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Some Road Impressions of the Dunelt 300 c.c. Two-stroke as a Sidecar Machine.
WHEN long before last Olympia, we first inspected the drawings and experimental engines of the Dunelt big single two-stroke, we felt that, with perseverance and careful study on the part of its designers, it would eventually materialise as a serious sidecar proposition.
It was exhibited at the last Show, and created a great deal of attention as a passenger machine at a popular price. The exhibit served merely to test the potential demand for such a machine, and no orders were accepted, as the makers had still certain difficulties to overcome production.
In the meantime, encouraged by the interest manifested in its design, the experiments were continued, and two machines were entered in the A.C.U. Six Days Trial. One of these was fitted with a sidecar, and was in the hands of a driver who had never before competed in a trial.
A Wonderful Performance Recalled.
We confess that when we saw the machines at Darlington, on the eve of the start, and learned that the sidecar outfit had only a 12 to 1 ratio for its lowest gear, we thought its chances of securing a "gold" were small. It was the first time a two-stroke had taken a sidecar through a Six Days Trial - and the most difficult Six Days to boot - yet both machines secured the much prized gold medals.
This trial proved that a 500 c.c. air-cooled two-stroke was a practical proposition as a sidecar machine, and was at least the equal of a 4 h.p. of the four-stroke type. In fact, it is worthy of note that 4 h.p. four-stroke sidecar machines were originally entered, but were withdrawn when the severity of the route became known.
At Olympia this month, the Dunelt will again he exhibited, but this time as a commercial proposition.
Recently, we were afforded an opportunity of trying the sidecar outfit on the road. The machine itself being one of the Six Days mounts, with many thousands of miles to it credit. The Sturmey Archer gear box was still in the position adopted hurriedly for the big trial, that is above the bracket designed for a difFerent type of gear box which was fitted to the machine at the last Show. In the production model the more conventional position below the bracket had been decided upon.
It may be remembered that the Dunelt differs from the orthodox three-port type engine in one important respect. It has stepped cylinder am piston which increase; the displacement in the crank case. This provides better scavenging and, in consequence less tendency to four-stroke at low speeds and under light loads.
The "two diameter" piston also has the advantage of being cooled on both sides of its wall by the entrance of the cool gases. Whether or not this is one of the reasons why the engine runs so cool the fact remains that it will apparently run indefinite without overheating, and without giving trouble through distortion.
This is certainly an important point gained, as although the Dunelt is not the first big single air-cooled two-stroke, it is the first to be placed on the market in this country, after it has been proved equal to all the tasks expected from a machine of its size.
The Effect of Appearance.
One's first impression of the Dunelt from an appearance viewpoint is the apparently enormous size of its cylinder. This is due to the double diameter piston referred to above. Then its inclined position in the frame, and the angle of the intermediate frame tube give the machine the appearance of being somewhat high at the front.
These latter points, however, are only first impressions created because the machine departs from conventional outlines. It is not, however, so heterodox as to create unfavourable comment. It is merely different, and therefore attracts the eye.
Once in the saddle, any suggestion of unconventionality disappears, and one gets an impression similar to that obtained on a big twin or 4 h.p. single with large tanks. One has no doubt that the Dunelt is a fully-fledged sidecar machine, and when the engine is started there is little of that uncertainty of impulses usually present with a two-stroke. The machine we tried certainly four-stroked a little, but scarcely any more than some flat twins have eight-stroked; in fact, when running idle, the occasionally four-stroking reminds one of a twin of which one cylinder is occasionally cutting out.
Once under load., however, all four-stroking ceased, and; with the even torque of a flat twin - without a flat twin's mechanical sounds - the machine gets away smoothly and without any "fuss." A strange carburetter, with lever opening in the reverse direction from the one we had just left, a totally different riding position, different location of the brakes were all points were against favourable impressions at once, in a hundred yards we were quite at home on the machine.
As regards the power of the engine its performance in the big Trial is sufficient to show that in this respect it was all that may be expected from an engine of 500 c.c. capacity.
