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 A wide variety of bogie-based suspension designs were introduced during the early days of tank design, but few used the concept of a common spring under compression in the fashion of Carden's original concept. For instance, the American vertical volute spring suspension (VVSS) shares many design notes but uses two separate springs compressing vertically against the bogie framework rather than each other. In contrast, the late-war horizontal volute spring suspension, or HVSS, has more in common with the Horstman design, using a volute spring in compression between the two crank arms, as well as a shock absorber above the spring. HVSS was a major feature of the M4A3E8 model "Easy Eight" ('easy' being the US Army phonetic code for 'E' at the time). Mechanically, the design also 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. Applications The name "Horstmann suspension" was sometimes 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 (now Horstman Defence Systems) whether of the bogie type, torsion beam design, hydrogas, hydropneumatic or other. In the research of United States Army Materiel Command and in newer university