Horizontal Twin From Harley-Davidson

New Middleweight Sport Model Has Unit Power Plant, Final Gear Short Drive and Enclosed Chain-Numerous Examples of Over Designing

From the February 1919 issue of Motorcyclist magazine

Milwaukee-Representing the final evolution of three years’ development during which several models were built, tested, rebuilt and, in some cases, discarded, and a fresh start begun, the new Sport model middleweight twin announced by the Harley-Davidson Motor Co., marks a radical departure from all previous designs turned out by this organization. While its existence has been known to the trade for a long time, and its eventual appearance was well discounted, the adoption of the horizontal type of motor by the House of Harley-Davidson was an announcement of real surprise to the general public. Incidentally, it has the honor to be the first post-war motorcycle to make its debut.

Aside from some minor features, it may be said that this new Harley-Davidson not only is brand new throughout, as compared with the other models of the line, but it also introduces not a few original and unconventional fashions in motorcycle design as the specifications and illustrations well indicate.

Motor-Horizontal opposed, four-cycle type. Bore, 2/34 inches; stroke, 3 inches. Piston displacement, 35.64 cubic inches. Rated horsepower, 6. Brake horsepower developed on dynamometer, 7 1/2 Roller bearings throughout. Balanced crankshaft. Outside flywheel, enclosed. Horizontal tandem valves, interchangeable; 45-degree bevel seats. Valve opening, 1 1/4 inches in the clear. Unit power plant.

Carburetor-Standard Schebler; ¾-inch outlet, with super-heated manifold.

Ignition-Berling magneto.

Spark Plugs- Splitdorf metric.

Lubrication-Full automatic system for entire power plant unit, supplied by gear driven plunger pump. Spray from breather outlet piped to final drive chain.

Clutch-Enclosed multiple disc, running in oil bath, and gear driven from crankshaft. Thirteen steel discs, 7 plain, 6 with Ray-bestos inserts; 48 sq. in. of contact surface; 860-lb. spring pressure. Left pedal control.

Gearset-Three-speed, sliding. Ratios, 5 to 1 on high; 8.33 to 1 on intermediate; 13.87 to 1 on low. Reduction from motor to transmission, 2.58 to 1; from transmission to rear axle, 1.94 to 1.

Final Drive-Enclosed roller chain. Housing quick demountable and provided with adjusting window. Automatic lubrication.

Road Clearance-5 ½ inches.

Wheelbase-53 ½ inches.

Weight-256 lbs., empty.

Frame-Double bar, trussed keystone type; 1 1/8 inch tubing, 12 and 16 gauge. Forged fittings. Armor plate under motor.

Fork- Double truss type, with triple helical springs. Triple fork stem. Balanced caster steering action.

Tires-Firestone non-skid, 26x3 inches; standard CC rims. Average mileage, 6-7,000.

Fuel Capacity-Three gallons gasoline; two quarts oil.

Speed-Average, 52 m.p.h.

Brake-Double acting, external band; 7-inch dia., l-inch face. Right pedal control.

Saddle-Mesinger, mounted on Ful-Floteing post. Height from ground, 28 ½ inches.

Starter-Foot pressure type, on left side. Sector and pinion action, direct on crank-shaft. No connection with transmission.

Control-Ignition and gas regulation through twist grips and flexible cables.

Adoption of the double-opposed type motor has given the Harley-Davidson engineers a free hand in design exploration and they have taken full advantage of it. This trend not only is apparent in’ the fundamentally new construction that has been evolved, but also in the harmonizing of past Harley-Davidson practice with the departures necessitated by the basic difference in motor design between the double-opposed and V types.

It is but natural that most of the fresh ideas with which the Sport Model abounds, should be concentrated in the motive power component. For instance the unit power plant is not a new thought in motorcycle engineering, but its application to the “flat” twin is original in this instance. So many advantages are offered by unit power plant construction that it is a cause for wonder that it long ago has not become standard construction for all types of power two-wheelers

While on the first models the motor flywheel is only partially enclosed, it will be wholly enclosed before production reaches any quantity. Incidentally this flywheel is a pressed steel job, which insures that there is no danger of it flying into a thousand pieces due to centrifugal strains set up.

The two-throw crank is an especially neat job, drop forged, of course. The crank pins are 1 3/8 inch diameter and the rods are not split on the lower ends but slip over the cheeks to their proper positions. They are held in place by locking rings. The crank throws are drilled and mounted with counter- weights to neutralize vibration. This is one of the secrets of the velvetiness of running and smoothness over a wide throttle range. This motor will “idle” exceptionally slow without thumping. Big roller bearings on the rod ends insure long life and minimum friction at this heavy duty point.

Rear cylinder cooling has been achieved to complete satisfaction. One of the methods used was slightly less compression in the rear barrel. It is asserted that absolutely no overheating will be experienced.

New Harley-Davidson Sport Model

Taking cognizance of the very poor quality of the average gasoline of today, the builders of the Sport Model have paid attention to easy starting. There’s a unique manifold with a “hot spot” that prevents condensation and assures a quick firing mixture. The manifold is a single casting with twin passages and a “cross-over” from the exhaust to the intake, whereby exhaust heat warms the admission charge. The attachment of the manifold is a particularly clever piece of work.

Probably the closest approach to 100 per cent automatic and efficient lubrication is achieved in this model. A simplified edition of the Harley-Davidson plunger pump circulates the oil to the gears, whence it goes into the crankcase and eventually the spray is exhausted on the drive chain through the breather. The pump stroke, of course, is adjustable. There’s also a hand pump.

Much Clutch in Small Space

Harley-Davidson has scored a distinct triumph in the new clutch. Its a multiple disc type, running in oil. Compactness to the ‘nth degree is apparent and this permits the clutch being tucked away in a very small space. While discs are small in diameter, there are thirteen of them and an 860 pound spring furnishes the grip-some grip. This “little” clutch has 48 square inches of contact surface against 56 inches on the big twin.

The internal helical gear drive from the crankshaft to the main shaft of the transmission is another innovation. These gears are especially husky for the hard work put on them and run quiet as a watch. Meshed with them are the cam action, ignition and oil pump drivers.

Harley-Davidson is the second big maker to carry the short drive through gears and use only one chain in the line. The system has obvious advantages to recommend it. A real chain case, that is quick demountable, and that permits chain adjustment without tearing everything apart, renders this unit especially attractive. It steps up chain drive to its highest efficiency.

In the keystone frame and truss forks with plunger spring shock absorber, Harley-Davidson has made two departures from its previous designs in these units.

The low-slung motor affords a delightful ease of balance and handling in hard going as well as on boulevards. The short-legged man will especially appreciate the low saddle position, 28 1/2”from the ground.

Workmanship, standardization of details and equipment, all qualify to the well-known high standards of Harley-Davidson. Production of the Sport Model for this season will be limited to 20 per cent of the total factory output, and from the dealer enthusiasm manifested at the district conferences, the available supply will be many times oversold.

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