Two-Wheel Drive Harley-Davidson XS
Perhaps one of the rarest Harleys in existence is the Model XS. Only three of these prototype military motorcycles were constructed in 1942, and of those three, only one is known to remain. That single example resides in the Harley-Davidson Archives and was found in original condition with all of the standard military gear still attached.
The XS was a purpose-built sidecar rig designed for possible use in the North African campaign during World War II. Instead of starting with a standard Harley-Davidson WLA, the factory based the XS on another experimental military motorcycle: the Harley-Davidson XA, short for Experimental Army.
The XA was Harley-Davidson's answer to the German BMW R71. It used an enclosed shaft drive and a side-valve flat-twin engine, two features that made it better suited for harsh desert conditions than a conventional chain-driven 45-degree V-twin. The standard WLA was already doing heavy work for the U.S. Army, but sand, heat, and long-distance desert use created problems that the Army wanted to solve.
A shaft drive kept more of the sand and grit out of the final drive, while the XA's opposed-cylinder engine cooled more evenly because both cylinders stuck out into the air stream. The XS took that same basic idea and pushed it even further by adding a powered sidecar wheel.
Mechanically, the XS used the XA engine and drivetrain as its foundation. From there, Harley-Davidson added a drive system that sent power to the sidecar wheel. A transverse shaft and bevel-gear arrangement drove the sidecar wheel, creating a two-wheel-drive motorcycle-and-sidecar combination intended for loose sand and difficult terrain.
That is what makes the XS so unusual. Harley-Davidson built plenty of sidecar motorcycles, commercial trikes, police machines, and military WLAs, but the XS was something different. It was not simply an XA with a sidecar attached. It was a purpose-built military prototype that used the sidecar wheel as part of the drive system.
This is the only time Harley-Davidson ever produced such a two-wheel-drive motorcycle system. It was an experimental answer to a very specific wartime problem: how to move through the deserts of North Africa with more traction and less maintenance than a chain-driven motorcycle outfit.
Unfortunately for Harley-Davidson, neither the XA nor the XS made it into regular production. By the time these prototypes were being tested, the military situation had already changed. The North African campaign was winding down, and the Jeep had become the Army's preferred light cross-country vehicle. Compared to a specialized motorcycle-and-sidecar outfit, the Jeep was more versatile, could carry more cargo, and was already becoming the standard answer for rough military transport.
The XS remained a dead-end prototype, but it is one of the most fascinating branches of the XA story. The standard XA showed how far Harley-Davidson was willing to move away from its traditional chain-drive V-twin formula when the Army asked for a desert motorcycle. The XS shows that the factory was willing to go even further, adding a powered sidecar wheel in an attempt to create a true two-wheel-drive military motorcycle.
Harley-Davidson explored other XA-related ideas as well, including the later XA-powered Servi-Car prototype. None of those projects entered production, but together they show how much experimentation was happening inside the factory during World War II and the immediate postwar years.
Only three were built. Only one is known to survive. For that reason, the Harley-Davidson XS sits in a very small category of factory-built Harley-Davidson prototypes: machines that were not production motorcycles, but still show how far the company was willing to go when the Army needed a different kind of motorcycle.