Powermeter question

Discuss light weight issues concerning road bikes & parts.
robertbb
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by robertbb

TobinHatesYou wrote:
Mon Apr 09, 2018 3:20 am


An auto zero is a +/- of the constant from the original manual zero within a define range.
I'm having a hard time finding out how this actually works, but plenty of sources say this is problematic and better left switched off if your head unit allows it.
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Calnago
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by Calnago

Bingo!
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by Weenie


TobinHatesYou
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by TobinHatesYou

robertbb wrote:
Mon Apr 09, 2018 5:49 am
TobinHatesYou wrote:
Mon Apr 09, 2018 3:20 am


An auto zero is a +/- of the constant from the original manual zero within a define range.
I'm having a hard time finding out how this actually works, but plenty of sources say this is problematic and better left switched off if your head unit allows it.

It seems auto-zero is partly why P2Ms are more reliable against temperature-based drift now than before, so again I don't see why you would turn it off unless you are running a fixed-gear bike that can't coast anyway.

robertbb
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by robertbb

TobinHatesYou wrote:
Mon Apr 09, 2018 6:40 am
robertbb wrote:
Mon Apr 09, 2018 5:49 am
TobinHatesYou wrote:
Mon Apr 09, 2018 3:20 am


An auto zero is a +/- of the constant from the original manual zero within a define range.
I'm having a hard time finding out how this actually works, but plenty of sources say this is problematic and better left switched off if your head unit allows it.

It seems auto-zero is partly why P2Ms are more reliable against temperature-based drift now than before, so again I don't see why you would turn it off unless you are running a fixed-gear bike that can't coast anyway.
I have a software engineering background and have done a bit more reading.

As far as I can tell, the "auto-zero" function on a head unit will cause the head unit to send an instruction to the PM to run it's default re-baseline code (the same code that gets run when doing a manual re-baseline), in this case while riding. I still don't fully get how this would work given a proper re-zero should be done without any weight on the pedals, but putting that part aside this seems to be a hack that would have been useful for PM's that had less mature active compensation built-in for environmental changes/drift during the course of a ride.

Modern PM's - certainly the quality ones - will constantly check the data of their own accord and compensate as necessary without needing to be actively told to do so by the head unit. They still should be manually zero'd before every ride and it probably can't hurt to also do if you stop for coffee, bathroom, etc.
It's ALL about the bike.

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Calnago
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by Calnago

There ya go... basically in a nutshell the errors introduced through an auto zero during the ride can be greater than the errors incurred as a result of natural drift (what you’re trying to correct for in the first place). That’s why there’s a lot of advocacy for turning off the autozero function. I’ve tested it both ways and decided to leave it off for those reasons. The natural drift was very small. And I have no idea how an on the fly autozero function could even possibly work with a pedal based system. Also, the Garmin head units used to not even have the ability to disable the autozero function. They have since added that feature for this exact reason. If while out for an all day ride say, and you experience an extreme change in weather conditions you can always do a manual reset at some point if you want. At least you know when and where you reset it that way.
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TobinHatesYou
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by TobinHatesYou

robertbb wrote:
Mon Apr 09, 2018 7:22 am

I have a software engineering background and have done a bit more reading.

As far as I can tell, the "auto-zero" function on a head unit will cause the head unit to send an instruction to the PM to run it's default re-baseline code (the same code that gets run when doing a manual re-baseline), in this case while riding. I still don't fully get how this would work given a proper re-zero should be done without any weight on the pedals, but putting that part aside this seems to be a hack that would have been useful for PM's that had less mature active compensation built-in for environmental changes/drift during the course of a ride.

Modern PM's - certainly the quality ones - will constantly check the data of their own accord and compensate as necessary without needing to be actively told to do so by the head unit. They still should be manually zero'd before every ride and it probably can't hurt to also do if you stop for coffee, bathroom, etc.
Auto-Zero is toggled on or off by a head unit. Probably a value of 0 for off and 1 for on...stored in the PM. What it doesn’t do is require a head unit to send a command periodically to zero midride.

I agree that manual zeroing should be done at the start of a ride, but like someone mentioned earlier, auto-zero functions only modify the manual zero by a small amount. Also P2Ms have a much lower polling frequency than SRMs and are much less likely to skew during the auto-zero process anyway.

robertbb
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by robertbb

I'm stuffed either way, I can't find a way on my Edge 500 to turn auto-zero (i.e. mid ride) off.
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ms6073
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by ms6073

robertbb wrote:
Mon Apr 09, 2018 11:18 am
I'm stuffed either way, I can't find a way on my Edge 500 to turn auto-zero (i.e. mid ride) off.
Assuming you have updated your Edge to newer firmware versions that support Auto-Zero, then for the Edge 500: Settings > Bike Settings > Bike 1 (or applicable bike profile)* > ANT+ Power > Sensor Details > Calibration

Image
Michael - The Anaerobic Threshold is neither...

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Calnago
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by Calnago

Yeah, make sure evertyhing is updated.... as being able to disable the autozero function was an added feature to the Garmin head units. I don't know exacgtly when they did it or to which head units, but it's certainly a current feature going forward and has been for some time.

