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Stages power meter reporting lower than expected wattage

63K views 119 replies 32 participants last post by  thisisthebeave 
#1 ·
I have both the Powertap G3 and Stages. Friend of mine also has a Stages, and before this he also had a Powertap (but he no longer has his Powertap).

anyway, we both tested our Stages power meters against my Powertap G3, and both Stages meters (his and mine) report about 11% - 13% lower in wattage (normalized power) reading than my Powertap G3. And although he no longer has his old Powertap, he also acknowledged that the reading of the Stages is lower than his old Powertap.

Even though the numbers between the Powertap and Stages are different, but the Stage is still consistent (precise). Precision of the Stages (as shown by its graphs) mirror the precision of the Powertap. Still, it does bother me that the accuracy between the Stages and Powertap can be at 11% - 13%.

Does anyone who has a Stage can verify this observation when compared to other power meters I'm most definitely curious.
 
#2 ·
I would have to put both on one bike to compare heads up which I have not done. I can say on my typical rides everything I have seen indicates they are pretty close, not 11-13% apart - that's huge. My basis for saying about the same is average power over 1-2 hour rides same course similar rides and also max efforts up some longer climbs like 7-10 minute 4-5% grades.
 
#3 ·
I have a Stages on my Commuter bike and Powertap on my Road bike and overall, the Stages shows a bit lower power but not at the levels you are talking about ... more like 2% - 3%.

With that said ... my power imbalance can be chalked up to leg strength imbalance.

The big question is whether it's consistent with it's readings? If it's consistent, you can still use it for accurate training because you can set up your training zones with it and get the same basic feedback as you get from your Powertap. The only difference is your zones will be a little different from one meter to the other.

There are no accuracy standards for power meters at this point ... so, though they can post accuracy numbers ... what's the baseline measuring tool they are using? Until standards are set up and accepted by all power meter manufacturers there will be variances between brands ... but again, as long as they are consistent, they are a usable tool.
 
#5 ·
There are no accuracy standards for power meters at this point ... so, though they can post accuracy numbers ... what's the baseline measuring tool they are using?
Most manufacturers use some variation of known force and known speed as their baseline measuring tool. You can do the same thing to determine how accurate your power meter is. The easiest way is to do a static torque check. A 10% difference in power is pretty large so that should be easily detectable. You can also do a dynamic power check, though it's more hassle. Basically, you find a steep hill of known length and elevation gain, then you weigh yourself on an accurate bathroom scale while holding your bike with everything on it and wearing what you're going to be wearing for the test. Then you climb the hill at least twice (possibly more depending on your boredom threshold), once slowly at low power and once as quick as you can at high power. You want a steepish hill so that your overall speed is low enough that aerodynamic drag doesn't much matter even during your high power run. If you weighed yourself accurately and you know the elevation change accurately you can check the accuracy of your power meter(s). There are more details but that's essentially it.
 
#4 · (Edited)
I've had the Powertap and Stages for awhile now, but never actually compare them, figuring that the two should be close. But the my buddy said that his Stages reads way lower wattage than his Powertap (which he had sold) and so we decided to test both our Stages against my Powertap G3.

- We tested on the same bikes
- we used 2 Garmin 500 headunits (with the lastest Garmin firmware)
- then we used a Garmin 500 and a Garmin 510 headunits (with the 500 connected to Powertap, 510 connected to Stages)
- we calibrated the power meters

And we came away with very similar results, that both our Stages is 11% - 13% below the Powertap. So our test verified my buddy's initial suspicion that prompted us to do this whole Stages vs Powertap comparison thing.

Very unlikely that both our left legs weaker than the right.

While the power chart of the Stages is consistent and it's precise, and still can serve as a training tool, but because the power discrepancy is off too much, it makes training based on %FTP using both power meters to be difficult. I mean 13% off is huge, and it messes up your %FTP target.

Also, when you're riding with another person and you're trying to match his speed on the climb and he gives you his W/kg, you're trying to train to match his W/kg. Well if the Stages is off too much, then your goal of W/kg is gonna be off too if you use the Stages.

There may be no power meter standard, but this discrepancy is too much. Something is not right here. True that precision is more important than accuracy when you only have ONE power meter to work with. But when you're training using multiple power meters, then accuracy has to be at least close within 1% - 2%, or else you're having to guess how much %FTP should you be pumping out.
 
#14 ·
PT is zeroed by removing the battery (per Powertap tech), reinsert the battery, and then letting the Garmin head unit calibrate it. That is about as much as an end user can do as far as calibrating/zeroing the PT. If the strain gauges of the PT are somewho messed up, then I guess that's something end user can't fix.
 
#24 ·
Stages has an iOS app you can download which let's you do static torque testing I thought?
 
