Thursday, August 21, 2008

Power meters

The latest fad in cycling is called the power meter. It is a rather fancy and expensive gadget that is de rigueur for anyone taking the sport seriously. There are three popular power meters on the market. The earliest, and the most expensive unit is called the SRM power meter. It uses sensors in a specialized crankarm-spider combo. The SRM unit has three versions but almost anyone uses the professional (intermediate) version. There is an amateur version that is cheaper and a science version that is more expensive. The more expensive the unit, the more gauges it has.

Next comes another German unit, called the Ergomo. It uses the spindle in the bottom bracket and features an optical, non-friction pickup. Ergomo is about half the price of the SRM professional. Third in line is PowerTap, a system that uses strain gauges in the rear axle as its pickup. Unlike SRM and Ergomo, PowerTap is tied to a special wheel. On the other hand, moving it from one bike to the next is as easy as switching the wheel. Depending on the type of wheel you choose, PowerTap is about the same price as Ergomo or somewhat more expensive.

Finally, Polar, the maker of heart rate monitors, offers a rather complex method that detects vibrations in the chain. That requires a more delicate setup with a sensor on the chainstay. It also requires additional sensors measuring cadence to be placed elsewhere. The main advantage of the Polar system is its price. It is half as expensive as the Ergomo and less than a quarter the price of SRM.

Despite the differences, all four systems perform about the same provided they are installed correctly. As mentioned before, the Polar system is the most finicky and the one that is most likely to develop problems.

Since powermeters are hardwired -the new SRM is wireless- there are few problems with reliability and reproducibility. Battery life is not great but acceptable. Apart from power, all systems display a variety of other useful information. That includes speed, cadence, heart rate -using a Polar strap-, calories burned, and other derived variables.

All devices store data at regular intervals for download and later analysis. Most come with, or are compatible with software packages to analyze training data and make comparisons with others.

For accurate results, powermeters need to be calibrated often. Few cyclists bother to do this, and in some cases it is easy to see why. Calibration procedures can be quite tricky and time-consuming. In general, the effect on data collected is not that great, but for those who rely on fancy data analyses and cross-compare with others, calibration is a must lest the results be meaningless.

One thing a power meter user quickly finds out is that estimating power under normal conditions is quite easy. Power output is a fairly steady affair for a given rider and much of the data collected overlaps to an amazing extent with previous runs on similar courses.

Because power is so easy to estimate based on speed and a few other parameters, iBike recently introduced a power meter that is not only a lot cheaper but also quite accurate. Rather than measuring power, it estimates it using a few key measurements such as speed, cadence, wind, and a few simple inputs such as rider and bike weight. Tests under real life conditions have shown it to be very close to SRM, Ergomo and other direct-measurement devices, in all but the most rare conditions.

Think before you buy. A power meter is, for now at least, a very expensive gadget that will provide minimal additional information. A great way to estimate your power output and to detect any improvements is to ride a flat course for a set distance and measure your time. Or take a long steady uphill at an even grade. These will give you a very accurate way to estimate your progress without having to pay $500 or more for a meter. And if you need to know your power, you can use a simple web-based calculator to get a fairly good estimate.

A great rule of thumb is that if you ride 20mph steady in no wind, you are generating about 175W. At 22 the output jumps to 200-220W.

Happy riding!

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