Power Meter Calibration Differences

harringtonp

For anyone who has owned more than one power meter and used them a lot, have you noticed big difference in power readings ?

Have owned a Power2Max for years and used it loads (and its still going strong). Bought a 4iii precision recently which I use on the race bike and keep the P2M on the training/winter bike (use to regularly swap the P2M between bikes).

Anyway on steady Z2 spins the 4iii reads 10+ watts lower. This isn't scientific as I cannot use them both together. However when you've done the same route for years and compare other metrics such as speed and heart rate, differences soon become apparent.

And when racing difference are even bigger. Attached is an image of a Strava segment of a club race in Shannon I've done several times over the years. It shows 13 power readings, 12 with the P2M and then last nights with the new 4iii. Last night was second fastest in the list with HR normal race levels and I'm certainly not lighter than previous years. Yet power is 25+ watts lower. That's 10% !

Its only numbers but it does mean that for any interval work you have to rediscover the numbers for the particular intervals.

ED E

If I'm not mistaken you're comparing a right side spider with a left side crank? A left leg right leg discrepancy could account for a largish chunk of that delta.

tunney

I have had multiple SRMs over the last 15 years.
Computrainer, Neo, Kickr as trainers.
Also Powertap P1s and Powertap hubs.

Once you factor in things like drivetrain loses, location of where reading is being made and percentages of accuracy I found all bar the kickr within expectations.
The Kickr was pure junk and I got rid of it.

Your 10% difference of power etc is not really scientific, as you say. Things like wind, road surface, temperature, tyre pressure, drivetrain will affect how you were moving.

Its less about calibration and more about accuracy.

The P2M has an accuracy of 1-2%.....

After having written all the above I realised that you have not said if you had the single or dual 4iii precision ???

Macy0161

harringtonp wrote: »

Its only numbers but it does mean that for any interval work you have to rediscover the numbers for the particular intervals.

I don't have loads of insight, but the TrainerRoad podcast always says new ftp test for new power meters.

fwiw my left sided 4iii matched my cyclops Hammer power, but that's all I had to compare it with (well also the Elite virtual power, but that was way off).

harringtonp

I am comparing a right side spider with a left side crank and the left leg right leg discrepancy is undoubtedly a factor.

It's not scientific but blatantly obvious from examining the image above that the 4iii reads way lower than the P2M. This is not an isolated case either, every time I use it numbers or lower.

tunney

harringtonp wrote: »

I am comparing a right side spider with a left side crank and the left leg right leg discrepancy is undoubtedly a factor.

It's not scientific but blatantly obvious from examining the image above that the 4iii reads way lower than the P2M. This is not an isolated case either, every time I use it numbers or lower.

You aren't comparing a right side power meter with a left side one.

Yes the spider is on the right and measures the power - however it measures the total power, not a single side. SRM, Quarq, P2M all do the same

The premise of a left sided power meter is flawed. It is not simply double the left and thats the total.
Not everyone generates the same from each leg, and certainly not at all wattages and efforts.

So the 10% is really more that the P2M measured your power while the single sided 4iii guesses it.

harringtonp

Implication there is a large imbalance rather than them being calibrated differently

Daroxtar

My FSA Powerbox (rebadged power 2 max) measures lower than my smart turbo. Similar to about 200w but after that there's a steady creep, about 10% difference so 400w on the FSA is closer to 450 on the turbo but strangely in a flat out sprint the FSA usually shows a higher maximum output

tunney

Daroxtar wrote: »

My FSA Powerbox (rebadged power 2 max) measures lower than my smart turbo. Similar to about 200w but after that there's a steady creep, about 10% difference so 400w on the FSA is closer to 450 on the turbo but strangely in a flat out sprint the FSA usually shows a higher maximum output

Whats the smart turbo?

harringtonp

Interestingly the one area where the 4iii seems to read higher than the p2m is sprinting

doozerie

For what it's worth, my dual power meter pedals show my (averaged) left/right power balance as 45%/55% at its worst. If I was using a single-sided power meter then my estimated average total power could obviously differ from reality by quite a bit - it would potentially show 90% of actual power if mounted on left, or 110% if mounted on right.

