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The Christie suspension is a suspension system developed by American engineer J. Walter Christie for his tank designs. It allowed considerably longer movement than conventional leaf spring systems then in common use, which allowed his tanks to have considerably greater cross-country speed. The system was first introduced on his M1928 design, and used on all of his designs until his death in 1944.
Christie advocated the use of lightweight tanks with long range and high speed, designed to penetrate enemy lines and attack their infrastructure and logistics capabilities. His earlier designs in the 1920s were hampered by poor cross-country performance due to limited suspension capability. The major problem he faced was the limited vertical space for springs to move in: for a 25 cm movement it might need 50 or 75 cm of vertical space for the spring and strut, and his small designs did not offer such space.
In the late 1920s he devised a better solution. The solution was the addition of a bellcrank, which changed the direction of motion from vertical to horizontal. The road wheels were individually mounted on a pipe that could move vertically only, at the top of which the bell crank rotated the direction of motion to the rear. Springs were mounted on the end of the crank, and could be as long as needed, lying along the inside of the hull. The result was a substantial increase in range of motion, from only some 10 cm in his original designs, to 25 cm on the M1928, 35 cm on the M1930, and 60 cm on the M1932. The most famous Christie-based tanks, the Soviet BT tank series and the T-34, used coil springs mounted vertically (on the BT) or at a slight angle from vertical (the T-34).
Another feature of Christie's designs was the "convertible" drive: the ability to remove the tracks for road travel, allowing for higher speeds and better range, and reducing wear on the fragile caterpillar track systems of the 1930s. In one public test 1931 in Linden, NJ, Army officials clocked a Christie tank attaining 104 mph, making it the fastest tank in the world: a record many believe it still holds. To allow this, Christie used large rubber-rimmed road wheels, with no return rollers for the tracks, although wartime shortages often led to the use of steel wheels, which gave an uncomfortable ride. As with many track designs with center guide teeth, dual wheels were used, allowing the guide teeth to run between them. By 1939, the Soviets found that the BT tank's convertible drive was an unnecessary complication which occupied valuable space in the tank, and the feature was dropped in the T-34.
Because large road wheels and "slack track" are characteristic of the Christie suspension, other designs with these features are sometimes misidentified as such. The real Christie suspension was used only on a few designs, notably the Soviet BT tanks and T-34, the British Cruiser tanks, including the A13's: Cruiser Mk III, Cruiser Mk IV, Covenanter, Crusader, Cromwell and the Comet, as well as some experimental Polish and Italian designs and the modern day Israeli Merkava tanks.