Harley-Davidson XR-750
An XR-750, number 16, through the dust at Scioto Downs. Ohio
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. [ 1 ] The XR-750 went on to become the winningest race bike in the history of American Motorcyclist Association (AMA) racing. [ 2 ] [ 3 ] [ 4 ] [ 5 ]
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. [ 6 ] 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. [ 1 ] [ 7 ]
Rule changes obsolete KR racers [ edit ]
2.75USqt (2,600ml)
The AMA Grand National Championship Class C rules, introduced in 1933 and revised in 1954, had an equivalency formula limiting flathead. or sidevalve, engines to 750cc (46cuin) displacement. while more modern overhead valve (OHV) engines could be a maximum of only 500cc (31cuin). [ 8 ] Over time, this displacement advantage kept the older flathead technology on the track and discouraged a broader field of competitors. At least 200 homologated examples of a model had to be built and made available to the public. [ 8 ] The flathead Harley-Davidson KR series had dominated Class C racing, but by the late 1960s BSA. Norton and Triumph had little market for 500cc OHV motorcycles, and there was increasing pressure for a single displacement, without reference to valve configuration. [ 8 ] The public was buying 650cc (40cuin) and larger displacement British bikes, and they would prove to be competitive, given the chance. [ 8 ]
With the British marques gaining influence in the AMA, in 1969 new rules were established that there would be one maximum displacement for dirt track racing, 750cc, with no regard for valve type, though the 500/750 OHV/sidevalve split was kept for the time being in road racing. OHV engines began to dominate racing, in spite of Mert Lawwill ‘s efforts to delay the inevitable on his flathead Harleys, and the KR bikes were a decade out of date and could no longer compete. [ 8 ]
Development [ edit ]
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. [ 8 ] The iron cylinder. aluminum head 748cc (45.6cuin) V-twin four-speed engine of 1970 was based on the mass production Sportster XL dating to 1952, but with modified heads and cylinders, a magneto instead of generator, and improved oiling. [ 8 ] The frame and the running gear were held over from the KRTT racer, with a Ceriani front fork and two Girling rear shocks. [ 8 ] The fuel tank, fenders, and rear seat/fender combination were fiberglass. with a snap down seat cover over a foam cushion. [ 8 ] [ 9 ] 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$ 19,433. [ 9 ]
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. [ 8 ] 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. [ 10 ] 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 straigher path to the rocker arms. [ 11 ]
XRTT road racer [ edit ]
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