power2max wrote:
Hi Eric, thanks for your message.
It's tough for me to diagnose over the distance, but I will try my best:
First about the wattage "dancing around": the power2max measures 50x per second and updates every second. During pedaling power fluctuates very quickly due to the nature of pedaling. Every power meter smoothes data in some way, as completely unsmoothed data would not be readable on the screen - it would fluctuate too quickly. It's possible that the different smoothing of the two power meters make it appear that the wattage are changing relative to each other. For example on my Wahoo Kickr the power displayed changes much more slowly than on the power2max, due to said differences in smoothing. If you look at 10s average that should be less so. This is not what I meant, I'm sorry If I didn't explain it well. Let me give a more specific example: I will often do multiple intervals indoor on my *trainer*, the goal to hold a specific wattage for every interval. I use a Kurt Kinetic Road machine and have my reading 3s Average. When I used my *powertap*, I would go into the big ring, 3rd gear at a cadence of 94 and be at 165 watts (or within a 1-2 watts). Assuming I stayed at 94 cadence, my power would be 165 watts, all the intervals were consistent.
With the power2max, when I do everything the same above (big ring, 3rd gear at 94 cadecnce) I have found the wattage will stay steady, but it is sometimes higher or lower after a calibration. So for example, after a calibration is will be at 157 steady. Then later after a calibration it could be at 169 steady, 172 etc... it will change after calibration. A couple times I stood up and stretched my calves and then it was 180+ watts. I have found once I hit about 20-30 minutes into the ride it seems to stabilize more.
I will confirm this is not a scientific test, but with my powertap it just seemed to be consistent and make sense, with the power2max the math just doesn't add up for me sometimes.
Second about zero offset calibration: every time you stop pedaling (cadence = 0) the unit waits for the torque value to stabilize and sets the zero. It has an algorithm to prevent false zero offsets which is difficult to trick. A very safe way to conduct a zero offset calibration is with the pedals stationary for 5 seconds (to be on the safe side) and the freewheel clicking - which guarantees there is no tension on the chain. If you prefer to do it manually you can get a Garmin to tell you the zero offset it is using by conducting a manual zero. If there is tension on the chain or the pedals are turning it will return an error. This kinda stinks, because I frequently stand up to stretch my calves. Now I'm always wondering if I screwed up the calibration.
Currently I am indoors on the trainer, the trainer is consistent resistance so I have an idea when it seems off and can recalibrate. Now that the weather is getting nicer I will be outside, no way will I be able to tell if it's off.
My overall feeling is it is accurate for an entire ride, but I have this nagging feeling that I need to keep calibrating so I can offset any screwups.
I hope this helps!
Best
Nicolas