Truck and Trailer

I have a problem with joints, any kind of joints.

I made a working truck and a working trailer.

(working on android)

I connect the truck and the trailer using this scheme:

Are there any configuration that I have to make? Any ideas?

Why not just use a single hinge joint between trailer and truck?

Why so many joints?

You could probably also make the joints stiffer.

how do i make the wheels turn