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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]]>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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]]>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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]]>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) 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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]]>Wheel rotational accelerations are very small, especially at steady state, although the oscillations about the mean are a bit larger in the initial seconds of a hard acceleration (but that’s the case no matter the chainring shape). I went through this in a couple of my own blog items:

http://alex-cycle.blogspot.com.au/2015/01/the-sin-of-crank-velocity.html

http://alex-cycle.blogspot.com.au/2015/01/accelerating-sins-crank-velocity.html

SRM Powercontrols from PCV to PC7 enabled 2Hz recording rates. PCIV before that enabled 10Hz recording rate, although (as with 2Hz) it would only update power values once a crank revolution was completed. IOW the power file would step up/down more frequently than 1Hz, but each step would last X tenths of a second, with X being the whole number of tenths of a second it took to complete a pedal stroke.

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