Power Meter Differences

I have an Elite Direto and a Stages Left Sided PM. The Elite measures about 15/20W lower. Both are consistent.

I understand a PM closer to the source and that bypasses things like the chainset are going to measure higher.

My question is two fold:

Am I right in my assumption that the Stages is more accurate as it’s closer to the source?

Is there a way to manually adjust the Elite to match the Stages?

FWIW I use the Stages as it’s my outdoor PM but do Zwift races with Family & Friends and don’t want to receive an unfair advantage if it’s not representative of my actual ability.

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After stuffing around with the same dilemma (less the Elite brand specific trainer) I’d say both are equally accurate.
It’s good to know crank/pedal for training. It’s good to know cassette/hub for actual output.

No idea with the Elite.
I’d be super impressed if the stages allowed for a manual offset or scale function. Maybe in the future?

Any discrepancies in your left vs right leg will be magnified with left only.

As for Zwift, pick the one with the higher power numbers of course :rofl:

The next time you do a Zwift ride, record the power of both separately. Pair the Elite to Zwift. Pair your Stages to your Garmin. Afterwards, go to Zwiftpower, use the Analysis function, and upload the Garmin file. Yes, you’ll need to have a ZP account first. The Analysis will show you how close or far the Elite is to the Stages.

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I just loaded a few sets of rides I did this past week into Zwiftpower. Two rides I recorded power with both a Powertap wheel and Stages L-only (gen 2), and one ride I recorded with a Saris H3 and Stages. For each ride the Stages measured about 20w higher. I know that the Powertap has been reasonably accurate in the past as I have paced hill climb time trials with it (without unexpected results).

20w does seem to be a pretty big discrepancy. Drivetrain losses will account for some of that. I do tend to have pretty severe pain on the right side of my body (back, hip, knee) which could cause an imbalance in my case, although I didn’t feel particularly bad on the rides I recorded. The difference in power does bother me enough that I’m considering ditching my stages entirely.

You can see my data here: https://zwiftpower.com/analysis.php?set_id=46984
Go to “All Files” to see all 3 sets. Note that the first one had a long dropout on the Stages which affects the averages (artificially narrows the gap). You can select sections of the graph with the mouse to see averages over particular sections.

No.
First of all, you will have drive train losses, so it is to be expected that your Elite measures lower power numbers than your Stages. Depending on how well you are maintaining your drive train, you should expect about a 5 % loss in your drive train. If you haven’t oiled or waxed your chain in while, this could be higher. So at 250 W that alone is about 12.5 W.

Then your Stages is left-only, so if you have an imbalance of, say, 2 % (51-to-49) and your left leg is weaker, this gives you another 5 W. So just taking these two factors into account, we are already at 15-20 W (if you are doing 250 W).

Another point is accuracy. Elite’s power meters are quite famous for their accuracy. In their high-end trainers, they are the best on the market. Nevertheless, you have an error that is typically 1.5-2 % for each power meter. You add that on top and you easily can see that this may account for another few watts.

So I think everything is working properly in your case. If you want to slightly inflate your power numbers in Zwift races, I guess you can choose to believe your Stages power meter. But for training, I’d probably stick to the Elite since it’ll allow you to do single-leg drills. (I have a left-only power meter, too, and single-leg drills get a bit wonky — I just ignore power numbers and look for consistency.)

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I’m happy to check my ego at the door for Zwift races and use the Elite but I often read that I should use the same PM indoors and outdoors? Which would be the stages.

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Sure, that makes sense.
Perhaps you can have your Elite report its power numbers via Ant+ to your head unit, so you could in principle also have its power numbers on screen in case you want to do single-leg drills or some such.

First of all, same. :smiley: So I don’t think you’re experience is unusual…you can probably find a few other anecdotal confirmations of your situation on this forum if you look hard enough.

Second, your stages PM is an array of orthogonal strain gauges stuck on your crank arm…the Direto has an optical torque sensor. So there are two very different instruments at work. I suspect you’ll see the difference between the two close substantially if you add some additional warmup time before you spin down the Direto & zero out the Stages. I think Elite recommend a 10 minute warmup. Take 20 minutes. See if that helps.

But the difference between the two will never be zero. It’ll always be something like 3W (for a super efficient well-lubed drive train with an optimum chain line) to 12W.

Thanks to all who have taken the time to reply.

I suspect you’ll see the difference between the two close substantially if you add some additional warmup time before you spin down the Direto

Is this spindown the calibration performed in the My Elite Training App?

What exactly does this do, how would it reduce the differential?

Thanks.

The torque sensor in the Direto essentially measures the distance between an array of metal tabs around an axel. As torque is applied to the axel there is some measurable ‘twist’ than can be measured this way. Very handy…you just need to back out all the confounding losses associated with the Direto ‘drive train’.

