Power Meter Calibration with weight offset

I’ve read that triathlete make sure to turn of auto calibration and calibrate their power meters without their shoes attached.

I wonder why. For example, if I calibrate with shoes attached with cranks in the vertical position, wouldn’t that just add an offset so that the power meter thinks there is an extra force pushing the pedals away/towards the axle. This force results in zero torque with regards to the axis of rotation, thus zero power reading difference?

Additionally, pedal bodies for pedal-based power meters have different weights (Garmin Rally). Pedals themselves have different weights for crank-based power meters. Thus, why would extra weight from say, a shoe, make any difference?

For me, this is important because I am using flat-pedal adaptors on my Favero Assiomas. The way I have it setup, it is quite difficult to remove every time (it’s a long story)

Has anyone tested this with their power meters?

I was thinking of doing some back to back testing to see what the difference.

Having the shoes installed is misuse and not proper application of the zero offset procedure. It may be a minimal difference (if it even shows in the final results), but it is still wrong from a conceptual level.

I understand it breaks the rules in the instruction manuals, but what I want to know is why it is conceptually wrong. I think if the cranks are vertical during calibration, then any extra weight has zero difference. The weight adds an offset force, but it’s not contributing to torque around the bottom bracket axis.

I think that in practice, if the cranks are not perfectly vertical, then any weight (including pedal body, pedal, attached shoes/adaptors), would affect sensor accuracy. Maybe because pedal bodies/pedals are light enough, that getting the cranks close enough to vertical gives negligible sensor inaccuracies? However, shoes are not that much heavier, maybe 200 grams for one side, compared to maybe 100 grams for a pedal? So why is it such a big no-no? I may be misunderstanding, but that’s why I’m asking and explaining my thought process :slight_smile:

Edit: apparently I was wrong. Corrected by @RChung below.

Pedal power meters do need to know orientation, which is why they’re supposed to be calibrated at a particular point in the circle (typically, either at 12 o’clock or 6 o’clock). Shoes or shoe attachments can slightly change the “pivot point” so you may not get zero force vectors in the intended direction–but unless the pedal attachments add a bunch of stack it probably isn’t very large. Still, for proper calibration, you should remove them.

As an aside, early models of the SRM had only 2 strain gages and you could get pretty different torque readings around the crank circle.

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Is the issue with the orientation during “calibration” in effect for all the major power pedal models? IIRC, the main brands (Powertap, Garmin, Favero) use different arrangements of the gauges etc, with the Vector 3 being perhaps the most resistant to this added error?

My un-expert assumption is that the “calibration” value has a large multiplying effect as part of whatever calculations are done using it in order to output the power value, so even a small change caused by the added weight of the shoes has a not-insignificant effect.

I should not write about stuff I apparently don’t know enough about. Thanks for correcting me.

This is not something I have seen stated by any pedal power meter maker. None of these list an orientation requirement that I can see.

With respect to a defined orientation, crank arm power meters are the ones I have seen with that orientation requirement.

Am I missing something here?


Edit to add some other models I just remembered:

Moral of the story seems to be “it depends” when the question is whether power meter pedals require specific crank arm orientation for proper calibration (actually zero offset). It appears the more modern power pedal designs (sorry SRM :stuck_out_tongue: ) have improved and eliminated the crank arm orientation requirement.

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Very much in line with my surface level understanding of these meters… With the requirement being more stringent on crank based meters because of how they are manufactured (basically sticking the magic bits onto a flat plane on the inside of the crank arm)

You’re right, other comment is wrong. The strain gauges in the pedals have no way of knowing where on the circle they are.

It’s crank based power meters that need to be vertical.

You linked to a Power Meter City post. Here’s their detailed instructions on zeroing torque: How to Zero Your Power Meter - Power Meter City

Note that there, they recommend orienting the Assiomas at 12 o’clock (and the Vectors at 3 o’clock).

The Garmin Vectors also needed to be oriented – at least, the first generation, with the pods. I confess I’m not as familiar with the podless versions.

Pedal power meters need to know when the pedal forces are tangential to the pedal circle and when they’re radial (for example, pressing straight down when the crank is at 3 o’clock produces different vector forces than pressing straight down when the crank is at 6 o’clock). The Vectors were named “Vector” because they had to figure out the force vectors in different orientations. Pedal power meters now use accelerometers to determine their position but since they also use the accelerometers for cadence you can see that they’re a little “noisy” if you try to use accelerometer cadence to calculate gear ratio. (This is also an issue for crank spider power meters that have the option to use either a reed switch or magnetless accelerometer cadence – the reed switch cadence gives clearly more accurate data than the accelerometer cadence).

Crank arm power meters absolutely do need to be oriented since they (typically) don’t have a full rosette of strain gages (the exception is the Infocrank). And, as I mentioned above, even early crank spider power meters needed to be checked in different orientations. They typically don’t need that now since they use more strain gages now than they used to.

Getting back to Assiomas, they claim they don’t need manual zeroing, but I’ve seen data files from them (admittedly, on fixed gear track bikes) that are cleaner when one does a forced manual zero.

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I got a response from 4iiii that the weight of pedal/anything-attached will not affect power measurement as long as it is calibrated(zero-offset) in the 6/12 o’clock orientation.

The Favero Assioma manual (Rev20 EN-IT-DE version) from Favero’s website says to put cranks in the vertical position. I don’t know how, but they do figure out the orientation; as others stated, it’s necessary. 1) they need to so that they can calculate power using the tangential force. Radial force results in 0 power. 2) They report cycling dynamics’ power phase.

Favero told me having different weight isn’t allowed during zero-offset though.

I believe regardless of brand, calibration should be done with cranks vertical.

If it’s not in the vertical position, then there would be either a clockwise or counter-clockwise torque from the weight of the system. Having cranks vertical means there is zero tangential force from the weight of the crank/pedals/attached-stuff, so this would be the proper zero-offset.

In the vertical position though, having additional weight could bend the crank “in/out” (towards/away from bottome bracket). But I would assume the strain gauge orientation would account for that, since that also happens during pedaling and that force accounts for zero power.