Blue Mule: 1947 Harley-Davidson UL Bobber

Speed Style

The need for speed is an age-old quest. 'Blue Mule' is a surviving example of a late 1940s 'bob-job' speed-machine. Now referred to as bobbers, bob-jobs evolved from 'cut downs' —the race-modified Harley-Davidson J Series motorcycles of the late 1920s. Along with the J Model, the stripped-down AMA Class C racing motorcycles of the mid-1930s also served to solidify the style of post-WW2 bobbers.

Ducktail Fender

The bobber design philosophy is a simple one that has stood the test of time. Bobbers got their namesake from having clipped fenders, especially the rear fender where the front portion was sliced away so as to rotate the whole assembly forward. The result is a sort of 'ducktail' —a style that reappeared as a Motor Company standard in the early 1980s.

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Trend Setter

Like hot rods, bobbers surged in popularity after WW2 with returning servicemen looking for another kind of fast action. In the pursuit of speed, bob-jobs were lightened by removing all the unnecessary accessories, adding a single seat, and shortening the fenders. What started as a no-nonsense approach quickly became a custom movement that served as the springboard for chopper trends in the late 1950s and onward.

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Aftermarket Cubes

The beating heart of Blue Mule is an 80-cubic-inch Flathead V-twin with side-valve configuration. Blue Mule's low compression engine, replete with aftermarket pistons and cylinders from S&S, is an upgrade over the standard 74-cubic-inch UL motor of the era. The U-series was introduced in 1937 as a replacement for the V-series —the primary Harley-Davidson 'Big Twin' since 1930.

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Oil Loss

The Harley-Davidson U-series motors featured a recirculating oil system, a vast improvement over the total loss oil systems employed up until 1936. The U-series oil was pumped through the engine and back to a separate oil tank via an oil return line instead of to the ground. Previous systems had used split tanks, with one side as an oil tank and the other a fuel tank.

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Bang Blow

Fuel and air are delivered to finned aluminum alloy heads using a single Linkert butterfly-style carburetor on the primary side. Having the carburetor on the kickstand side produced fuel leaks on the UL models. Learning from experience, Harley-Davidson switched sides on the overhead valve Knucklehead motors. Spent exhaust is expelled via a single fishtail muffler.

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Door Opener

Blue Mule's 80-cubic-inch Flathead is hooked up to a stock 4-speed transmission with a primary chain drive. A hand-shift gate on the left tank holds the shifter lever topped with a crystal doorknob. The clutch is engaged and disengaged with a rocker clutch foot pedal —otherwise known as a 'toe and go' system. A drum-mounted sprocket on the rear wheel applies the engine's rotations to the ground via a drive chain.

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Drum Standard

Non-hydraulic brakes were standard issue technology in the 1940s. The front drum brake is a cable operated system with the lever in the conventional right-hand position. The rear brake is actuated via a series of adjustable rods —a system still used today by many manufacturers. The stylish alterations to Blue Mule's front fender are clearly visible in the above image.

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Hard Roller

The only real suspension on the Blue Mule is the stock Springer front fork. The classic hardtail set up is compensated for by a sprung solo seat on a telescopic tube, which is supported by a pivot between the gas tank and seat. Hard ride or not, this custom 1947 UL speed machine promises thrill rides for any seasoned rider —even seven decades after this Sky Blue UL rolled out of the Milwaukee shop.

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