Harley-Davidson KR

The Harley-Davidson KR, also called the KR750, was a V-twin flathead racer built for flat track competition. Its engine displaced 45.125 cubic inches, and the bike was produced from 1953 through 1969. A faired version, the KRTT, was used in road racing. Upon its debut, the KR swept American motorcycle racing. It was succeeded in 1970 by the XR-750, which went on to a long and victorious career in U.S. racing.

AMA Class C racing was established in 1933. The Great Depression had slashed consumer spending, leading to low race participation and sluggish motorcycle sales. The year before, Harley-Davidson fielded the only factory team, with a single rider, Joe Petrali. He won every AMA national title in 1935, thanks largely to a lack of competition and his own skill. With AMA rider membership falling, the two major factories—Harley-Davidson and Indian—gained greater sway over the organization. They used this influence to create a uniform racing class with a low entry barrier.

Unlike the many incompatible classes in car racing, this new class used the same bikes across multiple disciplines: road racing, quarter-mile (short track), half-mile, and one-mile ovals. Specialized hillclimbing machines and the large 74-cubic-inch open-class bikes used in TT (or TT Steeplechase) racing were excluded. Class C rules aimed to make racing accessible to the public, not just well-funded teams and factories. Homologation rules and bike inspections ensured that amateur riders could buy the same bikes the factory teams used, along with any speed parts from the factory catalog.

The displacement formula allowed side-valve (flathead) engines up to 750cc (45.77 cubic inches). An equivalency rule limited more advanced OHV engines—mostly from UK imports—to 500cc (30.51 cubic inches). From Class C’s start in 1938 until the KR arrived in 1953, Harley-Davidson relied on two models: the 27 hp WLDR, sold with lights and fenders so amateurs could ride it to the track, prepare it, and race; and the race-only 35 hp WR (1941–1952), the KR’s direct ancestor.

Development of the 742 cc Model K street motorcycle began in 1950. UK brands Triumph, Norton, and BSA were entering the U.S. market in greater numbers after World War II, thanks to reduced import duties and Britain’s need to boost exports. The new K and KR adopted technology from these European imports: hydraulic front forks, a rear swingarm, a four-speed transmission, unit construction, and a hand-operated clutch instead of a foot pedal. Alongside the street-going Model K, the KR racer was developed for Class C competition.

Motorcycling writer Kevin Cameron noted that Harley-Davidson was “outside the mainstream of engine development in general, to say nothing of the racing mainstream.” He pointed out that the company’s first production motorcycle with overhead valves (OHV) came one year after the last AMA TT race won by an OHV engine; overhead camshaft (OHC) had already become the only competitive technology in TT racing. American brands Harley-Davidson and Indian favored flathead engines because American riders traveled far longer distances over rougher roads with lower-octane fuel.

They needed low-RPM torque more than the higher-RPM power of OHV engines. Additionally, the cheap Ford Model T had dominated the U.S. car market since World War I, keeping motorcycling a recreational niche. In Europe, motorcycles remained mainstream mass-market transportation, with sales and profits that funded ongoing technological investment. After World War II, lighter, faster British imports poured into the U.S. as the UK desperately sought foreign currency to repay war debts. Harley-Davidson and Indian had to respond quickly to a newly competitive, fast-changing market.

Indian chose to meet competitors on their terms, imitating the OHV parallel-twin design of the British imports. Technical hurdles proved too great, and the new engine couldn’t save the company from its demise in 1953. Harley-Davidson played to its own strengths, creating a pushrod OHV V-twin in 1936 that kept a separate transmission. The new unit construction appeared on the 1952 Model K, which still used a side-valve engine.

Cycle World road-tested a 1963 KRTT alongside a Harley-Davidson Sprint TT racer. They recorded a top speed of 142 mph, 0–60 mph in 5.8 seconds, and a quarter-mile time of 14.1 seconds at 97 mph.

In 1956, every Class C race was won by a Harley-Davidson KR. From 1955 through 1969, KRs took 12 of 15 AMA Daytona 200 national championships. A 1957 KR appeared in the 1999 Guggenheim Museum’s The Art of the Motorcycle exhibition in New York, and a 1963 KR was shown in Las Vegas. The AMA Motorcycle Hall of Fame Classic Bikes collection includes two KRs: a 1959 model, the last bike raced by three-time AMA Grand National Champion Joe Leonard, and Mert Lawwill’s 1969 KR750.