The power curve of the engine shows that at 2,400 r.p.m. over 8 h.p. has been obtained on the test bench. This is remarkably good, better perhaps than may at first be appreciated, is in the majority of engines the peak of the curve is reached at higher speeds - speeds which are seldom used in a motor cycle. For a motor cycle, at all events, an engine which will give good power at the lower speeds is better than another which may develop a greater power at much higher speeds.
The Dunelt engine develops 5 h.p. at 1,400 r.p.m. and 6 h.p. at 1,600 r.p.m., and as these are the powers and speeds used under normal conditions, such results should be regarded as very satisfactory.
Incidentally, the average 1,000 c.c. twin at about 2,700 r.p.m. generally develops about 13 h.p. So far as retaining its power is concerned, we have seen many engines tested, and must say that the Dunelt maintained its maximum in a manner quite as satisfactory as four-stroke engines.
In fact, we have been surprised at the length of time this big two-stroke has run all out under the adverse conditions existing in a shop. Probably it is owing to the satisfactory reading at the lower end of the curve that the makers favour comparatively high gear ratios, and in riding the machine we have the impression that the top gear was a little too high for sidecar work, notwithstanding that a very satisfactory speed was obtained on the level. This, however, was only an impression, since we had just dismounted from an 8 h.p. twin, and were on a strange machine.
Probably other riders have noticed that, having become accustomed to one machine, it requires some time to be able fairly to judge the speed, power, and tune of a strange mount. Especially is this the case when one leaves a four-stroke and tries a two-stroke.
A Comparative Test.
After having again taken over our own machine, we accompanied the Dunelt exponent, Capt. Dudley Barton, who rode the solo machine in the Six Days, and who now took the Dunelt with empty sidecar, when the big two-stroke proved to be superior to the twin, which had twice the capacity.
As a solo machine, too, the Dunelt should be satisfactory; but in making it a dual purpose mount the designers appear to have considered that the larger number will be used with a sidecar. It has quick detachable wheels, interchangeable all round, including the sidecar, and we understand it will be marketed with a spare wheel, so it will be seen that it is as a sidecar outfit that the greatest demand is expected.
Smoothness and Economy.
The engine is particularly smooth in its running, for which its large outside flywheel is partly responsible, while as for flexibility, exhaust noise, and petrol consumption it compares favourably with a four-stroke of its power. With an automatic carburetter 60 m.p.g. is being obtained with sidecar and passenger, but the makers hope to improve upon this in the production model, which we hope to test more fully at a later date.
In the meantime we are very favourably impressed by the general performance of the machine, and congratulate Messrs. Dunford and Elliot upon the successful issue of their long series of experiments.
The Motor Cycle November 4th, 1920. Pages 522, 523, 524
The Olympia Show, 1920
Dunelt. (Stand 138.)
4½ h.p.;, 85x88 mm. (499 c.c); single-cylinder two-stroke; drip feed lubrication; Cox Atmos carburetter; M-L chain-driven magneto; three-speed Sturmey-Archer gear; chain and belt drive; Hutchinson 26in. x2½ in. tyres. Price £115, with sidecar and spare wheel £150.
Dunford and Elliot (Sheffield). Ltd., 95, Bath Street, Birmingham.
It is satisfying to note that the Dunelt is making good, striking out as it does on new and hitherto untried lines, and yet with a definite reason for its divorce from Dame Convention. To combine power for sidecar-work with a low initial cost is the raison d'être of this machine, and we think that the makers have achieved their aim.
Cooling, or perhaps more correctly overheating, the inherent bugbear of the big two-stroke engine as a type, has been overcome by using a two diameter piston, allowing, a 50% greater charge than is ordinarily possible, and providing a flow of air round the piston trunk. Numerous little details have been improved since last year. The compression release valve now has a flexible tubing connection to the main exhaust pipe. The adoption of split roller-bearing big ends is the only internal modification of importance. Two thumbscrews allow the primary chain cover to be quickly detached, while all hubs have knock-out spindles. The cylinder, it will be remembered, lies almost diagonally in the frame, which certainly keeps the large, somewhat bare, outline from obtruding in the super aesthetic eye.
The machine immediately impresses one as sound and sturdy, yet compact and symmetrical. Evidently the lessons of the last Six Days Trials where the Dunelt gained two "golds" have not been lost on the designers.
The Motor Cycle, December 2nd, 1920.