4x4 Hinge demo help?

Hey, I tried to recreate the 4x4 hinge demo and it was not successful. The wheels are in place on the box going before the object hits the ground but as soon as that happens, things go everywhere! The same thing happened when I deleted the wheels off of the demo “vehicle” itself and copied the logic bricks as the previous wheels were before.

What is causing this and how can it be fixed?