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Sublimis / UrbanBiker

Urban Biker issue tracker and Knowledge Base
https://urban-bike-computer.com/
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What assumptions does the app use for air density and for drive train loss? #131

Open ghost opened 3 years ago

ghost commented 3 years ago

I am still trying to adjust the power parameters to get the results I want, and I am getting very close. I actually have a spreadsheet for some of my rides where I estimate power from first principles and compare it to the results Urban Biker gives. I am using the formula from the Computational Cyclist website [https://www.gribble.org/cycling/power_v_speed.html]. Urban Biker is giving results within one percent to my spreadsheet when I ride at constant speed. (I use a metronome to time my pedal strokes to keep my speed as constant as possible.)

I am using the app with an indoor stationary bike with GPS turned off. The only sensor I have is a speed sensor. The app says that my phone does not have barometric altitude or thermo altitude. I suspect that the app is using a constant for air density that is hard-coded in the app. Based on my testing, it looks like that value is around .075 lb/ft^3. Can you tell me what it is exactly?

Also, is the app assuming a value for drive train loss? This is the loss in power as it is transferred from the pedals to the wheels due to friction on chain/gears, and referred to as Loss_dt in the equation in the link. Based on my testing, it looks like the app is assuming drive train loss of about 2.8%. Can you tell me what the exact value is?

Thank you for indulging my very geeky interest here. I am well aware that even if I were to get the power parameters so that the app calculates power "perfectly", it will still be an estimate based on the fiction that a stationary bike has speed and travels distance, and then the calculation of calories from power is only an estimate too. But I am having fun playing around with it . . .

Thanks! Lisa

ghost commented 3 years ago

Developer, may I have an answer to these questions? What assumption does the app use for drive train loss? What assumption does the app use for air density (when there is no altitude sensor)?

Thanks! Lisa

Sublimis commented 3 years ago

Hi Lisa, sorry for the late reply. No worries, we're all geeks here 🙂

When altitude is unknown (GPS turned off, that's your case), it's taken as 0 meters above sea level. When temperature is unknown (frequent case), it's taken as 15 °C. Air density is then computed using a standard barometric formula. Thus it's not quite a hard-coded constant, although if the above conditions are met (no altitude nor temperature), it will in fact be constant.

However, for a stationary bike it's probably best not to fiddle with air density at all, and exclude air drag from the power/energy calculations altogether. You can do this by simply entering 0 for the "air drag area" parameter (Cd*A).

Urban Biker does not take into account drivetrain losses when computing power. We think it would be too small a correction to an already very noisy data to be of any significance, but we're open for discussion. Instead, all generic losses are covered by a more general concept of "thermal efficiency" when computing calories only (not used when computing power).

Thanks for the feedback!