Stages Power Meter Compared (my best DC Rainmaker impersonation)

[savechief]

Just before doing some intervals tonight, I decided to throw my Stages power meter (Ultegra 6800, Gen 2) on the CAAD10 that lives mostly on an Elite Drivo trainer. Some people on WW are very happy with their Stages power meters, despite some early issues, while others completely dump on them. Hopefully this data will be interesting and perhaps will help some people decide to buy (or stay away from) Stages products.

Comparison #1 (35 minutes, 3x5' intervals with 20s bursts at the beginning of each minute during intervals, 5' RBI, Resistance mode)

Elite Drivo with Trainerroad
Power Metrics (per Trainerroad):
- Average = 206W
- Normalized = 230W

Stages Power Meter with Garmin Edge 520
Power Metrics (per Golden Cheetah):
- Average = 206W
- Normalized = 229W
- xPower = 226W

Plots are forthcoming, but the DC Rainmaker Analyzer tool doesn't seem to be ingesting my .tcx files right now, so I'll have to figure that out. I also haven't really had a chance to digest the data and come to any conclusions. Hopefully in the next week, I'll be doing a ride with both the Stages power meter and a Powertap G3, after which I'll plot that data.

[TobinHatesYou]

Accuracy isn't the only issue with Stages PMs, but it is one of them.

- Battery life is seemingly random. Sometimes I get 120 hours of pedaling time, sometimes way less. It doesn't ever come close to the 200 hours claimed.
- Gen1 enclosure problems. Mine failed out of warranty and sweat corroded the electronics. Luckily they serviced it and upgraded it to a Gen2 for free outside of warranty.
- The Gen2 enclosure doesn't really seem that much better. The O-Ring always falls out when I change the battery, which means it can pick up dirt or other debris that might compromise the seal.
- With my L/R balance of 52/48, it just isn't going to be as accurate as a full or dual PM.

My Stages is basically used for indoor training on a spare bike so I can confirm the numbers I'm seeing from my Cycleops Hammer. For outdoor riding, I use Garmin Vector 2s, and will eventually upgrade to Vector 3s.

[savechief]

Why do you believe that the left/right balance that your Garmin Vector 2 pedals is providing is actually accurate? I've heard just as many, if not more, complaints about Garmin Vector power meters as I have Stages power meters. Is your left/right balance consistent through a broad range of wattages?

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[wowamj]

stages is one power of one leg so it is not exctly
i love vector2 that is good and good

[gewichtweenie]

Avg power over long duration isn't that insightful for comparisons. Try doing different chunks at different wattages and different RPM , you might see if there is a separation, and if there is whether its trended

[savechief]

I'm hoping to do a MAP test with the Drivo and Stages sometime soon.

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[youngs_modulus]

Avg power over long duration isn't that insightful for comparisons.

Oh, but it is. If one meter reads high (or the other reads low) you'll pick it up in average power. The longer the duration, the easier it is to pick out a difference.

[Marin]

The real question is - how accurate do you *need* your power meter to be?

The only reason I would want something more accurate than my Stages is for doing Aerolab regression for CDA measurements. For everything else, not.

[gewichtweenie]

Avg power over long duration isn't that insightful for comparisons.

Oh, but it is. If one meter reads high (or the other reads low) you'll pick it up in average power. The longer the duration, the easier it is to pick out a difference.

Entirely missing the point. I've seen different drift at different power and different cadence. For example if one we're doing high cadence drill, 1 reading will be very predictably higher (or lower) compared to the other.

If doing a long riding session covering a wider range of cadence (e.g. faster on flats and lower on higher resistance/simulated hills), the differences will average out to 0 at on the full ride figures. But the reading on the simulated hill section will have drift

[youngs_modulus]

I have no doubt you’ve seen one meter read higher than another, but cadence isn’t the reason. It can’t be.

I’m about to explain why, but I should warn you that I design scientific instruments for a living, so I think about this subject way too much.

TL;DR: Cadence can’t explain different readings because of the way power is calculated from what these meters actually measure (torque). While there are a lot of reasons one meter might read higher than another in the short term, if the two readings converge over the course of a ride, the meters are effectively reading the same torque, and that’s what matters.

