Superbike of the Roaring Twenties: The 1929 Harley-Davidson JDH
With a racing pedigree, the JDH is a rare speedmachine from a bygone era.
The JDH is a race bike turned street machine. Powered by a detuned factory race engine housed in a standard JD chassis, the JDH is arguably the first ever 'superbike.' Harley-Davidson produced the JDH for two years from 1928 to 1929, and in very limited quantities. It is believed that there are less than 1,500 authentic examples — most in museums and private collections.
Introduced in 1915, the J Model was a thoroughly modern motorcycle. As Harley-Davidson's most expensive machine, it featured a 61 cubic-inch F-head engine, three-speed transmission and electric lighting. The 74 cubic-inch JD was launched in 1920. While the JD proved popular, enthusiasts craved the elusive race-only dual cam engine. It remained out of reach for almost a decade, until 1928.
The JDH had an important and lasting effect. Starting in the mid-1920s, Harley-Davidson pushed the idea of nation-wide motorcycle clubs and encouraged dealers to sponsor these clubs. Active clubs meant increased sales, especially for the sport models. Yet, enthusiasts still preferred the Indian 101 Scouts and Excelsior Super X. This bias led Harley-Davidson to release the dual cam JH and JDH.
Harley-Davidson began developing the dual cam racing engine in 1919. The idea was to have two cam gears, with two lobes per shaft, instead of all four cam lobes mounted on one gear shaft. The dual cam engines could rev higher and made for a fast and reliable race engine. The 74 cubic-inch JDH cylinders are identifiable by the continuous fins around the exhaust valve area, which increased cooling.
The race engine was available as the JH 61-cubic-inch and JDH 74-cubic-inch version. The dual cam setup acted through tappets to actuate the overhead intake valve pushrod and the exhaust valve. The intake valves had double valve springs and the springs on the exhaust valves were uncovered for increased cooling. Lifter blocks on top of the right case helped route oil back to the crankcase. The pistons were domed magnesium alloy producing a compression ratio of 6.5:1.
The J Models were offered in a sports package, called the Sport Solo. These machines featured a narrowed gas tank and eighteen-inch wheels. The package also included shorter handlebars that were more like the bars on modern bikes and less like the wheelbarrow-like bars on earlier motorcycles. The Sport Solo package was applied to many of the dual cam JH and JDHs.
Along with the hot motors, Harley-Davidson wisely introduced the front brake in 1928. As a unique feature, 1929 J Models were factory equipped with distinctive dual headlights. The catalog boasted this feature among other improvements as, “Two bullet headlights for safer night riding, generator output controller, new electrical panel with a built-in ammeter, clear-the-way horn, and the silent, but efficient, four tube muffler.”
The JDH was not a cheap motorcycle and a tuned JDH could not run on the low-octane fuel available at most pumps. JDH owners had to purchase or concoct, expensive high-octane fuel. All Harley-Davidsons of the era were painted in army surplus olive drab, but for a price, dealers offered optional colors. Despite the costs, the JDH was worth it and stands at the apex of a truly exciting and 'roaring' decade.
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