Motorcycle: Twins - Early Twins
Date: Sunday, October 31 @ 23:03:22 UTC
Topic: Motorcycle, Motorcycles, Motor Cycle
With the single having amply demonstrated its ruggedness, the twin that evolved from it ought to have been the same - only more so. Like the singles, the twin was single-speed with belt final drive, one-piece heads and barrels with horizontal "beehive" finning. Capacity was 53.7cu in (880cc), although several development engines of varying capacity had been built and even raced during the previous two years.
Yet the Model D proved remarkably troublesome, with a mere 29 built in 1909 and just one in 1910. Harley-Davidson appears to have blamed its shortcomings on its automatic inlet valves, although other sources suggest slippage of the belt drive was at fault. Either way, when the re-engineered twin returned in 1911, it had both mechanical inlet valves and a simple but sturdy belt tensioning device which could be operated on the move by the rider's left hand.
Although still designated the Model D, other changes included a slightly reduced capacity of 49.5cu in (8 lice), from a bore and stroke of 76.2 and 88.9mm respectively, the same as the 1904 single. Beehive finning had also given way to vertical cylinder-head finning. At $250, the asking price was the same as the twin of 1908.
By all accounts, the revised twin proved far more dependable than its troublesome predecessor. Although little faster on the flat than the 30-inch single, it climbed hills far better - not least because the belt tensioner allowed the rider to maintain drive. Even so, a welter of improvements followed year on year. 1912 brought a new frame and a free-wheel clutch assembly in the rear wheel that freed the rider from the need to kill (and re-start) the engine if wishing to halt the machine. The same year saw the introduction of the first "one-litre" Harley, the 61-inch X8E, which also featured chain final drive. Although the two capacities were briefly produced in tandem, production of the smaller twin was dropped for 1913. The same year marked the debut of the Model G "Forecar", a 61-inch twin with front-mounted luggage box — a sort of back-to-front precursor of the Servi-Car.
By this time, the engine was almost unrecognizable in its internal details from the original twin. Relatively exotic alloy steels were employed in high-stress areas of the engine, such as chrome vanadium steel for the beam connecting rods, while roller and ball crankshaft bearings were widely used. A separate three-quart (2.8 litre) tank held oil for the total-loss lubrication system (used oil was either burnt by the engine or drained by hand from the crankcases). Like most contemporary engines, there was no oil pump: good old gravity and flailing engine parts moved the oil around the engine's internals and all the major bearings were of "self-lubricating" phosphor bronze. An auxiliary hand-operated oil pump had become standard in 1912, crudely metering about 25 "drops" of oil per minute. Until this time, all Harleys had been single-speed, but in 1914 the Model 10F and Forecar debuted the two-speed rear hub. This astonishingly intricate device comprised no fewer than five bevel gears, yet was replaced by a true three-speed gearbox after only one year.
A measure of the twins' success is that, by 1915 the Harley-Davidson range comprised just two single-cylinder roadsters but six twins, including the three-speed Model K "Stripped Stock" - a race replica for its time. Among six "Speciality" twins were the even hotter "Fast Motor" K12 and out-and-out racers such as the KRH. This elaboration continued throughout the 61-inch F-head's astonishingly long career.
So comprehensive was its evolution that, by the time it was discontinued in 1929, almost no part from the original twin would fit.
SPECIFICATIONS: 1911 MODEL D
Engine: F-head V-twin
Capacity: 49.48cu in (811 cc)
Transmission: single speed, leather belt drive with freewheel
Power: around 7bhp
Wheelbase: 56 1/2 in (1,435mm)
Top speed: around 60mph (96kph)
This article comes from Free Vin Check, Get Vehicle History Report, Free Car History, Used Car History, Auto History, Free Vehicle History, VIN Number Check, Car History, Lemon, Check
The URL for this story is: