Rigidbody isn't working properly

I am trying to make a skateboarding game, but the rigidbody goes crazy and seperates itself from the others.

[33568-screenshot+on+10.11.2014+at+10.53.37+pm.png|33568]

[33569-screenshot+on+10.11.2014+at+10.58.50+pm.png|33569]

The wheels and the entire deck + trucks have a mesh collider and a rigidbody attached.
Why are they seperating? Do i need to forcibly join them together?

You need to use joints, experiment with which one works best for this game