Shall we go motor-bicycling?

The indications are that if a lot of us want to go places after the war we assuredly shall

Which line of thought has prompted Cecil Clutton to review modern motor-cycle design in another of his inimitable and valuable articles, giving, also, some consideration to what the future has in store. – Ed.


Never having been a motor-cyclist before the war I have at least to thank the Axis for introducing me to the sport, and I certainly do not intend to be without a two-wheeler again.

It is a pity that motorists and motor-cyclists have moved in such entirely separate orbits, because each could have learnt so much from the other. So far as the motorist is concerned, he has expressed a reasonable dislike for the nasty, noisy, oily, clattering, power-operated cycle; but such strides were being made when the war cut across development that it is well worth glancing at the state of things. Some of the lessons to be learnt are distinctly humiliating.

Design between the wars was largely dominated by the remarkable prestige of the T.T. races, and right until the eve of hostilities the unblown single-cylinder held that particular field unrivalled. The single-cylinder engine, especially of high power output, is not a very nice thing, but it is worth seeing to what a remarkable pitch it was developed, notably by Norton and Velocette. Structurally, everything was astoundingly strong, and the T.T. Norton, with a 94-mm. stroke, was expected to turn over regularly at around 6,000 r.p.m., giving a piston speed of 4,000 f.p.m.! It was reckoned that the engine speed would never fall below 4,000 r.p.m. during the race and the power output below that point was poor. Needless to say, very special rods and forged pistons were needed in order to make this sort of thing possible. Maximum speed was about 124 m.p.h. at 6,700 r.p.m. on the T.T. course, and the power developed was 50 b.h.p.; 100 b.h.p. per litre is fairly startling from an unblown engine, especially with such a large cylinder, and, in addition to the high revs., steps were taken to make the engine breathe to the best advantage.

The inlet valve was opened 60° before t.d.c., relying on the area of depression around the exhaust valve to suck in new mixture, although the piston was still rising. This, of pourse, goes far beyond the scope of normal valve overlap, which is merely arranged to get the valve fairly well open when the induction stroke starts. For this reason, the expression of overlap in degrees only can be misleading, as it takes no account of the rate of acceleration of the valve. But to continue with the Norton, the incoming mixture had the added merit of cooling the exhaust valve, though the valve ports had to be arranged so that too much mixture did not disappear down the drain. The exhaust valve was then held open till 55° after t.d.c. to make full use of the extractor action, which, in turn, was strengthened by the megaphone exhaust pipe. The gas travelling through the induction pipe at some 150 m.p.h. naturally possessed no small momentum, especially in an induction pipe of reasonable length, and its force was such that it would continue to drive mixture into the cylinder against a rising piston to such an extent that the inlet valve could be advantageously held open until 75° after b.d.c. The inlet valve was therefore open for 315° in all, and the overlap amounted to 115°.

Cooling valve seats by air only under such conditions was quite an undertaking, and fins became of massive proportions. The use of sodium cooled valves was reckoned to facilitate an increased compression ratio of 0.7 to 1, while hairpin valve springs reduced the weight of moving metal and also kept the operative part of the spring away from the heat. Overhead camshafts were essential.

To keep an engine of this kind up to operative engine speeds naturally required very close ratios, and those of the K.T.T. Velocette are fairly typical, giving 5.05, 5.55, 7.3 and 9.6 to 1. These machines, no less triumphant in the 350-c.c. class than the Nortons in the Senior, ran with a compression ratio of 8.75 to 1 on 50-50 fuel, and the 5.05 to 1 gave 116 m.p.h. maximum at 7,400 r. p. m.

In 1938 and 1939 it became apparent that the old order was being ousted, and if the more advanced racing designs of those years are to be the production models of the future, then motor-cycles will definitely "have something." The modern motor-cycle may be said to date back to the 1920 A.B.C. of Mr. Granville-Bradshaw's creation, and a truly remarkable conception it was. In the days of single cylinders, single speeds and belt drives, the A.B.C. appeared fully fledged with a horizontally opposed 400-c.c. twin, four-speed gearbox, sprung rear wheel, and built-in weather protection. Silent to an extent hardly surpassed, it was an immediate success, and had it been more skilfully handled from a commercial point of view and developed mechanically to overcome its teething troubles, it might well have grown up into something even better than the B.M.W., which was originally copied from it. As it was, it went under in the slump of the early twenties. A remarkable feature of the machine was that, despite its refinements, it weighed only 250 lb., as compared with the fearful overall weight of the modern motor-bicycle.

