THE OVERHEAD VALVE DOUGLAS. Some of the Outstanding Constructional Features of a Machine which has done so well in Races and Trials.
GOOD wine, it is said, needs no bush, and it is surely equally true that a good motor-cycle needs no praise. At any rate, there is a certain amount of satisfaction for the pressman who sits down to write
a few notes about the Douglas machine, in that its fame relieves him of some of the work, that which otherwise might be drudgery. I mean the delving into the murky past—well, just past, if you like—for where a machine is not too well known, it seems to be expected that the scribe shall waste a good deal of his time and spoil a considerable area of fair white paper, explaining how it came to be and whence. With the Douglas this is not necessary. Even the veriest newcomer to the motor cycling fraternity knows something at least about the Douglas, knows of that special feature of its construction which won it fame at the outset, which fame has, by various devices, been maintained intact ever since. In the case of the Douglas, therefore, any too persistent harking on its past, or concerning the fact that it has a twin horizontal opposed engine, more commonly and popularly known as a “fiat twin,” will most surely be met by the suggestion that such matters come into the same category as that sad news item about the death of the late lamented Queen Anne.
Not Concerned here with Performances.
So much for the past history, and for the outstanding features of all Douglases. The more up to date items of its performances on road and track are equally taboo from an article which is supposed to deal only with the mechanical details which, the driver apart, do so much to make the machine capable of achieving those successes. We come, therefore, after this long preamble,
to the meat ; and the meat, in this instance, is the overhead valve I.O.M. o.h.v. model Douglas Motor Cycle.
First as to the size of its engine. The machine I have in mind is the 3/ model, which has an engine of 68 mm. by 68 mm. bore and stroke. The cubic capacity of the two cylinders thus dimensioned is 494 c.c., so that the machine itself comes within the 500 c.c. class. The transmission, which includes a three-speed gear box and disc clutch, is all chain, and that, to my mind, is enough of the bare bones of the specification to set us on our way towards discussing the really interesting features.
First as to the ingeniously arranged overhead valves. They are overhead, in the sense that they are in the cylinder head with their own heads pointing to the piston. Actually they are set at 45 degrees to the centre line of the cylinder, and are horizontal, merely because the cylinder itself is horizontal. This allows of the maximum possible diameter of valve being used, and, moreover, as may be seen by reference to our illustrations, affords the most efficient design of cylinder head. They are provided each with a pair of springs, and are operated through pushrods from the cam shaft, through rocking levers, the pivots of which are most ingeniously and automatically lubricated through the medium of wicks.
The pistons, which we also illustrate, are of special form and are designed to be of minimum weight, but maximum gas tightness, while providing ample surface to carry the load which is placed upon them by the work which they have to do, without bearing upon the walls of cylinder with an intensity of pressure which will inevitably result in excessive wear of those walls. Reference to the illustration will enable the reader to realise that the actual piston is, in some sense, hardly more than a skeleton, so ruthlessly have its skirts been cut
away. At the same time, it should also be noticed that the portions cut away are only those which are not needed to carry the load, that is to say, those at the side of the connecting rod, while above and below that rod, where the thrust comes, first above and then below, there is ample area to sustain it. That is not the only novel feature about this part of the engine. As a rule, in pistons as ordinarily constructed, the bosses for the support of the gudgeon pin are cast to the walls of the piston and the connecting rod comes between them.
In this case, apparently with the object of increasing the amount of wall which can be cut away at the sides, an entirely different construction has been adopted. The boss for the pin is mounted on a strong web or rib which is cast to the interior of the head of the piston. The gudgeon pin is secured to this central boss, and the connecting rod, which has a forked end, embraces this boss, taking hold of the gudgeon pin at each side. Besides the advantage already named, this construction is also useful as it allows the overall length of the gudgeon pin to be reduced, thus not only decreasing the weight of the reciprocating parts, but also reducing the bending stress in the pin itself which can, therefore, again be made lighter for the load which it has to carry, than is usually possible or safe.
The Crank Shaft.
