The Harley-Davidson XA, for Experimental Army, was a flat-twin shaft-drive motorcycle built for the US Army during the Second World War. The Army wanted something like the BMW R71, about 1,000 XAs were made for evaluation, and the project ended when the Jeep became the Army’s general-purpose vehicle.
Harley-Davidson XA at a glance
| Purpose | Experimental Army motorcycle, 1942 |
|---|---|
| Request | a BMW R71-like machine with a flat twin and shaft drive |
| Engine | air-cooled flat twin, side valves, longitudinal crankshaft |
| Gearbox | four speeds, foot shift, bolted to the engine |
| Final drive | shaft |
| Front suspension | springer fork in 1942, Harley-Davidson’s first telescopic fork in 1943 |
| Rear suspension | plunger-type |
| Frame | steel tube, built to take a sidecar |
| Built | about 1,000, for evaluation only |
| Rival | Indian 841, built to the same request |
History of the Harley-Davidson XA
During the war the US Army asked Harley-Davidson to design a motorcycle much like the BMW R71 used by German forces, with shaft drive, a boxer engine and the other features that made the BMW reliable and easy to maintain. Harley-Davidson was already building the WLA with its traditional 45-degree V-twin, but the Army wanted the one thing the WLA did not have, shaft drive. The company therefore produced the XA, whose engine and drivetrain followed the side-valve BMW R71 of 1938 to 1941 rather than the overhead-valve R75. The War Department had asked the leading American makers in 1941 for a heavy military motorcycle with shaft drive; Harley-Davidson drew up the XA and Indian presented the 841.
About 1,000 XAs were built for evaluation. By the time production began it was clear that the Jeep was the Army’s general-purpose vehicle of choice, no further XAs were ordered, and the less advanced but cheaper WLA was considered sufficient for the limited roles left to motorcycles. In 1943 the leading-link fork of the XA was replaced by Harley-Davidson’s first telescopic fork.
The flat-twin layout had real advantages. Harley-Davidson built a new side-valve boxer four-stroke with a longitudinal crankshaft, which gave a low centre of gravity, suited the shaft drive because only one bevel gear was needed at the rear wheel, and balanced better than a V-twin. According to the Motorcycle Hall of Fame Museum, the large cooling fins kept the XA’s oil about 38 degrees Celsius cooler than a standard Harley 45, whose transverse V-twin could not get air to the cylinders so easily. The same BMW R71 became the basis of the Soviet IMZ-Ural and KMZ-Dnepr M72 from 1942 and the Chinese Chang Jiang CJ750 from 1956, though their parts are not interchangeable with the Harley-Davidson. The XA engine was also tried in other roles: an overhead-valve 45-cubic-inch version went to Detroit Wax Paper as a generator set, 49-cubic-inch overhead-valve engines with gearboxes went to Willys-Overland, blower-cooled units were tested for tank generators in 1943, and there was even a Servi-Car prototype called the Model K using the XA engine for the 1946 model year.
Design and features
- Flat twin: a boxer engine with a longitudinal crankshaft, unlike any other Harley-Davidson of the period.
- Shaft drive: the feature the Army wanted and the WLA could not offer.
- Cool running: exposed fins kept oil far cooler than on the transverse V-twin.
- First telescopic fork: fitted to the XA in 1943, before it reached the big twins.
Harley-Davidson XA versions
- XA (1942): the springer-forked production batch of about 1,000.
- XA (1943): the same machine with Harley-Davidson’s first telescopic front fork.
- Experimental engine uses: overhead-valve generator and Willys-Overland versions, and the Model K Servi-Car prototype.
The motorcycle the Army chose instead is described in the article on the WLA, the three-wheeler that nearly received the XA engine in the article on the Servi-Car, and the big twins of the era in the article on the FL. More are in the collection.
Video: Harley-Davidson XA review
Video by KrasMoto on YouTube.
Sources
- Harley-Davidson XA, English Wikipedia: development, production, cooling, related projects
- Harley-Davidson XA, German Wikipedia: Army request, engine and gearbox, chassis
.jpg){kind=link}
