For most of the American marque's 119 years in existence, calling any Harley-Davidson a road racer has seemed pretty much a contradiction in terms.
Sure, the Motor Company’s Italian Aermacchi subsidiary won a quartet of 250/350cc GP World titles in the mid-1970s courtesy of Walter Villa, but the same kind of badge-engineering today would have Charles Leclerc racing a Fiat in Formula 1, not a Ferrari, and Aleix Espargarò would be challenging for the MotoGP World title on a Piaggio, not an Aprilia. Until the arrival of the VR1000 Superbike in the 1990s, the involvement of Harley's Milwaukee home base in road-racing was pretty desultory.
Of course, there was a reason for this: they didn't need to. Till then, Harley only did what they called 'pavement racing' whenever they had to, not out of choice, and only once the AMA started running a token handful of such races as part of the Grand National series, as a sop to the growing European participation in US racing (basically, British – Triumph, BSA and Norton). Even then, they kept the dice pretty much loaded in Milwaukee's favour for quite some time, i.e. the Daytona 200 was more dirt than Tarmac until it came off the Beach onto the Speedway in 1961, and the annual Laconia Classic at Loudoun took place on a tight point 'n' squirt track that V-twins weren't disgraced on. And that was about all the road-racing that took place at National level in the USA till about 1970.
Considering those Harley dirt riders who opted not to give the hard stuff a miss and actually tried to score AMA points in such events did so on the very same flathead 45ci motorcycle they used in TT races on the dirt (meaning it had the useful addition of rear suspension and a pair of brakes, compared to dirt oval spec), this was just as well. It meant they weren't discouraged from riding with a foot-down cornering style stubbornly similar to their approved half-mile technique!
In 1969 the AMA changed the rules for US National Championship racing – dirt and/or pavement alike – to allow 750cc OHV engines, which had previously been limited to 500cc (vs 750cc side-valvers, aka KR Harleys). The iron-barrelled XR750 (K = side-valve in Harley numerology, X = overhead-valve) duly made its debut in 1970, but the Sportster-based race engine was chronically unreliable, with severe overheating problems that were frequently terminal. It survived to win a single AMA National road race in the hands of Mark Brelsford at Loudoun in 1971 – and of course Cal Rayborn's brilliant Match Race display in Britain over Easter 1972 was on an iron XR – but it was a flawed design, and H-D management knew it.
Harley knew what had to be done, and legendary race manager Dick O'Brien was the man to do it. With designer Peter Zylstra, he produced an all-new aluminium XR750 race engine which, for 45 years after its 1972 debut, still dominated US Grand National dirt-track racing until 2017, when the Grand National Championship was rebranded as the American Flat Track Championship. During this time, Harley’s XR750 earned no less than 37 AMA Grand National
