A Krauss-Helmholtz bogie (Krauss-Helmholtz-Lenkgestell) is a mechanism used on a steam locomotive, where a carrying axle is connected to a coupled axle via a lever such that when the carrying axle swings to the side on going round a curve, it causes the coupled axle to move sideways in the opposite direction. In this way the radial forces during curve running are more or less evened out on both axles, so that riding qualities similar to those of a normal bogie are achieved and wear and tear reduced on wheel flanges and rails. The bogie is a type of pony truck and was named after the locomotive firm of Krauss and the engineer, Richard von Helmholtz.
By contrast a Bissel bogie is independently installed in the frame, and sideways guidance of the locomotive is achieved by elastic forces. The distribution of these forces is not tightly defined and, in addition, they are dependent on the curve radius.
Because the advantages of a pony truck come into play particularly on tight curves, the Krauss-Helmholtz bogie initially appeared on branch line, Lokalbahn and narrow gauge locomotives. One of the first locomotives of this type was the Bavarian D VIII. On this tank locomotive the bogie was located at the rear; however in the majority of cases it was at the front or - if the locomotive had to have equally good riding qualities in both directions - at both ends.
Later this pony truck arrangement was also adopted by the DRG standard locomotives (Einheitslokomotive) of the Deutsche Reichsbahn, e.g. on the ten-coupled classes: 44, 45, 50 and 85. An exception was the Class 84, that was fitted with Schwartzkopff-Eckhardt II bogies or Luttermöller axle drives.
The tender locomotives of classes 41 and 45 only had a Krauss-Helmholtz bogie at the front; the trailing axle was housed in a Bissel bogie. The tank locomotives of Class 85, like some of the Class 64 and 86 engines, had two Krauss-Helmholtz bogies.
Even the electric locomotives of Reichsbahn classes E 04, E 17, E 18 and E 19 were fitted with comparable pony trucks, known as AEG frames (AEG-Gestell). Because the axles had external bearings, the lever linkage also had to be on the outside, a characteristic detail of these locomotives.