The name ballast is derived from the nautical term (see sailing ballast) describing heavy material added to a vessel to improve stability. With a semi-trailer, the weight of the trailer presses down through the fifth wheel and adds ballast. In the case of a ballast tractor, the drawbar only transmits a horizontal force to the load, which is supported separately. Ballast is added over the driving wheels to increase the available tractive effort. The weight increases the friction between the tyres and the road surface. Without such ballast, the tractor would be unable to overcome the inertia of a heavy trailed load, and its wheels would rotate without generating forward motion. Ballast tractors tend to have high power engines that provide lots of torque, even at low speeds, which is more imp