ONE of the most interesting light-weights which has yet been produced is the latest model Sparkbrook, to be known as the "Spark."
Primarily intended to be produced in quantities at a low figure, it is the frame construction which rivets attention. At first sight it would appear that a loop frame of normal construction is employed, but a closer scrutiny reveals many unusual features. To begin with, pressed sheet steel lugs are used throughout. Even the head lug, a particularly fine piece, of work, is a one-piece pressing, welded at the edges of the webs. The main frame consists of the head, tank rail, and a loop member, to which stout pressed steel engine bearer plates are brazed. Bolted to the rear engine bearer plates are the chain stays, the back stays being detachable from both chain stays and seat tube. These stays have flattened, ends, but are reinforced where the swaging takes place by stout inner liners of steel tube.
Quite an unusual feature is incorporated in the stand, for this is so arranged that the light tubular legs receive the minimum bending stresses, since they lie directly between the chain stay and the ground when the machine is raised.
Two sheet steel straps position the legs, and one of these lies on either side of the rear fork-end lug, taking a bearing on the outside of the retaining bolt boss and not on the bolt itself. The front fork is of the well-known Triumph type in so far as it is pivoted at the bottom of the head lug, but plain bearings are employed on the pivots, a feature which is emphasised by the designer, who claims that the slight amount of friction at this point is beneficial in its damping action.
Planned tor Production.
Both compression and rebound springs are fitted at the top of the fork, and the amount of pressure is adjustable. Before leaving the frame it should be mentioned that each side of the chain stays, back stays, and spring forks is identical, and this simplifies replacements as well as lowering production costs.
Footrests are carried from the engine plates, and on the cross-bar are mounted a pair of brake pedals, one on each side of the machine. These pedals operate Ferodo shoes in the belt rim, one being placed above the chain stay and one below.
All brake gear is arranged in such a manner as to clear a gear box and primary chain should such a device be fitted. The brake shoes are interchangeable, and a single return spring suffices for both brakes.
Again composed of pressings, the tank is of tubular section, with a recess along the top to fit snugly round the tank rail; projecting lugs are formed at each end, which bolt up to the steel webs of the head and seat-pillar lugs.
Before joining up the two tank pressings, a steel bottle is inserted, and this serves to contain lubricating oil. The cap of this bottle is screwed down on to a leather washer, so as to be absolutely air-tight, and carries a ball non-return valve and an adapter to suit a normal tyre pump. A tap and sight-feed arrangement are fitted to the neck of the bottle. Thus, after filling up with oil, a few strokes of the tyre pump will supply sufficient pressure to empty the tank of oil gradually when the tap is turned on. It has been found in practice that the oil compartment is so completely air-tight that, even after several days, or even weeks, no diminution of pressure is traceable.
Fitted with the well-known Villiers two-stroke engine and flywheel magneto, the little machine will sell at thirty-eight guineas, with direct belt drive. At this attractive price the machine is thoroughly well equipped, 26x2¼ in. Dunlop tyre's being fitted. Sensible mudguards, a sturdy little carrier, and good quality saddle are included in the specification, and the finish is black, with Sparkbrook green tank and flywheel cover.
The well-known Sparkbrook lightweight models will continue to be sold either with single gear, two-speed, or two-speed and kick starter; and, in addition, a new model fitted with the 350 c.c. J. A. P. engine and a three-speed gear will be listed.
The MotorCycle, October 1921, p517
Perhaps the most interesting machine in the Sparkbrook display will be the new Villiers-engined Spark. Having a frame in which no castings are employed, and a steel tank incorporating an air-pressure lubrication system, the little machine bristles with novelties, and is well worth a close study. The Spark will be sold as a single-geared or clutch model, but the standard Villiers-engined Sparkbrook models are retained, and may be fitted with the Sparkbrook two-speed gear and clutch. Another new model, having a 350 c.c. J.A.P. erigine and three-speed Sturmey-Archer gear, will also be shown.
The Motor Cycle, November 24th 1921