The Pursuit of Chassis Rigidity

It’s all about connecting the steering head and swingarm pivot as directly as possible.

Norton Feartherbed motorcycle frame
Norton Feartherbed motorcycle frameCW

We know that the late Antonio Cobas was the originator of the now widely used twin-aluminum-beam motorcycle chassis. It has been very successful especially in high-powered machines because it most directly connects the two most heavily loaded points on the chassis—the steering head and the swingarm pivot.

The twin beams begin at the steering head and run diagonally, as straight as possible, to the swingarm pivot. The straightness and directness of this structure make it possible to provide maximum strength at minimum weight. Usually the engine is used as a stressed member as well.

This idea of direct connection between head and pivot is not new. After the Irish McCandless brothers showed in 1950 the value of the steel-tube twin loop chassis (the “Featherbed” Norton, see above photo), which provided wide-based support to the swingarm, an enterprising English sidecar racer, Colin Seeley, saw that it could be improved. He was able to achieve superior stiffness at a considerable weight savings by running two of his chassis tubes almost straight from head to pivot. His “Seeley-Matchless” machine went on sale to racers in July 1966.

Thirty years earlier, Husqvarna engineer Folke Mannerstedt stiffened the chassis of his roadrace bikes by adding a straight member on either side, running from the bottom of the steering head directly to the rear axle plates (these were rigid bikes).

1935 Moto Guzzi IoMTT race action
1935 Moto Guzzi IoMTT race actionCW

Even before that, Englishman Frank W. Cotton in 1922 took his novel chassis concept to the Isle of Man TT races. He knew that the strongest structures are built from straight, not curved, members. Cotton motorcycles were noted for what in those days was called "good steering." In those days, that meant freedom from wobble and weave, which was the result of their construction as a system of triangles. The man who has been called "motorcycle racing's first superstar," Stanley Woods, got his start on a Cotton.

Through the 1920s, when a motorcycle chassis design showed misbehavior at speed, one of the standard remedies was to “add more stays.” Stays were braces, run back to the axle plates, often at three levels. The top stays began at the top of the saddle tube (the vertical tube behind engine and gearbox, with the saddle at its top). Bottom stays began at the bottom of the gearbox mounting plates. And, when necessary, so-called “torque stays” were added at intermediate level, originating at the top of the gearbox mounting plates. The famed Brough Superior V-twin tourers were given “doubled stays” for a total of 12 such members.

Why didn’t efficient, stiff designs like Cotton’s catch on sooner? I suspect that, in the days of rigid frames, riders perceived their added stiffness all too well. When Stanley Woods put an end to the rigid frame idea by winning the 1935 Senior TT on a swingarm-suspended Guzzi 500, a rich new menu of design choice was presented to all.