| rdfs:comment
| - 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
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| abstract
| - 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 important. Ballast tractors are often fitted with heavy duty hub reduction axles, or high reductive gear boxes to increase torque at the wheel & consequentially this reduces the travel speed. Therefore heavy duty ballast tractors tend to have low maximum speeds. It is commonplace to find ballast tractors with a torque converter and modern units have retarders with extra oil coolers among other things as large loads also take a lot of stopping on steep decents. A ballast tractor can be either a modified tractor-unit, or the truck itself can be built from the ground up to be a ballast tractor. The tractor-unit variants tend to be heavy versions of readily available commercial models; such models have increased chassis strength to accommodate extra ballast and drawbars. A ballast tractor that is built as a ballast tractor tends to be the heaviest class of on-highway trucks. In some cases, the chassis weight alone of these trucks can be greater than their axle configuration legal GVW for a normal truck, so require special permission to use the roads. In the UK they operate under STGO regulations. Ballast tractors tend to have their ballast located across their driving axle. The ballast on multi drive axle machines is designed such that the desired axle load per axle is achieved to avoid overloading a particular axle. Modern ballast trucks, which tend to be modified heavy duty versions of normal tractor-units, tend to have two or more driven axles (such as 8x4 configuration). Heavier ballast tractors tend to have all wheel drive. The heavier variants of ballast trucks, which tend to live their lives with permanent ballast (or high weights), have strong heavy chassis. Having a ballast truck with a high curb weight means that the truck may be already beyond the maximum permitted GVW and requires special permission even in this state. An advantage of such a machine allows ballast to be added when the demand is there (such as moving a heavier load).
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