Fun with Power

This past weekend I did a 40K time trial (bike race), and for the first time I used a Powertap wheel for the race, allowing me to see and analyze my power during and after the race.     As it often happens, I went out too hard and suffered on the last half of the race, but what I did leave with was some great data for training and racing in the future.   Here’s the stats from the race:
My weight: 195lbs
My bike weight: 19.5 lbs
Race distance: 40K
Average watts: 362
Elevation gain: 0 ft (out and back course)
Average speed: 27.7
Finish time: 53:40

Now, here’s what I can do with all this data using the website http://analyticcycling.com/ForcesPower_Page.html
The first thing I need to do is convert my average speed and weight (of me and my bike) into metric units since that’s the units used for the calculator.
So my bike weight + my weight in kilograms = 97.3… so I’ll throw that in the calculator.  And my average speed was 27.7 = 12.4 m/s, so I’ll type that in…

The calculator starts out with all default values that you can then change as needed.

I can leave Air Density at sea level (the default value) and the coefficient of rolling resistance at asphalt road (default value).    My average cadence was 90 and crank length is 180mm.     The default value for ‘Effective Frontal Area’ is .5 meters squared, which calculates to 339.5 average watts; meaning that if my actual effective frontal area was exactly .5, I would have had to average 339.5 achieve 27.7 mph (12.4m/s).   But I actually know that in order to achieve 27.7 mph average I had to average 362 watts, so clearly my frontal area is more than .5.     So I just start filling in higher values into ‘Effective Frontal Area’ until the calculated average watts equals 362.     The value ended up being .539 meters squared.

This is the calculator with all my info for the time trial.

So now that I know my ‘Effective Frontal Area’ I can figure out what average wattage I need to produce to achieve any time.   So I was curious what sort of wattage I would need to do to win the time trial all out.   The overall winner of the time trial did a 50:40, averaging 29.3 mph or 13.1 meters/second.  I simply type in 13.1 meters/second into the calculator with my other existing info…

Averaging 13.1 m/s with my drag requires a massive 421 watts.

And find out that I need to average 421 watts over the 40K course to average 13.1 (29.3 mph) and finish with the same time as the winner (50:40).   I’ve got some work to do…

The great thing about this is that I basically figured out my drag numbers without going to the wind tunnel.    This means that I can figure the required wattage for any time I want to achieve.

A picture from the end of the TT. My frontal area is .539 meters squared.

So let’s say for instance that I want to see what’s it’s going to take for me to break the course record at the TNT time trial.    The TNT record is 22:57 (28.7 mph or 12.83 m/s).  So typing that into the calculator (assuming I don’t change my position at all… which I won’t), shows that it will require me to average 398.1 watts to get that time.  Hmmm….

My TNT prognostications

Knowing this also really allows me to dial in my training as well.  If I have a wattage goal, I can train my power intervals with much more specificity.  Training at or above my functional threshold is the best way to improve my functional threshold.  So now I know that training with intervals at ~360-400 watts is going to give me the best bang for my training buck.

For the record, I train with a Powertap Elite + powermeter and a Joule 2.0 head.  Check out Kevin and my review of the Joule here http://gearwestbike.com/about/joule-2.0-review-pg444.htm.

3 thoughts on “Fun with Power

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