Calibrating Power Meters with known weights

It’s often said that for day-to-day training, the important property of a power meter is precision (i.e. repeatability of measured values) and not absolute accuracy (i.e. correct value). I do agree but … what if you had several bikes with power meters fitted to each (okay, that’s actually another reason to swap a hub- or pedal-based power meter!) or wanted to review your long term performance changes 10 years from now?

Some power meters allow to check their absolute measurements after doing a zero-reset (example: Pioneer displays force in [N]), some even allow you to specify a scaling parameter after checking (example: Vector displays torque in [Nm] and allows to store a scaling factor in the pedal to correct their output). Some, like the older P2Ms, unluckily don’t do any of this.

Garmin has a manual on the internet for the recommended procedure. Although they mention the difficulty of measuring a heavy weight of over 10 kgs to the required precision, in their example they are using a large weight, and hanging that from a pedal requires hanging the bicycle high up in the air while attaching the weight … nothing I’d be keen to try.

One alternative could be to just use a calibrated weight that’s used for checking scales, which looks like this:

power meter calibration weight 2This one here is a 10 kg weight (which I admit is a bit on the light side, even as a light weight cyclist with a not too high maximum power number; 10 kg is equivalent to between 150 and 200 W at cadence 100 for ideal completely round pedaling or probably about 50 W at cadence 70 for typical not-round pedaling) accurate to plus minus 1.6g (guaranteed for one year by the manufacturer), which is far above the accuracy needed for this procedure. A 20 kg weight would only measure about 25% more in height/width/depth each and still be compact enough to measure both tangential force (as seen in the picture with a horizontal crank) or radial force (with the crank in upward position) with the wheels on the floor.

Together with the metal hardware like shackles to mount the weight to the pedal, measured on a extra precise kitchen scale, the total weight was 10184.5g plus minus 2.2g or 0.02% accuracy. With power being linear to force and torque, that’s more than accurate enough. (Sorry for the blurry smartphone picture.)

power meter calibration weight - small hardware on kitchen scale

My results for Vector2: Expected 16.485Nm (for crank length 165mm), measured 16.81Nm on right and 16.44Nm on left.

My results for Pioneer: Expected 99.91N, measured tangential 102N / radial -102N on right and 98N / -102N on left.

In both cases, that’s about 2%, which means that without any other error, the final power values could be within 2% error.

Thoughts:

The big question here is of course: even a slight cadence error of 1 rpm will set this off largely, so a 2% error of the final power value is actually unlikely.

I might better use not just a heavier weight, but actually several different weights.

DCRainmaker reported that the yet to come Watteam Powerbeat will use a plastic bag that fills with an exact amount of water to act like a accurate weight. If that works, that’d be nice, although, hanging like 10 kgs = 10 liters doesn’t seem very practical.

Somewhat related: Wahoo used to sell, and probably now rents a weight for calibrating the power meter inside their KICKR trainer whereas Tacx claims their new really direct drive trainer is calibration-free. (The KICKR was only half direct between chain and trainer, but still had a belt driving a flywheel, while the new Tacx is doesn’t really have a flywheel and is completely electronic producing a virtual feeling of inertia by electronic control.) The principle behind the Tacx I guess is that calibration is not necessary if you can control or measure electric current very accurately. A more simplicistic view could be: for a rotation sensor you’d either have a magnet switch or some self-calibration using accelerometers and gravitational force, so why having to calibrate a power meter, isn’t that just poor engineering?

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6 thoughts on “Calibrating Power Meters with known weights

  1. I’ve done the same type of measurements. Quick comments:

    1) It is worth noting that the weight of the pedal/crank is included in the hanging weight. To account for this, record the response for several weights and use a linear fit to extract the response (slope) and pedal/crank contribution (intercept).

    2) On my Pioneer L/R system, I found nearly identical results to yours (L is about 2% low and R is about 2% high) for the tangential response.

    3) Pioneer radial force should have been near 0N for each case (L and R). It is odd that yours were -102N. In my own case, I found the L radial response was consistent with 0 N (< +/- 3N) but the R radial response appeared to register about 10% of the (purely) tangential load. I'm not sure what to make of this but it could be a limitation of the more complicated mechanics of the R crank/spider yielding an overlap in the radial and tangential strain responses at the strain gauges.

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    1. Thanks for the comments – and happy to learn.

      1) My assumption for the Pioneer was that pedal and crank weight (in the case of the Vectors just the pedal weight – especially as the pedal body can now be swapped to Shimano Ultegra) would be “taken care” of by the zero offset calibration and the force displayed is any additional force working on the pedal. Will think about this…

      2) Thanks, good to have some comparison data.

      3) Yes, the radial force is usually close to zero in horizontal crank position. The radial values I gave were measured independently from the tangential ones and in (nearly) vertical crank position. I had nothing to measure and lock the position of the cranks, so I found it more convenient to read the maximum value while slowly moving the cranks fore and aft, and applying this method, I reasoned I should measure tangential in horizontal and radial in vertical crank position. Not sure what to make of your left-right difference…

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      1. Thanks for the clarification – I understand your radial measurements now. If you get a chance could you measure the tangential and radial components simultaneously with a hanging weight and the crank oriented horizontal? I’m curious if you find the reported radial components differ between the L and R as I did.

        FWIW, I simply held the opposite crank steady and horizontal (by eye) to make my measurements. A few degrees of error will have essentially no effect on the tangential reading but could yield a few % error in the radial reading (since cos(small angle) = 1, sin(small angle) = small angle). Along the same line, the radial component I observed on the R would require to about 6 deg of misalignment in the R strain gauge (or my estimate of horizontal (unlikely, I think)).

        Thanks!

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      2. Sure, I will check the next time I make the calibration test. (Though, at this moment, I do not expect that much of a left-right difference as there wasn’t in vertical orientation…)

        Agree with your reasoning about error. Thinking now that it might be useful to measure both tangential and radial in all four positions to check for symmetry as well and to also have numbers that are comparable between tangential and radial forces.

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  2. Where did you hang the weights for measuring the radial force on the Pioneer? Using a 25 lb weight, I got 113 Newtons across the dead center of my Speedplay X2 pedals, but 45 N hanging the weight on the axle right next to the crankarm.

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    1. Good point … admit it had crossed my mind to verify for different locations but eschewed the time to do so. It might be a bit difficult yo judge from the picture but the weight was hang in the opening of a Look-compatible pedal using a carabiner, so the weight was approximately below the center.

      It would seem to me that the forces that work on the crank differs depending on how far the weight is on the pedal axis (right next to the crank it would be more of a compression/elongation at 12/6 o’clock instead of flexing) but also depending on crank position (at 3/9 o’clock it is a twisting force), so a deeper investigation could check all these variables – but I would probably check only for locations within the width of the cleats. Hanging the weight directly next to the crankarm should be equivalent with hanging a smaller weight at the center due to a shorter lever, so I think it’s okay that the readings differ there.

      BTW, I believe this is where for example verve cycling claims higher accuracy due to placing the strain gauges “inside” their cranks, where pedal based power meters might at least be more symmetric through out their rotation, where hub and crank shaft based meters are nicest as they can measure just the rotational force and where spider based meters get close to that except for cross chaining. Though I don’t know the exact number and location of the strain gauges in the Pioneer the contact surface is relatively large, and I was surprised to see how little contact surface the 4iiii and other similar crank based power meters have.

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