Is The Affordable Power Meter Finally Here?

Published January 28, 2013 09:33PM

A new company in Boulder, Colo. may be bringing the ability to measure power to the masses.

SRM, PowerTap, Quarq, Look and others all make power meters that do roughly the same job. They all directly measure power—the rider’s output—with the goal of precisely dosing effort in training and races. Why create another system that does something similar?

We posted the following question to Inside Triathlon’s Facebook page: “Weigh in: Do you ride with a power meter or without? If not, why not?”

72 percent of respondents agreed: They cost too much. PowerTap has been the cheapest option, and its power meters start at $1,400. As reader Matthew Fleisher put it in a Facebook comment, “It looks like we have a consensus. No accessories that cost more than the bike.” Fair enough.

Measuring power is tricky, so the hardware is expensive, limiting the number of triathletes who benefit from this invaluable tool. And while PowerTap offers the most affordable model, it is built into the rear wheel, preventing the athlete from training and racing on different hoops. SRM ($2,045 and up) and Quarq ($1,600 and up) both cost even more.

Stages Cycling is breaking down the cost barrier, selling a power meter for $700-$900 (without a head unit, which runs another $200 or so, depending on the model) that can be used on multiple bikes or with different wheels.

Power meters function by measuring the torque applied to the bike multiplied by the speed at which it is rotating. The speed component comes from cadence, but force is the difficult aspect. To measure force, all direct measurement power meters use strain gauges to measure the miniscule amount of flex in the component. In the case of Stages, this means the non-drive side crank arm.

Stages’ system has one big drawback compared to other direct-measurement power meters—it only measures power created by the left leg. All the others record both legs. By ignoring the right leg, Stages looses the ability to measure total power as accurately. While they claim a range of +/- 2%, similar to the others, this only accounts for one leg. The error in accuracy introduced by only measuring one leg is greater. Accuracy, however, is arguably a power meter’s overrated attribute.

Accuracy is defined by Wikipedia as “the degree of closeness of measurements to the quantity’s actual value.” Precision, “the degree to which repeated measurements under unchanged conditions show the same results,” is what really matters. Spitting out the same power number from the same effort produced by the rider makes a power meter a useful training tool. Whether or not the numbers generated are accurate is less important.

Its other stated objective is to make power easier to use. “There’s a ton of complexity [to power measurement],” says Stages senior vice president Pat Warner. “If we strip it down to what the rider needs, can we achieve the core functionalities without that level of complexity?”

To do that, Stages created a system that just measures power. It can’t do right leg/left leg balance or any other esoteric function. Stages didn’t make a head unit (SRM and PowerTap offer their own), deferring to Garmin and other cycling computer specialists instead. The system doesn’t have to be reset frequently, known as zero-offset, to ensure accurate readings. Andy Lull, the engineer in charge of developing the power meter, says zero-offset needs to be done only “as often as you feel comfortable,” not before every ride, giving the impression zeroing is a mental comfort as much as a mechanical necessity.

Temperature has a big influence on the degree the crank arm flexes under the same amount of force, so every strain gauge-based power meter needs to correct for temperature. Stages calibrates its systems by measuring flex at two different temperatures before sending the power meters out of the factory, and Lull maintains this method gives the Stages Cycling meter the ability to adapt to any temperature without losing accuracy. This is one of many claims we will vet during a long-term ride test.

Embedding the strain gauges in the crank arm instead of the spider means Stages can only instrument alloy crank arms. Carbon is out. The company is creating power meters out of left crank arms supplied directly by Shimano, SRAM and Cannondale for road, track and mountain. For triathletes on the cutting edge of fit using short cranks, Stages has options as short as 165mm.

Although Stages has yet to ship a single power meter to a customer, their operation in Boulder, Colo. is fully functional—this is not a garage-based start up. They are a branch of a company that provides 24-Hour Fitness and Life Time Fitness gyms with stationary bikes, some of which are equipped with a different power meter. Stages is stocked with product and know-how. They’re new to the outdoor power meter, but they’re cycling engineering veterans.

The hardware is tiny. A plastic shell barely bigger than a thumb drive contains the battery, a thermometer, ANT+ and Bluetooth transmission devices and the strain gauges. The complete hardware package, not including the head unit, weighs less than 20 grams. Placing it on the frame side of the non-drive crank arm protects the hardware from banging against any foreign objects in a crash. ANT+ transmission allows it to work with current computers from Garmin and others, and Bluetooth is a hedge against future technology, preventing head unit technology from outpacing the power meter. The very first units are shipping this week, and we will start wear testing one at the same. A more detailed analysis is to come on Triathlete.com. If Stages lives up to the billing it created, training and racing with power is about to become a real possibility for a whole new sector of triathletes.