Motorcycle: Racers - RR250, RR350 1971 - 1976
Date: Sunday, October 31 @ 15:31:03 UTC
Topic: Motorcycle, Motorcycles, Motor Cycle
RR250, RR350 1971 - 1976
The twin cylinder, two-stroke racers built at Varese during the 1970s brought Harley-Davidson its sole successes in post-war grand prix road racing. These were state-of-the-art machines, broadly similar to the stroker twins produced by Yamaha and Kawasaki, with fierce powerbands, strident exhaust notes and performance belying their relatively small engine displacement. To cope with the narrow band of usable revs, the gearbox held six speeds - and might well have held more if this were not prohibited by FIM regulations.
The 250's 56.2 x 50mm cylinders were fed via rotary disc valves by twin 34mm Mikuni carburettors. The earliest examples were air-cooled but the factory soon switched to liquid-cooling, bringing far greater temperature stability and reliability during long grand prix races. Primary drive was by gear, to a multi-plate dry clutch and thence to the six-speed "box". Power output was prodigious (particularly by Milwaukee's four-stroke standards), with the 250 producing 58bhp at 12,000rpm and the 3.50 a dozen horsepower more at 11,400rpm. The 250's figure equates to over 232bhp per litre, compared to a mere 65 for the KR750 which had been Harley-Davidson's racing mainstay at the time RR250 development began. The chassis featured a conventional twin cradle of high-grade tubular steel. Italian Ceriani suspension graced the front end with a pair of English Girling shock absorbers at the rear. The whole package weighed just 2301bs (104kg). If the machine had a shortcoming, it was the use of drum brakes (albeit a double-sided twin-leading shoe affair at the front) when hydraulic discs were already in widespread use and far superior. Discs finally arrived in 1976, by which time the great Walter Villa had already taken two 250cc world titles.
With better brakes, he repeated the feat in 1976, adding the 350cc title for good measure. RRs also campaigned successfully in American road races, ridden by Gary Scott, Jay Springsteen, Cal Rayborn and others.
Engine: liquid-cooled twin cylinder 2-stroke
Capacity: 15. leu in (248cc)
Peak power: 58bhp @ 12,000rpm
Weight: 2301bs (104kg)
Top speed: 140mph (225kph)
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