New Crankarm Design: As the Crank Turns
– By Stephen S. Cheung, Ph.D.
Associate Professor, Dalhousie University
One of the beautiful things about the bicycle is its elegant simplicity. Most of us, with a bit of knowledge, can take apart a bike and put it back together. At the same time, there are always new technical advances and something that can be tweaked and improved.
The bicycle is endlessly fascinating because there are so many ways (unless you talk to the UCI) to improve its performance. Every few years, interest re-focuses on the optimal biomechanics of the pedaling motion. Back in the mid-80s, Shimano came out with the non-round Biopace chainring, with the theory of maximising power output throughout the pedal stroke. Good science or not, Shimano’s marketing misstep of filtering Biopace upwards from the lower-tier groupos rather than from the pros downwards ultimately doomed it. Then in the ’90s, Greg LeMond and Miguel Indurain popularized the use of long (180 mm for Big Mig!) crankarms to maximise leverage to the pedals. However, while long cranks are great on the downstroke, it also results in more inertial resistance from the leg during the upstroke.
To get the best of both worlds, an Italian research group has patented a new crank system (yet unnamed) that changes crankarm length throughout the pedal stroke, increasing it during the downstroke to maximise leverage and decreasing it during the upstroke to minimise inertial resistance. In the well-respected Journal of Biomechanics this month, Zamparo et al.(1) published a study comparing the biomechanics and energy requirements of this pedal system compared to a standard 172.5 mm Dura Ace crank. Seven highly-fit subjects pedaled at 60 rpm for 5 min at 50 W increments using each of the two systems. Interestingly, no differences were observed in energy costs between 50-200 W. However, oxygen consumption decreased and mechanical efficiency slightly but significantly increased in the 250-300 W range with the variable cranks. This follows the theory of an increased torque on the downstroke along with decreased torque on the upstroke. Overall, the authors extrapolated this improvement to an extra 1 km over a 1 h TT.
How well will this system play out one day in the local peloton? The improved efficiency at higher wattages suggest that the cranks would still maintain their benefits at higher cadences, though this still needs to be researched both in the lab and especially out in the field. Ultimately, the makers should learn a lesson from the Biopace experience and convince the pros to use the system first. This may be the case sooner rather than later, with reports of a small Spanish or Italian pro team willing to test out the system in the pro peloton next year. Stay tuned “As the Crank Turns!”
1. Zamparo P, Minetti A, and di Prampero P. Mechanical efficiency of cycling with a new developed pedal-crank. J Biomech 35: 1387., 2002.