What causing iBike Gen III to be reading too much Power

[DavidSkater]

My iBike Gen II has been working perfectly fine for months. Then, overnight, I began registering Watts that I know are far too high for my actual power output of each ride (registering about 25% too high). I can tell the iBike is off because my calories per mile for solo rides changed +25%, my peak watts on max efforts also increased about 25%, and my 20' max efforts also increased about 25% overnight. . . so, I know it an iBike problem. I'm not taking steroids

So, now for the past two weeks, my rides are consistently about 25% too high. I can always do a new calibration ride and set of coast downs. But, what on earth caused my iBike to change so suddenly overnight? My ride tilt is checked every ride, I didn't change any settings or do any new coast downs or calibration rides. Prior to this situation, the iBike was very consistently accurately measuring power output each ride.

Thanks

[rruff]

Sounds very consistent. Sure you didn't change CdA or something? How does your airspeed data look?

[pjboyle]

Have you looked at the profiles from the "good" data compared to the current to see if anything changed? What does the wind speed look like on the speed chart? Is it always more than the wheel speed (ie as if you are now always in a head wind)? If so, and assuming you are doing a wind offset prior to rides, it would sound like your wind scaling is off and you need to do a cal ride (I don't think you would need new coast downs). Did you have any wet rides where the head unit ingested water? some people have reported wind scaling shifts after such episodes. The good news if that if a cal ride produces a good profile for the current set up you can go back and change the profile for the previous rides with questionable data and recover good data.

PJBoyle

[DavidSkater]

Thanks for the ideas. I went back and compared files of older rides versus more recent and the Cda = 0.411 and Crr = 0.0064 were consistent.... I had made no changes. But, I think you are on to something when asked about the wind speed. Sure enough, on recent solo rides the wind speed is always more than the wheel speed (of course this is not true during large group rides). . . i.e. on the solo rides it looks like I am pushing into a strong head wind the entire time on a loop course even though I do a wind offset before each ride. So, I will try another calibration ride. In fact, I did have a wet ride around the time my problem started. I just figured that once you had a good calibration ride, the data would be valid forever???? How often does one need to do a calibration ride?

Thanks again for the quick response!

[Velocomp]

Please post a ride file showing the problem

[DavidSkater]

I've attached one of my recent solo rides showing what I believe to be too much power.

Not attached is tonight's 35 mile club suffer-fest. The iBike would have me believe I held 390 watts for 20 minute during my peak. . . . There is NO WAY I can hold that! I am not sending this file because it was a group of 40 riders with much drafting.

On a related note, rather than doing another calibration ride, could I merely re-send the profile from my last good calibration ride back back into my iBike? Or will this not accomplish anything because it is all the same data. Or did my ride in the rain actually now require me to do a new calibration ride?

Thanks

[Velocomp]

Sorry to say, your suspicions are correct: you aren't Lance...

Your ride file reveals a couple of things:

1)Assuming you had no net headwind, your wind scaling should be more like 0.986, not the 1.38 value you had. This knocks down your watts considerably.

2) Your Crr of 0.0062 is high, too. A better default value of 0.0055 is more realistic

3) Your CdA of 0.41 may be high, depending on your height.

I left CdA alone but changed WSF and Crr. Your watts are a much more realistic 204...

[DavidSkater]

Thanks. . . I'll give it a try. I still don't get how the numbers looked so consistently accurate for so long and then became so consistently bad.

[lorduintah]

There have been several posts addressing the CdA "correctness." Look around for them.

Set you iBike to Coast in the setup menus. Ride to a place where you can go downhill - fairly calm wind - and get up to around 20 mph or higher. Then Coast. If you are dialed in the reading of watts on this screen should go to near zero and hover around zero. If it is always positive or negative, then CdA needs some fixing.

To Do this in the best way (not guessing how much change is appropriate) - re-run the out/back (cal ride - 6-10 miles total, if you can) and 8 or more CDs (I personally like at least this much as variations are part of riding.)

You probably should also run an estimate Aero on your iBike - the value in the calculation there from height and riding position is not far off - so if you get something different, you may need to look closely at your values after the cal ride and CDs.

John has given you new estimates for Wind Scaling and Crr. If you need to, you can manually adjust the values in the advanced section of the software and tune a little more.

Tom