The BT-2 tank of 1932 a Soviet copy of Walter Christie's tank.
The Christie suspension is a suspension system developed by 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 and a lower profile. The system was first introduced on his M1928 design, and used on all of his designs until his death in 1942.
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 constantly hampered by poor cross-country performance due to limited suspension capability, and in the late 1920s he spent considerable time coming up with a better solution. The major problem he faced was the limited vertical space for springs to move in, for a 10 inch movement you might need 20 or 30 inches of vertical space for the spring and strut, and his small designs simply didn't have a place to put them.
A British Comet with Christie suspension
The solution was the addition of a bell crank, 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 massive increase in range of motion, from only a few inches in his original designs, to 10 inches on the M1928, 14 on the M1930, and 24 on the M1932. However, the most famous Christie-based tanks, the Soviet BT and T-34 series, used coil springs mounted vertically (on the BT) or at a slight angle from vertical (the T-34).
Another feature of his designs was the use of very large road wheels with no return rollers for the tracks. The idea was that the tracks could be removed for road travel, allowing for higher speeds and better range. In order to allow this, Christie pioneered the use of rubber covered wheels, typically with the tires on either side of the wheel with a slot in the middle to allow lugs on the track to run between them. This became a common feature of almost all tanks, although not to allow the tracks to be removed, but because it was found that the rubber dramatically increased the life of the tracks. Some Soviet tanks had to be produced without rubber due to wartime shortages. The all-steel wheel was unpopular with tank crews as contact with the metal track at high speeds set up harmonic vibrations that were noisy and unpleasant for those inside, and could cause damage to the T-34 itself by loosening parts. As rubber became available again, rubber-rimmed wheels were used in the first and fifth positions. With increased rubber supplies in 1943, the all-steel wheel was phased out.
The use of oversized road wheels and lack of return rollers is a common design feature of many tank suspension systems. For this reason it is common to see the term "Christie Suspension" applied to designs that don't actually use it. In fact the vast majority of these examples, notably the late-World War Two German and post-war Soviet designs both used torsion bar suspensions. The real Christie suspension was used only on a few designs, notably the Soviet BT tank and T-34 series, the British Cruiser tanks, including the A-13, Covenanter, Crusader and finally Comet, as well as some experimental Italian designs.
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