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Whence Came the Swingarm Frame?

Tuesday, February 13, 2018 16:45
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The first motorcycles were hard things, that shook like hell over the rough cobbles and horse-shit roads of the late 19th and early 20th Century.  Figuring out ways to absorb shocks and control a bike over bumps has been a challenge from the very early days of the industry.  Spring forks were adopted first, as a bouncing front wheel is a lousy way to steer, and sprung rear wheels followed in an amazing variety of configurations, most of them surprisingly ‘modern’.  Even in the ‘Noughts, Motobis had monoshocks, the air shock was invented on the ASL, and by the ‘Teens, Indians had leaf springs, Jeffersons had short links, Merkels had monoshocks, and a dozen variations of all these appeared on bikes around the world. The 1903 Thomas AutoBi, one of the earliest American manufacturers, with East Coast dealer Lincoln Holland Sr.   The rear swingarm suspension uses an enclosed mono-spring unit behind the saddle, and a sliding-axle front fork design. [Wright]By the 1930s, two principal types of rear suspension were common; swingarms with attached springs using friction damping (Moto Guzzi), and plunger frames, usually undamped, or with attached André multi-disc friction dampers for racing bikes (BMW, Norton).  Both designs have issues with coil springs: they absorb the energy of an impact with a bump, but they also release it right back! Thus various types of ‘damping’ have been tried to control this tendency, but the earliest shocks tended to boing and caused a new kind of handling trouble. Leaf springs are self-damping, but have their limitations as well. One approach to rear suspension on a 1935 Moto Guzzi Bicyclindrica GP racer; a triangulated rear swingarm with spring boxes horizontally laid beneath the motor, and André friction dampers below the saddle [‘Moto Guzzi da Corsa’, Colombo]The motorcycling world changed in 1936, although it took a while for the industry to realize it, when Velocette built 3 new GP bikes  with a swingarm rear suspension, and the world’s first true shock absorber units, or ‘shocks’.  The new chassis debuted in the 1936 in the Ulster GP, Isle of Man TT, and Continental GPs, and quickly cut lap times, and led to significant wins for Veloce, even with their relatively outdated single-cylinder motors, which competed against increasingly fast and sophisticated supercharged, multi-cylinder racers from Moto Guzzi, BMW, and Gilera. Another solution; the ‘plunger’ frame, as used by BMW and Norton, with the rear axle held between two springs for compression and rebound suspension. [The MotorCycle, May 3 1936]Where did the idea for ‘shocks’ come from, who invented them, and who made them are seldom-asked questions, but Velocette historians Dennis Quinlan and Ivan Rhodes have done some digging, and came up with a conclusion.  The Development Engineer at Veloce Ltd, parent company of Velocette motorcycles, was Harold Willis; an unsung genius of design (as well as description) whose work changed the course of the motorcycle industry.  He loved a good nickname, and the terms ‘knocker’, ‘double knocker’, and ‘electrified dirt’ (for magnesium) are examples of his amusing shorthand.  In 1928, Willis also invented the positive-stop foot-operated gearchange that nearly every motorcycle in the world uses today.  Willis had a plane (‘Clattering Kate’, a deHavilland DH60 Moth) that he flew regularly to clear his head literally and figuratively – if he ever felt a cold coming on, up he’d go. Harold Willis with his beloved biplane, ‘Clattering Kate’ [‘Velocette: Technical Excellence Exemplified’, Rhodes]According to Charles Udall (Chief Designer for Veloce in the 1930s), Harold Willis came up with the idea for motorcycle rear suspension units after observing the latest aircraft landing gear with oleo-pneumatic units (called Oleo landing gear) made by the Dowty Company of Gloucestershire.  These were springless gas/air units that used a clever valving system to push oil into a pressurized chamber, and became progressively ‘harder’ as the oil compressed the air inside.  