I think my imbalance varies according to effort too, tends to work closer to 50/50 as I work up the power zones, though I suspect it goes out again if sprinting.

harringtonp

From precision

---

"Thank you for reaching out.

We understand that customers also want to compare their efforts to those of others running different power meters. There are a number of reasons for slight discrepancies between your Power2Max and your PRECISION. Power meters that measure at different locations in a bike's drivetrain are almost assured to not match up exactly. Power imbalance from left and right legs output is very likely a cause, since most people are a few percent stronger in one leg than the other. Since PRECISION only measures your left leg, it cannot account for what your right leg is doing. Differences between the way that companies calculate power can also have an effect on slight differences in power measurements.

As I'm sure you're aware, the most important aspect of a power meter's function is the repeatability, so that you can compare your latest efforts to previous ones objectively. You can make sure that your PRECISION reads as consistently as possible by making sure that you zero your power meter a few minutes into each ride.

We do have a solution for you. if you go to the advanced features screen in the 4iiii app on your smart phone, you will see a Scale Factor feature. When turned on, you can provide PRECISION with a multiplier to adjust the power output. This was made available so that people with known left/right power imbalances could adjust PRECISION's output to be more accurate, since it only measures one leg. It can also be used in this scenario to match the output of PRECISION to a user's other training equipment, so that they have a seamless training experience on all of their equipment without having to apply any formulas, or do any post-processing to make things match up.

For example, if you think PRECISION is reading approximately 10% low, try a scale factor of 1.10 and see how that works.
The formula is: Output power = Scalefactor * (left leg * 2)

On Android phone - open the 4iiii app then click on the three small rectangles in the top right corner of the screen. Select enable advanced.

Click on the wrench that appears to enable the Adjustment Scale Factor

On iphone - Advance to Device Settings, and under PRECISION settings, there will be an option to set Scale Factor.

Please let me know if you have any further questions."

Daroxtar

tunney wrote: »

Whats the smart turbo?

It's a Tacx Vortex smart.

tunney

Daroxtar wrote: »

It's a Tacx Vortex smart.

Daroxtar wrote: »

My FSA Powerbox (rebadged power 2 max) measures lower than my smart turbo. Similar to about 200w but after that there's a steady creep, about 10% difference so 400w on the FSA is closer to 450 on the turbo but strangely in a flat out sprint the FSA usually shows a higher maximum output

The issue here and cause of the creep is tyre temperature.
I used to have the same with my Computrainer for the first twenty minutes of a session, then I'd do a roll down calibration and it would be spot on again. Do you do the roll down calibration after 20 minutes? I see the vortex has one as well.
Also trainer specific tyres help too.

Another issue can be tyre slippage meaning differences too, more likely temperature mind.

Macy0161

doozerie wrote: »

For what it's worth, my dual power meter pedals show my (averaged) left/right power balance as 45%/55% at its worst. If I was using a single-sided power meter then my estimated average total power could obviously differ from reality by quite a bit - it would potentially show 90% of actual power if mounted on left, or 110% if mounted on right.

If the power meter is consistent, does it really matter? I only really use power when training or analysing after the fact. I'm happy enough with the limitations of my left only power meter to be honest, particularly when it matched the total "power" of my Hammer. Or rather I can't and couldn't justify the cost of total or dual power meters.

doozerie

Macy0161 wrote: »

If the power meter is consistent, does it really matter? I only really use power when training or analysing after the fact. I'm happy enough with the limitations of my left only power meter to be honest, particularly when it matched the total "power" of my Hammer. Or rather I can't and couldn't justify the cost of total or dual power meters.

Whether it matters is very much a personal thing. For me it mattered enough that I was wiling to pay the extra for dual power. I wanted to know if I had an imbalance, and to what extent if so, and I was willing to justify the extra cost for that reason.