The crank shaft is of necessity different to the majority as familiar to motor cyclists, if only because of the disposition of the cylinders, and the arrangement of the crank throws. The problem was to provide, in a crank shaft of this kind, which, for strength and reliability, should be in one piece, for its equipment with ball or roller bearings, both as to the big ends of the connecting rods, as well as the main bearings. In the end this object was achieved in a most ingenious manner. The crank shaft is first made quite simply, with plain narrow slab, crank arms small enough to allow the big ends, and double row roller bearings for those big ends to be threaded over them. Detachable balance weights are then slipped over the two outer crank arms, as shown, and their attachment is cleverly designed to serve also as a means of retaining the whole assembly, roller bearings, as well as connecting rods, securely in place.
Compact Design : Disposition of the Clutch.
One of the most striking things about this model Douglas is its extreme compactness. At first glance one is inclined to doubt the existence of a gear box at all, and it is only on closer inspection that it is discovered behind and above the engine, neatly mounted on the top of the engine crank case. Before dealing at any length with the gear box, however, I must refer to the Douglas clutch. It is worthy of special mention, if only for the fact that it is on the engine shaft, instead of on one of the slower gear box shafts, which is so often the case. The advantage of having it on the engine shaft is that, since it is revolving so much more quickly, it needs not be so strong to transmit the same power as it has to be when it is carried by a gear box shaft which is seldom moving at more than half the speed of the engine, and which, therefore, necessitates clutch plates of twice the strength to make up for the loss of power resulting from the reduction in speed.
A Car-Type Clutch.
In its disposition of the clutch, the Douglas but emulates, of course, the universal practice amongst cars. There is a further parallel in that the type of clutch which is fitted is the same as that which has proved to be so successful and fool-proof on all kinds of cars, and even on heavy lorries, motor-‘buses and char-a-bancs. It is of the disc type in which the driven part of the
clutch is simply a steel plate which, when the clutch is engaged, is gripped between two rings of ” Ferodo ” or similar material. These friction rings are pressed together by light springs. When it is desired to release the clutch, positive means are operated to separate these discs and rings, and overcome the resistance of ,the springs.
Only Six Parts to the Clutch Proper.
We are fortunately able to reproduce a sectional line drawing of this important part of the Douglas. On reference to that illustration, it will be seen that there are only six parts to the clutch ; the flywheel, which, besides carrying out its proper function of maintaining the smooth running of the engine, also serves as a body, or part casing, for this clutch. There is a back plate, which, in the drawing, is the deep ring which is secured to the flywheel by countersunk screws ; the centre plate, which is slightly dished, and is secured to the driving sprocket : and the pressure plate, which is the one upon which the springs act. The back plate, and the pressure plate, also serve to carry the ” Ferodo ” rings to which reference has already been made.
Ease of Adjustment : Simple Operation.
The clutch springs, it will be observed, are carried in a concave ring which is supported by a nut on the outer spindle of the clutch. To adjust the pressure of the springs and the pressure between the plates of the clutch, to alter its load capacity, it is only necessary to screw up this nut. Disengagement is effected through the medium of a Bowden type cable, which rotates the operating cam shown on the drawing, and thus separates theplates.
Accessibility not Overlooked,
The layout of the engine and gear box is, as has been stated, very compact indeed, and it is good to note that this neatness of design has been attained without any sacrifice of that essential : accessibility. The mechanism of the gear box is similar to that of a car, and the top speed of the three which it provides is direct. First and second speeds are engaged by sliding the teeth of adjacent wheels into mesh, one with the other ; the
top gear is engaged by means of dog clutches. Those teeth which have to engage endwise are specially prepared to enable engagement to be effected with the maximum of ease.
The frame, which is depicted on another of our illustrations, is a model of strength, being almost pyramidal in form, a shape which has been known throughout the ages for its inherent strength. The forks are well known for their capacity to control the machine without having any adverse influence on the steering.
These are the outstanding points of the I.O.M. Douglas. There is no need for us to give the specification in detail. Any reader can obtain that, and any other information he desires, simply by applying direct to the makers, whose address is, of course, Kingswood, Bristol.