Air is a perfect, progressive, frictionless spring medium as it’s compressed; the ‘Oleo legs’, as they were called, function exactly like modern ‘air shocks’ and ‘air forks’ do today, as they’re effectively the same design, updated. The first Dowty Oleopneumatic shock absorber units fitted to a 1936 factory GP Velocette chassis, specially modified as an experiment. It worked! [The MotorCycle, May 3, 1936]Willis visited the Dowty company (with, perhaps, Udall and the son of Veloce founder Percy Goodman) to discuss making a miniature version of their Oleo shocks, suitable for a motorcycle, and they built, apparently, an initial 6 sets in 1936.  These same shocks were sold as standard on the Velocette MkVIII KTT production racer from 1937-1950, but the first 3 chassis built to adapt to these units (stamped SF1, 2, and 3) were a little different from the production versions.   After some sketches by, presumably, Willis and Udall, and probably Phil Irving too (who was employed by Veloce at the time), the triangulated, rigid rear frame section of a Velocette racing frame (the MkVII type) was cut away, and a tubular cross-member welded to the saddle tube upright.  The ends of this cross-tube had a pair of steering head bearing cups (with loose balls!) welded on, and a pair of tapered legs fabricated for the swingarm, connected through the tube by a splined shaft that locked each leg in position.  The end of the tapered legs held a casting for the rear axel, and a clevis fitting to hold the bottom of the Oleo unit.  An upper, bolted-on frame member to hold the seat, upper shock mount, and rear render mounts, was fabricated, and apart from details making manufacture much easier, this is essentially how every swing-arm motorcycle made afterwards, all over the world and to this day, was laid out. Three Velocette spring frames were built in 1936 by modifying the existing rigid racing Velocette frame; here the steering head cups are clearly seen as bearings for the swingarm [Quinlan]George Dowty, the inventor of Oleo-pneumatic landing gear for aircraft, was knighted for his services to the British aviation industry.  He initially worked at pioneering British aircraft firm Avro, and moved by the mid-1920s to the Gloster Aircraft Company.  In 1922, Dowty presented a paper to the Royal Aeronautical Society exploring the subject of oleo-pneumatic undercarriage design, and in 1926 delivered a second paper, “Aircraft Alighting and Arresting Mechanisms”, followed by articles (Feb.1929) in The Aeroplane and Aircraft Engineering. He was unable to convince the aircraft manufacturers to take up his on landing-gear suspension, so he struck out on his own, forming the Aircraft Components Company in Jan. 1931, from which the huge Dowty Organisation followed, which is still a major player in aviation. Stanley Woods at the 1936 Isle of Man TT, aboard his 350cc DOHC ‘dog kennel’ racer with the new swingarm rear end, and the front Borrani aluminum rim he brought back as hand luggage from Italy! [Hockenheim Museum]Dowty was happy to supply the experimental motorcycle-sized Oleo shock absorbers in 1936 for Veloce, and the following year went into limited production for the rear units on the production Velocette MkVIII KTT.  Following WW2, they used the same principles to build ‘Oleomatic’ front forks for motorcycles, as used on Velocettes in 1948, as well as on Scotts and Panthers. As the gas seals for the Oleomatic forks were prone to failure after tens of thousands of miles of hard use, all these factories designed hydraulically-damped telescopic ‘spring’ forks along the lines set down by BMW in 1935.  This became the industry standard, but ‘air forks’ and ‘air shocks’ are technically superior, and became the standard for performance motorcycles from the 1970s onwards.  All because of a brainwave by a rather eccentric genius in Birmingham named Harold Willis, and the willingness of an equally visionary George Dowty to try something new.  Hats off to you, gents! The 1949 Velocette MkVIII KTT production racer in the author’s collection; surely among the most beautiful racing motorcycles ever built. [Special thanks to Dennis Quinlan for his original article on this subject, and Pete Young for his further research into early rear suspension, and a posthumous thanks to Stephen Wright for his brilliant research into early American motorcycles; please find a copy of ‘The American Motorcycle: 1869-1914’ and be amazed and enlightened!]

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Source: https://thevintagent.com/2018/02/13/whence-came-the-swingarm-frame/

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