touch the ground with all four wheels at the same time.

Reproduces with: 5.3.6f1; 5.4.4p3; 5.5.2p2; 5.5.2p3 (spin 2); 5.6.0f1 (spin2); 2017a4;