Calibrating iAero vs. SRM - Disappointing or normal results?

[cantona2k]

Hi,

I am a brand new iAero owner and tried calibrating the device over the weekend.

For the last half year i have been in the lucky situation to use a SRM of a friend who didn't need it temporarily.
As the time for returning it is near, I decided to go for the iAero and use the "overlapping" time before returning the SRM for some tests and calibration.

On Saturday morning i did the coastdowns and 4mile rides and everything looked quite ok.

Immediately after the 4 mile ride i did a "testing" ride to compare the iAero vs. the SRM.
This was before i connected the iAero to my PC for finetuning, naming etc. the profile settings.

The results i got from the 30km test ride were ok (?), the iAero averaged at 192,3 vs. the SRM at 186,4.
Here are the details...

Dist: 28,2 km (0:57:16)
Climbing: 281 m
Min Avg Max
Power 0 192,3 701 W <-- iAero
Power 0 186,4 546 W <-- SRM
Aero 0 NaN 0 W
Rolling 0 NaN 0 W
Gravity -685 -23,7 286 W
Speed 0,0 29,6 52,6 km/h
Wind 0,0 33,2 59,6 km/h
Elev 123 194 273 m
Slope -6,2 -0,35 6,2 %
Caden 4 74,9 99 rpm
HR 103 144,0 176 bpm
Aero: 0,344; Fric: 6,60
84 kg; 12.07.2008 09:36
18 degC; 1011 mbar

After the ride i downloaded the files and calibrated the profile with my CD's & 4-mile-ride.
For getting better results I assumed it would be good to take the SRM 4-mile-ride results into the calculation.

Here are the results of the calibration:

Calibration results

calibration1.jpg (62.83 KiB) Viewed 4858 times

This morning i went for a second 90min ride.

Now i got quite contrary results which aren't okay for me...

Even during the ride by having the devices next to each other i noticed the iAero getting much lower power output, especially when accelerating (e.g. short peaks of cycling standing e.g. after a red light).

SRM vs. iAero, ride #2

srm_iaero.jpg (180.23 KiB) Viewed 4856 times

In the end the SRM averaged at 176,8 vs. the iAero at 163,4.

Especially considering the "missing" peaks, this is not what I expected and surprised me quite a bit...

Here is the detailed csv file.

iBike_07_13_2008_1108_43_km_CR.csv

detailed ride #2 results

(585.77 KiB) Downloaded 312 times

Any input or ideas of what i could have done wrong is greatly appreciated!

I can definitely see the potential of the iAero but the results today made me a bit pessimistic of what to expect...

Regards

Chris from Germany

[rruff]

Even during the ride by having the devices next to each other i noticed the iAero getting much lower power output, especially when accelerating (e.g. short peaks of cycling standing e.g. after a red light).

If the iAero is reading low regularly, then it is a matter of calibration and can be addressed. I don't know how you incorporated the SRM data in your 4 mile... but I don't see the point in that. Anyway, I wouldn't expect the iBike to respond as quickly to a sudden surge since it must compute power from your rate of acceleration and the resistive forces... ie it needs to collect a few seconds of data. I'm not sure what if any smoothing the unit does in processing, but this could contribute to a difference as well. I know that in sprints that last awhile I'm getting numbers of over 1000W so I figure they aren't far from reality. Do you have a particular reason for wanting to know a 1sec power pulse?

[coachboyd]

OK, not sure if this is going to be the recommended way to do this at all, but I like to play around.

Your wind scaling is a bit off, maybe you didn't have a perfect wind offset when you did your calibration ride or something weird happened. So, I changed your wind offset to .98 from the .88. This changed your cda to .327. Not knowing what you look like on the bike I have no way if telling if this is more accurate or not.

I also changed your crr to a more real world number of .0065 (what I have found for clinchers on pavement). I did this by changing your friction number to 7.782. This keeps your riding tilt (which looks very good) the same, but changes your crr number.

After making these changes I was getting min--avg--max values of
Power------0--176.8--534 SRM
Power------0--179.1--488 Ibike

The difference in the max watts between the two could be due to side to side rocking motion of the ibike underreporting watts a little bit due to a combination of centrifical forces and the handlebars tending to slightly point down when they rock back and forth ( I might get corrected on this one, guys?)

The power comparison graph looks pretty good when I change these numbers, although this is obviously NOT the way to permanently fix the problem. Just my way of playing around and maybe coming up with some numbers that get you closer to what the SRM says for if you redo your 4 mile O&B ride.

[travispape]

Boyd had some good ideas on this one. If you increase the Crr to 0.61 and look at the power meter comparison graph, things start looking pretty close to what I would expect. You are seeing some higher peaks in the SRM data, but they do look like they could be corresponding to places where you are still standing when your bike speed is in the 20 mph range. Note that the iBike and iAero rely on you riding in your nominal position as your wind speed gets large enough to be significant. If this this is the explanation of your power discrepancies, you will just have to decide if you can live with the small errors in the iAero caused by standing (which messes up both your aero and riding tilt). And like Boyd suggested, maybe you have a swaying motion that contributes?

But I'm not completely sure that the discrepancies are due to standing or swaying. I've been riding with iAero (+ 2 iBikes) + PT + Quarq for months now and I can say that your instant-to-instant power discrepancies in your data are a little larger than what I see. I'm almost thinking that there is something "jittery" about the SRM data for some of your 10 s efforts, but I don't have any explanations to offer.

Keep in mind that we are getting close to splitting hairs on this. After raising your Crr to a more reasonable value, your iAero agrees with your SRM data very well for both the agregate numbers and the instantaneous numbers.

Travis

[cantona2k]

well, first of all sorry for replying to all your great input that late.
i have been in china on a business trip for a week and ibikeforums.com is banned in china for whatever reason

anyway back to topic...
i really thank you a lot for leading me into the right direction!

so would your proposal be to redo the coastdowns AND the 4 mile OB ride?
or just the OB ride (can you do the OB only)?

i have some time over the weekend (and still have the SRM) so i will be able to give you an update sunday the latest!

best regards
chris