Harley-Davidson UL Machine Gun Flathead Prototype
This appears to be Harley-Davidson’s answer to the U.S. military’s interest in mobile machine-gun firepower. The prototype was based on a 1940 Harley-Davidson UL, the 74-inch side-valve big twin commonly referred to as a flathead. Harley’s earlier military Model U work shows the company was already willing to adapt big-twin flatheads for military use, but this armored machine was a very different animal from the smaller 45-inch side-valve motor used in the WLA during World War II.
Mechanically, the motorcycle was still basically standard-issue UL: a 74 cubic-inch side-valve V-twin, a tank-shifted four-speed transmission, and chain final drive. The military idea was not to redesign the engine from scratch. The idea was to take a big Harley flathead, add armor, add a heavy gun carriage, and see if the result could put a Browning machine gun where soldiers needed it.
A 1940 UL Flathead Under Armor
The UL was Harley-Davidson’s big flathead. Compared with the 45-inch WL and WLA, the UL gave the project more displacement and torque to work with, but it also started with a heavier motorcycle before any armor, ammunition, gun mount, or sidecar weight was added. This particular machine is best understood as a prototype rather than a normal production UL variant.
Armor, Visibility, and Airflow
The most striking feature of this prototype is the amount of armor plating Harley added. The rider was protected by a metal fairing with just a small porthole in the front. That might be usable when riding in a straight line on flat ground, but just imagine trying to maneuver over rough terrain while looking through that tiny window.
The front tire was protected by an extended front fender, and another plate protected the front of the motor. That may have looked good from the standpoint of stopping incoming fire, but it was probably not the best design for a motorcycle powered by an air-cooled motor. The UL depended on airflow over the cylinders and heads, and the extra plating would have made cooling and low-speed handling even more questionable.
The Gun Carriage
The gun carriage was a great piece of engineering. Most likely, Harley’s engineers started with the frame from a Package Truck, which had a higher payload than a standard sidecar. This rig would need to carry quite a bit of extra weight, including the gunner, the machine gun, armor plating, and of course lots of ammunition, so the heavy-duty suspension used on Package Trucks made sense.
To that package-truck-style frame, Harley added a custom seat and sidecar-like body, along with the machine gun mount and front armor plating. Notice how the front plate has a curved profile that looks like it was shaped to deflect incoming fire rather than simply stop it head on. It gives the sidecar a very different look from a delivery box, but the load-carrying logic behind it is easy to see.
The Browning M1917 Machine Gun
The machine gun chosen for this prototype was a Browning M1917. As the name would suggest, this heavy machine gun went into production in 1917, which meant it saw action at the very tail end of World War I. The M1917 was a .30-caliber belt-fed gun, and by the time this motorcycle was built it was capable of firing about 600 rounds per minute.
The barrel’s odd shape is due to the water jacket that surrounds the barrel, keeping it cool during extended firing. That was a huge advantage for a fixed machine-gun position, but it also made the gun a lot to ask of a motorcycle sidecar. The gun, water, ammunition, mount, armor, and gunner all added up quickly.
A Mobile Gun Position, Not a Charging Motorcycle
Looking at the armor and forward-mounted gun, it certainly seems like Harley designed this motorcycle to attack the enemy head on. I still don’t think they envisioned it charging across the battlefield with its gun blazing. The rig would have been slow and unwieldy, and there is no surviving documentation showing some clever recoil-management system or deployed firing legs. The practical use would probably have been closer to a mobile firing position: ride it into place, stop, and use the motorcycle as a way to move the gun without making someone lug around the 100-plus-pound M1917.
Of course, if they used the M1917 in the picture below, weight would not have been an issue.
Earlier Machine-Gun Motorcycle Ideas
Harley was not the first company to explore the idea of putting a machine gun on a motorcycle or sidecar. During World War I, motorcycle and sidecar machine-gun units were used by several armies to move men and guns faster than foot infantry could manage. Between the wars, European armored-motorcycle experiments also tried to combine a motorcycle, armor, and a mounted gun into one compact fighting vehicle.
Those comparisons help explain why this UL prototype is so interesting, but they also show the problem with the whole idea. A motorcycle is useful because it is light, fast, and able to go places larger vehicles cannot. Once it is loaded with armor, a gunner, a water-cooled Browning, ammunition, and a reinforced sidecar frame, it starts to lose the very advantages that made the motorcycle attractive in the first place.
Why the Machine Gun Flathead Stayed a Prototype
Unfortunately, this motorcycle never made it into production. Like Harley’s two-wheel-drive XS, the shaft-drive XA, and other experimental military prototypes from this period, it looks like a fascinating answer to a very specific problem, but not a practical machine for widespread service. The armored UL was heavy, the rider’s visibility was poor, the air-cooled engine had more plating around it than it wanted, and the Browning M1917 brought the weight and supply needs of a real heavy machine gun along for the ride.
Meanwhile, the lighter Harley-Davidson WLA became the practical wartime motorcycle. It could handle dispatch, escort, military-police, and general utility work without pretending to be an armored gun platform. That is probably the best way to view this machine gun flathead: not as a failed production model, but as a great example of how far Harley-Davidson and the military were willing to push motorcycle design before the war settled on more practical answers.