Daily Slideshow: The Evolution of Harley Flat Track Racers - KR750, XR-750, XG750

1. KR750

A 741.68cc side valve (or flathead) 45°, air-cooled, V-twin engine powered the Harley-Davidson KR750, which produced 50-57 hp and gave the 320 lb motorcycle to a top speed of 125 mph - pretty quick for a motorcycle of that time. Creature comforts were minimal so, while there were front telescopic forks, there were no brakes and only later versions had rear suspension. It was produced from 1952 until 1969 and, although its technology was considered outdated even for the time, it dominated racing in the United States. This was, in part, due regulations in AMA Class C racing which limited OHV engines to 500cc and side valve engines to 750cc, allowing the KR750 to stay competitive against more advanced European motorcycles. It was derived from the Model K street motorcycle, which was introduced due to increased competition from UK brands such as Triumph, Norton, and BSA. Technology from these European competitors was used on the K and KR, such as hydraulic front fork suspension, four-speed transmission, and a hand, rather than a foot, operated clutch control. 

From 1938 until the KR750 was introduced in 1952, Harley-Davidson competed using the WLDR, which produced 27 hp and had lights and fenders. So, the riders could ride it to the track, race, and then take it home. 

>>Join the conversation about the KR750, XR750, and the XG750 right here in the forum.

2. A Big Success

The KR750 was a success on the track right from the start. In fact, in 1956 the KR750 won every Class C race. And, if that isn't impressive enough, 12 out of the 15 AMA Datoyna Championships between 1955 and 1969 were won by KR750s. These sensational statistics didn't go unnoticed - there are two KR750s in the AMA Motorcycle Hall of Fame Classic Bikes collection, one 1959 and a 1969 model.

>>Join the conversation about the KR750, XR750, and the XG750 right here in the forum.

3. XR-750

The Harley-Davidson XR-750 was produced from 1970-1985 and went on to be, arguably, Harley's most famous flat track racer. Although the XR-750 ceased production in 1985, it went on to win races for many years to come. The XR-750's 748cc 45°, air-cooled, V-twin engine produced 82 hp, which propelled the 295 lb motorcycle to a top speed of 115 mph. There were Ceriani telescopic forks up front and twin Girling rear shocks, while a rear disk brake provided the stopping power. The XR-750 was developed in response to a change in AMA Class C racing rules, which stated that, regardless of valve type, 750cc would now be the new maximum displacement for dirt track racing. This meant that the KR750, which by this time was nearly a decade out of date, could no longer be competitive. Although Harley's racing manager, Dick O'Brien, and his team had limited finances and time, they created one of the most successful Harley-Davidsons in history.

>>Join the conversation about the KR750, XR750, and the XG750 right here in the forum.

4. An Evil Machine

To be competitive Harley needed a flat track racer with an OHV engine, but rather than develop a brand new engine, they developed their existing OHV racing engine - the Sportster-based 900cc XLR magneto-equipped race engine. This engine was modified to meet the 750cc limit by shortening the stroke and using shorter connecting rods. It proved to be a great decision, as the XR-750 went on to win more races than any other motorcycle in AMA racing. The XR-750 found more success off the track when Evel Knievel started jumping it in 1970. He set the world record for the longest motorcycle jump in history when he jumped 14 greyhound buses for a total distance of 133 feet. This record stood for almost 25 years.

>>Join the conversation about the KR750, XR750, and the XG750 right here in the forum.

5. XG750

The XG750 is Harley-Davidson's latest flat track racer and, not only is it the nicest looking flat track racer they've ever produced, it's arguably one of the nicest Harleys ever. It bears a resemblance to the street legal XR 1200 and the old XR-750 but, being the first brand new flat track racer released by Harley-Davidson in 44 years, it's a different animal entirely. The XG750 is powered by race-modified Revolution x 60°, liquid-cooled, fuel injected, V-twin, rather than the 45° air cooled engine Harley is most famous for. This engine was originally created for the Street 750. A steel tube frame cradles the engine, Showa suspension provides the dampening and a rear disk brake brings the XG750 to a halt. Performance figures are unknown but they're expected to be up considerably on the production model's 56 hp. The XG750 should have no problem picking up speed, weighing just 300 lb, which is the legal limit for the GNC1 class.

>>Join the conversation about the KR750, XR750, and the XG750 right here in the forum.

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