Are online bike calculators considered accurate?



HillyGoat

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Sep 19, 2010
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http://bikecalculator.com/veloUS.html

I always thought the online bike calculators were pretty accurate. Now I wonder. Today I did 40 minutes of high tempo riding, 234 watts averaging 21.8 mph. No wind, flat terrain. Plugging in the required parameters (158lb rider, 15lb bike, hoods, clinchers) in two different bike calculators both say it would require 264 watts to average that speed. Can anyone explain why a 30 watt discrepancy between the calculator and my PT? Is it more likely my PT is off or might the calculators be in error?
 

daveryanwyoming

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Oct 3, 2006
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It's much more likely that the calculators are in error as they are forced to make some big assumptions particularly regarding:

- Rider position and CdA including how much you moved around and how much CdA varied during your ride
- Air density on a particular day which can vary substantially based on temperature, humidity, and of course altitude
- Road roughness, tires, tubes and Crr
- Wind conditions and wind variation around the course you rode including things like effects of passing traffic

Actually I'm surprised the online estimates got you within 30 watts, that's pretty close considering the opportunities for error.

Definitely torque zero your PM before rides and it never hurts to perform a static torque test on a power meter if you're uncertain about accuracy but I wouldn't ever trust an online power estimation calculator over an actual on the bike power meter.

-Dave
 

HillyGoat

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Sep 19, 2010
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There was no wind, riding position is accounted for by the calculator (hoods), no appreciable traffic wind, roads normal good asphalt, tire CRR variation is only a few watts for the bulk of tires, air was quite dense (Sea level at 50 degrees) which would require even more watts. I say the variables mentioned could not account for much error on this ride. I have used the calculator several other times and it has always closely correlated with my PM. Maybe I need to do another torque test.
 

daveryanwyoming

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Oct 3, 2006
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Well it never hurts to torque test your PM but I'd say you're putting way too much faith in the inputs to the power estimators.
 

HillyGoat

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Sep 19, 2010
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Maybe so, only because I have compared the online calculations to numerous other efforts including time trials and hill climbs and they have always been amazingly close (within a percent or two max) to the actual results
 

daveryanwyoming

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Oct 3, 2006
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Originally Posted by HillyGoat .

Maybe so, only because I have compared the online calculations to numerous other efforts including time trials and hill climbs and they have always been amazingly close (within a percent or two max) to the actual results
I'd say you're very lucky then if you're getting agreement right down to the accuracy of commercial power meters while relying on default values for things like CdA, Crr, and rho. That definitely has not been my experience though I'll give you hill climbs tend to agree pretty well as long as they're reasonably steep and it's not too windy. But given the wide range that something like CdA 'on the hoods' can vary between riders it's amazing how well the programmed defaults have worked for you.

FWIW, over the past five years I've performed aero field tests on more than thirty days both for my own positioning and a number of other athletes. In some cases we had wind tunnel derived CdA data or reproducible prior field tested CdA values for their baseline positions and utilized portable weather stations and or local airport NOAA data to determine air density. Even on the calmest days there's measurement noise and hourly variation in air density that is measurable in the resulting data sets and something as simple as a passing car can result in scrapping a test run based on data that won't converge. Which is why it seems so surprising that you can get away with the default values provided in the on line calculators and see such close agreement to your power meter. But I guess if it's always tracked that closely for you in the past I can see why you'd wonder what's different now.

Good luck,
-Dave
 

gudujarlson

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Aug 30, 2012
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For what it's worth, my limited personal experience is that the online calculators consistently underestimate my power during a flat TT by 5-10%. I have a Power Tap and I have never checked its accuracy myself. I've never tried to measure my CdA, but I suspect it is not good because I don't have an aggressive riding position due to the lack of thoracic flexibility. I have measured the resistance of my training tires (Specialized Armadillo Elite) vs race tires (Michelin Pro4 Standard Course) and found about a 5% difference on the trainer.