Friday, November 20, 2015

The Mighty Harley Davidson JDH

1929 Harley-Davidson JDH
Claimed power: 29hp @ 4,000rpm
Top speed: 85mph
Engine: 1,207cc (74.7ci) air-cooled IOE 45-degree V-twin, 3.424in x 4in bore and stroke, 6.5:1 compression ratio
Weight (wet): 408lb (185kg)
Fuel capacity: 4.75gal (18ltr)
Price then/now: $370/$45,000-$65,000

Is the Harley-Davidson JDH the first Superbike? When motorcycle magazines started talking about Superbikes in the late Sixties they were big-bore motorcycles with speed and panache, bikes that broke quarter-mile times and turned heads with equal ease.

The JDH has been the stuff of legends for over 80 years. It weighed about 408 pounds, was powered by a 74-cubic-inch V-twin and was good for 85mph in standard trim — 100mph if you matched the manifold to the cylinder heads and knew how to tune the beast. Based on factory racing designs, a JDH would blow away almost everything else on the road, two wheels or four, when it was introduced in 1928. In the late Twenties, if you had a need for speed, a JDH was the cat’s pajamas.
The bike that became the Harley JDH took shape in the period around World War I, a time when Harley-Davidson’s archrival Indian concentrated on selling bikes to the American expeditionary forces, starving its dealers and leaving the field open for Harley to expand. In 1915, Harley sold over 16,000 motorcycles, mostly 61-cubic-inch twins. Good sales led to improvements in the product. An electric headlight and taillight were offered as an option, as well as Harley’s first 3-speed transmission. The oiling system was improved, and Harley guaranteed its twins would develop 11 whole horsepower.

The valve gear on Harley’s 1915 V-twins was inlet over exhaust, with the intake valve operated by a cam lobe in the crankcase via a long pushrod and an exposed rocker to the valve, which sat atop a valve pocket cast into the cylinder. The exhaust valve, located in the bottom of the valve pocket, was also moved by a cam lobe, but with a much shorter pushrod. This inlet-over-exhaust top end was messy — oil mist got over everything from the exposed valves. Yet it worked, and worked well, and Harley used this system for the next 15 years.