Motorcycle: Racers - XR750 Flat -Tracker, 1970 - Present
Date: Sunday, October 31 @ 15:34:48 UTC
Topic: Motorcycle, Motorcycles, Motor Cycle
XR750 Flat -Tracker, 1970 - Present
To many people's eyes — and ears — the XR750 dirt-track machine is the most handsome, the raunchiest and the most purposeful piece of kit ever to rumble out of Milwaukee. Certainly no other Harley-Davidson has enjoyed so much competition success over so many years as the seminal XR. After almost two decades plugging along on the side-valve KR twin, the arrival of an overhead-valve replacement in 1970 must have been keenly anticipated. Yet its debut year was to prove a deep disappointment when Mert Lawwill and Mark Brelsford could manage only a disappointing sixth and seventh in the AMA championship. Just 12 months earlier, Lawwill had won the title on a side-valve KR.
The problem was the new engine's iron cylinders. They were heavy and conducted heat much less efficiently than aluminium components. This in turn meant that the motor's compression ratio had to be drastically reduced - to around 8:1 — if fatal breakdowns were to be avoided. With lumpier pistons, the iron XR had proven fast - but it just couldn't last the distance. In 1970, race boss Dick O'Brien admitted to a lowly 62bhp at 6,200rpm. In June 1971, light alloy replaced iron, and the XR750 hasn't looked back.
On dirt, Mark Brelsford took the XR to the
1972 American Number One in its maiden season. At Easter the same year, the great Cal Rayborn whipped all comers in the annual USA v
Britain road-race series. the heart of the XR is that rarest of a, a short-stroke Harley-Davidson. "Iron-head" XRs used what were tially de-stroked 883cc Sportster es, retaining the road machine's 72 mm bore but with a shorter 82mm
stroke. The 1972 engine's cylinders measured 79.4 x 75.8mm - by comparison, the side-valve KR measured an ultra-long-stroke
70 x 97mm. This, allied to a much stronger crankshaft, permitted more revs as well as a substantially higher 10.5:1 compression ratio made possible by the use of light alloy. The result was a dramatic rise in power and reliability.
Shortly after Rayborn's Transatlantic road-race triumph (which was, ironically, on an iron-head engine), peak power was reputed to be around 80bhp at 8,000rpm. Even so, for many years even the light alloy XR ran dangerously hot — road-racers employed two oil coolers yet were worn out after only 200 miles (320km) — until the internal oil circulation was improved.
At the outset, carburation was by twin 1 1/2in (36mm) Japanese Mikuni instruments (often larger on the road racers). Sparks were provided by a
Fairbanks-Morse magneto, an unreliable system sometimes dubbed "Can't Get Worse" by unhappy riders. The lightweight, high-grade steel frame used Ceriani forks and paired Girling real-shock absorbers. Dry weight was a feather-light 2911b (132kg), although the road racers, with a fairing and brakes, were somewhat heavier. Almost 30 years of detailed development have left the XR engine little changed visually, but without equal for the demands of dirt-track competition. These days, power is around 95bhp at 7,800rpm, with a strong spread from 4,500 to more than 8,000rpm. On one mile (1.61km) ovals, this is good for a top speed of around 130mph (210kph), although it's the way that the big twin finds grip that really sets it apart from the others on the field. True, the XR soon became obsolete as a road racer, but has gone from strength to strength on the dirt tracks of the United States.
In the hands of men like Brelsford,
Gaiy Scott, Jay Springsteen and Scott
Parker, the XR has simply swept away all before it, taking three-quarters of all the subsequent AMA championships.
Engine: ohv V-twin
Capacity: 45.8cu in (750cc)
Power: around 95 blip
Weight: 2901bs (132kg) Top speed: 130 mph (210kph)
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