Wind Resistance Formula

Anything about Power (watts)

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ericreid421
Posts: 4
Joined: Tue Jan 15, 2008 3:28 am
Location: Seoul, Korea

Wind Resistance Formula

Post by ericreid421 » Thu Jan 24, 2008 9:17 am

I recently tested my Fortius (2.0.1 software, 0.10.4 motorbrake) against my SRM power meter, and I found a considerable discrepancy. After doing some research I wonder if the Fortius is taking wind resistance into account or not.

I used a calibrated motorbrake w/ 120psi in the tires and about three turns on the brake. I got readings for cadence, HR, speed, and watts on both the computer and the SRM on the handlebars.

I found that the Fortius was consistently higher than the SRM, and the faster I went the bigger the difference. at 100W Fortius was 115W and at 250W Fortius was over 300.

I did the math and found that at 25mph, for a person of my weight (6'2", 175lbs), the wattage on the Fortius (300W) was exactly what I would need to put out to do that speed OUTSIDE... while the SRM was 50 watts lower for the same speed, indoors, with no wind resistance.

Am I doing something wrong, calibrating wrong, or does Fortius not account for wind resistance? I need to know because I plan on racing with power this upcoming season.

mcorn
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Location: USA, Bellingham, Washington

Post by mcorn » Thu Jan 24, 2008 4:03 pm

What goes in to the Fortius speed and power algorithm is a bit of a secret as there are no detailed published specifications. Tacx doesn't even have a published spec on ratings for continuous and peak wattage. My suggestion is that you use your ability to accurately determine wattage and heart rate to measure your progress.

I think what is most important for training is being able to obtain consistent relative measurements over time.

If you want to know how that will translate into real world performance, you can go to a couple of good quantitative cycling websites:

http://www.analyticcycling.com/QCHome_Page.html

http://www.kreuzotter.de/english/espeed.htm

Both of these sites have very detailed formulas and do a good job of laying out the assumptions.

ericreid421
Posts: 4
Joined: Tue Jan 15, 2008 3:28 am
Location: Seoul, Korea

Post by ericreid421 » Thu Jan 24, 2008 4:16 pm

thanks. I've been going around bragging that I'm doing X watts at Ironman pace, so now I'll have to go back and humbly admit that I'm actually only doing X-50 watts at IM pace. Your advice is good.... consistently improving numbers at a certain HR over time.

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