Power Meters, Part One

posted Feb 28, 2010, 5:05 PM by Donald Vescio   [ updated Sep 12, 2011, 8:10 AM ]

Power meters have been the big buzz for the past five years or so in cycling, replacing in the minds of some coaches and athletes the use of a heart rate monitor.  Essentially, what a power meter does is to measure that amount of work being done on the bike, usually in units called "watts."  The more watts produced, the more work that is being done; unlike heart rate, which can be influenced by a number of outside factors (fatigue, weather, hydration, etc.), watts are remarkably stable--300 watts of effort is 300 watts of effort, whether you are tired, rested, going up hill, or pounding into a headwind.

It's important to understand that while power meters measure the work done on the bike, heart rate monitors measure stress placed on the body. While an athlete can be very successful relying exclusively on either technology, it is our belief that both power and heart rate data collected in tandem offer the most complete performance picture.  There are a number of excellent power meter guides available for novice and expert cyclists, perhaps most notably Allen and Coggan's book, Training with Power.  But unless you are willing to make a commitment to review and analyze your data on an ongoing basis, a power meter becomes little more than a remarkably expensive speedometer for your bike.

What follows is a summary of our thoughts and recommendations regarding bicycle power meters.  In part two of this series, we will outline some specific ways in which power meter data can be used to optimize your training and aerodynamic performance.  We have extensively used five of the major power meters available on today’s market; the sixth, Quarq, is new to the market and hasn’t been tested as of yet.

Power meter data has two primary uses: tracking performance over time; setting a target pace goal when racing or training.  Of the two, we find that tracking data over time is most useful, as it helps quantify whether you are improving or not.  An important collateral use for power meter data, assuming that the meter is consistent (see below) is that it’s an excellent tool for optimizing on the bike aerodynamics.

Power data can be correlated to physiological stress over time; on the other hand, TRIMPS also does this, too, based on HR. It's important not to discount HR and perceived effort when training and racing--power data only provides part of the information that's critical for assessment (you will hear others argue to the contrary).

 If I race with a power meter, it's only to collect data for later review--I don't actually view the data during the ride (most of the time, I stick the power meter head under my seat).  Power meter data can be used for pacing--e.g., "I will ride for one hour at 400w," but PE/HR can provide roughly equivalent pacing feedback. 

There is a huge difference between accuracy and consistency. A power meter might give an accurate reading at any given moment, but not be consistent over time (drift in values, etc.).  A power meter might not be all that accurate (am I really putting out 300w?), but it can be remarkably consistent over time (in which case you're comparing relative values).  If I can’t have both accuracy and consistency, then I’d opt for consistency every time.

Personally, I value consistency rather than accuracy, except for very limited applications, such as training for match sprints on the track: "What is my absolute peak power and five second power data for the next six efforts?"

Regarding power meters…

The SRM professional model is perceived to be the gold standard of power meters, but it costs ~$3K.  It is a crankset-based power meter; this means you can use any wheel that you want.  The newer versions can be swapped between bikes relatively easily.  SRM power meters do require periodic vendor maintenance; they can be user calibrated.

Powertap—This is just as accurate, by most counts, as SRM, though users are locked into a specific rear wheel, as this is a hub-based system.   It is very easy to use and install; for racing, just put a cover over your training wheel, and you'll have a setup as fast as any disk. (There are lots of studies on this).

Polar--I really like this power meter, though its initial setup is a little fussy.  Once it is setup properly, though, it is as accurate as a Powertap. Go with the CS600 version--it's a lot easier to install than the first generation version, and there are fewer problems with data drops (which all power meters have, at times).  Also, you can use the computer head as a watch for your run.

iBike--actually my favorite, as it is low-cost and bomb-proof. Some claim that iBike is only a power estimator, and not a power meter, but all power meters estimate power in their own ways.  iBike is easy to install and to move from bike to bike; remarkably consistent (and even accurate) if calibration is done with some care.  The only issue is that it can be challenging to mount it on some aerobar combinations. 

Ergomo—Ergomo’s manufacturer recently has declared bankruptcy and support for their units is very limited.  Ergomo determined power by measuring the amount of twist that occurred in the bottom bracket axle; because of its design, it measured the amount of power generated by the rider’s left leg, and then doubled it to establish total power. For most riders, power is evenly distributed on the leg and right legs; for those with significant muscular imbalances, the power readings could be suspect.  There were reports of problems with the unit’s consistency (see above), and installation was extremely challenging in practice.

Don't forget the software.  Polar has the best native software; third party software that works with any of these meters include CyclingPeaks (~$90) and SportTracks (free open source--very, very good and probably more flexible--and useful for triathlon--than Cycling Peaks).

What do I use?  I mostly train by HR and PE; periodically, I mount a SRM to collect session data to track performance over time.  I like the SRM because I can use it with my race wheels or training wheels; also, I like it because I can tuck the computer head under my seat so that it's out of the way (the iBike requires the computer head to be mounted on the front of the bike to see clean air flow).  I'd use the Polar, but it's hard to set up properly on my Cervelo, which has a combination of really short stays and an overly large chainring.  If I was starting out and wasn't sure that power-based training and racing was for me, then I'd get an iBike or Polar; if I were starting out and had lots of disposable cash, then I'd get a SRM professional, which carries excellent resale value.  If I were looking to spend less than for a SRM and didn't mind using the same rear wheel for racing and training, then I'd consider a Powertap.

Power meters are no magic cure for slow bike splits; used correctly, though, the data that they provide can make your training much more efficient and effective, which is the topic of the next part of this series.