If the cost of dual had been too high for me then I would certainly have settled for single-sided power. In reality, for my very limited ability and potential, training with single-sided power would be just as effective as training with dual power, but for some people I think the benefits of dual power are significant and single-sided power could potentially impact the effectiveness of their training (if they were unaware of an imbalance and therefore not correcting for it). In the same way I think that some people can train just as effectively with no power meter at all, but I'm not one of those.

The reason for my post above was not to question the value of single-sided power meters, it was to point out that a difference in readings between a dual power meter and a single-sided power meter does not necessarily mean that the latter is actually incorrect or faulty as such (in its measurement of the actual power output of that one side, that is) but that it might simply point at an imbalance on the part of the rider. And to point out that, certainly in my case, that imbalance can be significant.

From a training point of view I certainly consider consistency as the most important aspect of any power meter, whether an individual meter is consistently out by 1% or 99% doesn't really matter as far as I am concerned (so long as my ego can live with seeing figures consistently lower than I expected or hoped for!). However, if you are swapping between two or more power meters then it becomes a real pain if you have to mentally (or via software) adjust for significantly different readings for each, even if each of those meters is consistent in itself.

It could even hamper your training if you get that adjustment wrong between meters. At the very least you really need to measure your FTP per power meter, which is not a major pain but it's an extra hassle that I'd personally avoid the need for if I could, but others may well be perfectly fine with that.

tunney

Macy0161 wrote: »

If the power meter is consistent, does it really matter? I only really use power when training or analysing after the fact. I'm happy enough with the limitations of my left only power meter to be honest, particularly when it matched the total "power" of my Hammer. Or rather I can't and couldn't justify the cost of total or dual power meters.

The terms "single sided power meter" and "dual sided power meter" are misleading.

There are power meters, and then things that are not power meters. To be a power meter its required to measure the power (torque, cadence, and whatever else).

The balance of legs output is not consistent across effort, duration or even day to day issues. So something that doesn't measure the actual power will never be consistent.

harringtonp

doozerie wrote: »

However, if you are swapping between two or more power meters then it becomes a real pain if you have to mentally (or via software) adjust for significantly different readings for each, even if each of those meters is consistent in itself.

It could even hamper your training if you get that adjustment wrong between meters.

This is the crux of the issue. Lower numbers in particular mean getting new numbers for intervals, higher isn't so bad. If I was doing 6x3min on the road tomorrow using the p2m I'd be looking at averaging 330-340 for each interval. If I was to try this with the 4iii I suspect I would blow after 2 intervals. And that's the same for every interval length. And when I blow a session I then have to figure out if and how I can rescue it as there is a strong urge to just bail.

Lumen

harringtonp wrote: »

This is the crux of the issue. Lower numbers in particular mean getting new numbers for intervals, higher isn't so bad. If I was doing 6x3min on the road tomorrow using the p2m I'd be looking at averaging 330-340 for each interval. If I was to try this with the 4iii I suspect I would blow after 2 intervals. And that's the same for every interval length. And when I blow a session I then have to figure out if and how I can rescue it as there is a strong urge to just bail.

Can you anticipate and therefore avoid the blow up using HR? Or does that remove the whole advantage of a power meter?

doozerie

harringtonp wrote: »

This is the crux of the issue. Lower numbers in particular mean getting new numbers for intervals, higher isn't so bad. If I was doing 6x3min on the road tomorrow using the p2m I'd be looking at averaging 330-340 for each interval. If I was to try this with the 4iii I suspect I would blow after 2 intervals. And that's the same for every interval length. And when I blow a session I then have to figure out if and how I can rescue it as there is a strong urge to just bail.

In the circumstances I think that the simplest (and cheapest) option is to measure your FTP for each power meter, and train to percentages of each FTP rather than to specific power/wattage figures.

