Putting Power to the Pedals

Power is the rate at which work is performed or energy is transmitted, or the amount of energy required or expended for a given unit of time. In the cycling world, power is measured in watts (w). The watt is the SI derived unit of power, equal to one joule of energy per second. Power to weight ratio (w/kg) is one of the most critical aspects to being a successful bike racer. The more power one can generate as they pedal relative to their body weight, the faster that person will be as a cyclist. For example, an average person can usually generate a power to weight ratio of about 2.5 w/kg for a 1 hour effort, whereas a Tour de France contender can turn out 6.5 to 7 w/kg for the same amount of time. Hence, cyclists, are always struggling to strike the appropriate balance between their body weight and their musculature.

I, being the bike nerd that I am, have a Powertap hub and computer on my road bike that instantaneously measures my power output and it stares me in the face as I ride. I download data to my computer and crunch numbers to see how I am doing and my workouts are structured around hitting certain power objectives. I can very accurately track how my fitness is improving or decreasing by doing standardized tests over a certain course and comparing my average power for the same route. Power is more accurate than time or speed, because it eliminates variables such as wind, tire pressure, mechanical problems, etc. As the saying goes, power is absolute.

I have always wanted to be able to have a power meter on my mountain bike, but there was never a really good option because Powertaps were not compatible with disc brakes, SRM seemingly ignored the MTB market, and Ergomo, well, it has some serious bugs and is not compatible with triple chainring cranksets found on MTBs. So, I've never had good data to analyze mountain bike races or how to structure my training to better perform during races.

For 08, however, there several manufacturers that are releasing viable options. One setup that I'm lusting after is combining a Garmin Edge 705 gps unit with a SRM crankset that measures power generated at that crank rather than the wheel. The GPS gives you all the cycling computer data (speed, distance, etc.) as well as topographic information such as rate of ascent/descent, pace, and other numbers to crunch. The SRM crankset communicates wirelessly with the Garmin computer providing power data.

08 SRM MTB Crankset


Garmin Edge 705


Also, word on the street is that Powertap is going to release a disc-brake compatible rear hub in early 2008. This unit would not be compatible with the Garmin, but it would still give power information, which is the most critical. Plus, it will likely be a slighlty cheaper option than a SRM setup.

Lastly, there is a new company on the block called Quarq. They are going to be rolling out a chainring spider mounted power measurement device that is adaptable to cranksets with separate chainring spider assemblies. It is not clear to me whether this unit will work with a triple chainring crankset though.

If one of these options comes to fruition, this year is going to be even more fun. I can't wait to be able to crunch my MTB data and use it to get faster.

Blue Star (aka Bike Nerd)