The Knucklehead’s roots date back to the summer of 1931, when Harley-Davidson began developing a new approach to its power plant, seeking to produce a cleaner-running, more efficient modern motorcycle. More flowing design elements were introduced into Harley-Davidson’s lineup in 1925 and had become an industry priority in the cash-strapped 1930s when styling and paint helped carry the load that performance R&D once carried. Though its tried and true inlet-over-exhaust “F-head” V-twin platform served the company well for a couple of decades, the accessible and sturdy side-valve or “Flathead” engine became Harley-Davidson’s standard platform by 1930. The new prototype was built around a 65 CI side-valve V-twin, with a twin down tube, double-loop frame, lower seating position, instrument panel on the tank and refined Air-Flo design. Though reliable and fast, side-valve engines suffered from inefficiencies, and The Motor Company’s experience transitioning its lineup in 1930 hadn’t exactly gone seamlessly. Overhead-valve technology was nothing new by 1931; both Harley-Davidson and its long-time rival Indian had success with OHV engines racing dating back to the 1910s. Harley-Davidson’s legendary Wrecking Crew of the early- 1920s dominated the sport onboard the brilliant OHV designs of William Ottaway. Still, very few companies had found commercial success in offering an OHV engine in a consumer motorcycle. So, in late 1931, Harley-Davidson pivoted from its initial approach, “IF THERE WAS EVER A SINGLE MACHINE THAT TRULY CAPTURED THE LIFESTYLE OF MOTORCYCLING, THE HEART OF THE HARLEY-DAVIDSON BRAND, IT WOULD BE THE KNUCKLEHEAD.” dropping the 65 CI side-valve to develop an all-new, road-ready 61 CI OHV. Slated for release in 1935 but delayed by Depression Era logistics and limitations, the “Sixty-One” Model E was only previewed at the annual dealer convention that November, with the first production not leaving Milwaukee until 1936. Its heart is a 61 CI (989cc) overhead-valve V-twin with hemispherical combustion chambers, efficient single camshaft, dry-sump (circulating) oil system and distinctive rocker boxes, the source of its Knucklehead moniker. The 40 HP EL engine was torquey and fast and, once mated by a duplex chain primary to Harley-Davidson’s new 4-speed constant mesh transmission, could hit nearly 100 MPH from the factory. A double-downtube, truss-reinforced frame with heavy castings boasted an all-time low seat height of just 26 inches and was joined with a new tubular and strengthened fork design. The refined, streamlined tank and fenders complemented the new Art Deco lines of the EL, while modern features like the mechanically actuated ammeter and oil pressure gauges updated the lighted dash. Harley-Davidson’s proven standard equipment, like the hand- shift and rocker foot clutch, crash bars, toolbox, jiffy stand, hinged rear fender, sprung seat post and ride control damper completed the Knucklehead package, setting a standard in performance and design that defined the very concept of a modern cruising motorcycle. MECUM.COM // 81
