Zenith Motorcycles

Zenith Bradshaw Flat-Twin 1921

The latest model Sporting Zenith, engined with a very striking flat twin designed by Mr. Granville Bradshaw.

Zenith Bradshaw Flat-Twin 1921

The cylinder head of the Bradshaw flat twin. R. Interior of the cylinder head.

An Unconventional Flat Twin Engine.

A Bradshaw Power Unit to be fitted to Zenith Machines.

As might be expected, the new engine from the hands of Mr. Granville E. Bradshaw is full of very interesting features. Mr. Bradshaw is nothing if not original, and in the design of the medium-weight engine under consideration he has certainly excelled himself. There is nothing freakish in the engine, yet it is full of originality. His aim has been to produce the highest possible power for the engine capacity, and, notwithstanding the many special features, manufacturing possibilities have not been ignored. Mr. Bradshaw has worked on the theory that the air-cooled engine is in effect largely an oil-cooled engine, and therefore if the engine is to be kept cool the lubrication system requires special attention. Recent experiments have proved that the amount of heat dissipated by the crank case is relatively large, and Mr. Bradshaw has set out to keep the crank case cool and to keep the pistons cool, and claims that, having done this, a considerable step towards perfection in air-cooling has been taken.

Partially Concealed Cylinders.

Contrary to his usual practice, Mr. Bradshaw has made use of cast iron cylinders. These take the form of liners pushed into the aluminium crank case and (clamped in position. The reason for this choice of cast iron is that it is found to be more satisfactory than steel! when aluminium alloy pistons are employed. These pistons are of quite plain design. They posses the usual gudgeon pin fastening as found in aero practice, which has been for a long time adopted in A.B.C. engines, and are provided with a single piston ring at the top and a scraper ring at the bottom.

Practically none of the cylinder projects beyond the crank case. The orifice into which the cylinders fit is bored a few thousandths small, so that they are held firmly in position by the clamping together of the two halves of the well-ribbed crank case casting. Since the induction pipes form part of the crank case casting, each cylinder flange has a port in it registering with the inlet port in the crank case, and this in turn registers with the port in the cylinder head proper. The cylinder head is a nice piece of design, although it is not hemispherical in shape. Like every other part of the engine, it has been carefully thought out.

Between each valve there is a deep recess down to the actual thickness of the head casting, so that there is no unnecessary metal in the hottest part of the head, and there is a clear passage of air between the two valves. The actual bolts through the cylinder head clamping it on to the cylinder serve also to support the spindle on which the valve rockers work. These supports being adjustable may be made use of for the purpose of taking up the wear, while the push rods are provided with the usual adjustment.

Dual Pump Lubricators.

Internally, careful attention has been paid to the lubrication of the engine, which is effected by means of two pumps driven off the timing gear, and situated at the base of the crank case. Both these pumps are of the gear type, and are placed side by side in a single gun-metal casting consisting of three parts. Owing to the design of these pumps, which are driven off the crankshaft, incorrect reassembly is impossible. The small pump delivers oil from the tank to the hollow crankshaft, while the larger pump pumps [sic] the oil back again to the reservoir. Since forced lubrication under high pressure is employed, plain phosphor-bronze bearings are used for both small and big ends, and the oil, after circulating through the drilled crankshaft, is ejected on to the camshaft, which carries all four cams. This camshaft lies below the engine, and splashes the oil over the timing gear case which is used to radiate the heat.

The Mainshaft Bearings.

So far as the mainshaft is concerned, the driving side has one roller bearing and one ball bearing running in a steel housing, provided with an oil-retaining washer. The other side of the crank shaft runs on two plain bearings, the shaft at a point adjacent to the flywheel being 30 mm. in diameter. Next comes a sharp taper to take the flywheel, and then a square shaft for the fitting of the Gradua gear.

Amongst the most interesting features in the design are the induction pipes, which, as we have said before, are cast in the crank case on the side opposite to that on which the flywheel and distribution gear are situated. This renders the magneto accessible. The new single-lever motor cycle model Zenith carburetter is to be standard on these engines, and is bolted to the centre of the induction pipe at a point where a V-shaped projection is formed in the casting in order to direct an equal quantity of mixture to each cylinder. The carburetter is provided with a lateral adjustment, so that if there is any irregularity of firing the distribution may be corrected by a slight movement to the right or left. Yet another important feature in the design of the induction pipe is the fact that each pipe rises from the carburetter - a fact which has been lost sight of by most designers of flat twins.

Induction Pipe Design.

This means that there is never an accumulation, of unvaporised petrol adjacent to the inlet valves. The valves themselves (of the overhead type) are of ample size, and each one is larger than half the bore of the cylinder. The valve mechanism is entirely controlled by straight rods, and is, We are assured, absolutely silent.

An ingenious form of magneto coupling has been fitted which clips on to the magneto shaft, and is provided with a dog engaging with the shaft, so that the magneto can only be reassembled in the right place.

Essentially a Sporting Mount.

This engine, which is made by E. T. White and Co., 1, Albemarle Street, London, for its designer, Mr. G. E. Bradshaw, will be a feature of the exhibit of Messrs. Zenith Motors, Ltd., at Olympia, where it will be seen built into a specially-designed frame, the whole proving to be an ideal sporting mount. No clutch will be fitted, simply the Gradua gear, so that there are as few complications as possible, and yet an adequate means of altering the gear ratio to suit varying conditions is provided.

An experimental racing model has been on Brooklands track for some time. An oil tank is situated in front of the engine, in which the lubricant is cooled before returning to the oil delivery pump. The Gradua gear lever projects through the tank, and is connected by bevel wheels to a long rod running underneath it, on the rear end of which is the chain wheel connected to the rest of the gear mechanism. This machine, which is practically a replica of the type which will be sold to the public, has already shown great promise on the track, while the appearance of the new model will undoubtedly create a sensation.

The Motor Cycle November 25th, 1920. pp 650-651

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