That would mean that a 3min interval of, say, 120% FTP would require the same physical demand for each power meter. Well, maybe not exactly the same of course due to the different methods of measuring power, but hopefully "close enough" and certainly closer than using the same target watts for both power meters.

That same approach would be needed even if you had two dual-sided power meters whose measures differed.

8kvscdpglqnyr4

Could you borrow a PM from a friend? Or even better, get 2 PMs?
I have a PowerTap wheel and I've calibrated it against Garmin Vector pedals and a Rotor crank PM all at the same time on 1 bike.
All 3 PMs were within a couple of watts of each other across all powers.

harringtonp

Lumen wrote: »

Can you anticipate and therefore avoid the blow up using HR? Or does that remove the whole advantage of a power meter?

Yes on the turbo I can generally tell from HR how I am doing during the intervals. For example when looking at 3min intervals I can usually judge with the HR on the 4th whether I'll get 5,6 or 7 done. Basically once HR gets close to max I usually have one more in me. But by doing 3 min intervals to a known power on the P2M I know from the outset I am looking at 5-7 reps which is what makes a decent session for me given time constraints etc.

And even with the P2M I have different numbers for turbo and road in my head but they have been built up over time. And I will probably do the same with the 4iii.

BTW I haven't done a proper FTP test in years. If you use power meters a lot and maintain a fairly constant level of fitness during the year they are of limited value.

doozerie

harringtonp wrote: »

BTW I haven't done a proper FTP test in years. If you use power meters a lot and maintain a fairly constant level of fitness during the year they are of limited value.

FTP tests are of limited value if you assume that all the power meters that you use are equally well calibrated all the time, equally as accurate as each other, and all consistent over time, and they often are not some or all of these.

Even in terms of training, basing your training on your FTP is only reliable if your FTP is accurate for each training block. It can and will change with circumstances (too much training, too little training, age, quality of recovery, stress, ... - some short term factors in there but some long term ones too). I reckon that if you are very effective at assessing your state of fitness then you can have some degree of feel for whether your FTP has deviated from the last time you tested it, but a regular FTP test seems like the most reliable measure still, especially if you are invested in a power meter as a fundamental part of your training since it makes training so specific/targetted.

FTP tests are certainly not perfect, the result of a test can itself be skewed by circumstances, but if you do them regularly enough then you'll minimise the effect of any skewed measure. Like many people I'm sure I don't like FTP tests, but I see them a being a necessary part of training with a power meter.

Crocked

Anyone ever notice power differences when comparing a power meter to a smart trainer depending on what chainring they are on?

I have a neo I use for training and a typeS P2M on the bike. Previously I didn't care too much how accurately they lined up to each other as I do an indoor ftp test for the neo and then an outdoor one for the bike/P2M.

This year I picked up a second P2M so save the faffing about switching the TypeS between bikes. So I've done some testing to compare each one back to the neo as a reference to see if the two P2M are comparable. The TypeS reads low when in the small ring compared to neo but then reads high when in big ring.

The NGeco tracks very close to the neo in small ring (2-3w higher which you'd expect) but then reads 10-12w higher when in big ring.

Anyone come across anything like that. Ideally I'll get a loan of some power pedals to figure out which one is off, thought it was strange though.

Should say I think all the tests have been in ERG mode so that might be causing some of the issue?

doozerie

Re Erg mode, if you are measuring FTP via a ramp test then I'm inclined to think that Erg mode won't skew things at all. If doing a more conventional FTP test though then I think it might affect the results.

Re big versus small chainring, I've read some very determined claims online that you'll get different power readings for the same "level of effort" in big versus small chainring on a turbo trainer. I've certainly read convincing conclusions from tests that measured different power losses between big and small chainring due to differing amounts of chain articulation, the wattage differences tend to be in single digits, but the other (less convincing to me) claims I refer to have talked of bigger differences.

