Turning Runkeeper Cycle Logs into Watts

A good enough approximation is (10 x velocity x slope x mass) + velocity^3 / 8

velocity in metres per second (divide kmh by 3.6, divide mph by 2.25)

mass in kg (bike and rider)

slope in percent

output in watts.

Example, a 70 kg rider on a 7 kg bike doing 18 kmh up a 10% grade:

(10 x 5 x 0.1 x (70 + 7)) + 5^3 / 8 = 5 x 77 + 125 / 8 = 401 watts = 5.7 watts per kg. That's Cadel Evans territory.

In keeping with my all numbers, all the time philosophy, I decided that what I really need to be able to do is to turn a watts number. Of course, the way to bankrupt yourself correct way to do this is with a gizmo on your bike like this one:


If you want a non-wired version of something like that it'll set you back about $1,000.

So I figured, there's got to be a math way to do it. And I googled, and found this cool formula:

A good enough approximation is (10 x velocity x slope x mass) + velocity^3 / 8

velocity in metres per second (divide kmh by 3.6, divide mph by 2.25)

mass in kg (bike and rider)

slope in percent

output in watts.

Example, a 70 kg rider on a 7 kg bike doing 18 kmh up a 10% grade:

(10 x 5 x 0.1 x (70 + 7)) + 5^3 / 8 = 5 x 77 + 125 / 8 = 401 watts = 5.7 watts per kg. That's Cadel Evans territory.

Sweet. So now I just have to get from runkeeper data to this formula. Here's my most recent AWESOME ok for a beginner ride:

And then I realized that since I always bike a loop, the whole first part of that equation dissapears and we get simply velocity^3/8. Which honestly seems like a drastic oversimplification, but what the heck, let's give it a shot. 2:33 min/km needs to be converted to meteres per second for this formula, which converts to 153 seconds/km. Divide by 1000 to get .153 seconds/meter, and invert to get 6.535.

So, after this simple conversion, the internet is telling me that my power output for this ride was: (6.536^3)/8 = 35. Which is pitiful. Also, I don't believe it.

So, what's wrong? Well, for one, there's no account for resistance, size of bike, wind resistence, rolling resistance etc. So, I check out http://bikecalculator.com/wattsMetric.html. There, entering in the same numbers resulted in 97 watts for the ride, which is closer, but there again there are problems. Primarily, there is no way to indicate the elevation changes in the ride. As the runkeeper data shows, the road goes up and down significantly. The uphills suck, and the downhills rock, but do they offset each other? Not likely. But I'm out of time and space, so that will have to be the topic for another post!