AMA C Class Racing
In the 1920s and early 1930s the American factories spent small fortunes racing highly developed prototype machines. To reduce the cost of racing for ordinary riders and to try and level the playing field, the American Motorcycle Association (AMA) introduced Class C racing in 1933, based on production motorcycles. As the Great Depression hit home and racing budgets were slashed, the factories moved into C class racing which became the premier US racing class, and fuelled an intense rivalry between HarleyDavidson and Indian. C class machines were production based sidevalve engines with a maximum displacement of 750cc, or overhead valve engines with a maximum displacement of 500cc. The rules were amended in the early 1950s banning overhead camshaft machines, after Norton successfully raced Manx Inters at Daytona from 1948 to 1951. The AMA Grand National Championship series featured four different types of races; the Mile and Half-mile held on oval dirt tracks, TT (turning track) also on dirt but with bends and jumps, and circuit racing on sealed courses. After the failure of Indian motorcycles, the Harley-Davidson factory dominated the series until the early 1960s. British manufacturers then got into their stride, with BSA and Triumph winning most of the national titles from 1963 to 1970. In 1969 new regulations were introduced allowing 750cc overhead valve engines. KR Series Harley Davidson Harley-Davidson introduced the ‘middleweight’ K series in 1952 to counter the growing popularity of the lighter and faster British motorcycles. The 750cc side-valve K-series engine was based on the bottom end of the previous WL engines, with the same bore and stroke (2-3/4” x 3-13/16”), compression ratio of 6.5;1 and aluminium cylinder heads. However the rest of the machine was a radical departure from its predecessor; the new unit construction crankcase housing a 4 speed, right hand foot change, gearbox and primary transmission. The all-new frame had hydraulically damped swinging arm suspension and hydraulically damped telescopic front forks. Producing 30-horsepower in a bike weighing 400 pounds, first-year K-models were not much faster than Harley’s 600-pound Panhead. Harley-Davidson quickly introduced the KR and KRTT racing models to replace the ageing C Class WR racers. The factory KR dirt track racer had a bolt-on rigid frame which allowed riders to switch back and forth for different types of track competition. The KRTT used a version of the new road going K series frame and cycle parts. Besides high-performance engine parts, hubs, rims, brakes and six gallon tank were available as factory racing parts. The factory also produced two further KR variants; the KRM, a desert racer that was only made for a couple of years and the KHR an 883cc version for use in some events that allowed up to 900cc side-valves. The road-going K series was superseded by the OHV 888cc Sportster in 1957, but production of the race-only KR-models continued until 1969. The race department built small batches of machines each year, keeping some for themselves and selling the rest to sponsored riders and dealerships. There was always a shortage of new machines, so how Alf Groves in New Zealand was allocated a new machine is a real mystery.
Over its eighteen year factory racing career the KR engine was gradually developed, with improvements being made available to riders each year to keep pace with the smaller OHV opposition. Dick O’Brien, head of the Harley-Davidson racing shop, recruited tuner and camshaft expert Tom Sifton, gas-flow guru Jerry Branch, and others to help develop the engine out of all recognition. The factory even developed different camshafts to suit each of the racing disciplines. The factory also issued a handbook detailing how to bring the machine up to the latest standard and recommendations on how to set up the machine for each of the disciplines.
The final factory version of the KRTT 750 for 1968/69 had a new “lowboy” road racing frame and twin carburettor conversion. Dick O’Brien has stated the greatest power they ever achieved was 58bhp, despite wild rumours of much larger outputs. In 1964 the AMA lifted a ban on fairings in road racing, and the factory eventually produced an excellent road racing fairing and streamlined seat for the lowboy frame that added 6mph to the top speed over the previous fairing. The new fairing and seat were painted in the distinctive Harley-Davidson orange, black and white colour scheme that is still used to this day. The KR 750 was superseded by the XR750 in 1970 but the KRTT remained competitive in road racing whilst the early iron engine XR750s were redesigned with better aluminium heads and barrels. Cal Rayborn rode an outdated KR 750 “lowboy” in the 1972 Trans-Atlantic Match Races against some of the finest riders in the world on their latest factory machines. With no experience of the English tracks Cal won three of the six rounds and tied for the overall title.