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Elliptical Chainwheel
Shortly after the development of the chain-driven "safety " bike in the late 19th century, it occurred to many people that there could be some benefit in making chainwheels of elliptical shape, rather than round, as a way to tailor the instantaneous gear ratio to the position of the rider's legs at various parts of the pedal stroke.
The most obvious way to do this was to put the long axis of the chainwheel perpendicular to the crank. This was intended to accomplish two things:
- Provide a higher ratio when the cranks were horizontal, when the rider can apply the most power to the pedals.
- Provide a lower ratio when the cranks are vertical, to get the cranks through the "dead center" position as quickly as possible.
This design looks good on paper, based on a static analysis of the pedal forces, but it turns out to be hard on riders' knees. This is partly because the ratio is too high during the "power" phase of the pedal stroke, encouraging the rider to push too hard.
In addition, this system causes the legs to "snap" as they change direction too fast at the top and bottom of the stroke. It is my belief that this is the major reason this type of elliptical chainwheel is harmful to riders' knees.
In the 1980s, Shimano developed a non-round (but not exactly elliptical either) chainring called "Biopace ®" that was oriented in the opposite direction, so the long axis was roughly parallel to the arms. This was based on a dynamic analysis of the riders' pedal stroke, considering speed as well as force.
See my separate article on Biopace Chainwheels.
