Wobbly-web wheel

The wobbly-web wheel is a form of metal disc wheel where the disc is 'wobbled' into spokes. This provides a stiffer, lightweight wheel.

Wobbly-web wheels are best known through their iconic use on Lotus racing cars of the late 1950s and 1960s.

The theory of a disc wheel depends on their behaviour as a stressed skin structure. Their materials are assumed to be , but flexible. Such a skin is stiff against tensile loads in the plane of the skin, but flexible for loads perpendicular to it. A flat disc is thus strong for loads in the plane of the disc and also for torques. It is flexible for axial loads, perpendicular to the disc plane and also for nutating, wobble of the axle at an angle to the main axis.

A coned disc also becomes stiff against axial loads, as the web is no longer purely perpendicular to the axis of the wheel. Unlike a flat disc, there is no direction where the whole of a coned disc would be perpendicular (and thus flexible) to it. A drawback of a simple cone is that the wheel now becomes wider, along the axis. The wobbly web effectively "folds" a coned disc wheel into pleats, making it narrower again, but retaining the axial stiffness. This pleated disc is also stiffer against nutating forces. A similar feature, a 'sine wave wall', is sometimes encountered in architecture where a straight wall is made stronger against sideways forces by corrugating it.

Key features of the wobbly web wheel are that they are designed as a modification of the disc wheel with additional shaping, rather than a spoked wheel with merged spokes. They can be manufactured by either pressing from sheet steel, or by casting in light alloys. To avoid stress risers, the webs are formed into smooth curves, rather than sharply defined spokes.

When cast, foundries prefer a disc which has a consistent wall thickness, as this makes their shrinkage behaviour simpler to control. The Lotus wheel's peculiar shape was arrived at deliberately, by keeping this consistent wall thickness, for ease of manufacture, and folding it to achieve the varying stiffness distribution required across the radius, to meet the performance needs.

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