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The Harley-Davidson XA (Experimental Army) was a flat-twin, shaft drive
motorcycle A motorcycle (motorbike, bike, or trike (if three-wheeled)) is a two or three-wheeled motor vehicle Steering, steered by a Motorcycle handlebar, handlebar. Motorcycle design varies greatly to suit a range of different purposes: Long-distance ...
made by
Harley-Davidson Harley-Davidson, Inc. (H-D, or simply Harley) is an American motorcycle manufacturer headquartered in Milwaukee, Wisconsin, United States. Founded in 1903, it is one of two major American motorcycle manufacturers to survive the Great Depre ...
for the US Army during
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...
.


Development and production

During WWII, the U.S. Army asked Harley-Davidson to design a specification of a motorcycle much like the BMWs used by German forces, with
shaft drive A drive shaft, driveshaft, driving shaft, tailshaft (Australian English), propeller shaft (prop shaft), or Cardan shaft (after Girolamo Cardano) is a component for transmitting mechanical power and torque and rotation, usually used to connec ...
, a
boxer engine A flat engine is a piston engine where the cylinders are located on either side of a central crankshaft. Flat engines are also known as horizontally opposed engines, however this is distinct from the less common opposed-piston engine design, wh ...
, and several other features that made the BMWs exceptionally reliable and low-maintenance machines. Harley was already producing the WLA, based on its traditional 45-degree V-twin but the army specifically wanted the one feature that the WLA didn’t have: shaft drive. So the company produced the XA, whose engine and drivetrain were based on the flathead BMW R71 (1938-1941) versus the overhead valve (OHV)
BMW R75 : ''For the 1970s 750 cc motorcycles see BMW R75/5, BMW R75/6, or BMW R75/7'' The BMW R75 is a World War II-era motorcycle and sidecar combination produced by the German company BMW. The BMW R75 stands out by its integral two-wheel drive des ...
(1941-1946). 1000 XAs were produced for evaluation. By the time production had begun, it was clear that the
Jeep Jeep is an American automobile marque, now owned by multi-national corporation Stellantis. Jeep has been part of Chrysler since 1987, when Chrysler acquired the Jeep brand, along with remaining assets, from its previous owner American Motors ...
was the Army's general purpose vehicle of choice. No more XAs were ordered after the initial order of 1000. The less advanced but cheaper WLA was considered sufficient for its limited roles. In 1943, the XA's leading link fork was replaced with Harley-Davidson's first telescopic fork.


Similar motorcycles in other countries

From 1942, the BMW R71 would serve as the basis for the IMZ-Ural and KMZ-Dnepr M72 (U.S.S.R./Russia), and from 1956 the Chang Jiang CJ750 (China) which likewise pressed it into military service though parts should not be considered interchangeable across marques despite visual similarities.


Cooling

According to the American Motorcyclist Association's ''Motorcycle Hall of Fame Museum'', regarding the World-War-II-vintage Harley-Davidson XA, which has an opposed twin engine — ''"Mechanically, the large cooling fins stuck straight out in the breeze, reportedly keeping the XA’s oil temperature cooler than a standard Harley 45."'' The latter was a transversely mounted V-twin, in which the air stream cannot reach the cylinders, especially the rear cylinder, as efficiently as on longitudinally mounted flat twin. The use of "transverse" and "longitudinal" here follows motor car convention based on crankshaft alignment. However, with motorcycles the convention is based on width - hence cylinder orientation. Therefore, a flat or v-twin with left and right cylinders is transverse but with front and rear cylinders it is longitudinal. File:1942 Harley-Davidson XA.jpg, 1942 Harley-Davidson XA on display at the
California Automobile Museum California Automobile Museum is an automobile museum located in Sacramento, California. It has a collection of over 150 classic cars, race cars, muscle cars and early models displayed throughout of museum space. The mission of the California A ...
File:Harley-Davidson XA engine.jpg, Harley-Davidson XA engine


Related projects

Though the XA motorcycle was "dead" as far as the military was concerned for full-blown production, both the side-valve and overhead-valve versions of the XA engine were explored according to Jerry Hatfield in his book "Inside Harley-Davidson" 1990 published by Motorbooks International. A overhead-valve version XA motor was sent to Detroit Wax Paper for testing as a generator set. OHV XA motors of , with transmissions, were sent to
Willys-Overland Willys (pronounced , "Willis" ) was a brand name used by Willys–Overland Motors, an American automobile company, founded by John North Willys. It was best known for its design and production of World War II era and later military jeeps (MBs) ...
for testing as well as two blower-cooled XA motors for generators to be tested on tanks in 1943. There was even a Servi-Car prototype made called "The Model K" using the XA engine instead of the G motor planned for the 1946 model year.


See also

*
BMW R75 : ''For the 1970s 750 cc motorcycles see BMW R75/5, BMW R75/6, or BMW R75/7'' The BMW R75 is a World War II-era motorcycle and sidecar combination produced by the German company BMW. The BMW R75 stands out by its integral two-wheel drive des ...
*
Indian 841 The Indian 841 was a motorcycle designed by the Indian Manufacturing Co. for desert warfare. It pioneered the drivetrain configuration later popularized by Moto Guzzi, having a longitudinally mounted air-cooled 90-degree V-twin with shaft drive ...
*
G-numbers This is the Group G series List of the United States military vehicles by (Ordnance) supply catalog designation, — ''one'' of the alpha-numeric "Standard Nomenclature Lists" (SNL) that were part of the overall List of the United States Army w ...
*
List of motorcycles of the 1940s This a listing of motorcycles of the 1940s, including those on sale, introduced, or otherwise relevant in this period. Motorcycle * Acme motorcycle (1939–49) * AJS 18 *AJS 7R * AJS Model 16 * AJS Model 20 *AJS Porcupine *Ariel Red Hunter *A ...


References


External links


''Army Tests Experimental Shaft Driven Motorcycles''
September 1942, ''Popular Science'' {{DEFAULTSORT:Harley-Davidson Xa XA Motorcycles powered by flat engines Military motorcycles Motorcycles introduced in the 1940s