The truck weighs about 10 tons and the log 15 tons.
I used Truss Physics for the tires: http://forum.unity3d.com/threads/released-truss-physics-for-unity3d-soft-body-simulation-plugin.349503/
I also used Truss Physics for the Front Leaf Springs, for the Rear Suspension Airbags, for the Rear Leaf Springs and for the Frame so it can twist.
I am trying to make a physics based logging truck.
For more info about Unity Engine visit the official website: https://unity3d.com/