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I have a 4iiii single sided crank PM which in most time data fields is pretty much bang on with my kickr (20mins,10 mins,5mins,1min,30s). At 15s it just starts to wonder slightly and at 5s and 1s it is between 5-10% different to the trainer. Obviously these times relate to sprint efforts so do they become more inaccurate at high speed or could my trainer be reading wrong? I dont think its related to L/R imbalance as its on my weaker side. A small example was kickr hit 1070w max and the 4iiii read 1184w. The weird thing is i only had one gear left on the bike and i was in 53/12 gear turning at 120rpm and i am sure i have hit higher in lower gears. I just feel the kickr is not right but i always assume that would be more sccurate than a single side pm.
It could easily be sampling differences here - if you imagine it's averaging over 3, 5, whatever seconds to get the power. Depending when the start/stop of that interval is, you might get different answers. The trainer and meter will not be synchronised. Even if both meters were 100% accurate you'd get different values. They are both "right" but different.
You'd expect to see it for these short time intervals - especially when the power is varying rapidly.
Euain, yes that is a very valid point i forgot about. I would still expect to see the max wattage to be closer though as that will be a very small data point not reliant upon synched readings.
I think your max is still going to have to be averaged over a small period of time. I'd guess you're still going to have to average over one or more revolutions of the pedals.
It would be very hard to get an meaningful instantaneous power reading (you need torque and rpm) and the discrepancy between the two doesn't seem that large.
There is also the fact one is at the cranks and the other is at the rear wheel / rear hub. You'd expect the measure more power at the crank - but I'm not sure how much you lose through the transmission. Clean/oil your chain?
Euain, yeah i expect a little difference with them measuring at either end of the power chain but not that much. Chain is nearly new as is the cassette and i use Molten speed wax so pretty low friction. The only real issue this brings up is if i have to verify power data for zwift, it just looks wrong.
It could easily be sampling differences here – if you imagine it’s averaging over 3, 5, whatever seconds to get the power
If it's a sampling difference I'd expect one to read more than the other on one day, then maybe the other way round the next. Does that happen?
A discrepancy due to the PM being single sided? When you're flat out maybe you're relying more on one leg than the other?
More likely to be something with the really short measurement duration though.
I'm just about to jump on the turbo so I might try something similar.
Thought I read somewhere that trainers lose a little in the drivetrain, not sure it would account for that kind of difference though.
Possibly reaction time of the Kickr?
Chain drive between 98% and 94% or something last time I checked, depending on chain line.
I get similar, peak power from my crank based Stages is usually higher than my Kickr, it just seems easier to create a power spike on a crank based system when you stomp on the pedals for a sprint.
I always assumed it's as the crank based one is measuring your input into the drivetrain whereas the Kickr is measuring the output. There's a bit of inefficiency in the drivetrain but it also maybe smooths out the spike of stomping on the pedals.