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Late in 1944 Zahnradfabrik of Augsburg produced a PzKpfw IV Ausf H fitted with hydrostatic drive, apparently as a result of a request made by the Waffen SS in July 1944. This hydrostatic drive system was known as Thoma. It dispensed with a mechanical transmission and gearbox and instead the tank’s engine, a standard Maybach HL120 TRM, was linked directly to a pair of high-performance pumps. Oil was forced from these pumps into oil motors, one of which powered each track through reduction gearing. The engine and drive mechanism powered the tracks through small rear sprockets, an unusual feature in German tank design.
The driver controlled the volume of oil passing through the pumps and hence the speed of each motor, allowing him to control the speed and direction of the vehicle. By re-routing the oil flow it was possible to run the vehicle in reverse. Along with the hydrostatic drive, turret traverse and elevation of the gun were also achieved by hydraulics. Dispensing with the manual transmission provided extra space in the front of the hull. According to a drawing of October 1944 this would have allowed sloping front armour to be incorporated into the final design.
1) U.S. Army Ordnance Museum, Aberdeen Proving Ground, USA
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182
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This vehicle, the only prototype believed to have been built, was recovered from a factory in Germany after World War II and had not been used in combat. After the war it was taken to the United States where it was thoroughly tested by Vickers of Detroit. Vickers had been working on a similar design for US armoured vehicles, and produced a test report on the vehicle in April 1946.
It can be seen that the original Ausf H chassis used in this vehicle has been extensively modified. The engine compartment is entirely new, it is smaller and does not extend back the full length of the vehicle. Instead it slopes from behind the turret down to the final drive housing, and has three large cast air intakes on either side. Some form of auxiliary motor silencer has been fitted on the left side of the hull. The vehicle has sprockets front and rear, though only the rear ones were powered, and neither type are of a standard PzKpfw IV design...
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