Thank you for the infoPensarethebest wrote:Back when horse drawn wagons were used, the conection between the steerable axle on the wagon and the wagon resembled a wheel. The part became known as the "5th" wheel. Now the connection between the powered unit and the trailer is called the 5th wheel because that is where the vehicle pivots...like the old pivoting joint on the wagons.
There is always a good stroy behind those sayings.