As tracked vehicles exist for their mobility across obstacles, they usually have no difficulty in doing this (however, see the "Power for loading" section below.) The load then drives up the ramps and onto the trailer under its own power. The simplest means of loading the transporter is with a pair of hinged ramps at the rear. However, heavier loads may require more axles, and steering gear may be needed at the front of the semi-trailer. Lowboy semi-trailers are commonly used to keep the load's center of mass low, as they have a low horizontal bed, with a gooseneck that rises up at the front to connect to the tractor unit.Īs with drawbar trailers, it is simplest if all the axles of a semi-trailer are concentrated at the rear, away from the tractor unit. A strong metal post or kingpin on the trailer fits into a socket or ' fifth wheel' on the tractor. The simplest semi-trailer is "half of a trailer", having wheels at the rear only and an articulated connection to the tractor unit. They are only mobile when connected to the correct tractor unit, which can limit their use in recovery. Semi-trailers cannot be moved on their own, or pushed and pulled by ballast tractors. This avoids the need to ballast the tractor, making a greater proportion of the total weight available for the load. By using a semi-trailer instead, some of the load's weight is instead carried by the tractor. The ballast tractor for a drawbar trailer must be comparable in weight to its load if it is to have traction, which means that the total weight of the tractor-trailer combination might be as much as twice the useful load. The German Sd.Ah.116 trailer of World War II had a steersman's position on the rear bogie. The outermost four axles had Ackermann steering for their wheels. The Cranes trailer had a frame split into two sections of two and three axles, with a carrying bed above them. Placing the wheels at the ends also allows the chassis to dip down into a "well", giving a lower centre of gravity during transport. This makes them more complicated to manufacture. Those with axles throughout their length must have suspension that allows the axles to move and also allowing some steering.
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The end-wheel designs have the advantages of better ability to cross rough ground and steering more easily. Others, such as the 70-ton Cranes trailer used to carry the Second World War-era Tortoise heavy assault tank had five axles, spaced along the length of the trailer. Some designs, such as the 1928 Aldershot design, grouped pairs of axles at each end of the trailer. Other attempts have been made around 1942 with heavy-trucks as known for the Dart T13 Tank Tractor. Tank Recovery Unit T3 including Dart Truck Tractor T13 and Trailer T28