Another early adherent of the "multi" was that wonderful machine the Scott, with its vertical twin, 500-c.c., watercooled, two-stroke engine, and which, happily, is still with us after continuing basically unchanged since its introduction around 1907. The Scott might almost be said to hold the same position in the two-wheeled world as the Bugatti does among cars.

The first machine to drive the Norton off the field was the B.M.W., still showing unmistakable evidence of its A.B.C. parentage, with a 500-c.c., horizontally opposed, supercharged twin engine, in unit with the gearbox. Final drive was by shaft and bevel, and both wheels were sprung on plungers, similar to the Lancia arrangement. Discussion was rife – and yet is – as to whether the gyroscopic action of a powerful horizontally opposed twin is, or is not, an impediment to roadholding; but whatever may be the case, the motor lashed out no less than 7.5 b.h.p., which enabled it to execute neatly delineated rings round the Norton, without bothering very much how swiftly it was capable of changing direction; 150 per litre on 50-50 fuel is, again, a performance at which designers of racing cars may well hang their heads, but, even so, it was readily apparent that a mightier still was invading the field from the direction of Italy, where the four firms of Benelli, Bianchi, Gilera and Guzzi were rapidly bringing the multi-cylinder racing machine to a state of perfection.

Before considering them, however, it is well to record that some English manufacturers were busy stamping down the grass which tended to grow under their feet, and the Triumph vertical 500-c.c. twin won immediate popularity as a production machine. Even better was the 350-c.c. vertical twin, primarily developed as a W.D. mount, but nipped off by the Midland blitz before it could be put into production. Of this we shall most undoubtedly hear more after the war.

The Square Four Ariel made a notable success in the four-cylinder field, though the porting and cooling put it out of court for racing purposes. But those who have ridden one of these machines solo will have little doubt that something of this kind is the motor-cycle of the future. The 1,000-c.c. model (65 x 75 mm.), using a compression ratio of only 5.5 to 1, would attain 98 m.p.h. on a final drive ratio of 4.5 to 1 and 88 m.p.h. on the 5.7 to 1 third. Some performance figures of this machine, taken by Motor Cycling, are quite interesting. Acceleration from 10-30 m.p.h. in top gear took only 5.2 secs. and 30-50 m.p.h. no more than 5 secs. A standing quarter was accomplished in 16.8 secs. Braking figures are also interesting, because they give separate results for front and back brakes, which, of course, is seldom possible with a car. Thus, both together arrested the vehicle from 30 m.p.h. in 29 ft.; front brake alone took 39 ft. and rear wheel alone no less than 60 ft., despite a spring heel.

The overall weight of the Ariel was the massive one of 466 lb., a nasty thing to have to push about in the garage!

A particularly superb four-cylinder, which was shown at the 1938 Show, and of which an experimental model is now believed to be motoring about with marked success, is the Freddie Dixon designed Brough "Golden Dream" (what a horrid thing to call a motor-bicycle!). This extremely handsome machine had a horizontally opposed, four-cylinder, 71 x 63-mm., 996-c.c., air-cooled engine, with two crankshafts above each other and geared together. Unit gearbox, shaft drive and a plunger type spring heel complete the picture. This disposition of engine ensures equal cooling for all cylinders, while the geared-together crankshafts eliminate all gyroscopic action, in whatever direction. One hopes to hear more of this machine after the war.

In the racing field, A.J.S. and Velocette were the English pioneers. The A.J.S. people produced a V.4, 500-c.c. supercharged, water-cooled machine, with twin o.h.c. to each bank, and while the engine was rather ragged and possessed features which were not ideal, the model, nevertheless, had the honour of being the first to lap the Ulster Circuit at over 100 m.p.h. As Joe Craig is now with A.J.S. something fairly startling may be expected from them after the war, and the effect of his presence in their midst is already evident in their latest products.

What seems a better design, which was not ready in time to run in the 1939 T.T., though it showed up well in practice, was the supercharged vertical twin 500-c.c. Velocette, and this may still make its mark, as fantastic speeds were mentioned in connection with it. Not its least interesting feature was pneumatic rear suspension. This was also fitted on the successful "350" in 1938 and 1939 and was evidently a tremendous asset on the T.T. circuit, so that it can be regarded as a fully tried system. The great difficulty when springing the back end of a motorbicycle is to ensure proper lateral rigidity, the least flexing being fatal to success. The plunger system is probably the simplest and most popular, and it also makes it possible to fix things on to the frame. But it limits motion to 2" or 2 1/2", and for racing it is likely to give way to a pivoting system such as the Velocette.

