Coach Conversations: Allen Lim and Why PowerTap?
Unlike 15 years ago when power meters were just beginning to appear on the market, consumers now have a litany of options and feature sets to chose from. Those options include the location on the bicycle, the cost, weight, and sensor design.
While this can make it hard to decide between power meters, for me, there is only one thing that really matters when investing in a power meter and that's accuracy. My reasoning is that if a power meter isn't accurately measuring power then it's not a power meter — it's just a piece of decoration.
With that in mind, there are only two power meters that have been scientifically validated as accurate. They include the PowerTap and the SRM 1, both with the longest histories in the power meter market, but both with very different designs.
Almost every power meter on the market, including the SRM and PowerTap use strain gauges to measure power. The strain gauges in the SRM are placed in the spider of the crank while the strain gauges in the PowerTap are held within a tube inside the hub that connects the free hub body with the wheel. While strain gauge location might seem like a trivial difference, it's not, as there are some inherent benefits of a torque tube over all other designs.
By creating a torque tube within the hub the measurement of power is isolated from outside forces that might affect the measurement of power like road vibrations, frame flex, and chain angle. This isn't the case with strain gauge systems that use other locations and in real world conditions this difference can have an affect on accuracy and reliability. The other benefit of a torque tube is simplicity, as there are only two things that are need to be measured – the twisting or torque around the tube and the angular velocity of the wheel. By definition, power output is torque multiplied by angular velocity and the PowerTap, because of its unique torque tube design, is the only power meter that measures power by that definition. Although torque tube designs also exist for the bottom bracket, they only translate power through the left crank arm, excluding forces from the right crank arm, which may be significantly different depending on the individual. Finally, by placing the torque tube between the free hub body and the wheel, the torque tube is unloaded while coasting, creating a zero load reference for the wheel to continuously and automatically recalibrate. This is especially important in real world conditions because changes in temperature can change the zero offset of a strain gauge and if not compensated for real time can lead to inaccurate measures.
Ultimately, the reason why I chose PowerTap, is because it's scientifically valid measure of power with an inherently simpler and straight forward design that make it more robust in real world conditions. While other feature sets like a minimal (100-130 gram) weight penalty, ease of use, incredible track record, exceptional customer service, and new and improved hub design are also benefits, the bottom line is that the PowerTap is the one power meter that I know I can always be 100% confident is giving me the right data.
Dr. Allen Lim is a sports physiologist, cycling coach, and a founder of Skratch Labs, a manufacturer of performance hydration mixes and the world's first active nourishment company. Beginning his coaching career with Jonathan Vaughters' TIAA-CREF cycling development team, Lim developed a method of testing for biological markers of performance enhancing drugs that led to cycling's Biological Passport. Lim was director of sport science for Garmin and RadioShack cycling teams and is the only American scientist to have worked and cooked for teams at the Tour de France. Lim has worked with dozens of top American cyclists to improve their performance and nutrition.
1. Duc, S., Villerius, V., Bertucci, W., & Grappe, F. (2007). Validity and reproducibility of the ErgomoPro power meter compared with the SRM and Powertap power meters. Int J Sports Physiol Perform, 2(3), 270-281.