Cycle News - Archive Issues - 1970's

MilestoDes of progress, Pari 2 By Geoffrey Wood Photos courtesy of " Mot or Cycle". London. England Motorcycle design down through the years has had a complex progress. As noted in Part I, most of the really significant steps forward had their birth on the world's classic racing courses. What was learned on the race track was often incorporated in to This 1939 Rudge 4·valve single used an idea proven by Rudge in t he 1930 TT races. Honda picked up the idea in 1959, a nd now most four-stroke racen and a few roadsters are using the 4·valve design. NSU f ielded this streamlined 125cc racer in 1954. In 1952 and 1953 t he fin t head and dolphin fa irings had bee n t ried wit h success in internatio nal road racing. Whe n Tri umph de signed t heir vertical twin " Speed Tw in" in 1938, th ey revol ut ionized half of motorcycling. Th is 1965 engine d iffers from the or igina l design with its un it construction and alternator powered elec t rics. • 44 the production roadsters - proving that racing does improve the breed. In Part I, I presented ma ny of the most important technical improvements that have occurred. This time , I will p rese nt so me of t he less n o tic eab le but equally important milestones of progress. Take the u se of the alu minum alloy piston, for instance. In the ear ly days of raci ng, pistons were made of steel. The steel piston was very heavy . of course , which places great stress on the crankshaft and lower end bearings. Failures were common. The steel piston also tended to run hot, whi ch limited the compression ratio that could be used . One of those most suffering from the steel piston was the 192 1 AJS 350cc single that turned over at the remarkable speed of 6500 RPM . The British marque found a way around this problem with their new forged alloy piston on their 1923 models, which was much lighter, dissipated its heat better and was less prone to crac k. Within a few years the whole industry ad op ted th is practice. Another improvement in p iston design came in 1931 when Norton tried the slipper de sign on th eir works racers. This piston was as strong as the full skirt type, ye t it was much lighter. The early AJS racing single was also responsible for another idea - this one being the use of an inlet valve larger than the exhaust valve. The result was more power. In the early side-valve engines the larger valve would have yielded little power gain, but the new OHV AJS with its hemispherical head was a natural for the big valve approach. Another new idea was th e use of hairpin valve springs on the 1925 Sunbeam singles. This design allowed equal valve control with less spring pressure, and then in later years its flat profile allo wed the use of steeply inclined inlet ports on big sin gles. Valve con trol also took a big jump with the 1933 works racin g Velocettes, wh ich featured eccentrically mounted r ocker arms to adjust the valve clearances. This reduced the weight of th e valve gear and allowed higher revs with less fear of valve float, and it is still a popular practice today. The idea of "tuning" the exhaust system for more power did not enter the scene until 1932 when the British Rudge four-valve singles appeared with big megaphones on their exhaust pipes. An increase in power at higher engine speeds was obtained at the sacrifice of some bottom end power. Twenty years later Moto Guzzi improved on the idea with their reverse-cone megaphone, which added a bit at the lower revs . This megaphone bit may not see m very important for roadsters but it di d make designers aware of the pr inci ple involved so that today we have mu fflers with more power at lower decible rea dings. A nother race -pro ven idea was the use of alu min um a lloy fo r heads an d cylinders. T he ea rly cast-iron engines ten ded to run rather ho t, wit h their h ea t dissipation ra te limi ting the a mount of compression t ha t co uld be use d. Velocette tried an alloy head on the ir 1932 racers, which allowed the use of a higher compression ratio to produce more power. This was fo llowed by alloy cy linders in 1933 on the Norton works racers, with these ideas gradually working their way into the roadster field. Vet another ra ce-proven idea was the fou r-valve head , p ioneered by Rudge in the 1930s. Rudge won many races with their pushrod engines, but the idea died out un til 1959 when Honda first ca me to the Isle of Man with their 125cc twin-cylinder racers. It took Hon da a few years to build a winner, but when they did all of Europe began to go back to the old Rudge idea. Honda then went one step further and began using the idea on some roadsters in the early 19 70s - the advantage being ligh ter valves that will not float as well as a gain in the spread of power that is available. The re were, of course, so me design improvemen ts that did not e minate fro m the race track. On e of the most sign ifican t was the 1938 Speed Twin fro m the Triu mp h factory . T his 500cc model had a vertical twin design in which the pistons rose and fell in unison . The advantages were ligh ter pistons, sma ller valves, more revving ability, less vibration t han a single, an d a compact design. The Speed Twin revolu tionized the roadster field , and after the war it gradually put the big single to rest. Oddly enough, the vertical twin design has been very unsuccessful on the rac e courses of the world. The handling advantage of the single was lost, yet the new four-cylinder models were so much faster. During the I950s, England lost her sales leadership, first to Germany and then to Italy. These countries also took command on the road racing Grand Prix courses, with one significant item being the new 125cc Mondial racers for the new lightweight class in international racing. The Italian "double knocker" single revved to 10 ,000 RPM and ran 100 MPH, which staggered the world with performance from such a small engine. The trend was set and a whole host of small-engined buzz bombs made their debut in racing, to be followed with small roadsters that had high revving engines and a surprising performance. The post-war trend to lightweight road models was on its way . In t he 500cc class, the Gilera "four" also took over from the Norton single, followed by many multis over the next two decades. This trend to more than two cylinders became manifest in the roadster field during the late 1960s, with the Honda 750 four proving that a fast, reliable , smooth, and beautiful "four" could be produced at a reasonable cost. Rider comfort was another area where racing improved the breed, with the mu ch ign o red seat finally receiving some at ten tion by the British New Imperial re cord models in the mid-1930s. It was left to the 1948 AJS 350cc 7R road racer to really get the dual seat show on the road, which was followed one year later on their road models. The old solo saddle was slowly put to rest. Improved rider comfort came through use of streamlined fairings. Probably the first use of a fairing was way back in the late 1930s when the supercharged Gilera-Rondine Four appeared with a crude "dolphin "-type fairing in a race in Ita ly . After th e war it was left to NSU and Mo to Guzzi to develop and prove t he merits of improve d air pen etra tion, an d wi thin a few years some slee k fairings began to be seen on road models. T oday, many serious road r ide rs would n ever think of blasting down th e freeway wit h ou t first getting th at fa tiguing b last of wind off th em by u sin g one of t he many fairin gs on th e market. The progress of m o torcy cle design down through the years is obviously tigh tly in terwoven wit h the sport of motorcycle racing. One cannot help b ut wonder, if there had been no racin g, where would we be now? Raci ng has had a tremendous impact on improving the breed, and perhaps the best part of this whole story is that the research on the race tracks is still going on today . •

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