Collapsing structure (mesh import physic simulation)

Hallo, I’m a newbie in Unreal… I’m following some tutorial to create a very basic vehicle simulator but I got stuck at the very start… I designed my own vehicle on SpaceClaim exported chassis and wheel as STL and reassembled them in Blender as an object with correct size, placement and orientation (saved as .fbx).
When I try to import the fbx file it only recognize a single bone (probably because of the size of the vehicle) but follwing tutorial hints I forced the creation of a ‘single convex hull’ for each component.
The result is apparently correct:


After that I just assigned the weight to each component and locked the constraint on swing angles of wheels (it’s will be a skid steering vehicle).

When I start the simulation I got this:

How can I fix it?

After some more readings I found I misunderstood some concept … bones from the original mesh are imported correctly even if they are *small *and almost non visible they are all set in the center of the body (whatever it means).
The convex hull, one per component, is of course the collider bounding. Even with this mind clean up I have not idea how to fix the problem. The same behaviour it’s not limited to the simulation inside mesh editor but it also occour during game run.
BTW I tried to change and simplify the chassis and wheel meshes but I always got the same result, I tried also to import wheen and chassis as single static mesh and connect them trough their actor but the final result didn’t change…