PowerPod calibration for TT bike/wind scaling

[Carlos13]

I spent a fascinating day yesterday setting up my PowerPod. I mounted it on my TT bike in a similar position to the new TT garmin combo mount, between the arms, using the standard PowerPod mount and a 3T computer bridge spaced out with old tyre. It's extremely secure. The PowerPod wind port is roughly level with the base bar. I have several cm of risers fitted so my arms and hands are well above. I did the initial 1-100 calibration ride as part of a 5 mile out and back and went back to base to have a look. I was also using a DFPM recording on a different Garmin. The initial power comparison between the two was quite good. However, I noticed that the calibration was using 0.369 for CdA, and it had defaulted to 1.4 for wind scaling. The most striking thing was that the wheel speed/air speed chart was showing the wind speed below the ground speed for both the out and back sections of the ride, which was not correct.

So I decided to do a 10 mile out and back ride and then check the calibration. It suggested a wind scaling factor of 2.1, which is well above the 1.4 and 1.7 numbers I have seen quoted. If I used this, along with the suggest CdA of 0.369, of course the power was way too high. But if I used my Aerolab-derived number, of course as you'd expect the power was a very close match to the DFPM (A Power2Max). So Using wind scaling 2.1 "corrected" the wind, in that generally it looked as I expected, headwind where I expected, tailwind where I expected. Nice.

Having saved these settings to the PowerPod I exactly repeated the 10 mile out and back. Short term power over a few seconds did not appear to closely match the DFPM, but that is only anecdotal. Across the whole 30 minute ride, average power for the PowerPod was 2w higher than the DFPM, and normalised power 5w higher (the latter being consistent with the more "jumpy" power trace exhibited by the PowerPod, IMO). When pedalling at around 200w, occasionally the PowerPod would jump to 4-500w for a few seconds and at other times it would read zero. But the overall average was pretty good. However, even with a wind scaling of 2.1, the ride is showing lower wind speed than air speed for almost all the ride, out and back. I'm sure that's not correct. Please can you give me an idea what might be happening? I can post any of my ride files if necessary.

Also I cannot merge my power data into this last ride. For some reason I get a "memory full" error in the Isaac software when I try to merge, and the regular Isaac menus disappear. I have tried this a number of times, including rebooting my Mac in between.

Thanks in advance!

[Velocomp]

Please post your ride files, including any ride files you have for your DFPM.

[Carlos13]

Attached ibr files. First one is the out and back at 1504 that I managed to add the DFPM data to. Second one is the later, virtually identical out and back that I was unable to add DFPM data to.

[Carlos13]

The zip file is not a zip file, it's a .fit file (.fit could not be attached). So please just remove the .zip extension. It's the DPFM file for the 1909 .ibr file attached in the previous message.

[Carlos13]

Photo of the PowerPod as mounted to my TT bike

IMG_0997.jpeg (157.83 KiB) Viewed 5125 times

Photo of PowerPod mounted on my TT bike.

[Velocomp]

I get the same LabView error as you. Likely the .fit file is somehow corrupted.

Your other file shows a lot of "noise" in the PowerPod file. I cannot tell how you've attached your PP to your aero bars, but I am concerned the mount is not stiff enough.

[Carlos13]

Hi John,

I imported the .fit file into Golden Cheetah no problems, so I exported it as GPX and have attached it to the ride, and attached that ride here.

I will think about the mount. I tightened it as much as I dare, given the 3T computer bridge is plastic (or at least clad in plastic). I had to space out from the bridge to the PP mount using old tyre. When I first fitted it I was happy it was secure, but now there is a certain amount of flex even if I tighten it further. So I think I'm going to have to wait for the TT mount.

Has anyone successfully fitted the PP to TT extensions any other way? I don't mind buying the TT mount at all - so long as I can get it all to work the product offers incredible value for money - but I guess it's going to cost me $$$ to get one of the mounts shipped to the UK by itself.

Thanks

Carl.