Zenith Motorcycles

Today in Motorcycle History

Zenith 1919 347cc Flat Twin

Zenith 1919 347cc Flat Twin

The 2 3/4 h.p. Zenith flat twin, embodying the Gradua gear below the crank case.

Zenith 1919 347cc Twin, Dismantled

The new Zenith engine dismantled.

Zenith 1919 347cc HO Twin Engine

Timing gear of the Zenith flat twin engine.

Zenith 1919 347cc Twin, Rear Brake

A diagram (not to scale) of the rear brake adjustment.

Rear Stand

Zenith 1919 347cc Transverse Twin Rear Stand

Zenith 1919 347cc Twin, Saddle

Patent laminated spring saddle, showing height and adjustment and method of disconnecting.

Zenith 1919 347cc HO Twin Decompressor

The compensating valve-liftmg mechanism on the Zenith flat twin

A 2 3/4 h.p. FLAT TWIN ZENITH.

38.73 mm. x 63 mm.. 347 c.c, Gradua Gear.

WE have known for a long time that Zenith Motors, Ltd., had a 2 3/4 h.p. flat twin on the stocks, and as early as May this year we had a short run on this attractive little solo mount, which behaved admirably. A study of its general arrangement reveals the fact that its designer, Mr. F. W. Barnes, has evolved an ideal motor bicycle of medium weight for the motor cyclist - young, middle-aged, or old - who needs a machine which is lively, comfortable, and flexible. As regards simplicity and efficacy, the Gradua gear is unassailable, and it is not surprising that it is adopted in this entirely new model. The manner in which the main Zenith features are retained, while the machine is adapted to accommodate the new horizontal opposed twin-cylinder engine, is extremely neat and ingenious. The new engine is remarkably well finished and of distinctly pleasing appearance, and follows standard practice throughout.

A Neat Power Unit.

The cylinders are clean castings, having the valves at an inclination so as to improve the design of the combustion space_and to allow free circulation of air round the valve ports and between them and the cylinder. The tappets are of large dimensions, and have large heads which come in direct contact with the cams, which are solid with the camshaft, and are designed so as to give a rapid lift and closure of the valves and yet to allow the latter to work silently. Both the crank and camshafts run on Skefko self-aligning bhall bearrings. The engine is as well-finished internally as externally; all important parts, such as crank-shaft journals, are hardened and ground, and the connecting rods, which are provided with split big ends, are machined all over.

Lubrication is by splash, an ordinary hand pump being used. The oil pipe is connected to a union on the crank case, and the lubricant, after entering this, is directed into two channels, one leading into the crank case and the other into the front cylinder, a hole being drilled in the base of the latter which registers with the channel referred to above. This system secures an equal supply of oil to both cylinders.

The frame of the new model is well worthy of comment, as the usual down tube and saddle tube are in duplicate so as to leave room for the cylinder heads.

The engine is bolted to a cradle which rests on a short tubular frame member; beneath it is an aluminium casting holding the Skefko ball bearings in which the chain-driven countershaft runs, while below this is the rectangular silencer, out of which leads a short exit pipe.

Novel Brake Operation.

In the general design of the machine it is easy to see that ample provision has been made for accessibility, and the cylinders may be removed without dismounting the engine from the frame. Considerable ingenuity has been displayed in the design of the new belt rim brake, and it is clear that it was no easy matter to actuate a rod controlled brake, which the latest pattern now is (a Bowden wire was used in the earlier Zenith models), on a wheel which is capable of a certain amount of longitudinal movement. This feat, however, has been successfully accomplished. The brake shoe is mounted on a flat rod, the rear end of which is anchored to the back wheel spindle, while the forward end is attached to the expanding pulley mechanism. The brake shoe lever has a slotted end which slides on a flat rod anchored to the rear of the chain stays and actuated by a pedal and toggle mounted on the near side footrest. It is an excellent piece of design, and allows the brake to be worked positively, no matter what the position the rear wheel occupies.

We have previously referred to the Zenith patent saddle, which is so effective that it gives equal comfort to that afforded by a spring frame. This, we may remind our readers, consists in suspending the saddle on laminated quarter-elliptical springs, while the saddle peak is mounted on a similar pattern spring, the forward end of which is provided with a roller working in a specially designed recess attached to the top tube. The forward end of the saddle may be lifted out of its recess and the whole saddle may be tipped backwards, so that in wet weather the actual seat may be kept dry in the event of the driver having to leave the machine standing in the rain. Means of adjustment are also provided to the forward spring so that the tilt of the saddle may be altered to suit the rider's requirements.

The rear of the frame is of particularly neat design; a spacious and substantial carrier is fitted, and the leverage of the stand is so arranged that the machine may be jacked up with one hand. Instead of being clipped on to the rear of the back mudguard, the stand is secured by a special clip attached to the end of the luggage carrier. Points of convenience have been carefully thought out.

An efficient shield protects the gear-operating, mechanism, while a front guard keeps wet and mud from the transmission. Altogether the new Zenith strikes us as being an exceedingly practical mount, which should be very popular during the coming season amongst those who require a light and fast mount of the sporting variety.

The 4-5 h.p. and 8 h.p. twin-cylinder Zenith-Jap models embody several of the novel features we have just mentioned, but otherwise do not depart to any great extent from standard practice, embodying as they do the well-known J. A. P. engines and the well-tried Gradua infinitely variable gear.

The Motor Cycle, October 30th 1919. p490