powermeter pedals as the control and compare

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Amateur recreational hobbyist cyclist
https://www.strava.com/athletes/337152
https://vimeo.com/user11846099

Pedals are the real answer. But if you don't feel like buying pedals you can compare HR if you have a second wheel-on trainer that you can use the powertap on.

It's not going to be hugely important tho unless they are far enough apart for you to easily notice. Indoor and outdoor thresholds can be pretty different. And I assume the powertap will be used for outdoor and the trainer indoor, but I could be wrong.

https://www.highnorth.co.uk/...r-ftp-vs-outdoor-ftp

I'm yet to be convinced.
My Garmin pedals read about 10% higher than my H3 (ok, they are LHS only but it is my weaker leg) but were fairly similar to the PT I've got sitting in the rafters. That used to correlate pretty well with the Drivo that I had, but the H3 seems to read low (or everything else is high)

I think there's something in establishing relevant zones for both given that the pedals are what you'll use outside.

Could you use a weight hung on a pedal?

Something along the lines of: set cranks horizontal, hang weight from pedal, release and record power measurement. Repeat but with the wheel off and bike on the trainer. Theoretically, the same force is being applied with measurement in the same(-ish) location.

Might not be a long enough duration or a large enough weight to be meaningful but could be a starting point of comparison.

To be clear, when I suggested using powermeter pedals as a control, i was referring to dual-sided PM pedals. Single-sided pedals that then estimate power would not provide an appropriate control.

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Amateur recreational hobbyist cyclist
https://www.strava.com/athletes/337152
https://vimeo.com/user11846099

You could compare a threshold test on the trainer then on the road (or wheel on trainer) with the PT wheel.

With this method you only know the applied force but not power. In addition, what bike computer / app to use to get a static force or torque measurement?

When one knows force, one knows power as long as one also knows velocity.

Power = dot product of (linear) force and velocity,
Power = dot product of torque and angular velocity.

Torque is just force applied at a distance to a fulcrum; and angular velocity in unit of radian/s is easily converted from rpm.

SRM report slope as Hz/Nm and offset in units of Hz. In effect, the offset is indirectly reporting torque. And from a set of offsets generated from respective torque inputs, one obtains a linear regression yielding the slope and offset when zero torque is applied. This is the entire underpinning behind static calibration.

What complicates static calibration for the H3 (or indeed any smart trainer giving power reading at the hub) is that it's quite difficult to apply static force to the hub without the hub rotating. For crank-based PMs and powertap, one simply applies the rear brake when hanging an object with a known mass (usually from a pedal), and one can obtain the static force applied.

Without further contraption, do this on a smart trainer, and the chain will rotate. The hub on the smart trainer has to be held still somehow in order for the static calibration to work.



Graph x-coordinate is torque (in Nm); y-coordinate is reported offset (in Hz)

There is the simple practical problem that bike computer normally only interpret ANT+ data for power.

In a static test the ANT+ data from a PM is usually interpreted as zero power and the force or torque measurement value, even if it is indeed in the ANT+ Messages (it is indeed in the wheel torque ANT+ messages from a powertap hub, but not in the power only messages from the same hub) is not registered or shown on the display. With the right computer (for example a Joule for a powertap hub) one can use the calibration / zero function to somehow overcome the problem. But it is not that funny to click the zero button and wait until the PM returns a zero value, often in a cryptic dimension. How such an approach works with a Saris trainer? I don`t know.

I also don`t know if Bluetooth LE PM data usually contain torque in a static test. At least Assioma pedals do broadcast the applied force to the dedicated smart phone app.

With SRM PM (and a SRM PowerControl helps too) it is different, here ANT+ calibration messages are sent automatically for some time after one stops pedaling. And as you pointed out the sent offset in Hz is linearly related to the static applied torque.

Why don't you simply use the pedals for both indoors and outdoors and even if you do know that they are reading 10% higher (you could also correct by playing with the crank arm lenght) you know what your numbers should be on race day.

I have the H3 too, not sure if it reads high or low but seem to match my Assioma pedals (have done comparison testing a few times) which I use for race day so it works well for me.

Don't know much about the Garmins, but it is well known that PTs read high.

At least on the Garmins I used, live offset number is reported in the "calibration" (actually offset) screen. That's sufficient to hang a mass off of a pedal, notice the new offset shown, and enter it into MS Excel...

I mean, that's pretty much how I can tell that a SRM has a dying battery. The offset is all over the place and never settles.