Low CdA values ?

[Graham]

I used Aeropod for the first time this year with firmware 7.06. It seemed a lot more stable and repeatable than previous releases. If I am correct in my analysis with "Isaac", which is showing my CdA to be in the region of 0.160. Surely this can not be correct? I am confident that the profile parameters are correct and my DFPM is reading correctly. I used 4 helmets during the test

Helmet 1 - 0 to 8.7 miles
Helmet 2 - 8.7 to 19.1 miles
Helmet 3 - 19.1 to 29.4 miles
Helmet 4 - 29.4 to 39.8 miles

In the 40 mile ride file, I have merged the GPS data from the Garmin file, I used an out and back course of a section of the local TT course which is a slightly rolling course. The cal ride was OK with no issues but it did take several attempts to get AP to pair with my DFPM (SRM) and Speed Sensor (Garmin), could see the sensors straight away on the Garmin head unit. I have 3 files from today's ride - 1 mile, 2 mile and 40 mile.

John if you could give my data a look over please and see what CdA values you get for each helmet and if you can see any reason for my low values. Thanks Graham

[Velocomp]

SRM is COMPLETELY DIFFERENT from other power meters.

All other power meters calibrate inside the sensor. SRM calibrates inside the receiving device--in this case AeroPod.

I agree your CdA numbers are too low. This is because the SRM calibration in AeroPod is wrong.

It's critical to follow the SRM calibration instructions precisely (I am not saying you did not...)

One of our other customers reports that this sequence works with AeroPod (step 6 is new...)

1. turn the wheel to switch on the speed sensor
2. rotate the crank to wake up the SRM DFPM
3. switch on the Garmin Edge 1000 (or other similar device)
4. wait for 5 seconds (not sure if this is actually needed but doesn't hurt either)
5. switch on the AeroPod (green LED flashes, then goes off)
6. *issue a manual zeroing (calibration) of the SRM via the Garmin Edge 1000*
7. start riding

We don't have an SRM, so give the above a try and see if that fixes things.

[Graham]

Thanks for the reply John

I did calibrate the SRM via the Garmin before calibration ride with AP, and I don't think the SRM cal was wrong as my perceived effort felt right for the numbers and speed that I was doing as I have rode this piece of road many times and the condition were good

If the SRM calibrates (zero offset) in the receiving device, in this case Garmin 520, how is that transmitted to the AP ?

To give really low CdA as I am seeing would the power data not have to be way off?

After several attempts of using AeroPod I still have not be able to make any informed decisions about my equipment choices

Graham

[Velocomp]

The calibration adjustment is done in the bike computer head. That is, the raw data transmitted by SRM can be "off", but after a calibration is done, an internal correction factor is applied inside the bike computer head to correct it. This is different from EVERY other power meter. In other power meters, when a calibration is done the correction is applied in the sensor, such that the data transmitted is correct.

In this application AeroPod is acting as a bike computer; that is, the SRM calibration factor is applied inside AP. It is entirely possible that your Garmin can be properly calibrated against your SRM, but your AP not.

Pre-calibration, SRM can be pretty far "off", which would cause low (or high) CdA.

We have had other users use AP with SRM with great success, and I am confident that once you get your calibration correct you will too.