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Yet More Power Meter Musings

Posted by Bruce Hildenbrand on Aug 13, 2009 10:53:48 PM

I have written about the use of power meters several times in the past year. I still ride with one and am still finding ways that it helps my riding. As I said before, I don't race anymore so I am not looking for something to either help me with my intervals or to analyze my performance on race day. What I am looking for is something, at a much higher level, to tell me the difficulty of a ride I am doing and how I am performing.

 

Again, I am looking something at a very high level. I don't really do "structured" training, per se. I figure out a ride of a certain length with a certain amount of climbing and I go and do it. So, I am not looking at my sustained power up a specific climb. I am looking for a more general measure of how I am riding on that day. But, for the "how am I riding that day" to have some meaning, I also need to figure out how hard the ride was from a terrain/environment perspective.

 

I think I have found a good indicator of both how I am riding and how difficult the ride actually was. My Power Tap power meter has a setting which displays the total energy expended on a ride, expressed in kilojoules(kj). On the flats, the harder I go, the more energy I expend. Unfortunately, on climbs below about 12mph climbing speed, you expend the same amount of energy for a particular climb regardless of how fast you go. This is a bit of a digression, I will get back to that later.

 

What I do after a ride is to divide the total energy expenditure by the number of miles ridden. Today, I expended 3186 kj on an 86-mile ride giving an rating of 37kj/mile. This was a hilly ride, my total climbing was 7500 feet. Two days ago I burned 3000kj for 80 miles for 37.5kj/mile. Again this was a hilly ride with over 6500' of climbing. Last Saturday, I burned 4000kj in a 110-mile ride for 36kj/mile. This was another hilly ride with 7500 feet of climbing.

 

So, my empirical data seems to indicate that somewhere in the neighborhood of 37kj/mi is a good number for me on a hilly ride. As a bit of comparison, on my flatter rides, I get somewhere around 20-25kj/mi.

 

Unfortunately, this type of measurement is still not ideal. Clearly, for me, I burn a lot more energy going uphill than on the flats. But, because you can't really change the energy expenditure by going faster on a climb, this number is more an indication of two things. First, it is a good indicator of the amount of climbing you have done. Secondly, it is also an indicator of how fast you go on the flats. For a given ride, if I go harder on the flats, I will expend more energy. Well, duh?

 

So, if you want to compare how you were riding while doing a particular ride, going faster, or slower, on the flats and flatter sections affects the kj/mile. But, when comparing different rides, the more climbing, the more energy expenditure per mile, unless of course you can ride on the flats at the same watts you put out on the climbs.

 

Everything here is still a work in progress. Stay tuned for more.

 

Bruce

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