Why Harley-davidson Used Four Camshafts For Pushrod V-twins

The Evolution Big Twin-engined motorcycles for illustration Fat Boys and Road Kings spent 18 years utilizing a azygous four-lobed camshaft, while nan Sportster's Evolution centrifugal — and its ancestors for illustration nan K-model and Flathead motor bid — utilized a radically different cam architecture. They utilized 4 individual camshafts, each pinch its ain gear, meshed together and spinning successful a synchronized movement. To nan uninitiated, nan full setup feels unnecessarily overengineered. But successful nan world of pushrod-driven engines, 4 cams were a necessity to execute cleanable valve alignment and stableness astatine precocious rpm. This quad-cam obsession dates backmost to nan precocious 1920s, erstwhile Harley-Davidson engineers were looking to compete pinch nan blase overhead-valve single-cylinder engines coming from Europe. (Even Indian jumped connected nan action, heading to England to find retired much and helping to substance nan Harley vs Indian rivalry.)

By giving each valve its ain dedicated camshaft located straight beneath nan lifter, engineers could execute thing valuable successful nan world of pushrod engines: straight-line geometry. Pushrods thief transmit nan cam mobility to nan lifter, moving nan valve. In a modular azygous Evolution Big Twin V-twin, nan pushrods person to snake up from a cardinal constituent astatine awkward angles to scope nan rocker arms. This creates side-loading connected nan lifters. By moving nan cams outward and giving each valve its ain pushrod, Harley engineers achieved parallel pushrod geometry. This allowed nan rods to move successful a perfectly vertical plane, allowing nan motor to rev higher and much reliably than its single-cam siblings.

To understand why Harley-Davidson stood pinch this creation for astir a century, we request to understand valve perspective geometry. In a high-performance V-twin, nan extremity is to make nan intake and exhaust valves arsenic ample arsenic imaginable while keeping nan combustion enclosure efficient. This often results successful a "hemispherical" aliases steeply-angled valve arrangement. When you person a azygous camshaft placed heavy successful nan "V" conception of nan crankcase, nan pushrods are forced to recreation astatine utmost angles to meet those valves. At precocious rpm, these angled pushrods tin messiness up valvetrain lash and affect valve timing accuracy. By utilizing 4 cams, Harley engineers were capable to position nan cam lobes straight nether nan lifters for each cylinder. This allowed nan power from nan cam lobe to beryllium transferred successful a direct, consecutive statement to nan rocker arm.

This layout gave engineers much precise power complete valve actuation timing. Since 4 cams dispersed nan load better, you tin comfortably usage high-lift valves for your Sportster's title engine. As each cam is an independent unit, a tuner tin theoretically alteration nan cam timing connected nan rear cylinder without rubbing nan timing connected nan beforehand cylinder. Additionally, the four-cam creation utilized aircraft-style roller tappets, likewise to radial engines. These rollers nutrient little clash than nan flat-tappet designs. When you harvester straight-line pushrod geometry pinch low-friction roller tappets and independent cam timing, you get an motor that's reliable and highly capable.

The modulation from nan flathead "K-Model" to nan overhead-valve Sportster successful 1957 was an important validation of Harley-Davidson's four-cam theory. While nan Big Twin engines for illustration nan Knucklehead, Panhead, and Shovelhead were designed for torque and long-distance cruising, nan Sportster was designed for sporty riding. The four-cam architecture allowed for a overmuch narrower motor profile that made it much commuter-friendly, which was important for a motorcycle designed to return connected nan likes of Triumph and Norton. 

The modern Harley-Davidson Sportster S is simply a paradigm shift now usies a liquid-cooled overhead-cam Revolution Max engine. In fact, Harley arsenic a full has moved successful a very different guidance from nan Sportster's gear-driven, four-cam architecture. Take nan Milwaukee Eight motor arsenic an example, which is currently the astir precocious air-cooled Harley motor to date. It uses a chain-driven azygous camshaft that moves 4 valves connected each cylinder acknowledgment to nan usage of 4 pushrods driven by 4 chopped cam lobes. In nan end, nan early for these engines is much refined and businesslike — but it arguably isn't arsenic cool arsenic a four-cam Sportster motor.