AeroPod, PowerPod and Newton

My experience is that tight fitting arm and leg warmers do not affect CdA very much. A windbreaker on the other hand is a different story: 10% or more change in CdA. I learned the keeping the windbreaker completely zipped up to the neck helps limit the increase. I suppose this is because doing so av...

I've found battery life temperature dependent, maybe 10 hours winter and 40 hours in summer.

My experience with the iSport, if you want the most accurate CdA calibration you can get, don't bother with a coast down calibration. Instead, set Crr to a fixed value and coast down a hill at 35mph. Do multiple runs and set CdA so to get near 0 watts when coasting. Of course across multiple coasts ...

If CdA = Aero/Wind Scale, what is the relationship between Fric, Riding Tilt, and Crr? I think Velocomp has not disclosed the relationship. My best guess is that it is something like this: Fric = total weight (kilograms) * acceleration of gravity (9.81 meter/second^2) * (Crr (unitless) - tan(riding...

Thanks rruff. Is it possible to check wind scaling on an iSport?

Yes. Watts are increasing with warmer temps. I looked up standard air density: at +5C it is 1.269 kg/m^3, at +30C it is 1.164. Ratio of 1.164/1.269 is about 92%. Interestingly, my summer/winter Aero ratio 0.406/0.439 is also about 92%. I wonder if this is just a coincidence? Maybe my unit's temperat...

Has anybody noticed a longer term drift in their calibration? Up until May I was using an Aero of 0.439 and getting nearly zero watts on the coast screen at high speed - 35 to 40 mph. By the end of May, coast watts had gotten larger so I reduced Aero to 0.428. By the first of July coast watts had in...

There is no reason to spend a long time trying to get a small wind offset value. Anything around 0.5 or smaller is fine. The reading displays dynamic pressure in units of pascals. To estimate the effect of a nonzero wind offset on power, use the forumla Dp*CdA*Vb where Dp is the offset and Vb is bik...

Power attributed to Crr at a particular speed can be easily estimated as 9.8 * weight(kilograms) * speed(meter/second) * Crr.

Examples:

Weight 90kg (198lbs), speed 8.8m/s (20mph), Crr = 0.0025: Power = 19watts
Weight 90kg (198lbs), speed 8.8m/s (20mph), Crr = 0.0055: Power = 43watts

Yes, I use the coast screen also for this purpose. But when I'm putting out fair bit of power at 35mph and see the display obviously 100 watts too high or too low, its easy to conclude that Aero needs a tweak. Maybe when I'm using the coast screen my position is not quite the same or maybe not pedal...

Regarding how important an accurate Crr is for correct displayed power, it usually isn't that important. At my weight, a 20% Crr change from 0.005 to 0.006 adds 3 watts when climbing at 8mph and adds 9 watts on the flats at 24mph. Getting CdA and Ws correct is much more important for moderate and hi...

daidnik, the stuff above is my best guess. I could be wrong. The iBike's pressure sensor reads something proportional to dynamic pressure (kilogram/(meter second^2)). Multiplying its reading by wind scaling gives actual dynamic pressure. Multiplying actual dynamic pressure by Cda (meter^2) and by bi...

This is my understanding on units, power terms and iBike coefficients: iBike Aero coefficient (meter^2) iBike Fric coefficient (newtons or kilogram meter / second^2) Cda (meter^2) Crr (unitless) wind scaling (unitless) riding tilt (degree) but can be converted to unitless radian as used below (multi...

I have an isport and have been also working to get good results. Your higher Fric on profile 2 is consistent with its higher Crr. Fric is a function of weight, Crr and the component of tilt due to stem/fork flex. Can you switch rear wheels, do a couple more coast downs, and see if you see consistent...

Snow? It was 95F here in the San Francisco bay area yesterday. If you are getting good power numbers in the first few minutes of your ride by not doing a trip reset, then the iBike's current calibration must be well matched to your initial riding conditions. And if you get bad numbers initially when...

It seems to me that this problem should be solvable. Tweak the calibration riding tilt number determined by the calibration ride that is used for the first 5 minutes so that it roughly equals the average of the dynamic riding tilt numbers determined after the first 5 minutes.

Switch to the setup rEL screen, version is displayed as 3.0x. Battery lifetime is no more than 10 hours on my iSport at those temps. Two or three times longer at higher temps. ibikesports.com sells batteries in bulk for a good price.

With a total rider/equipment weight of about 90kg, a Crr change from .005 to .006 when climbing at 8 mph is about 3 watts and on the flats at 23 mph is about 9 watts. I do get the sense occasionally on fast rough roads that speed drops slightly for the same level of effort. The iSport should read sl...

It looks to me that if Profile 2 has not had a calibration ride done, then its wind scaling equals 1. So switch to Profile 2, enter Est CdA parameters, and the resulting Aero value equals the CdA computed for those parameters. So it is easy to make up a chart relating these parameters to CdA if you ...

I've used Est CdA to tweak my calibration so that high speed coasts reads nearly 0 watts. I'd like to do the same with Profile 2 for a different aero situation (a windbreaker), but when I switched to Profile 2 and entered the same data, I get different Aero and Fric values. It is as if Profile 2 has...

I am planning to upgrade my iSport soon and want to make a note of my current calibration before the upgrade in case it gets lost. I have Aero, Fric, Crr. I know how to compute riding tilt from Fric, Crr and weight. I need help finding CdA and wind scaling. Would someone use the Est CdA screen on th...

OK, I agree with you now about your 10/30 and 40 example. Your Va^2*Vb make sense now. Relative wind may be the same, but work is proportional to displacement which, over a fixed period of time, clearly differs, so power must differ too, by a lot in this case. Thanks.

When I tweak CdA on the iSport, Aero changes. I believe wind scaling stays the same. There is actually no way on the iSport to display the wind scaling value nor to confirm that it has either changed value or stayed the same. I believe the only way to change wind scaling on the iSport is to do anoth...

The original post is great. This debate should not take anything away from that excellent post. I probably should have started a new thread.

I am still claiming my original post. Let dynamic pressure reading from the hardware be Dp. Then Ws*Dp = 0.5*Da*Va^2. Substituting in your original Pa = CdA*0.5*Da*Va^2*Vb I get Pa = CdA*Ws*Dp*Vb. It follows that an error in Ws can be compensated by a tweak in CdA. On a different subject. In your or...

Are you saying that CdA*Ws*0.5*Da*Va^2*Vb is not correct? If not, Ws has to appear somewhere. Where?

If as you say wind scaling is a calibration constant applied to dynamic pressure (call it Ws) then aero power is CdA*Ws*0.5*Da*Va^2*Vb. Is this what you mean? If so, then what I claim in my first post is correct: Because CdA and Ws are multiplied in this expression, a proportional change to CdA can ...

Aero is the product of CdA and wind scaling, but only the CdA relates to power. If you change the CdA when the wind scaling is really the one that is wrong, then your power reading will be off. For instance, a CdA of .30 with a windscaling of 1.33, and a CdA of .40 with a windscaling of 1.00 yield ...

On the iSport if Aero is off I have found that rather than do another coast down I get better results by changing Aero via the manual CdA screen. It does not matter if the error is due to a bad wind scaling or a bad CdA or both. Since Aero is the product of both, its value can be tweaked by changing...

I agree. Redo the calibration ride and check wind scaling and riding tilt and then redo coast downs. Looking at your first set of numbers Aero 0.294, Fric 4.645, CDA 0.294, CRR 0.0047, note that Aero equals CdA. This indicates that wind scaling is 1. On the other hand Boyd's number have a wind scali...