I wonder whether the differences you are seeing has anything to do with inconsistent measuring of cadence though? The reported power figure is calculated by torque multiplied by cadence so variability of either torque or cadence will obviously affect the power figure. The reason I focus on cadence is because I've had trouble with my Neo spiking to crazily high cadence figures at times. This happens in my power zone 1 primarily, tends not to happen at higher zones, but when I raised the issue with Tacx they basically told me that cadence figures reported by the Neo are calculated ones, it doesn't have a conventional cadence sensor built in, and essentially I've just got to live with its unreliability (my word, not theirs). Not very reassuring for such a premium product. I was left with little faith in the ability of the Neo to correctly determine cadence generally.

As an aside, since then (a few months ago) I've been using my bike's Garmin cadence sensor as the source of cadence measurement, ignoring the Neo's cadence figures, and my reported speeds for turbo sessions are noticeably lower than before. Not that it matters as such, I don't care what theoretical speed I'm doing when stationary, but it leaves me wondering how the Neo determines speed too.

tuxy

harringtonp wrote: »

This is the crux of the issue. Lower numbers in particular mean getting new numbers for intervals, higher isn't so bad. If I was doing 6x3min on the road tomorrow using the p2m I'd be looking at averaging 330-340 for each interval. If I was to try this with the 4iii I suspect I would blow after 2 intervals. And that's the same for every interval length. And when I blow a session I then have to figure out if and how I can rescue it as there is a strong urge to just bail.

Have you tried changing the scale factor like 4iiii support recommended?
It would be interesting if your numbers come closer to matching up after that.

harringtonp

tuxy wrote: »

Have you tried changing the scale factor like 4iiii support recommended?
It would be interesting if your numbers come closer to matching up after that.

No not yet. Before changing the scale factor you need to have a figure. And the impression I get is that the differences are variable depending on the type of riding. So it will take a while.

Crocked

doozerie wrote: »

Re Erg mode, if you are measuring FTP via a ramp test then I'm inclined to think that Erg mode won't skew things at all. If doing a more conventional FTP test though then I think it might affect the results.

Re big versus small chainring, I've read some very determined claims online that you'll get different power readings for the same "level of effort" in big versus small chainring on a turbo trainer. I've certainly read convincing conclusions from tests that measured different power losses between big and small chainring due to differing amounts of chain articulation, the wattage differences tend to be in single digits, but the other (less convincing to me) claims I refer to have talked of bigger differences.

I wonder whether the differences you are seeing has anything to do with inconsistent measuring of cadence though? The reported power figure is calculated by torque multiplied by cadence so variability of either torque or cadence will obviously affect the power figure. The reason I focus on cadence is because I've had trouble with my Neo spiking to crazily high cadence figures at times. This happens in my power zone 1 primarily, tends not to happen at higher zones, but when I raised the issue with Tacx they basically told me that cadence figures reported by the Neo are calculated ones, it doesn't have a conventional cadence sensor built in, and essentially I've just got to live with its unreliability (my word, not theirs). Not very reassuring for such a premium product. I was left with little faith in the ability of the Neo to correctly determine cadence generally.

As an aside, since then (a few months ago) I've been using my bike's Garmin cadence sensor as the source of cadence measurement, ignoring the Neo's cadence figures, and my reported speeds for turbo sessions are noticeably lower than before. Not that it matters as such, I don't care what theoretical speed I'm doing when stationary, but it leaves me wondering how the Neo determines speed too.

I haven't compared readings for a ramp test but am due one and will be recording both readings for it. the workouts I've compared readings in have been a mix of power level and mix of interval lengths.

