Harley-Davidson XA Experimental Army Motorcycle
During the first half of World War II, BMW produced the R71 motorcycle for the German Wehrmacht, while Harley-Davidson produced the WLA for the U.S. Army. Both motorcycles used side-valve, air-cooled twin-cylinder engines, but the R71 had a couple of distinct design differences when compared to the WLA.
First, the BMW used an enclosed shaft drive instead of an exposed chain to drive the rear wheel. Besides keeping out dirt, sand, and debris, shaft drives do not need constant adjustment and do not wear as quickly as an exposed chain. That made the R71 easier to maintain in the field, which was a major advantage for an army on the move.
The R71 also used opposed cylinders. Since both cylinders stuck out into the air stream, the engine could cool more evenly than Harley-Davidson's 45-degree V-twin, which always had more trouble cooling its rear cylinder. Once fighting moved into the deserts of North Africa, those advantages became important enough that the U.S. Army asked both Harley-Davidson and Indian to develop shaft-driven, desert-ready motorcycles.
Harley-Davidson answered with the XA, short for Experimental Army. The project borrowed heavily from the BMW R71, and the result was unlike anything Harley had put into regular production. Indian also developed a shaft-driven military motorcycle, but instead of copying the BMW flat-twin layout, Indian used a 90-degree longitudinal-crankshaft V-twin, similar in concept to a Moto Guzzi, for its Indian 841 model.
The XA used a 45 cubic inch, 740 cc, side-valve flat-twin engine with the cylinders sticking out on either side of the motorcycle. Power went through a four-speed transmission and an enclosed shaft drive. The basic idea was simple: build a motorcycle that could survive dust, sand, heat, and hard military use with less maintenance than a chain-driven V-twin.
Besides the flat-twin engine and shaft drive, the XA had many other improvements over the WLA. It used a foot-operated gearshift and hand clutch instead of the traditional Harley handshift arrangement. This allowed the rider to keep both hands on the handlebars, which was a major advantage when riding off-road or in sand. The XA also used a four-speed gearbox, larger 4.1-gallon fuel tanks, a larger-capacity battery, a radio-shielded ignition system, and an air cleaner mounted between the cylinders for better protection.
The improved cooling of the flat-twin engine allowed the XA to run much cooler than the WLA. Period sources and later histories commonly repeat the claim that the XA ran about 100 degrees cooler than the V-twin WLA. The lower center of gravity also made the motorcycle easier to handle, especially compared to the taller feel of a conventional Harley military V-twin.
The XA was originally designed with a leading-link front fork in the same general spirit as Harley's military and big-twin front ends of the period. In 1943, Harley-Davidson fitted some XA machines with the company's first telescopic front fork. That detail alone makes the XA historically important, because telescopic forks would later become standard motorcycle equipment, but they were still new territory for Harley-Davidson during the war.
Harley-Davidson received a U.S. Army contract for 1,000 XA motorcycles in 1941 and ultimately produced 1,011 XAs for testing purposes. Some sources cite slightly different totals, usually in the range of roughly 1,000 to 1,023 machines, but the XA was never a regular production motorcycle in the same sense as the WLA. It was built for Army evaluation, not for civilian sale.
Although the XA performed well and met many of the Army's requirements, the military motorcycle program changed direction quickly. After testing both the XA and Indian's 841, the Army decided that the Jeep was a better all-purpose military vehicle for cross-country work. The Army also continued to buy large numbers of WLA motorcycles, which were cheaper, familiar, and already supported in the supply chain.
Several other projects were proposed for the XA motor, including a 1,000-pound mini Jeep called a Peep, a blower-cooled version used to power a tank generator, and a new Model K Servi-Car. The Peep project makes an interesting companion to the XA story because it shows Harley-Davidson experimenting beyond motorcycles as the Army shifted toward small four-wheel-drive vehicles. In the end, the XA engine was not used for any of those projects in production, and Harley-Davidson went back to focusing on 45-degree V-twin engines.
The XA also led to a few related prototypes. Harley-Davidson built the two-wheel-drive Harley-Davidson XS prototype, which was essentially an XA with a powered sidecar wheel. Only three XS prototypes were built, and only one is known to survive today. After the war, Harley also experimented with an XA-powered Servi-Car prototype known as the Model K. That machine used the XA flat-twin engine in a three-wheel commercial chassis, but it never entered production.
The XA was not Harley-Davidson's only wartime experiment with shaft drive. Around the same period, Harley also built the TA shaft-drive Knucklehead trike, another rare military machine that showed how far the factory was willing to move beyond its standard chain-drive V-twin layout during the war.
The XA is one of the strangest and most interesting dead ends in Harley-Davidson history. It was designed to answer a real military problem, borrowed heavily from a German design, introduced features that were unusual for Harley at the time, and then disappeared almost as quickly as it arrived. No civilian version followed, and no later Harley production model used the XA layout. Today, surviving examples are rare, and the parts are difficult to find because the machine had no true civilian counterpart.
Harley-Davidson XA Specifications
| Engine | 45 cu. in. / 740 cc side-valve flat-twin |
|---|---|
| Bore / Stroke | 3.063 in. × 3.063 in. / 77.8 mm × 77.8 mm |
| Compression ratio | 5.7:1 |
| Top speed | 65 mph / 105 km/h |
| Power | 23 hp / 17 kW at 4,600 rpm |
| Ignition type | 6-volt battery and coil, no distributor |
| Transmission | 4-speed foot shift |
| Final drive | Enclosed shaft drive |
| Frame type | Tubular steel double cradle |
| Front suspension | Leading-link fork on early machines; telescopic fork fitted to later 1943 examples |
| Rear suspension | Plunger |
| Brakes | Drum brakes front and rear |
| Tires | 4.00 × 18 |
| Wheelbase | 57.5 in. / 1,460 mm |
| Fuel capacity | 4.1 U.S. gallons / 15.5 liters / 3.4 imperial gallons |
| Oil capacity | 2 U.S. quarts / 1.9 liters |