That’s what the spin down is for. Apply a certain amount of energy to the system, observe how long it takes the system to spin down or dissipate this energy, use that to calibrate true torque input to the OTS.

Ok, so one source of error there is how much the losses of the Direto ‘drive train’ change with temperature & how different steady state temperature is from just regular room temp. As you use the trainer the components warm up and as they warm up that can jazz the OTS numbers.

I’m hypothesizing that maybe the Elite 10-minute warm up is not sufficient to achieve steady state temperature in your trainer. Anecdotally, a longer warm up can sometimes help.

For fun, zap that sucker with a heat gun for a little bit. Spin it down. Do some zwifting. Let us know how it goes for you.

Wow, what a detailed reply. I don’t think I’ve done a spin down since I got it (don’t shout at me!).

Will try that on my next ride.

Don’t underestimate the single side versus total discrepancy. I’m a consistent 55/45 L/R rider. Doesn’t matter if it’s a 15 minute or 15 hour effort. Always 55/45.

When I finally bought a dual side power meter and smart trainer (which are consistently within 1% of each other), it took me a little while to wrap my head around the 10% drop in my power across the entire curve. Not good at all for the ego, not good at all.

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This is what I think is my main issue. A little from the drivetrain but mainly left leg dominant.

You should definitely do a spin down calibration. There should be something in the Elite app on your phone. As @Brennus mentioned, a spin down at the start of the ride vs one say at the end of the ride will yield diff results. Most manufacturers will recommend 10-15min of riding before doing a spin down. When I owned a Kickr1, I would do a spin down at the end of a ride. Then, it would be good for over a month (regulary compared it w/my bike PM). So, I would do a spin down every 3-4 wks at the end of a workout to maintain things. My trainer and bike were stationary. If I were always setting up and packing up the trainer, this would not have worked.

As for the L-only, here is something from a while back for you:

No, if both are properly calibrated (in the respective factories), they should measure power within the given accuracies. Measurement method is completely secondary.

Power loss in the drive train is not a source of error. The Direto should measure the power that arrives at its cassette accurately and not put in any fudge factors to compensate for (unknown) drive train losses. Also, AFAIK the Direto’s optical sensor does not require any temperature compensation. And since room temperature is pretty much constant, that shouldn’t be a factor for the stages either.

Maybe. But let’s think this through. The OTS has some bearing in it. They are sealed & lubricated. The Direto has a belt and another couple bearings. Does it make sense that the collective loss of all those components would be the same when you walk into the room as it would be after 20 minutes of riding? (If your answer is yes, then you need read no further)

Ok…sort of ‘summing the forces’ at the OTS there is the power supplied from the rider/bike (riders power less all drive train losses) and you’ve got the power supplied (or lost) in those Direto ‘drive chain’ components mentioned above. Do you think the sum of those two are what should be reported to your bike headset, or are you interested only in the rider/bike portion? (If your answer is the sum, you need read no further)

If you’re still reading, then I don’t need to keep writing.

Same experiment for you. Zap that sucker with a heat gun, do your spin down. Wait a couple hours. Hop on Zwift and do some racing. See if you notice a difference. (You might not! I’ve never raced on Zwift so I don’t know)

Also, get the datasheet for the OTS & give it a read. It’s a nifty bit of kit.

Stages crank-arm length setting is adjustable via the Android app. I haven’t recorded against a 2nd power meter to see how this affects reported power yet.

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I’ve been wondering this lately as well. I’ve got Assioma Uno pedals and my wife has a 4iii left side crank meter. We both zero offset before each ride. We ride side by side on a lot of backroads with barely any cars and I’m typically doing 30 more watts than she is in all almost all wind conditions, both riding on hoods. I’m 82 kg, she’s about 40 lbs lighter than me so must be the wind resistance from my larger frame?

  • If by “frame” you mean your body, then yes, that is very likely.
  • Also consider that clothing choice (specifically how tight fitting it is) can have a HUGE impact on wind resistance and related power.

Obviously other variables like tires, tire pressure, drivetrain cleanliness and condition, bearing condition and other factors will all play a part. Many small differences can and do add up, but body size and the wind resistance on mostly flat roads will be a big delta. Any elevation change will also have an impact that is notable.

We both wear bibs and race fit jerseys so its definitely not the clothes.

Our bikes’ drivetrains are very clean and similar wear, I clean both at the same time regularly, she runs 85psi compared to my 95psi, both conti GP5000 25mm.
I’m 5’11, she’s 5’9" but 40lbs less than me so my body frame is much bigger, I’m a pretty lean 82kg so bigger shoulders, hips etc. Gotta be the wind lol.

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