here you go, its a lot to get through
https://forums.unrealengine.com/showthread.php?48843-Horrible-Car-Physics
basically,
there is something wrong with the way suspension springs are calculated, meaning its all too easy to send a 30+ tonne vehicle flying into space from mounting a curb for example.
wheel on wheel collision is also guaranteed to send vehicles pinging round the map, there are countless other minor issues that probably stem from the same root cause.
as for trailers, the physx vehicle uses a single bone/single rigid body so a trailer would have to be something of your own construct that is attached somehow to the vehicle.
take a look at boredengineer’s tanks for a good alternative to the standard vehicle, it also includes one with a trailer.
best of luck, you will need it if its a vehicle based game.