Adjusting Elliptical or Ovoid Chainwheels

Early Origins

Elliptical chainwheels or elliptical chainrings, or ovoid rings or whatever you like to call them have been in and out of vogue every decade or so for more than a 100 years. [1,2] They come into vogue because of the logic of taking advantage of the extra leverage the crank has half way through the stroke and they go out of vogue because they just don't catch on. Results are inconsistent between riders and machines.

This article gives an explanation of why ellipticals are finding success and crucially gives a method for ensuring yours are bolted up in the correct place for you on your bike. The method was developed with recumbents in mind, but is generic and applies to any pedalling system with a chain or belt power offtake.

Recent Solutions

So what has changed in the last 5 years and why are elliptical chainwheels now being used at the highest level of cycling competition? The answer lies in applying a modern appreciation of the nature of the human physiology and performance as well as recognising the sinusoidal leverage function of the crank. And the other part of the answer is in providing adjustments so the timing of the peak of the ellipse can be fine tuned to the rider.

See Antony Galvan's bike fitted with RotorRings Q-Rings. Antony is California State Criterion Champion (2009)

See Maria Parker's record-setting Cruzbike Silvio recumbent fitted with RotorRings Q-Rings.

The power of the leg is not constant throughout the cycle, so the optimum timing of the higher gearing point on an elliptical chainwheel has to be found as a combination of both leg strength and crank leverage. If a rough function of the power of the human leg as it extends is multiplied by the sine function for the leverage of the crank, we can see graphically where maximum power is available and so gain insight for setting up the elliptical chainwheel.

The graph at left is based on:

1. The sine function of the crank as it moves from Top Dead Centre to Bottom Dead Centre, to indicate the leverage available.
2. The cosine of the leg angle as it extends to Bottom Dead Centre, the cosine being an approximation of the increasing strength available as the leg straightens.
3. The product of the above two functions, illustrating where peak power occurs during the cycling stroke.

 The above graph illustrates that the peak power (red line) occurs about 20° after the half way mark, or about 70° before Bottom Dead Center. This agrees with empirical evidence found across the Internet and with the q-ring patent by Spanish Rotor Cranks company (Rotor Componentes Tecnologicos S.I) US 2006/0211529.  The key insight by the Rotor Crank company is the importance of adjusting the crank position to take into account different riding styles and different kinds of bicycles. On the Rotor Q-ring, 35 holes are provided, giving 35 distinct positions in a full rotation of the chainring. Now if the chainwheel is given half a turn the timing of the ellipse is unchanged, but the holes will no longer match. The ring will have to be rotated another 5.14° to line up the next hole. This gives the ring 70 effective bolt up positions over 360°, or an adjustment range of 5.14° (360°/70). Its that simple principle of doubling the available adjustment positions that provides the basis to their patent.

Getting the Timing right

You would think that bolting up the elliptical chainwheel to the spider so the maximum effective gearing occurs at 70° before BDC would be an easy thing. But after seeing various bikes shops and individuals alike get it wrong, particularly on recumbents, Cruzbike has developed the following failsafe procedure.

Note A
1. Push on the pedal as far as it will go
2. Identify the first tooth of the chainring to engage the chain
3. Identify the tooth most aligned with the crank arm
4. Count the number of whole teeth between the two 13.2
5. Count the teeth on the chainwheel 44
6. Express as a percentage 30%
6. Express as a portion of 360 degrees 108
Note B
70° is right for most, with probably all riders in the range 60° to 80°

Conclusion

I have seen cases of bio-pace ellipticals being fitted to recumbents without making the corresponding adjustment for the different leg-chain angles with the effect that the rider gets a deleterious effect. But that doesn't mean that ellipticals don't work, it only means that particular elliptical chainwheel does not work on that bike with that rider when bolted in that position.

So there you have it. You will get a performance increase from using elliptical chainring, no matter what brand you buy, as long as you are able to get them into the right position. Many have found a new use for the Shimano biopace items when they can be clear about where the adjustment should be. The Shimano biopace has 5 holes giving 10 bolt up positions or 36 degree adjustment space. You might be lucky and land in your sweet spot - hopefully using this procedure will help you determine if you have.

[1] http://www.sheldonbrown.com/biopace.html

[2] http://www.highpath.net/highpath/cycles/ovals01.html