I have a very simple scene (in /Assets/testScene) which consists of a plane for the floor, a cube for a vehicle body, and 4 cylinders for wheels which are all connected via HingeJoints. There are no scripts in the project.
Can anyone tell me why the car wobbles and moves backwards/forwards even though I have not yet applied any forces?
The problem was two-fold. Firstly I had my model parented like this
-Chassis
--FrontWheels
----FrontLeft
----FrontRight
--RearWheels
----RearLeft
----RearRight
This in itself is fine, except for the fact that I had also accidentally placed a Rigidbody on the FrontWheels and LeftWheels objects. For clarity, you only need Rigidbody components on the main chassis and the wheels.
The second problem was the mass of the wheels. As they were only 1 unit of weight (Kg presumably) they moved very easily, increasing their weight to 10 did the trick. Now the vehicle is completely stable.