______________________

Ok; on to the gritty details:

Power is torque times angular velocity (omega). In other words, P = T ω. Omega is conventionally in units of radians per second, but that’s a straight unit conversion from RPM. (E.g., 100 RPM = 10.472 rad/s).

For two meters on the same bike, ω (cadence) is the same, because angular velocity is trivial to measure and doesn’t drift. So if the meters are giving substantially different readings, they’re necessarily measuring different torque values.

All meters do some signal processing to turn their strain gage readings into torque measurements, and the details of how they do that are part of what cause different meters to read differently at any given moment. For example, one meter may have a different thermal compensation algorithm than another, causing readings to diverge with temperature. Smoothing is a big deal in signal processing, and discretization error can affect the power displayed on your bike computer (though not as much at the meter) for short periods of time.

But the fact that two meters have measured a very similar amount of work done over the course of a ride (kiloJoules, which is essentially average power) means that they agree about torque (and they necessarily agree about cadence).

Going back to the formula that all power meters use to convert strain measurements to power: power is torque times cadence. If, at a given cadence, two meters disagree, that’s because they’re measuring a different torque. And if they’re measuring different torques beyond a certain margin of error, one or both is miscalibrated.

A power meter with a miscalibrated slope would be off by the same amount (proportionally) regardless of cadence, so you wouldn’t see of a difference there due to cadence. A power meter with a miscalibrated offset could read high at low torque and low at high torque or vice versa. But the error is torque-dependent, not cadence-dependent.

Keep in mind that most powermeters change their offset (their “zero”) multiple times over the course of a ride. Thermal compensation demands this. Meters try to do this when you’re not pedaling, ideally when stopped at a light. But many people load up their drivetrain slightly at lights. If you were stopped at a light with your left foot on the ground and your right leg resting on the pedal at 90 degrees to TDC, a Stages meter would get an accurate offset at that stop while a Qarq would get a slightly erroneous one due to the weight of your leg on the right crank. This would cause the two meters to give slightly different readings until the next zero.

The point is, there are many, many reasons why one meter would disagree with another, but cadence isn’t one of them. And if two meters agree over the course of a ride, they basically agree on torque, and since that’s what they’re measuring, you can’t really ask for more than that.

I bet you’re sorry you asked.

[youngs_modulus]

The real question is - how accurate do you *need* your power meter to be?

The only reason I would want something more accurate than my Stages is for doing Aerolab regression for CdA measurements. For everything else, not.

This.

The Chung method is one of the very few reasons to use an especially precise power meter. If you’re just trying to quantify the effects of your training, something like the Stages meter will give you good accuracy and passable precision. That’s all most of us need.

This is counterintuitive to many people, but a left-arm-only Stages meter can easily be more accurate than a $2000 SRM Science power meter. The reason has to do with the difference between accuracy and precision:



A properly calibrated Stages meter will be pretty accurate and precise enough. A poorly calibrated SRM Science (wrong offset and/or slope) will be precise, yes, but not accurate. A flaky temperature sensor would cause the SRM science to be both imprecise and inaccurate.

Properly calibrated, an SRM Science meter is going to be both accurate and precise, which is great. But you only need that when you’re doing things like calculating CdA from power files via Aerolab/the Chung method.

[Marin]

Which is why I'm considering picking up an old 10s Powertap just for aero tests. But I don't have enough time to do a good protocol anyway so I'm holding off.

[glepore]

Agree with the above re: Stages being "acurate enough". I'm fortunate enough to have had, over the years, Powertap, SRM, Stages and Quarq.

All of them have been fine for training purposes. The issues I've had with Stages are those already noted and not a big deal if you keep 2032's around. If you have battery life issues, its usually moisture (remove battery after rain rides). The only thing I've found to be a pain is the "settle" period for the Quarq after chainring changes (might be for all crank based meters, can't comment on SRM as I never changed rings).

My preference would be to do all of my training on one system as they all have their quirks, but even that is probably overstating it for rank amateurs.

[kulivontot]

In this thread:
Op shares data and 8 guys immediately hijack thread to go on rants unrelated to the dataset

[bremerradkurier]

Now I'm curious about how my Stages measures, if at all, when I'm in a track stand at a light-have to remember to look.