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A ballast tractor can pull and push extreme loads that would be beyond the strength of a normal tractor. It has a connecting device called a "drawbar" that allows it to get behind an object to push while still in drive. This is what sets it apart from an articulate tractor, which has to go into reverse to push a heavy load.
Not all ballast tractors are built as such; some are conventional tractors rebuilt to make them heavier and stronger. Their name is due to the practice of equipping these tractors with large quantities of heavy material, called ballast, placed over the wheels. This gives them excellent traction and power.
Ballast is typically placed over the axle. Heavier trucks tend to have all-wheel drive, and an effort is made to distribute ballast weight evenly over all the axles a tractor may have. Some ballast tractors have multiple axles and rival semi-trucks in size and weight.
Unlike big load-bearing semis and similar trucks, a ballast tractor can withstand severe stress because of its reinforced chassis. This allows two ballast tractors to be hooked together for greater pushing or pulling power. If the same maneuver were attempted using two large trucks, the trucks would likely eventually experience chassis fracture.
Despite their strength, ballast tractors are steadily becoming outdated and replaced by modern tractor-trailers. They are still best suited to certain tasks; for example, their unique design allows one tractor to pull a trailer while the other pushes it from behind. This combination of vehicles can still make safe turns and functions in a much more flexible way than a simple tractor-trailer does. In some circumstances, a ballast tractor may also provide more room for specialized trailers that don't fit onto modern vehicles.
With increasingly rigorous legislation, however, ballast tractors are gradually falling out of common use. They have powerful engines that require a lot of gas but are too heavy to move quickly. This makes them inefficient when it comes to fuel consumption and emissions.
In addition, the traditional ballast tractor exceeds current legal limits for gross vehicle weight rating. This means that ballast tractor operators must obtain special permission to drive their vehicles on most public highways. Many manufacturers have discontinued their ballast tractors because of these restrictions. Instead, they now offer smaller tractors or trucks with a slightly reinforced chassis that can be fitted with a ballast option when necessary.