Having briefly reviewed the British scene, it is now time to attend to the modern Italian school, which has achieved such phenomenal results, with air-cooled 250-c.c. singles, and transverse, in-line four-cylinders, of 250 c.c. and 500 c.c.

So far as single-cylinder "250s" are concerned the exhibits are provided by Benelli and Guzzi. The Benelli has a vertical twin-cam 57 x 97-mm. engine and develops the staggering figure of 30 b.h.p. at 9,000 r.p.m. unblown, equal to some 125 b.h.p. per litre – surely an all-time high. A radiator is incorporated in the oil circuit and, like all these Italians, an exceptionally large crankcase breather is provided. Equally all have, of course, a spring heel. This machine is so light that, in T.T. form, it can be lifted bodily off the ground by a man of fairly normal power output. The Benelli also appears in supercharged form, when it is credited with 45 b.h.p. on alcohol fuel. Very tame looking, by comparison, is the Guzzi, whose engine lies horizontal and has an external flywheel. It, too, is supercharged and, like the Benelli, has a form of balance tank, as the irregular gas-flow of a single makes it impossible to blow direct from the supercharger into the cylinder. The engine has a stroke considerably shorter than the bore-70 x 64 mm. – and gives out 44 b.h.p. at upwards of 8,000 r.p.m. It has taken the Class World's Record at 132 m.p.h. which few T.T. "500s" surpass.

Of the four-cylinder units, the Benelli is certainly remarkable, since its capacity is only 250 c.c.! As with Bianchi and Gilera, the cylinders are in line and set across the frame, despite which the overall width is not so great as a B.M.W., even with the larger 500-c.c. units and overhead camshaft drives to increase the width. The cylinder dimensions of the Bench are 42 x 45 mm. and the engine is inclined forward at an angle of 15°. The aluminium block has steel liners, twin o.h.c., specially tiny 8-mm. plugs and is water-cooled. The valves are 24 mm. in diameter and a Scintilla "Vertex" magneto is used. This engine gives 50-52 b.h.p. at 10,000 r.p.m.

This figure of 200 b.h.p. per litre was, indeed, almost equalled by the M.G. "Magic Midget," but whereas the M.G. only had to keep it up for a short, recordbreaking burst, the Benelli had to be capable of running through a 300 mile road race – and was! Had the war not cut short its final development it would almost certainly have had the legs of the Guzzi. The blower on this and the other Italian machines was a Rootes type.

The 500-c.c. Bianchi and Gilera are similar, having vertical, transverse, 52 x 58-mm. four-cylinder units with twin o.h.c., and both develop 80 b.h.p. at 7,500 r.p.m. The Bianchi is air-cooled and is thus somewhat lighter than the Gilera; it has a four-speed synchromesh gearbox. The Gilera was, nevertheless, the more successful, and towards the end of the 1939 season it had ousted the B.M.W. from the premier place, winning the Ulster, German and Swiss Grands Prix.

There seems little doubt that motorcycles somewhat after this pattern will become the sports and luxury machines of the post-war era, but if they are to be manageably light it is certain that manufacturers will have to make much more general use of light alloys in all possible ways. The French have already done much in this direction, as to gain the taxation benefits of "motorised cycle class" it is necessary that overall weight should not exceed 30 kilos. (roughly 65 lb.). For this purpose an alloy frame is almost indispensable, and here again the French are pioneers, since as early as 1896 a pedal cycle was made entirely of light alloys, except the front fork. A modern duralumin pedal cycle weighs only 17-20 lb.

Light alloys can also be used to a far greater extent than at present for engine accessories, and even all-alloy blocks are not outside practical politics, for the Cross engine has an alloy cylinder block with no liners, and after some 48,000 miles wear in the bores was only 4"! Special piston rings have to be used with a block of this kind. Alloy gears are also practicable to some extent, and the French Terrot reduction gears are duralumin to steel. As early as 1922 the A.J.S. which won the T.T. made extensive use of light metals, the clutch countershaft and rear wheel sprockets, being of duralumin.

As against these high-class products it may be expected that simple, robust single-cylinder engines, mounted in much simplified, and therefore lighter frames, will continue to be made for utility purposes. Single-cylinder two-stroke units of from 150 to 250 c.c., with overall weights of some 200 lb. seem a reasonable sort of thing to expect, and they could be marketed at a very modest figure.

But for performance models it will be seen that the motor-cycle has definitely become a sound engineering product, of ultra-modern design, of which the motorcar may well be envious, and which will certainly provide a thrill for its rider such as few, if any, cars can emulate. It will certainly never oust the car as one's favourite, but for sheer exhilarating performance one has got to hand it to the two-wheeler.