This is once agian a question that I am entirly serious about.  Does Optimus Prime in fact need a trailer?

For many fans and collectors the answer is a definate yes.  I have to admit, for a long while I too thought Prime needed a trailer.  I thought this for the most simple and obvious of reasons, he was a semi.  Semi's have trailers.  Or, do they?  To actually fit the definition of semi yes it does have a trailer.  A Semi would be defined as a tractor/trailer combo. 

So, was Optimus Prime a semi?  In fact,  usually he wasn't.  By my count there are 61 unique molds for Prime/Primal/Convoy (62 if you count E-Hobby's Orion Pax).  All of whom are often thought of as Optimus Prime by some loose standards.  Only 42 unique molds actually bear the name Optimus Prime.  Several of which were fire trucks, one was a shoe, and one was a Lamborgini (g2 go-bot).  None of these even have a true possibility of a detachable trailer with an independent cab.  Limiting the discussion to actuall semi alt modes brings the number down to 31 unique Primes.  That number drops to 29 because both G2 Hero Prime & Star Convoy, while semis, cannot detach their trailers from the cab protion of their alt mode. 

So we go from 61 to 29 possible trailers.  There are in that 29, in fact, only 12 true tractor/trailer combos.  Three of those trailers you had to buy seperatly (Masterpeice, Worlds Smallest, Transformers Junior Powermaster).  Re-referencing the previous question, was Optimus Prime a semi.  My answer is no.  Using the tractor/trailer combo as a definition in only 4 of these 12 instances does the trailer combine with Optimus for a "super" mode.  The other trailers generally turn into some version of a base mode.  In all of these cases Optimus Prime him self is the cab.  The trailer is an addition of some kind.

So out of 61 possible "Pimes" we have 4 where the trailer can be an integral part of the Optimus bot mode and still detach.  While I admit generally it looks odd to see a "semi" bob-tailing (running down the road without a trailer) in real life, the fact is it's not uncomon.  As my personal collection has grown, the number of Pimes that I have that are cab only toys has grown to the point that the trailered toys are the oddity.

If you insist that a semi-tractor needs a trailer, in most cases making a custom trailer would not be all that dificult.  Usually simply painting a non-tranformers semi-trailer and placing a correctly sized peg in the front would do.  For me, I'll do with out.  The extra trailers would simply take up valualbe shelf space anyway.

List of Primes with combining traiers
G1 Powermaster
TFJR 1 & 2 (basicly a wst Powermaster)
Armada
Energon (well the Prime Force combines and rides in his trailer)

Additional Primes with detachable trailers
G1
Pepsi Prime
Worlds Smallest Transformers (wst)
Masterpiece
THS-02 Hybrid
G2 (repanited G1)
G2 Laser
Machine Wars

semi Primes without a detachable trailer
G1 Star Convoy
G2 Hero Optimus Prime