http://en.wikipedia.org/wiki/Fifth_wheel_coupling
呢度有解釋:
The term fifth wheel comes from a similar coupling used on four-wheel horse-drawn carriages and wagons. The device allowed the front axle assembly to pivot in the horizontal plane, to improve turning.
第五輪呢個名來自有類似連接裝置的四輪運貨馬車
呢個裝置令頭擔係一個平面上轉動,令佢容易D轉