Power Meters / Powermeters (SRM, Powertap, Quarq, Ergomo, Vector, Stages, power2max, P2M, 4iii, InPower, Cinch)

First TT of the year today, for some reason my Quarq was initially reporting really low numbers. The Garmin auto-lapped at the half-way mark and the numbers jumped up to what I was expecting (>290w). I calibrated when unpacking the bike from the car, should I have calibrated again at the start line? I’ve never done so before

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I don't understand what the lap button has to do with the power calibration. I've never seen anything that suggests they're linked.

That’s what’s confusing me too; every metric is consistent throughout the TT expect power, which jumps up to expected levels at the 5mi (lap) mark.

I mean it was cold yesterday morning and warmed up quite quickly but would that temp difference (about 3 degrees) have resulted in this? Avg power recorded for lap 1 was 203w (essentially low tempo level) and lap 2 was 287w (c. threshold).

Will give it a steady run on the turbo this morning and if there’s any more inconsistencies then I know the meter is playing up.

I've not noticed temp differences like that (I'm pretty sure Quarq do temp compensation, it was only early P2M that didn't). I wonder if its auto-zero got weirded out somehow? Like, what triggers it? Could the Lap have triggered an offset zeroing? That would imply that its auto-zeroing isn't working properly though, given the jump.

Oh, was the auto-lap also at a turn, where you might've been coasting? I think Powertap used to do their zeroing when coasting so perhaps if you coasted around the turn, it zero'd and the Lap is a red herring. I'd need to read up on Quarq docs to see how/when it does the auto zeroing.

Which model is it?

"MagicZero is a feature of Firmware 10 and is available via the SRAM AXS App for all AXS and DZero power meters."

https://www.trainerroad.com/forum/t/sram-axs-quarq-powermeter-magiczero-vs-autozero/46708

https://www.sram.com/en/quarq/campaigns/magiczero-auto-calibration

"Typically, the MagicZero measurements occur when the bike is stationary, coasting, or slowly backpedaling."

So it is kinda similar to Powertap, but a little bit smarter by the sounds of it.

P2M does the zero offset when coasting too. Mine overreads versus my smart trainer until I let it coast for 10 seconds.

MagicZero

P2M does the zero offset when coasting too.

This must be it. Just checked the map overlay and the lap happens to coincide with a 2-3sec coast around the roundabout. Kind of clears it up, but why the weird low numbers before it auto-zeroed when the numbers in the warm-up beforehand were normal? As mentioned I auto-zeroed before warm-up when unpacking bike, should I have done so again before the race? Ive never done so before...

I can only guess the cold threw it off a bit. Did you warm up on the same bike?

Yeah same bike - weird.
Turbo sessions are done in the large shed so these kinds of temps are not unusual.

Was the bike itself at ambient temperature when you zeroed it? If it’d come out of the car the cranks and so forth may have still been warmer than when it reached the roundabout. Conversely, if the bike was on the roof of the car it may have taken until the roundabout to warm up to ambient.

Whichever way round it was, it’s annoying. Could you apply a correction factor to the pre-roundabout power, so your records are accurate?

Was the bike itself at ambient temperature when you zeroed it? If it’d come out of the car the cranks and so forth may have still been warmer than when it reached the roundabout

This might be it, and thinking about it, I haven't raced on this bike since Sept-19, and 'MagicZero' was in the version 10 (Jul-19) firmware. I didn't update until Oct-20, and all sessions since update have been on the turbo at ambient temps.

Whichever way round it was, it’s annoying. Could you apply a correction factor to the pre-roundabout power, so your records are accurate?

Good question, @Greenbank might know - Does a Garmin store the zero-offset number during an activity?

Does a Garmin store the zero-offset number during an activity?

Pretty sure I have been down this route before and it doesn't store it in the workout file.

Meh, thanks @danb . It wasn't a particularly great performance anyway so I'm going to leave it.
Will attempt more outdoor training on the bike prior to next race and see if it happens again.

Good question, @Greenbank might know - Does a Garmin store the zero-offset number during an activity?

No idea, but there's a boat load of stuff stored in a .fit file that isn't documented. If it's a .tcx file then I think the answer is no.

My PM clearly needs to be calibrated as I certainly didn’t average 240W for 3 hours today. Is there any easy way to adjust an activity? Mainly for TSS and load etc

In training peaks? You just edit the TSS

Strava

easiest way to do it is to download the .fit file from garmin or wherever, delete the strava, edit the fit file with this, then re-upload.

this would really be an intriguing possibility. i have been thinking about writing some software which attempts to make power meters a bit more comparable and easier to do historical reviews of your data and that'd be invaluable. you could maybe smooth the offset over time to adjust data, model l/r imbalance and apply that to single sided power meters, use tests of pms to estimate variance of pms by manufacturer, etc. plus you could estimate uncertainty for all of this so you could see if your data were significantly changing

Looks like the P2M issue I had with my old cranks on the grrl's bike was a battery connector issue. The little battery prongs looked a bit on the piss so bent them up a little bit and the bastard started working again. It was odd, because when it didn't work, the light would still flash green when I stuck a battery in, hence me assuming it had power. One green flash isn't enough, it needs to flash green multiple times.

Just realised I could try using the Zwift app on an iPhone and pair it against a Wattbike Pro that the local gym has a few of...

I could even take my laptop in (if it needed an ANT dongle) and do it that way.

Have booked a gym session for Wednesday and I'll try it out then.

Much cheaper than spunking £1800 on a Wattbike Atom for home (which I really don't have the space for!)

You can do this if it has Bluetooth

Anyone have a 4iiii left crank (HT2, 172.5mm) they want rid of?

You can do this if it has Bluetooth

Yep, I think it has the updated firmware. If not I just get to sit and do some intervals or the like as I know the wattbike can walk to my Garmin.

It's just annoying that it has taken me years to realise I could have done this whilst I've been trying to work out how to cram a wattbike (or similar) into a 1st floor flat and not piss off the neighbours (either with the vibration noise or the air mover noise).

I think it is actually the unit that has to have Bluetooth rather than just firmware. I'm not sure how what bike connects with your garmin? Shitty neighbour advice is the watt bike atom is very quiet and I personally wouldn't think twice about having it in a flat.