I had read some debate online regarding big v small ring and the RPE and did find that myself. All my indoor training throughout winter was done in erg mode and small ring with no outdoor rides in that time. It was noticeably harder to do the same intervals in big ring when I decided on a whim for ****s and giggles to go mad one night and use the big ring for a set a few weeks back. So much so I failed and bailed on a few workouts. This seemed to level off after mixing things up for a few sessions and I found my big ring legs if that makes sense. I'm thinking the neo and NGeco are reading ok, as they track very well in small ring and while a bigger diff in the big ring most of that can probably be explained by chain lines/indexing etc. TypeS seems to be way outside that though

I have noticed a difference in the cadence but my understanding was the neo didn't use cadence in it's power calculations so disregarded it as a cause. I haven't noticed any cadence spikes though and apart from being out by a couple of rpm cadence seems to track. Might be worth looking into further now you mention it though and stick a sensor on to check the p2m is reading cadence correctly. The Neo 2 apparently has proper cadence measurement rather than estimate as in the original.

doozerie

Crocked wrote: »

I had read some debate online regarding big v small ring and the RPE and did find that myself. All my indoor training throughout winter was done in erg mode and small ring with no outdoor rides in that time. It was noticeably harder to do the same intervals in big ring when I decided on a whim for ****s and giggles to go mad one night and use the big ring for a set a few weeks back. So much so I failed and bailed on a few workouts. This seemed to level off after mixing things up for a few sessions and I found my big ring legs if that makes sense.

Every time I try to get my head around this I completely fail, I might simply be missing something blatantly obvious. My brain tells me that if I choose a gear combination in big ring that is essentially the same as one in small ring, and I keep cadence the same in either gear (so that power output is the same, which means torque must be the same), then the effort I'm putting in must be the same in either gear. But I've encountered several online discussions where people were very adamant that even keeping cadence and power the same on the turbo, the big ring gear made for a much harder session.

When you tried your sessions in big ring did you use the same cadence as you'd been using in small ring?

Crocked wrote:

I have noticed a difference in the cadence but my understanding was the neo didn't use cadence in it's power calculations so disregarded it as a cause.

This fits with my experience too. Whenever my cadence, as reported by my Neo, spiked it never affected the power figures, even if the cadence spike lasted a relatively long time (30 seconds or more). I saw the same cadence spikes across two different applications so they were definitely coming from the Neo.

If the Neo had been calculating power based on torque and cadence, which is how I understand most power meters calculate power, then its power figures should obviously have spiked too. Which leaves me wondering how it actually calculates power. That hurts my brain too.

Crocked

doozerie wrote: »

Every time I try to get my head around this I completely fail, I might simply be missing something blatantly obvious. My brain tells me that if I choose a gear combination in big ring that is essentially the same as one in small ring, and I keep cadence the same in either gear (so that power output is the same, which means torque must be the same), then the effort I'm putting in must be the same in either gear. But I've encountered several online discussions where people were very adamant that even keeping cadence and power the same on the turbo, the big ring gear made for a much harder session.

When you tried your sessions in big ring did you use the same cadence as you'd been using in small ring?

Yeah I'd never even considered it a thing until I changed chainring and was wondering why my workouts went to ****e. I'll have to double check but cadence in big ring from memory tends to settle in around 90-92 whereas small ring i tend to settle in around 93-95 so not a massive difference. It definitely feels different though, small ring feels a bit heavier or less smooth in a lower cadence ( i used to have lower but I've naturally seemed to moved up to a higher cadence in the last year maybe due to the small ring workouts feeling a bit heavy at the lower cadence?). I'm guessing it's just flywheel inertia/speed or something along those lines but this kinda stuff is above my pay grade. I now mix and match rings a bit and I can do efforts in either ring but they still feel different to my legs/head. .

This fits with my experience too. Whenever my cadence, as reported by my Neo, spiked it never affected the power figures, even if the cadence spike lasted a relatively long time (30 seconds or more). I saw the same cadence spikes across two different applications so they were definitely coming from the Neo.

If the Neo had been calculating power based on torque and cadence, which is how I understand most power meters calculate power, then its power figures should obviously have spiked too. Which leaves me wondering how it actually calculates power. That hurts my brain too.

I've never noticed the neo cadence spike like that and thats the cadence I have showing on my workouts but can't remember ever seeing it look weird like you have had. I think the neo is using the magnets and the power to them to calculate the wattage so cadence to it isn't important. The powertap hubs estimate cadence too so likely they are calculating power in a similiar fashion?

