Powermeter question

[CallumRD1]

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?

[AJS914]

You should probably look for a power meter that allows the easiest calibration so you can calibrate it maybe 5 or 10 minutes after you leave the warm office.

For left side cheap, you'll be looking at 4iii, Pioneer, and Stages.

[TonyM]

The P2M are doing an automatic calibration as soon as you stop pedaling for a few seconds.

[Calnago]

Curious, can you disable the auto calibration in the P2M? There’s debate over whether you really want it to “autocalibrate” all the time. I used to have my Garmin head units set to auto calibrate and it would do that for my SRMs during the course of a ride. But as a result of looking at the pros and cons I decided I didn’t want the system to do it automatically, and I wanted to be in control of when a recalibration took place. Plus, I always want the pedals to be horizontal, still and unweighted when I recalibrate. What I do is calibrate before every ride but now I have the auto calibrate function off. If temperature changes drastically during a ride I may think about a recalibration if I stop but generally leave it. I like to check at the end of a ride occasionally (by doing a recalibration) if the temp has changed significantly from when I started just to see how much over the course of a ride it has actually changed. It’s usually insignificant.

[TonyM]

I don’t think you can disable it on the P2M type S as there is no P2M app or something like this.

Maybe on the newer P2M NG or NGEco?

[TobinHatesYou]

Midride automatic calibrations aren’t full zero-offsets. They fine-tune your last manual zero. There’s no reason to turn it off.

[Seedster]

I always recommend a crank-based meter like P2M or SRM. Given your budget limitations, there are very positive reviews of the new garmin Vector 3, which can be purchased in a one side only configuration.

[Calnago]

Midride automatic calibrations aren’t full zero-offsets. They fine-tune your last manual zero. There’s no reason to turn it off.

Please explain.

[c60rider]

I don’t think you can disable it on the P2M type S as there is no P2M app or something like this.

Maybe on the newer P2M NG or NGEco?

But there is a P2M app. At least there is for my NG as I have it on my phone from the google play store. It does have unreleased in brackets mind you plus I've never even got round to using it as my NG just works. The only information on it says the app can be used to change power meter settings and upload firmware updates. If I get a chance I'll have a look and connect it up see what it lets me do.

[claus]

For SRM the "auto zero" function requires that the value changes by no more than 4 Hz, hence it isn't the same as the initial (manual) setting.

[Geoff]

If you remember to do it, you can also re-zero the SRM on-the-fly once the bike gets to ambient temperature.

[MichaelB]

But there is a P2M app. At least there is for my NG as I have it on my phone from the google play store. It does have unreleased in brackets mind you plus I've never even got round to using it as my NG just works. The only information on it says the app can be used to change power meter settings and upload firmware updates. If I get a chance I'll have a look and connect it up see what it lets me do.

My understanding (from emails with P2M - I have an FSA Powerbox, which is essentially a slightly despecced NGeco) is that the app is to be used for getting updates and adding in the features/upgrades like Bluetooth, dual sided etc.

[robertbb]

I always recommend a crank-based meter like P2M or SRM. Given your budget limitations, there are very positive reviews of the new garmin Vector 3, which can be purchased in a one side only configuration.

Positive reviews of Vector 3? Have you read through the Garmin forums, or the comments on DC Rainmaker's page? They are causing a world of headaches for a lot of users - as a product it's a massive fail.

I took delivery of Favero Assioma Duo's late last week and they are everything I hoped they'd be, and have a LONG list of pro's over the Vector's.

Anyway, back to the original topic. Calibration is a one-time only process, done to each and every power meter during production to ensure its acuracy. Thereafter, a zero offset needs be done once before every ride to allow the power meter to clear itself of any residual data in its circuitry - think of hitting the "tare" button on a set of kitchen scales - and will also prompt the power meter to invoke any internal compensation required for temperature/humidity. The result of the zero-offset should always be, as the name suggests, zero. This will ensure that the numbers reported by the power meter to the head unit are clean. Any decent power meter will then self-check over the first 20-30 crank revolutions after a zero-offset, to ensure continued clean data output. Calibrating once prior to ride should be enough, and it should absolutely be done with crank arms vertical and stationary without being clipped in. It can't hurt to re-zero if you stop somewhere on your ride, particularly if the environmental conditions have changed significantly... it takes ~10 seconds.

NOTE: Some head units use the word "Calibrate" or "Calbiration" in their software - what they actually mean is zero-offset. Head units themselves do not calibrate power meters.

[robertbb]

Midride automatic calibrations aren’t full zero-offsets. They fine-tune your last manual zero. There’s no reason to turn it off.

Please explain.

See what I said in my post above. A zero-offset by its nature requires that there is no strain on the pedals; that's the whole point. What is done on-the-fly while you are pedaling (after your zero-offset) is just a fine-tuning of the data that is beingbroadcast to your head unit. Any decent PM does this, and it's done in the first 20-30 pedal strokes after the zero-offset, based on the algorithms in the firmware and how the firmware interacts with the strain gauges and other telemetry electronics.

[TobinHatesYou]

To elaborate. What's done at the factory is a calibration that determines the slope of the strain gauges at known torque values.

Say you have one PM that reads 0.0mm/mm, 0.01mm/mm, 0.02mm/mm at three known torque values. Say you have another that reads 0.0mm/mm, 0.011mm/mm, 0.022mm/mm. Two different slope factors need to be stored by the respective PMs. It is a multiplier.

A manual zero-offset is a tare button that zeroes out the strain inherent in the system (weight from pedals, cranks, etc. in a P2M.) It is a constant.

An auto zero is a +/- of the constant from the original manual zero within a defined range.