A manula auto-zero (or in Garmin speak, Recalibration), should absolutely be done at the start of every ride. The autozero function during a ride will fail if it detects a new autozero number adjustment beyond a certain range. It's basically saying "Hell, that new autozero number we just came up with cannot be right, so let's not even go there... we'll try to autozero another time when conditions are better and the guy isn't wiggling his feet on the pedals". There are condtions that must be met for the autozero to work. It's kind of a six of one, half a dozen of another situation, but for my money I've disabled the autozero function. You can test how far it's off yourself by experimenting on your own rides. Either way, I do believe the chances for incorrect data are higher if you let the unit auotzero during rides than if you just accept a bit of natural drift. An exception might be if I'm out for a long day, and the temperature change during the ride is extreme, then I might think about doing a manual "recalibration" if I stop, but that's usually as far as it goes... I just think about it :). It's just never been that significant.

At the end of the day... as much as I like to know how these things actually work, as long as I can see what I want to see, I'm good. The SRM's I have never skip a beat. And I've never once looked at any data to see how much drift is occurring. I used to have WKO+ some years ago and you really have to decide if you want to spend your time riding or anlayzing data with stuff like that. I use Training Peaks, and I like how I can select any section of the ride I want and see how I did. But it's still mostly for fun and interest.
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Bordcla
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by Bordcla

Since when are there any concerns with the accuracy of P2M?

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ms6073
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by ms6073

Calnago wrote:
Mon Apr 09, 2018 4:33 pm
It's basically saying "Hell, that new autozero number we just came up with cannot be right, so let's not even go there... we'll try to autozero another time when conditions are better and the guy isn't wiggling his feet on the pedals".
That must be a recent change as that was not the case when I still had auto-zero enabled on my Edge 520 back mid-2017. What I had observed that made me turn it off was apparently the auto-zero kicked off after briefly coasting on a downhill just as I got out of the saddle at the start of a short climb, resulting in a 1200+ hz offset and 0-watts displayed until I unclipped and ran another 'calibration' to 're-set' the zero-offset.
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Calnago
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by Calnago

Actually, I don't know if the Garmin autozero automatically rejects something if it doesn't seem right, but I think that's what the limited range of adjustment it can make is trying to accomplish. I don't have an SRM PC8 myself, as I actaully prefer the Garmin head units for all the other stuff it can do, but I am playing around with bike right now that does have the PC8 and geez... the stuff it has crammed into it is pretty impressive. Too bad they couldn't make it waterproof :). Still, I like the configurability of the Garmin screens, mapping and turn by turn navigation that the Edge 1000 offers quite a lot more than the PC8.

I've never experienced a huge discrepency like you just described, but it was more of a comparison of offset values at the end of a ride when autozero was enabled versus when it wasn't and comparing those values with the offset value that I started the ride with. Just thought I'd rather keep it disabled.
Colnago C64 - The Naked Build; Colnago C60 - PR99; Trek Koppenberg - Where Emonda and Domane Meet;
Unlinked Builds (searchable): Colnago C59 - 5 Years Later; Trek Emonda SL Campagnolo SR; Special Colnago EPQ

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Calnago
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by Calnago

Bordcla wrote:
Mon Apr 09, 2018 4:38 pm
Since when are there any concerns with the accuracy of P2M?
I don't think there really are any concerns, are there? These drift numbers, and whether to leave autozero enabled or turned off, I think are really splitting hairs. Be it in the marketing literature, or real life data junkies taking things to the nth degree, as long as you're getting reasonable numbers that are consistent and make sense, that's the biggest thing. Similarly, I have no use for left/right balance numbers. At some point, I just want to ride my bike and enjoy the scenery and if I get fitter along the way, then I'm a happy guy.
Colnago C64 - The Naked Build; Colnago C60 - PR99; Trek Koppenberg - Where Emonda and Domane Meet;
Unlinked Builds (searchable): Colnago C59 - 5 Years Later; Trek Emonda SL Campagnolo SR; Special Colnago EPQ

robertbb
Posts: 337
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by robertbb

ms6073 wrote:
Mon Apr 09, 2018 1:53 pm
robertbb wrote:
Mon Apr 09, 2018 11:18 am
I'm stuffed either way, I can't find a way on my Edge 500 to turn auto-zero (i.e. mid ride) off.
Assuming you have updated your Edge to newer firmware versions that support Auto-Zero, then for the Edge 500: Settings > Bike Settings > Bike 1 (or applicable bike profile)* > ANT+ Power > Sensor Details > Calibration

Image
Hmm. Garmin Connect tells me I'm on the latest version (3.30), but I don't see that option when I calibrate!

So weird (and frustrating)... any suggestions?
It's ALL about the bike.

RobertBB's CyclePlanner Spreadsheet:
https://weightweenies.starbike.com/foru ... 8&t=152263

by Weenie


bilwit
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Location: Seattle, WA

by bilwit

CallumRD1 wrote:
Sat Apr 07, 2018 3:23 pm
I'm looking to get a powermeter and I have a question. I'm looking for a reasonably affordable shimano compatable powermeter (probably just left side due to budget) but the most important consideration for me is accurate temperature compensation. I commute on my bike every day (in addition to proper rides) and keep my bike in an unheated garage and my heated office. Thus the powermeter will often see >40˚F (20˚C) temperature swings on a ride, be that a heated office (70˚F) into a cold winter night (20˚F) or a cool summer garage (50˚F) to hot summer day (90˚F) here in Colorado. Does anyone have any recommendations?
Sounds like you're describing Stages to the T. Should be a bargain these days if you get it used especially if you're using the last gen Shimano. Just make sure it's the "gen 2" version and that the firmware is updated when you get it via phone.

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