Harley-Davidson XR-750 (1970-1985)
With race victories in the thousands, the Harley-Davidson XR-750 is the most successful motorcycle in flat-track racing. It won the A.M.A. Grand National Championship in it's first year out, and has since won dozens more. Although production ended in 1985, the XR-750 continues to win races year after year.
The KR-750, first produced in 1953, was a purpose-built motorcycle to compete in mile, half-mile, and TT racing venues. The KR was powered by a 750cc side-valve (flathead) engine, whose design dated back to the 1920's.
In 1934, the A.M.A. set up rules to have fair and equal competition for all manufacturers, much like today's NASCAR. These rule changes gave flathead-powered motorcycles a 250cc advantage over their overhead-cam counterparts, which pitted the American 750cc flatheads against British 500cc overheads. This 'equivalency formula' remained for three decades until 1969, when new rules allowed any 750cc bike that was offered to the public eligible to race. With the KR-750 at a disadvantage. Harley-Davidson swapped the side-valve KR motor for a de-stroked XLR Sportster engine, and the KR-750 became the XR-750.
Using the bottom end of the Sportster XLR motor, Harley engineers achieved the desired 750cc displacement by decreasing stroke length from 3.81" to 2.983" and increasing the bore from 3.0" to 3.125". The 45-degree pushrod V-twin kept the cast-iron cylinder barrels and heads, with compression ratio set at a moderate 8.5:1. Sporting a peanut-shaped gas tank, the XR was sold without front brake or lights, and had high exhaust pipes running across the right-side of the bike. Overall weight of the XR-750 was about 320 pounds. In its first two years of production, approximately 200 were built.
It was the legendary Cal Rayborn who first brought the XR-750 to fame. In 1972, Rayborn rode an Ironhead XR racer in the British-American "Transatlantic" series. He won three out of the six events, tying British rider Ray Pickrell as the top scorer. Rayborn had no prior experience on any of these British race tracks. The XR ran well in cool weather on shorter tracks, but the cast-iron XL heads were prone to overheating in long events and caused reliability issues.
In 1972, a bigger bore, shorter stroke motor replaced the 750cc iron-head XR. Most everything in the motor was gone over and strengthened, including pistons, rocker boxes and rockers. New connecting rods were shorter and lighter. Compression ratio was raised to 10.5:1, allowing the engine to produce over 80 horsepower reliably.
New alloy heads with larger valves and better-shaped combustion chambers replaced the cast-iron XL heads. Ported and assembled by Jerry Branch of Long Beach, California (Branch Flowmetrics), they were then shipped back to Milwaukee for final assembly. Each cylinder received it's own 36mm Dell'Orto carburetor, with exhaust pipes running on the left-side to clear the carbs. With it's lighter weight and stronger engine, the XR dominated the flat tracks.
As more and more racers began using custom or home-made frames with the XR750 engine, Harley-Davidson stopped manufacturing frames for the XR in the 1980's.
Evel Knievel's Harley-Davidson XR-750
Stuntman Robert Craig Knievel, professionally known as Evel Knievel, rode a Harley XR-750 most of his career. After a succession of other brand motorcycles (350cc Honda, 750cc Norton, 650cc Triumph Bonneville, 750cc Laverda American Eagle), Knievel switched to Harley-Davidson in December of 1970. Sponsorship by the Motor Company lasted until 1977. His longest successful jump of 133 feet (Kings Island 1975) was made on his XR-750.
On December 31st 1967, Knievel's 150-foot jump attempt over Caesar's Palace fountains in Las Vegas ended in a hard crash. He fell off his motorcycle and hit his head on the ground, breaking his pelvis and leg. Evel quickly credited his Bell helmet for saving his life. Knievel was a proponent of motorcycle safety, and always encouraged fans to wear helmets while riding.
Today, one of Evel Knievel's machines, a red, white, and blue XR-750, can be seen in the Smithsonian American History Museum.