.

nilhg

doozerie wrote: »

If the Neo had been calculating power based on torque and cadence, which is how I understand most power meters calculate power, then its power figures should obviously have spiked too. Which leaves me wondering how it actually calculates power. That hurts my brain too.

The Neo couldn't be using your cadence to calculate power, it would have to be measuring rpm of the shaft the cassette is on and measuring torque somewhere in the same area, whether by a strain guage or some calculation based on the magnetic forces involved in the resistance unit

doozerie

nilhg wrote: »

The Neo couldn't be using your cadence to calculate power, it would have to be measuring rpm of the shaft the cassette is on and measuring torque somewhere in the same area, whether by a strain guage or some calculation based on the magnetic forces involved in the resistance unit

Thing is though, the formula for power that I've seen cited most often (from multiple sources) is: power = torque x cadence

Sometimes some static factor is cited as a multiplier on the right hand side of that equation too, I imagine some factor is always used but just not mentioned most of the time for simplicity.

The simplicity of the formula appeals to my simple mind. Take away cadence and that formula no longer works. And if you measure the rotational speed of the rear axle/spindle/cassette you are obviously not measuring cadence since different cadence will give you the same rotational speed at the rear for different gear combinations.

There is probably a simple explanation that I am missing here. Maybe the "torque x cadence" formula is itself a simplification of something more complex that is measurable directly within the heavily controlled environment of a turbo trainer i.e. maybe cadence is only used when you don't have access to the rotational speed of the wheel/flywheel? I guess that would fit with your suggestion above, and also how PowerTap hubs work as Crocked mentioned.

nilhg

doozerie wrote: »

Thing is though, the formula for power that I've seen cited most often (from multiple sources) is: power = torque x cadence

Sometimes some static factor is cited as a multiplier on the right hand side of that equation too, I imagine some factor is always used but just not mentioned most of the time for simplicity.

The simplicity of the formula appeals to my simple mind. Take away cadence and that formula no longer works. And if you measure the rotational speed of the rear axle/spindle/cassette you are obviously not measuring cadence since different cadence will give you the same rotational speed at the rear for different gear combinations.

There is probably a simple explanation that I am missing here. Maybe the "torque x cadence" formula is itself a simplification of something more complex that is measurable directly within the heavily controlled environment of a turbo trainer i.e. maybe cadence is only used when you don't have access to the rotational speed of the wheel/flywheel? I guess that would fit with your suggestion above, and also how PowerTap hubs work as Crocked mentioned.

The formula i was thought in leaving cert physics was power = torque x speed, so in a rotational system like a crank then speed is in rpm, or as cyclists say cadence.

That formula only applies if all the inputs are working on the same system so you can't take the cadence figure from your crank and apply it to the torque generated in the trainer, only the cadence/rpm of the trainer shaft will work.

AFAIK turbo trainers derive a cadence figure from tiny changes in the power applies as you go through your pedal stroke, ie they count the dead spots and divide by two so if you hit a sweet spot where your stroke is particularity efficient then it will struggle to report correctly.

harringtonp

Chatting to a club mate last night who has a stages which consistently under reads by 40W across a wide spectrum of power levels. Unlike me he was able to test it against a turbo with power and garmin vector pedals and saw the constant difference as he increased power.

I know of other riders who have had Stages that I felt over read (compared to my P2M) and I suspect difference versions and firmware can behave differently.

The leg difference was cited to him too from multiple sources but he did a watt bike test and there was little discernible difference.

When I think about how fickle the mechanism is I'm not at all suprised... you are measuring deflection at various points in the arc of the pedal stroke and trying to map these amounts to torque values

tuxy

Constantly under/over reading is fine for most people if you're training to power with just one PM the the numbers don't matter once it can be replicated.

dc rainmaker did do a tests on a one sided 4iiii and found it to be very accurate or at least as accurate as a one sided setup could be.
https://www.dcrainmaker.com/2015/12/4iiii-precision-review.html