At first sight, motorcycle frames may appear to serve two roles – one, as a structure supported by two wheels to carry the machine’s weight and to give free steering, while, two, continuously keeping the rear wheel precisely in the motorcycle’s centre plane. Simple? Well, no, not really. Other roles include stiffness to resist distortion, incorporate suspension system (if any), carry the front fork at an appropriate steering head angle, give desired ground clearance and seat/saddle height, carry its load of rider/often passenger/often luggage, for many designers look attractive and/or distinctive and achieve a compromise of being sufficiently strong but not too heavy.
While we think of early motorcycle frames being of cycle-type or built in the style of cycle structures, the world’s first production motorcycle, the Munichmade 1488cc twin cylinder Hildebrand and Wolfmüller (1894-97), had a bespoke open chassis designed and manufactured for it rather than the maker hanging its slow revving engine from a cycle frame. Designs have progressed from the cycle-like chassis employed by pioneer makers through a variety of forms including open, closed, cradle, double cradle, pressed steel, spine and trellis.
While wood and cast aluminium has been employed in part or wholly, the vast majority of makers favoured steel, aluminium or a combination of the two. Although there have been instances of casting frame members, for the most part makers made/make coherent structures by welding, brazing – often involving lugs/spigots – or bolting together. If the structure isn’t coherent, it may comprise two or more sections – main and/or subframe/s – bolted together.
Although the
