Bobby with unconventional chain rings at TOC '08 : Courtesy Eddy58
Last Sunday, as I watched the "Race to the Sun" Prologue TT on Versus, I observed Bobby Julich had the funky looking non-circular chain rings on his Cervelo. Okay, so whats new?
There seems to be a very comfortable relationship between him and O.Symetric. Julich reported once that he believed the chain rings gave him an extra 5 seconds per kilometer in his time trial runs.
Notice that in the beginning of this post, I said non-circular. Its safe to say that it is a custom made shape to Julich's biomechanical situation. There have been a few elliptical and oval models in the past. One of the best known initiatives was Shimano's oval Biopace of the 80's. Others who followed suit include the independently operated Power Cranks and Rotor Cranks with their elliptical Q-Rings. Another drastic idea was to use a spring mechanism with the circular crankset so that energy could be returned back at the dead centers to aid in power delivery.
In the 360 degrees of circular pedaling movement, there are two "dead centers", each occurring when the crankshaft is near the 6 o clock and 11-12 o clock positions. In these spots, it is difficult for the human body to produce much crank power since the tangential pedal force direction (shown below) is perpendicular to the preferred force direction of the legs. Thus, when one leg is vertically up and the other down, tangential forces cancel and a situation known as "Power Vacuum" occurs for a very short period of time.
Non-circular chain rings aim at reducing the dead centers by changing the shape of the pedaling path and manipulating the gear ratios so that a rider can minimize time spent in these dead spot and approach pedaling power potential.
Pedaling Forces : Analytic Cycling
It makes sense to me but I can't speak for these types of chain rings since I don't have them. It seems to me that they must be custom made to a rider's pedaling technique to provide any objective feel for higher power. Otherwise, any noticed variation is simply an unwarranted placebo effect. Other known issues are linked to nonrhythmic movement of the knee, front shifting and chain suck.
Apart from the mechanical side, there's also a small self-esteem issue for me to be maverick at the local circuit and ride strange looking stuff. I'm not Cipollini, but neither do I have the capacities of Julich. At the moment, I like what I use and prefer the idea of fully developing my upstroke phase to accelerate through the bottom half of the pedaling circle. This should account for any power variations at the dead spots.
What do you think of non-conventional chain rings? If anyone has had a direct experience, it'll be nice if you share with us some numbers related to the improvements or any other disadvantages you may have observed.