I’ve been using PowerTap P1s for a couple of years and it’s always nagged at me that the reading changes depending on my position on the bike, and perhaps how I’m pedalling.
Has anyone nailed this down as to whether it’s a real phenomenon for a start, and then what the cause is?
I use a dumb trainer, the resistance is fixed. I have to change gear to increase or reduce resistance.
If I’m doing 90rpm and the power meter says 180W, I can shift to aero position - still doing 90rpm at the same resistance - yet the meter reading will drop to 165-170W.
So before anyone leaps in and says “it’s normal to lose 10W in aero”, I can’t be…If I have the same resistance and the same rpm, I must be outputting the same watts, right?
I think I get the same still upright, if I change the ‘feel’ of my pedalstroke to be more stompy I see less power. If I kick over the top more, a more circular feeling stroke, I see more watts…but again this is the same rpm at the same resistance…am I going mad?
I used these same pedals, but tbh have not noticed different reading in different position. It is pretty hard to produce constant reading with each stroke, especially with single side PM. It is an interesting observation though. Would it make you feel better with that aero position probably saves you those 10 watts or even more, depending on speed. I also used power smoothing in TR to get more consistent readings as these were jumping too much, I think 3-5 revolutions average.
Reason I stopped using mine was slightly different, I noticed I started stomping on pedals to hit power targets and it completely ruined my smooth stroke and the speed as a result.
Peak power, power duration and power timing will all affect the reported power data, especially when you are measuring at the pedals. They are effectively more sensitive because they are closer to the power origin.
As such, it is entirely possible to hold a particular cadence at particular trainer resistance, while using different pedal strokes. Those differences will change the 3 variables mentioned above (Peak, Duration, Timing) and will affect the power that gets reported.
I can see the changes from seated pedaling when I alter emphasis at different points in the pedal stroke. It also changes a bunch when standing. This is all effectively “normal” and expected with pedal power meters.
This is one reason that people often use a longer smoothing value (maybe 7-15 seconds) for pedals as opposed to other power meter types (around 3-10 seconds) when the meter is in the crank, spider or even rear wheel. Again, the closer you are to the source, there is more tendency for the power to fluctuate more quickly.
I’d say you are seeing “real” data and can either live with it and possible learn from it for you needs. Or you can increase your power smoothing, that will likely mask some of that variation. This all assumes that some of what you are seeing is in a transition phase that will stabilize after some time, and when you settle in to a consistent pedal stroke at the new position.
One of the things I’d like to know is how the pedal differentiates between force tangential to the path of rotation and non-tangential forces. Clearly only the tangential component actually produces torque and the rest should be discounted.
In a crank arm based system the strain gauges can differentiate between the two since pure bending causes equal and opposite strains in the base material and axial causes equal strain. The differential between the two gauges is always equal to the bending.
There is likely a connection between the pedal (when zero offset to establish the resting angle) and couple that with the cadence sensing to determine pedal position around the circle.
The PowerTap app has a mode where you can watch the power during the cycle and see where you are applying power. So there is some method (even if my guess is wrong) that allows them to track (or reasonably predict?) the power during all phases of the cycle.
As such, I have no idea if or how it considers non-tangential forces?
I’d also be curious whether different trainers and the presence or absence of flywheel effect could have an impact on the reported power. If the power algorithms value force at particular points in the pedal rotation, surely this could be affected by the way that the turbo applies resistance?
I’ve been curious about this because I changed turbos around Christmas time, and my power values have been low (and relatively static) for a few months. It might be to do with my legs,/consistency, or to do with moving my position forwards on the TT bike, or it might be the trainer (Tacx Bushido Smart, compared to a Tacx Booster previously). The turbo feels like it stalls and I have a massive dead spot that I have to work extra hard to push over (until I get used to it). My legs felt really good when I rode outside on the weekend on the TT bike, so hopefully it’s not the legs!
I did an experiment ages ago on my dumb trainer where I could produce more watts by pulling on the upstroke and “pedalling in circles” - even though the cadence and the speed produced was exactly the same.
It convinced me that such techniques might result in putting more stress on the strain gauge, but not in a way that makes the bike go forward any faster.
I suspect the differences you’re seeing relate to this.
Assioma pedals are the same in that calibration needs to be done with the cranks vertically aligned. Calibrating with them horizontal should produce very low or zero power output. I’ll try that tonight…