page_title: Harley-Davidson XR-750
section_title: Development
hierarchical_section_title: Harley-Davidson XR-750 / Development
caption_attribution_description: English: Team Latus Harley-Davidson XR-750 at The One Motorcycle Show, Portland, Oregon.
context_page_description: The Harley-Davidson XR-750 is a racing motorcycle made by Harley-Davidson since 1970, primarily for dirt track racing, but also for road racing in the XRTT variant. The XR-750 was designed in response to a 1969 change in AMA Grand National Championship rules that leveled the playing field for makes other than Harley-Davidson, allowing Japanese and British motorcycles to outperform the previously dominant Harley-Davidson KR race bike. The XR-750 went on to win the most races in the history of American Motorcyclist Association racing.
  The XR-750 is associated with the careers of racers Mark Brelsford, Cal Rayborn, and Jay Springsteen, and was the favorite motorcycle of stunt performer Evel Knievel. Knievel used the bike from December 1970 until his final jump in January 1977. An XR-750 was included in the 1998 The Art of the Motorcycle exhibition, and one of Knievel's bikes is in the Smithsonian's National Museum of American History America on the Move exhibit.
context_section_description: With limited time and money in 1969, Harley-Davidson's racing manager Dick O'Brien and his team used elements of existing designs to put together a new OHV racer, but rather than start from scratch they decided to modify their existing OHV racer: the Sportster-based 900 cc (55 cu in) XLR magneto-equipped race engine with a 3.0000 in (7.620 cm) bore and a 3.8125 in (9.684 cm) stroke. It was too large to be legal, so a modified version was created using the same basic upper end parts (iron heads and cylinders) but with a drastically shorter stroke and shorter connecting rods to reduce the displacement to the 750 cc (46 cu in) legal limit.
   These iron head XR-750s of 1970–71 were prone to overheating (humorously called "waffle irons") as well as having insufficient power. The engine was again re-designed for 1972 with an all-aluminum head and cylinder package with bigger bore and shorter stroke for the same 750cc displacement. The frame and the running gear were held over from the KRTT racer, with a Ceriani front fork and two Girling rear shocks. The fuel tank, fenders, and rear seat/fender combination were fiberglass, with a snap down seat cover over a foam cushion. To comply with AMA homologation rules, two hundred examples were made and could be had upon request at Harley-Davidson dealers, at a price of US$3,200, which today with inflation would be about US$ 21,067.
  Not unlike other Harley-Davidson engines, the unit construction left and right engine cases split vertically, and formed four cavities: a center front crankcase, a center rear gearbox, a right side cavity gearcase  for the timing train, where the four camshafts are housed, and a left cavity for the three row primary drive chain. A row of four camshafts had also been used on the KR racers, inherited from the sidevalve Model WL, and even earlier Model D of 1929. While the single camshaft of other Harley-Davidson designs was cheaper to manufacture, and quieter, four cams allowed better performance, such as greater flexibility in adjusting the cam timing, and the short single camshafts are durable, and give the pushrods a straighter path to the rocker arms.
caption: XR-750 c. 2015: Engine left
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