That all looks normal. It looks consistent though, so I’d just compensate the number as everyone is advising.
Blockquote
The Kickr doesn’t guess power, it measures flywheel speed and infers power from the energy balance equation for the flywheel. The spindown allows your trainer to measure losses due to friction, which then enter the energy balance equation. Knowing the friction term, you can then accurately compute power from the energy balance equation for a given resistance. No guessing is involved here.
That - in terms of measurement - is a guess. It is an indirectly gauging of the performance. I think the flywheel rpms are measured via hall effect, which is pretty good. The strain is not a measurement, that is a based on an assumption of consistent performance from the spin down - you need a strain gauge to verify the strain assumptions. If the bearing performance degrades, you tilt the unit, the temp changes, etc, the flywheel resistance it predicts will be wrong. It then uses magnets to put additional load on the unit, it needs to ‘guess’ if the magnetic load is what it expects it to be, which would not be in your flywheel equation. It then has to flat out guess on your drivetrain efficiency to compensate for it to produce a number close to what you expect to see from your other PM numbers.
The difference here is the difference between one of those forehead contact-less thermometers and one you stick in your bum. The contactless can be thrown off quite a bit by any number of variables like skin color and glossiness. Direct measurement is much more accurate and consistent.
Blockquote @AussieRider did something much better than that, he recorded power with another power meter and used @dcrainmaker’s tool to compare the power numbers. He posted the power numbers, and looking at the data the agreement between the Quarq’s and the Kickr’s power numbers is almost uncanny. Even cadence is almost perfect.
I realized he was using the same cadence reading for both. The Kickr’s cadence numbers are a guess. The quarq will have a much better value, but it shouldn’t be equal to the pedal’s.
What I was looking to see with the gearing is if there was a load or rpm when the number falls off. That would indicate if there’s a mechanical defect.
It is no more a guess than “guessing” torque by measuring several resistances in a bunch of variable resistors we call strain gauges and temperature as well as cadence, and inferring power from that.
You can fundamentally compute power in different ways: work divided by time or force times distance in the direction of force or torque times 2pi times rpm. Most trainers use the first relation, most other on-bike power meters the third. From a physics perspective the first is the most fundamental, though.
You mention a bunch of factors in a trainer that change over time. That is correct, but no different than a power meter: both need to be zeroed regularly.
There is the guess word again: elementary signal analysis like Fourier analysis to determine the dominant frequencies in a signal (frequency = cadence) is not guesswork.
If you have a look at the numbers, then very often the small discrepancies are caused by the fact that the Quarq returned only integer values whereas the Kickr had a higher resolution and also returned fractional values.
Both units measure cadence directly, so they should give identical results within the margin of error.
Its definitely a thing. Here are 2 of mine from an indoor and an outdoor ride. The imbalance varies with fatigue, intensity, cadence and inside/outside. Probably the phase of the moon affects it as well. I went down this rabbit hole during the lockdowns and finally decided that for me single sided power meters were useless.
Yes at higher intensities Threshold + it’s near as damn it 50:50. In my case I suspect it’s something to do with slightly different leg lengths and/or different ankle flexibilities.
Similar with me. Perhaps standing up any difference in leg length is nullified. Who knows?
I have dual sided power meters, and I can see that out of the saddle almost always is exactly 50/50, but when in the saddle it changes depending on far too many things to figure out. One point to be made, Dr. Chung (inventor of the Chung method) simply says single sided power meters do not have the accuracy needed for that method. I think that trying to infer accuracy from a single sided power meter is a unfortunate errand you will never achieve. As has been said numerous times in this thread, so many things can get in the way of accuracy in this case. My opinion is simply do not expect accuracy from a single sided power meter. Even precision is a problem due to the extrinsic factors such as fatigue, pedalling technique etc. I gave up on single sided and bit the bullet and went dual sided.
With the Assioma app you can perform a static weight test. You need to accurately measure a weight of between 8 and 25 kg. Then, as you hang this weight on the pedal, you can let the pedal measure the weight to check the calibration and determine if a scaling factor is needed.
I did one. It’s within spec. I have wondered however, whether having wedges under my cleat offsets the pressure point on the Pedal and whether that throws off the Reading
It was within spec with or without the 10% factor?
Wedges should have no impact at all on the measurements, the sensor is further down and there is still the pedal housing between the sensor and the cleats.
I would and do trust single sided pedal power meter output over any trainer data. Speak to Assioma and Wahoo for their own views, which is what I did (Powertap and Elite) when I found my new trainer had a discrepancy from my pedals. Elite didn’t even contest the pedal data, they immediately realigned the trainer to match the pedal data.
As I said I also tested the Assioma’s against brand new quarq’s. Both the quarqs and trainer were 10-12% higher. So it’s not just the trainer. That said I don’t doubt the accuracy of the Assioma’s as they are single sided and it could very well be a leg imbalance. On top of that when I adjusted the power scale on the pedals up 10% they still didn’t match the trainer. That’s a bigger mystery. All the data is well documented in this thread along with links to the power files.
Well, I wouldn’t lump all trainers into a bucket because of a single manufacturer or experience like you had. Elite in particular has a less than stellar record for power data, and it’s one reason they pushed hard to implement and then market their OTS system. THAT is a decent concept and highly accurate system compared to nearly everything else they previously offered.
I have plenty of faith in my Kickr, Hammer and Neo trainers with testing against my PowerTap P1 dual power pedals. They are plenty accurate and reliable. I’d take most of them over a single-sided PM any day of the week.
I know that some people are strongly anti-singled sided power. I still haven’t seen the case made that dual sided is a game changer for training. If anything, crank-based seems to be the most reliable (srm, p2max, quark).
In the end most training is getting within zones:
Under LT1
At LT1
Between LT1 and LT2
At LT2 (ftp)
Above FTP
Any power meter gets you there with good, repeatable test protocols. The ranges are wide enough that you don’t need high accuracy. You mostly just need consistency.
That said, why can’t dual sided power meters be off? You could have one side be 2% high and the other side be 2% low. Both meters may be in spec but you end up with an artificial imbalance. Does Assimoma ship right and left sides that have been specifically calibrated to each other?
I’ll put myself in the ‘anti-single sided’ power meter camp - I would not bother with one at this point.
If a single-sided power meter is the only power meter you have, and are using it to set training zones, measure progress, etc. then I do think it is completely fine.
Once you start trying to compare two power meters (including smart trainers), then you get into real trouble throwing single sided meters into this puzzle (unless they are all single sided). If things match - great, you are done. If they don’t, you are now left with the question of whether the meters are off, or if you have a leg-imbalance. This is not answerable without L/R or total power data. Leg imbalances can change with effort, duration, etc. and can easily add 10-15% error.
There are plenty of problems getting multiple dual/total power meters to match, and trying to get single sided and dual sided to match is pretty much a fool’s errand.
