Single Sided Power Meter Rant

I’ve just done a quick test to try and plot how my Single sided power meter tracked against my Stac Zero Halcyon Trainer. As you can see at 244W (Stac) the Stages tracks well but a little high which I’m not to concerned about. However at 157W (Stac) the Stages alters quite dramatically with my cadence (60/70/80 & 94 rpm respectively). In short then the single sided meter is about as much use as a chocolate fireguard presumably due to my leg power imbalances.

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What exactly are your imbalances?

You will find variation between all brands of power meters. It’s hard to say that this is a single sides vs dual sided issue

I don’t think a single sided power meter will accurately measure power of any trainer as trainers use total power. I have found this to be the case with with two different trainers.

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Depends what you mean by accurate. Precise? Repeatable? Consistent with another PM?

A trainer measuring “total power” isn’t more accurate necessarily. What about the drivetrain losses that the crank based single sided PM doesn’t experience?

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It’s not about the difference in reported power between the two it’s the degree of variation of the Stages both in terms of intensity and when the intensity is kept the same, the cadence. I’m not saying that the stages is any more or less accurate than the trainer.

My reply about accuracy was not directed at you. What imbalances does your dual sided PM report?

And, to my original point, variation (even just looking at one PM in isolation) isn’t necessarily as simple to explain as “because its one sided”.

Your trainer output looks like it is being smoothed so you can’t graphically compare the two very well.

Are you asking why increasing cadence shows a bigger drift? Presumably you think at higher cadences your leg imbalance is more pronounced?

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I don’t have one. I did a Watt bike test sometime ago and was told/saw I had a leg imbalance thats all.
If I don’t have an imbalance then presumably the trainer is not working correctly and giving different resistances in Erg mode depending on cadence

Yes but by averaging the power over the individual 5 minute intervals you can see a pronounced difference

If the trainer is reasonably accurate then yes

I’m just trying to see what use my Stages would be on my outside bike given all these discrepancies. I’m starting to think I should have saved my money and just go of heart rate outside…

At this point all you’ve found is that you trainer power differs from your Stages and that the power difference varies with cadence. Before getting too in your head about it, I would try to get your hands on a third power source. Maybe a friend has a set of power pedals or something that you can borrow. That way you start to see if the issue is your trainer or the Stages. Without more comparison all you can reasonably say is “My Stages and Trainer don’t track well”.

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In the last month my overall L:R balance on rides has varied between 41:59 and 49:51. I’ve had a couple of left side injuries over the years that mean my left always tends to be a bit weaker and this gets accentuated either when I’m tired or when I’m not pedaling that hard. Muscle recruitment on the left seems to be better when I’m having to work hard and am reasonably fresh, e.g. TTs are typically close to 50:50.

My conclusion is that I should never buy a one sided PM!

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I must agree with the others here. I have compared several powertap hub (3), assioma pedals, quarq (2), kickr snap (2) and a 2nd gen kickr and rarely do they line up perfectly in all scenarios. It’s bad enough I don’t know who to trust. In your case how do we know it isn’t the trainer that varies?

In erg as you vary cadence (if you aren’t changing gears), then you are changing the speed of the flywheel - so things aren’t necessarily constant there either.

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I think I’ll try a similar workout but not in Erg as a similar thought had occurred to me.

It could very well be although i think the probability is that it is the single sided meter (I’m not having a go at Stages - that’s just the brand it happens to be)

Now repeat with sprinting, standing, high and low cadences etc. This is an inherent problem with doubling single side power because no one puts out equal power at all times in both legs

Given the trainer you are on, I would be inclined to believe the trainer is the issue, But you need a 3rd power meter to confirm.

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Assuming the OP is shifting to vary cadence at the same reported power output based on wheel revolution, why would a discrepancy in power attributed to the PM variation be due to the trainer?

That trainer is a Wheel on trainer that uses a magnetic unit to control resistance. It has no actual way to measure power output. the power it gives is strictly based off of power curve based on wheel speed and magnetic resistance. There isn’t any actual strain gauge to the trainer like the Stages PM has.

  • I am struggling to find the info, but AFAIK, the STAC uses a load cell (mounted between the trainer frame and the magnetic resistance unit) to actually measure force via deflection, and then calculate power from that deflection force.
  • If my memory is correct, it is a very accurate power measurement system and not at all like a virtual power curve estimated based upon wheel speed.

Found it: (They call it a “Strain Gauge” at first, then reference “Load Cell” later.)

The STAC Zero Powermeter works effectively on Newton’s laws: every action has an equal and opposite reaction.

The Zero magnet array applies a force to your wheel, slowing it down. But because of Newton’s laws, this produces an opposing force on the magnet array. The entire resistance unit is mounted on a strain gauge, which measures this force.

Atop the power pod is a reed switch that is watching for the spoke magnet to go by. When it does, it lets the reed switch determine your wheel speed.

Force in newtons (N) multiplied by speed in meters per second (m/s) results in power in watts (W), which is what all our cycling friends are interested in.

Does it need to be calibrated due to different wheel constructions or magnet distances?
It doesn’t! Because differing wheels or magnet distances will result in more or less resistance created by the trainer, they also result in exactly as much difference in the force felt by the strain gauge/load cell.

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I stand corrected, But I would still suspect the trainer. Need another power meter to confirm though.

It doesnt matter though. If wheel speed is the same it will report the same power value because the resistance is constant. By changing cadence and gearing to achieve the same speed the power reported by the stages should also be the same if power output is the same. The problem is not that the stages strain guages are inaccurate, its that people are not equally symmetrical across all power and cadences.

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See my experiences with a single side power meter here compared to when I bought the right sided Assioma pedal to compare:

I would also take with a grain of salt any previous readings on bikes with ‘dual side’ power that said you had no imbalance. When I compare my left right balance on my Quarq Dzero which is a very accurate power meter for total power, it tells me I am 51/49… As soon as I used power pedals with actual left right numbers, as opposed to ‘virtual’ left right numbers it showed I had a big imbalance and a single sided power meter for me always will elevate my power.

That said, if all you want is a power number to train to then there is nothing wrong with a single sided. Why don’t you just use the stages to control your trainer, and use it outside. That way you have consistent numbers?