The origins of the current Harley-Davidson XR-750 date back to the Great Depression. Until the early 1930s, the fastest, factory-backed motorcycles ridden by pro riders were single-cylinder 30.50 cu. in. Class A machines. Those bikes were too expensive to race in the Depression, however. Even the big factories like Harley-Davidson and Indian dramatically cut back support for Class A.
Faced with the need to either introduce new rules to control costs, or watch fields wither and die, the AMA Competition Committee published new Class C rules in 1933. Those rules allowed production-based 45 cu. in. side-valve twins to race against 30.50 cu. in. overhead-valve bikes, provided those machines had compression ratios of less than 7.5:1.
For the next 30 years, the governing body of American motorcycle racing pretty much unabashedly maintained rules that favored American-made motorcycles, or at least allowed U.S. manufacturers to field competitive machines with minimal research and development costs. By the 1960s, however, America's motorcycle landscape was changing while the rules stayed the same. Class C competition was by then a straight us-versus-all-the-rest-of-them fight in which 750cc, V-twin side-valve Harley-Davidson KRs raced against 500cc or smaller overhead-valve singles and twins imported from Britain, Europe and, increasingly, Japan.
|This minimalist KR from the late '50s betrays some creative drillwork. Every ounce counts!|
By 1967 (a boom year for U.S. motorcycle sales) Honda commanded a 50% market share, but there was still no Japanese representation on the MS&ATA (by then the 'Motorcycle and Allied Trades Association had somehow become the 'Motor Scooter and Allied Trades Association'. No matter what name it went by, it was the industry's main voice in the AMA.
The major Japanese importers formed their own trade association, the Southern California Motorcycle Safety Council. The name was innocuous, but when it attracted the interest of the British importers, too, the AMA realized that if it didn't level the playing field by changing the eligibility rules for Class C, the SCMSC would probably start sanctioning its own races.
The AMA Competition Congress met in the fall of 1968, to propose new Class C rules for the upcoming season. The 'import' bloc initially proposed raising the displacement limit for OHV motors to 650cc. Walter Davidson then countered with a motion proposing a limit of 750cc. This would seem to have been against Harley-Davidson's interests, but his thinking was that it would be easier for Harley-Davidson to develop a 750cc OHV race motor based on the existing KR design than create a new 650cc twin from scratch. That was put to a vote, and I can picture the 'import' delegates scratching their heads as they agreed to it. Then, Davidson immediately moved to delay the implementation of the rules until 1970 in order to give The Motor Company time to field a new bike. That was voted down. Those revised Class C rules still form the basis of the rules which now define racing motors in the GNC Twins class.
Walter Davidson might have envisioned a new OHV version of the KR (a production racer that had been made in both a flat track and KRTT road-racing versions for almost 20 years at that point.) But in fact, Harley's OHV racing twin was based on the 883cc Sportster street bike. Dick O'Brien, who ran the Harley race shop as a sort of personal fiefdom, and Pieter Zylstra (a H-D engineer) destroked a Sportster motor, producing an 'iron-head' XR for the 1970 season.
It was not, by any means, an overnight success. The top end couldn't dissipate the heat produced in racing. But Harley-Davidson cast alloy heads and made cylinders with a larger finned area. The alloy XR was a potent weapon on flat tracks, and the XRTT road-racing version was good enough for Mark Brelsford to win the (then combined) AMA Grand National Championship in 1972.
Harley-Davidson produced complete XR race bikes in limited quantities until 1980. By then, independent frame-builders were producing better chassis, and the factory saw no point in making and selling frames that weren't being used. In the late '80s, Harley stopped assembling XR motors, since by that time when teams took delivery of a new motor, they took it apart and tweaked it anyway.
|By the mid-'70s, the modern XR-750 had (twin shock) rear suspension and a rear brake, to go with those newfangled overhead valves. Its teething problems over, it would go on to dominate the GNC for 30+ years.|
Except for the few years in which Honda created its own version of the XR - and an even-shorter period in which a few Japanese manufacturers fielded flat track bikes powered by two-stroke road racing motors - the XR has dominated GNC flat track racing ever since. It's pretty much accepted wisdom that the same uneven firing order that gives the Harley its characteristic 'potato-potato' sound at idle gives it a natural 'big bang' firing order that helps it to hook up on a dirt track.
In future Blue Groove installments, I'm going to check in with some respected XR tuners to find out what other advantages the XR offers, and what has to be done to minimize some of its disadvantages - not the least of which is the rule specifying 32mm restrictor plates.