Motorcycle technology lagged far behind automotive technology since the earliest days of the internal-combustion engine. All that changed in 1936 when Harley-Davidson introduced the Model EL. For the first time ever, a company was manufacturing a high-performance overhead-valve engine for the masses.

And what an engine it was--in addition to bringing state-of-the-art technology to the motorcycling world--a work of art. Because of the shape of its rocker covers, the engine was given a nickname to match its looks: the Knucklehead. The technology used in this engine was so advanced that it laid the foundation for every future Harley-Davidson motorcycle, including the current models built in the 21st century. To this day, every cruiser-style motorcycle adheres to the shape of that original Knucklehead.