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.

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