Hi Jack,
I have prepared a procedure for setting up an elliptical ring. I wonder if you might test it and see what results you get?
Let me know if you find the procedure hard to follow.
Background
The huge majority of bicycles sold and available in the marketplace today are pedal powered, driving a pair of cranks to which a chainring is connected that carries a chain, thereby transferring power to a wheel.
The nature of a crank arm is that the pedal rotates in a circular path and the nature of the circular chainring is to provide a constant gearing through out the circular path of the pedals. By contrast the nature of the human body, specifically the power available from the leg is that it is available in non-constant pulses, since the legs have evolved through the act of walking and running which require a pulse like energy output from the body channelled through the foot’s contact points with the ground. Any able bodied human can readily identify the degree of extension of the leg that provides most power, by bouncing on the balls of their feet as a boxer might do. Indeed an extremely terse summary of the human body’s motive ability is summed up in the process of jumping.
Contrast then the short pulse loading that is natural in the body to the constant loading requested by the circular path of the pedal combined with a constant gearing load and it is immediately clear why throughout the history of the bicycle there have been numerous attempts to provide ovoid shaped chainrings which during the rotation of the pedal serve to alter the effective gearing and therefore the pedal loading so as to more closely match the natural ergonomic functioning of the human body.
The leg assumes a natural degree of extension in the process of bouncing on the balls of ones feet. Any attempt to bounce at any extension different to ones natural extension immediately feels awkward and weak and if continued would almost certainly lead to injury.
When introducing an ovoid chainwheel it is therefore critical that the peak gearing coincides with the extension of the leg that naturally delivers peak strength, not to where the leg has maximum leverage.
A recent patent addressing this phenomenon and meeting commercial success is US 2005/0022626 A1. The author of this patent also produced US 2006/0211529 A1, which implemented an ovoid chainring whose peak gearing point was adjustable to coincide with the leg’s peak power extension, whereas generally speaking previous implementations placed the peak gearing point at the position where the crank created most leverage, yet the leg was still far from its strongest extension thus ignoring the delicate realities of the human physique and leading in practice to knee damage. The peak gear of an elliptical chainring should coincide with the crank being about 20 degrees past the half way mark of the extension stroke of the leg. US 2006/0211529 A1 concludes that the momentum of the leg causes the peak capacity of the leg to occur past the half way mark of the leg extension where leverage is greatest. The nature of human physiology provides a possibly better explanation of the point of optimum timing.
