Lego set #8285 is a large Technic semi tow truck. It's a great set, and after finishing it I felt like trying to complement it by converting it into a tractor-trailer set.
Model overview.
The first thing I did was remove the tow truck tackle from the back and replace it with an improvised 5th wheel to hitch the trailer to. With that done I could measure the height for the hitch and make the bed and rear wheels for the trailer. The rest was relatively simple: Throw up SNOT walls all around, devise a rear door and add detail. Here are a couple of views of the finished trailer:
Other views.
Here's detail of the front of the trailer, where I tried to make an approximation of a refrigeration unit:
Refrigerator detail.
Closeups of the rear door in the closed and open positions - note door handle and rear bumper with tail lights and license plate:
Rear detail.
Roof removed to show detail of the door opening mechanism:
Interior door mechanism detail.
The rear door operates like a garage door. Turning the crank pulls on two parallel chains made of classic caterpillar links which have studs on them. The studs are affixed to the back of the horizontal slats that make up the door, and function both as the mechanical power to pull the door up and as the hinges to let the door flex from vertical to horizontal at the top rear corner. It's true that as the door is raised, the chains dangle down into the inside of the trainer - I could have avoided that by using a winch and string instead of gears and treads, but it wasn't a priority.
Actually I think the biggest flaw with this project is that the proportions of the rear wheels to the trailer feel a little wrong - the wheels look too small to me. It turned out that way because the 5th wheel is a little too high off the ground. Slightly larger wheels would fix it.