page_title: Tank steering systems
section_title: Controlled differential
hierarchical_section_title: Tank steering systems / Controlled differential
context_page_description: Tank steering systems allow a tank, or other continuous track vehicle, to turn. Because the tracks cannot be angled relative to the hull, steering must be accomplished by speeding one track up, slowing the other down, or a combination of both. The half-tracks avoid this by combining steerable wheels and fixed-speed tracks.
  Early steering systems were adopted from tracked work vehicles, generally using a clutch to reduce power to one track, causing it to slow down. These designs have numerous problems, notably when climbing hills or running at high speed, as the reduction in power causes the overall speed to slow. Delivering power to both tracks while turning them at different speeds is a difficult design problem.
  A series of more advanced designs were introduced, especially through World War II, that maintained power to both tracks during steering, a concept known as regenerative steering. Some also allowed one track to move forward while the other reversed, allowing the tank to spin in place, a concept known as neutral steering. The first really successful system was the British double differential design of 1924, which was copied by both the United States and Germany.
context_section_description: Differential braking systems are essentially an epicyclic transmission with a single fixed gear ratio transmitted through idler pinions. The controlled differential adds an extension to the rack holding the idlers, and puts a conventional brake on that extension. With the brake applied, the idlers are locked and the system works like a conventional epicyclic. When the brake is released, the idlers spin, reducing the speed of rotation on that side. This causes the output on that side to slow down by a fixed amount.
  The advantage to this design is that the brake is not being applied to slow the vehicle, it is simply clutching the second set of gears in or out. This means it does not dissipate energy, except in the brief period it is being applied or released. Smoother steering can be accomplished by partial application of the brake, but then it begins to dissipate energy like the braked differential. The main disadvantage is that there is only one turning radius. Like the standard differential solution, these systems are also subject to self-turning when travelling over terrain.
  The system was invented by Cleveland Tractor Company in 1921, and is sometimes known by the trademarked name Cletrac Differential. It was used by most French tanks in the inter-war era, as well as on the German Grosstraktor. It was most common on US designs, used on all light and mediums from 1932 until the end of World War II. Later uses included the French AMX 13, Japanese Type 61, and the widely produced US M113 APC.
caption: The M113 is the most widely used military example of the controlled differential system.
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