The majority of early motorcycles were produced by bicycle manufacturers. The technology associated with frame design and construction was adequate with these early machines which were relatively slow (often being restricted to speeds below 20 mph); however, as more power was produced, and the machines were allowed to go faster, it was necessary to improve the basic frame.
The basic bicycle frame was a diamond–shaped structure with tubes brazed into lugs. The lugs located the steering assembly at the headstock, the crank bearing case at the pedal location, and the rear wheel spindle. In addition, another lug carried the seat tube.
Motorizing the bicycle frame consisted of simply bolting or clipping an engine to the existing cycle frame. As these early machines generally didn’t have clutches or gearboxes, the manufacturers retained the pedals from the original cycle to assist with forward motion and starting. However, to give adequate clearance when pedaling, the frame had a high ground clearance which, combined with a high seat height to enable the riders legs to rotate, gave a high center of gravity.
Although the basic bicycle frame with an engine bolted in was adequate, initially, the design had some fundamental flaws. In particular, the sawn off finish of the head stock lug where the tubes penetrated it caused a concentration of stress with the inevitable result that the tubes cracked at the intersection. To negate this problem, manufacturers began to taper the lugs and, in some cases, fish tail them.
Motorcycle development between 1900 and 1910 was rapid. Purpose built frames became the norm to cope with the ever increasing power and therefore speed. And gone was the need for pedal assistance. Over the subsequent years, motorcycle manufacturers developed a design convention based on the type of motorcycle they were producing; however, there were exceptions such as Ducati with their triangulated multi-tube variants, and Greeves with their massive front bone design.
Single and Double Loop Frames
In the loop design, single or double tubes were fixed to the head-stock before wrapping (or looping) the engine and gearbox. This design has proven very popular with manufacturers and can still be seen on modern motorcycles. The continuous loops of this design illuminated, to a large extent, stress raisers created by joints (lugs) at the lower engine mountings, and at the swing arm (where fitted).
The loop design was sometimes referred to as the cradle frame and, where two loops were deployed, the duplex cradle. The most famous motorcycle to utilize this design of frame was Norton with their Manx racer, which had a duplex double cradle version.
The backbone frame typically utilizes a large section top tube (under the fuel tank) which, in some cases, doubled up as an oil carrying member. On this design, the top tube was the dominant load carrying member, often being fabricated from a large diameter steel tube with relatively thin walls (to keep weight to a minimum, which is especially important on this design with the tube being so high on the bike).
The frontbone design is generally associated with Greeves motorcycles. This design of frame was used on machines produced by this company for use in all branches of motorcycle sport including MX, trials, road racing and also some street bikes. Particularly noteworthy in the case of the Greeves design was the use of an aluminum (‘H’ section) casting for the frontbone which also incorporated the headstock.
Motorcycle manufacturers have used the exceptional strength afforded by triangulation in their motorcycle frames since the 1920s (Cotton and Francis-Barnet). Most well known user of this design is Ducati with the steel trellis. The Ducati frame uses a system of multiple tubes arranged in a triangle pattern.
Employing different materials for their constructions, the monocoque frame has been used by manufacturers of road racers, street bikes and scooters. The high strength to weight ratio of this design makes it ideal for motorcycle use. Famous brands to use this design include Velocette (the LE), and Ariel (the Leader and the Arrow). However, the design has been used extensively on many road racers such as the Krauser 50s and 80s used in Grand Prix racing.
Using the engine (and gearbox in some cases) as a stressed member of the frame (or chassis) has been used extensively on motorcycles. Manufacturers of motorcycles with this design of frame typically attached the headstock to the upper part of their engines; a separate frame carrying the rider and rear suspension is bolted to the rear.
The greatest volume of motorcycle frames produced is the step through type. Used for scooters (Vespa, Lam Lambretta) and many low priced commuter bikes (Honda ‘C’ series), the step through design is relatively weak, particularly during braking. However, the relatively low performance of machines using this design does not create a significant problem for the riders.