The principal design feature is that two or more wheels are mounted to a common framework. Each wheel is mounted on an L-shaped swing arm that forms a bell crank, so the upward movement of the wheel is turned into sideways motion of the top of the crank. The two arms may be mounted on a common pivot shaft between them, or more commonly, two closely spaced shafts. The two wheels share a coil spring running horizontally between the tops of the crank arms. Thus if one wheel moves up over an obstacle, the spring is compressed against the swing arm of the other wheel. An advantage to the Horstmann design is that the suspended weight is not placed entirely on the rising wheel: its paired partner will also increase its downward force due to the shared spring, spreading the load. In systems with fully independent wheels, it is possible for the entire tank to become suspended on one wheel, which is rare in the Horstmann case. Another advantage is that the spring may work both in compression and expansion, increasing the total travel of the suspension. From a maintenance standpoint, the fact that the spring connects at a single non-moving point and is otherwise self-contained makes it easy to remove and replace in the field. The location of the spring over the wheels also makes it quite compact, with little or no encroachment on internal hull space.
Comparable suspension designs
Mechanically, the design shares much in common with the Christie suspension, which also uses a bell crank to press on a spring. The main difference is that in the Christie each wheel is mounted separately, and the spring is usually mounted on or inside the tank hull. The longer spring allows for more controlled flexion and potentially longer throw. Christie suspensions are generally more difficult to maintain because the wheels and suspension are mounted separately, and a broken spring can be difficult to reach without removing the wheels. Externally, the Horstmann suspension appears similar to the Americanvertical volute spring suspension ; both pair the road wheels on a common mounting. The two systems are very different mechanically – the VVSS wheels are mounted on independent pivot points and each has a separate vertical spring. Late World War II American designs used a version of the Horstmann suspension that replaced its coil springs with the volute springs from the earlier VVSS. This was known as the horizontal volute spring suspension, or HVSS, and was a major feature of the M4A3E8 model "Easy Eight".
Applications
The name "Horstmann suspension" may be applied to any transmission system that has two opposed swing arms, no matter the type of springing between them. The name also refers to any suspension built by the Horstman company whether of the bogie type, torsion beam design, hydrogas, hydropneumatic or other. The Horstmann system was used on, amongst others, the following vehicles:
