Douglas Motorcycles

Today in Motorcycle History

Douglas 1921 494cc OHV

Douglas 1921 494cc OHV

Flywheel side of the new sporting type 3½ h.p. Douglas, a production model of a very successful speed machine.

Douglas 1921 494cc OHV

In addition to the new engine, the Douglas has chain drive, a new type of front brake, and an internal expanding rear brake.

Douglas 1921 494cc OHV

The rocker shafts at the cylinder heads are reservoir feeding the bearings by wicks.

Douglas 1921 494cc OHV

The exhaust valve lifter mechanism of the new Douglas engine.

Douglas 1921 494cc OHV

One of the aluminium oil reservoirs for lubricating the rocker shafts.

3½ h.p. Douglas 494cc Engine, 1921

The new power unit, showing the detachable cylinder heads, overhead valve gear, and induction system. The kick-starter is fitted at the right-hand side of the machine.

3½ h.p. Douglas 494cc Engine, 1921

Part sectional plan of the Douglas engine, showing the overhead valves and wide big end bearings which have double rows of rollers.


Advance Particulars of New Chain-driven 3½ h.p. Speed Machine with Overhead Valves.

EVER since Douglas motor cycles were first introduced, they have been prime favourites with the sporting element of the great motor cycling fraternity. Right from its inception, the speedy little "2¾" took a place in the front rank - in fact, it may be said lo have made smaller machines fashionable atHong both "speed merchants" and tourists of that day, for about that time a lightweight was not regarded very seriously as a practicable mount.

Further, the early Douglas machine will always be a milestone in the evolution of the motor cycle. It was fitted with the first practical "flat twin" engine, a type which has proved to be so satisfactory that it was destined to be the prototype of many similar designs produced in every country where motor cycles have been manufactured.

These two facts make any new Douglas model of more than usual interest, and when that new model has been designed as a sporting mount - and has been proved to be exceptionally fast - there is no doubt that it will hold large courts each day at Olympia during the Show next month.

Early Successes.

The engine has been on the track, the speed hill, and the racecourse in various forms during the past year. It formed the nucleus of the machine with which A. H. Alexander put up such good speeds in the Isle of Man. For some reason the Scottish speedman could not keep his belts on. In the Grand Prix chain drive was adopted, but the combination of a really "hot-stuff" engine and chain transmission was a new proposition to the Douglas experts, with the result that valuable data were obtained concerning shock-absorbers. It has been considered by those, competent to judge that Alexander would have won the Grand Prix but for a too enthusiastic spectator, who, after a roadside stop, assisted him to restart, and so caused his disqualification. His speed was infinitely superior to that of any other competitor, and many miles per hour faster than the actual winner. At the Gaillon hill-climb on the 10th inst., Alexander put up a very fine performance, securing four firsts and two seconds.

In the meantime various sample engines have been tried out at the hands of well-known riders, including S. L. Bailey, who broke several records in the 350 c.c. class at Brooklands. These were the flying five miles, 4m. 32s. (65.18 m.p.h.), and standing ten miles, 9m. 16 4/5 s. (64.55 m.p.h.). Bailey also secured four firsts in races with this engine. Alexander, too, secured two firsts at the Scottish speed championship meeting and at the recent Bristol open hill-climb. H. Thorpe, riding one of the new machines, gained nine firsts, three seconds, and one third, and a further six firsts in other classes in which the awards were not given, as the machine was ruled to be "not standard."

Flexibility and Acceleration.

This is all the more interesting because the engines which have performed so satisfactorily are uniformly good, and have required no "faking" - that is to say, that it is not a case of a star engine out of a batch, usually difficult to duplicate, and generally impossible as a marketable proposition.

The speed of the new Douglas engine lies in its design, and any difference next year between the speeds of S. L. Bailey and the private aspirant for honours in the field of high speeds will be one of tune and skill.

One astonishing thing about this engine is that, although it is capable of remarkably high speeds, it is quite docile for traffic work, and extremely flexible. Its acceleration is wonderful, and, sans silencer for track work, it has an exhaust note sharp enough to please the most meticulous critic who judges efficiency by sound.

Without doubt the new engine chiefly owes its efficiency to its overhead valves and its hemispherical combustion head. This, of course, is detachable, fixed by four studs, and having a flush joint with a copper-asbestos gasket.

Lubricating the Rocker Shafts.

The valves are of comparatively large diameter, and the pockets are entirely free from awkward corners which might impede the flow of the gases. Short in length and light in weight, the valves are set at 90° to each other, and are actuated directly by rockers mounted on vertical shafts carried in long bearings which are lubricated in a rather novel way. Over the heads of the standards containing the shafts, and forming a bridge between the two, there is a large aluminium reservoir containing oil. In this there is a long wick with its ends threaded into the rocker shafts which are drilled to receive them. From the interior of the shafts oil finds its way through a small duct to a groove on the outer surface and so lubricates the whole of the bearing. How well this system works was demonstrated to us on one of the several engines we inspected.

More or less conventional Douglas timing gear is embodied a transverse camshaft extending across the compartment below the magneto base. The tappets and push rods, therefore, are on the upper side of the cylinders. The former are adjustable and have spherical leads which fit into the cup-shaped ends of the push rods. A similar arrangement is also provided at the rocker ends of the rods, which incidentally are instantly detachable without the use of tools.

In the engines we inspected hand controlled drip-feed lubrication only was installed, but in the final model it is proposed to embody a mechanical pump at the head of the timing case which will give a constant supply through the drip, and the hand pump will be an auxiliary by which the rider may give an extra charge of oil when desired, without altering the adjustment of his mechanical system. Oil enters the engine at the base of the cylinders.

The cylinders are 68 x 68 mm., and on the greater part of the cylinder barrel are longitudinal fins, which diminish in area as they approach the base. The heads are amply provided with cooling surface, and have integral with them the standards for supporting the rocker shafts.

Double-row Roller Big Ends.

Cast iron pistons, with two rings are fitted, and, in order to obviate any tendency to whip, the connecting rods are of H section and rather wider than usual, but so carefully have they been designed that they are exceptionally light, being but 7 oz. each.

The big ends each have a double row roller bearing, and as the crankshaft is of the one-piece type, considerable ingenuity has been necessary in designing means for inserting the rollers, which are contained in split cages, with a two-piece retaining washer on one side.

Although the three-speed gear box is new in design, many of the parts of the present 4 h.p. gear box are interchangeable with those in the new model. The kick start mechanism is on the left side of the machine, but the starting crank has been brought to the right side by means of a well supported shaft.

The frame and detail work are very good - in fact, there appears to have been a really serious effort to "clean up" frame design generally. Of the semi-duplex type, the frame has mostly welded joints, but the head lug, which is very substantial and extends to the fork of the front part of the frame, is brazed. The rear part of the frame is exceptionally pleasing, and leaves plenty of room for cleaning purposes. The illustrations show the details better than any number of printed words, but one little detail, worthy of mention, is not observable. We refer to the method of fixing the engine and gear box. The power unit rests on the two longitudinal frame members which are bridged by two pieces of steel welded on the under side of the tubes. In each of these bridges there are two slots through which pass bolts from the base of the engine. By these means the engine may be slid to a limited extent in order to adjust the primary drive.

The gear box is supported from a similar bridge, but is not movable, the rear chain adjustment being effected by push screws. Among the many details of the specifications are 650x65 mm. tyres on interchangeable wheels, and a novel type of front wheel brake, which takes the form of a steel disc with an inverted V periphery upon which works a double-faced shoe.

This brake is very powerful, but sweet in action, and remains undisturbed when the wheel is removed. The rear brake is of the internal expanding type, and, with the transmission, is also undisturbed by the removal of the wheel.

One other little point will appeal to the rider. The guard over the primary drive chain is secured by spring clips, and so may be removed without the use of tools.

Low Riding Position.

Incidentally, the height of the machine is 1½ in. lower than any previous Douglas model, and, when stripped for a speed competition, it weighs but a little over 200 lb.

Altogether, we were most favourably impressed by this latest product of the Bristol firm, not only as a speed machine, but a sporting tourist's mount, and we heartily congratulate those responsible for its design and production.

In addition to the new sports model "3½," the popular 2¾ h.p. solo and 4 h.p. sidecar machines will be included in the Douglas 1921 range.

The Motor Cycle October 28th, 1920. Pages 506, 507

If you have a query or information about Douglas motorcycles please contact us