Forces of Creation

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A keynote of today’s Milwaukee-Eight is to deliver modern function without compromising identity. Despite its four valves per cylinder, low mechanical noise, and models with local liquid-cooling, it remains a 45-degree V-twin with pushrod valve operation, rolling bearings, and plenty of cooling fins. (Jeff Allen /)

The defining engine of ­American motorcycling for more than 100 years? The Harley-Davidson V-twin. The form of this engine we know today is as expressed by the Milwaukee-Eight Big Twin—still a large-displacement 45-degree V-twin turning on rolling bearings and covered with the fins that define air cooling.

Despite these traditional features, which have been retained over many years to satisfy the preferences of those who own and ride them, these machines meet the same standards for noise, emissions, and in-service durability that apply equally to the most recent of designs.

They are the product of 84 years of adaptation of product to ever-changing conditions of use. When the beloved “Knucklehead,” or EL, of 1936 came into being as Harley’s first OHV design, effec­tively doubling available power, many miles of America’s highways were dirt, speeds were low, and journeys were short. In those conditions, cylinders and heads made of iron were adequate to handle the resulting modest heat of operation. Absolute was the need for ruggedness with comfort, dictating a sprung seat, large-section low-pressure tires, and simplicity of construction.

As highways improved and speeds rose, riders could use more power to cover ground more quickly, so the Big Twin’s heads and cylinders became aluminum, just like those of the American combat aircraft engines of World War II—because aluminum transmits heat best. Three-speed transmissions were the reasonable choice in 1936, but as highway speeds rose, more speeds became necessary to combine strong acceleration from a standstill with comfortably moderate engine revs at cruise.

Every design with a long production history accumulates changes in response to new conditions. Harley’s classic XR-750 dirt-tracker showed the reliability value of forging each flywheel’s mainshaft in one piece with it, and this was in time adopted in the Big Twin as well.

The 10 years after World War II established British twins as strong competition. Harley’s reply was the K-model, introduced in 1952. Although its unit construction was a step forward, having side valves rather than OHV softened its performance. As the racing KR-750, however, it continued to win AMA races until 1969. (Jeff Allen/)

How do rolling bearings con­tinue to do the job in 2020, when so many engines of every kind now spin on plain journal bearings? US military jet-engine ball bearings in 1950 lasted only 300 hours, driving an urgent program of improvement that has tremendously elevated the reliability of all products that depend on rolling-bearing support. Today’s rolling bearings don’t look that different from those of 1936 because the changes are invisible, deep in the material and how it is prepared.

In 1957, iron OHV cylinders and heads were added to the basic K bottom end to make the first 883 Sportster. Hotter variants appeared regularly after that. The engines in this line retain the four separate, geared-together cams pioneered in 1929, which allow the two pushrod tubes on each cylinder to be parallel to one another. (Jeff Allen /)

In the 1980s, Harley told us, “We’re killing the noise so we can keep the music.” The noise ­signature of a vehicle can be dissected into all its parts, and Harley attacked those noise sources using modern scientific methods. Gear noise, bearing noise, panel vibrations—the sources of high-fre­quency sounds—were quieted by redesign, by manufacturing changes, or by added damping mate­rial so that more of the 45-degree V-twin’s signature exhaust sound could remain while meeting society’s legal standards for quietness.

This is the original Big Twin, the 61-inch EL, or “Knuckle­head,” of 1936, Harley’s first large OHV production engine. Improved breathing and high compression essentially doubled its power. Note that the pushrod tubes are angled and aimed so all can be driven by a single camshaft­—a feature that has returned in Harley’s latest single-cam Big Twins. (Jeff Allen/)

Old-timers I’ve known reminisce over the tricks they’d learned to get the most from Linkert and later carburetors. Two changes put an end to that. First, the experience of cities such as Los Angeles and London showed that air pollution from vehicles had to be rolled back. And second, if the motorcycle was to reach the greatest number of users, it would have to become as convenient to oper­ate as an automobile. Tinkerers love to tinker, but we all need to reliably get where we are going. Only digital fuel injection—already developed to high reliability by the auto industry—could meet foreseeable exhaust-emissions limits and deliver the turnkey, ­tinker-free performance that 21st-century riders need.

Although purists praise the quality of early Big Twins, the changes made over the years have enabled each successive new model to reliably give more power for longer as America’s highways evolved from dirt farm roads to interstates. Cooler-running aluminum replaced iron in cylinders and heads, to deliver increased durability. (Jeff Allen/)

Anyone who rides or drives in multilane rush-hour traffic knows that 85 mph has become the new cruising speed. Being unable to accelerate with traffic or unable to pass in the time available exposes a rider on a heavy machine to unnecessary danger. That has driven regular increases in engine displacement, and a move from four to five speeds, and then to six. Riders today want minimum engine vibration at cruise because all day in the saddle on a buzzing bike adds up to fatigue. This has made engine balance shafts a ­valuable sales incentive.

This OHV 1200 Sportster engine traces its four-cam valve train to 1929 and its compactness to the unit construction of the postwar side-valve K-model. Along the way, Harley’s hugely successful XR-750 dirt-track racing engine was developed from the Sportster in 1970. Note the layer-cake design of its block heads. (Jeff Allen/)

Will 150-inch V-twins soon define the norm? We don’t know, but Harley’s change in Big Twins from two valves to four per cylinder offers an alternative to displacement. The smaller, lighter paired valves of the Milwaukee-Eight can more reliably follow the cam contours that best combine bottom-end grunt with high torque for passing and on-ramp acceleration at 3,000 to 3,500 rpm. Rather than running out of breath after that, as two-valve engines do, the greater airflow area of four valves reduces torque drop-off after peak.

Here we see how construction of the 1200’s top end in layers allows valve service and/or head removal with the engine in the frame (the elements slide out sideways). Here the rocker covers have been removed to reveal the intake and exhaust rocker arms, which transmit the upward thrust of the pushrods into valve movement. (Jeff Allen/)

The more power we take from an engine, the more waste heat it generates. In the case of four valves per cylinder, this is most intense between the paired exhaust valves—a region that needs better cooling than air and fins alone can now provide. Liquid-cooling now provides this to H-D engines ­designated “Twin-Cooled.”

Crunching yearly production numbers suggests that some 5 million Big Twins have been made, growing with riders and their world at every step. Life is change, so this process of ­adaptation never ends.

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