Produced from 1970 through 1985, the Harley-Davidson XR750 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 Harley XR750 continues to win races year after year.
History of the XR750
The flathead-powered Harley KR750, first produced in 1953, was a purpose-built motorcycle to compete in mile, half-mile, and TT racing venues. The KR was powered by Harley's 750cc flathead engine.
In 1934, the American Motorcycle Association (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.
The A.M.A. '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 KR750 now at a disadvantage, Harley-Davidson swapped the side-valve KR motor for a de-stroked XL Sportster engine, and the KR750 became the XR750.
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" ad 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. A magneto mounted to the cam-cover supplied spark.
Weight savings included fiberglass seat/fender and peanut-shaped gas tank. Whereas a stock Sportster had a 1.65" backbone frame, the XR750 frames were a thinner, lighter 1" backbone. The XR750 was sold without front brake or lights, and had high exhaust pipes running across the right-side of the bike. Overall weight was about 320 pounds.
It was the legendary Cal Rayborn who first brought the Harley XR750 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. To make this feat even more impressive, Rayborn had no prior experience on any of these British race tracks.
In its first two years of production, approximately 200 examples were built. The XR750 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 Ironhead XR. Most everything in the motor was gone over and strengthened, including pistons, rocker boxes and rockers. Connecting rods became shorter and lighter. Compression ratio was raised to 10.5:1, allowing the engine to produce over 80 horsepower reliably.
XR750 Alloy Heads
New alloy heads with larger valves and better-shaped combustion chambers replaced the cast-iron XL heads. After being 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 its own 36mm carburetor, with exhaust pipes running on the left-side to clear the carbs. With its lighter weight and stronger engine, the XR750 dominated the flat tracks.
For long road-races, the Harley XR750 was fitted with either a 5 gallon or 6.5 gallon gas tank. A front wind fairing was also used for long circuit venues.
By 1975, the XR750 was no longer capable to compete on the road racing circuit against the lighter and faster two-stroke Japanese bikes. It did, however, remain King of the dirt-tracks for decades to come.
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 eighties. Engine production ended in 1985.
Evel Knievel's Harley XR750
Stuntman Robert Craig Knievel, professionally known as Evel Knievel, rode a Harley XR750 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 a Harley XR750.
On Any Sunday
The 1971 movie "On Any Sunday" follows the lives of motorcycle racers in documentary style. The competition is filmed in several different venues, including mile, half-mile and TT races across the U.S. Among the many riders in the movie is Mert Lawwill with an XR750 flat-track racer. Steve Mcqueen is also featured in the film.
XR750 Replica Build
The best way to build an Harley XR750 replica is to start with a pre-1974 right-hand shift Sportster engine. The closest frame choice (without making your own) would be one from a Harley-Davidson Cafe Racer, but these are far too rare and valuable.
The next bext choice would be to find a 1979 or later Sportster frame. To copy the original bike you would need to de-rake the neck and change the tail end. Both the XR750 seat/tail section and gas tank are still available, and at least one aftermarket manufacturer offers gas tanks and seats designed to be used with the 1986-2003 Evo Sportsters.