#46 ·
My stages is consistently about 10-12 watts higher than my 2014 Red quarq.
I dont know that one feels more accurate than the other. But theyre both consistent, and that's what counts.

if I had to guess, the reason is left-leg dominance. traditionally I have been right-legged in sports. Knee injury though has left left at lest looking more developed
 
#47 ·
A few comments.

1. Asymmetry of left-right power output is normal and it varies between individuals and for an individual due to various things. Many (but not all) become more symmetrical as they approach threshold power levels.

A 10% variance between Stages and a spider based power meter is totally plausible, especially when pedalling at low wattages.

For a start, a 45:55 left:right balance (or 55:45) at very low power is not uncommon.

Keep in mind that if you are actually 45%:55% left:right at a low wattage (e.g. 100W), then the Stages will report power as 2 x 45W = 90W, while your actual two leg power is 100W.

There's a 10% difference right there and that assumes both meters are equally accurate in measuring what they measure.


2. For a Powertap, you need to have several things checked to have confidence:

a. That you have checked and set the torque zero (which Garmin mislabel as "calibration"), AND

b. the torque zero is within specification (e.g. 512 +/- ~ 10 or so units), AND

c. that the torque zero does not fluctuate when no torque is being applied (if you see the numbers bounce about then you have a problem), AND

d. that the "slope" of the power meter is correct. IOW it's not enough to know the meter reports zero torque when there is no torque applied. You also need to verify that it correctly reports torque when a known torque is applied. It's this latter step that verifies a power meter's calibration.

I use the bathroom scale analogy. The bathroom scales might read zero when you are not standing on them, but that does not mean they will be accurate when you do stand on them. You also need to verify they correctly read a known weight.

e. while not an absolute necessity, it is also a good idea when validating the slope calibration of a Powertap to check its behaviour in different gear combinations.


While it's possible to verify the slope calibration of a Powertap (and most power meters), not all power meters permit you to adjust that slope calibration if you find it is incorrect. Powertap, Stages, Power2Max don't allow the user to adjust this calibration. SRM and Quarq do.
 
#48 ·
A few comments.

1. Asymmetry of left-right power output is normal and it varies between individuals and for an individual due to various things. Many (but not all) become more symmetrical as they approach threshold power levels.
I'm thinking even if you become more symmetrical as you approach threshold the Stages still probably isn't the right power meter for you.

You got any clients who use the Stages yet?
 
#51 ·
Speaking of checking the PT on different cogs of the cassette (and I agree with that) do you check both the outer and inner chain rings when testing a spider-based PM?

I remember that years ago Tobin used to hang the weight not from the pedal spindle but from a piece of chain hung over the chain rings. That's how he discovered that his SRM gave different readings "around the clock."
 
#68 ·
Typically I'll check both chainrings. Don't see many triple SRMs. Have tested MTB triple with Powertap.

I have one of tested my own units around the clock (partially), which you can do by clamping the bike in a bike trainer, and then rotate the bike around the rear axle, e.g. lift front wheel up onto a chair to get near 45 degrees, or vertically against a wall to get ~90 degrees, and even flipped right over on the bike's back. Obviously not the whole 360 degrees but enough to make a quick check and see if there are any anomalies over 180 degrees. Mine was consistent so I didn't bother again.

Given peak torque is either side of ~ 3 o'clock, then I figure that's where you'd want the least error if there are any variances around the clock, and that's what you check when bike is in normal position (although some do lift front wheel to give enough floor clearance to hang weights).
 
#54 ·
And yet no head to head reviews I've seen show more than a few percent difference at most between stages and any other meter. My powertap and stages results are so close I quit bothering to compare them.
 
#56 ·
From what I can see, the noise in the ant+ data transmission is at least as great as the differences among any of the direct force meters.
 
#58 ·
I haven't found that to be the case. I agree that the noise can be high but it's still possible to extract signal. Sometimes ferreting out the signal is a little harder, sometimes it's a little easier but, thus far, I don't think the ANT+ protocol has been the problem.
 
#59 ·
ANT+ is digital, so assuming the protocol is properly designed the transmission/reception is noiseless. The analog signals from the strain gauges and subsequent ADC will have noise, but there should be no additional noise after that.
 
#61 ·
In one of Rainmaker's reviews he shows records of data from the same power meter (an SRM IIRC) and same ride recorded simultaneously on different head units that differed more from each other than most of the different meters varied from one another. That's very telling. It's not noise exactly, but digital artifacts that are very significant.
 
#62 ·
I don't remember that but Greg Kopecky from Slowtwitch did a similar test with different head units recording from the same device. In part, I think that's more about the head unit than the ANT+ protocol -- ANT+ requires transmission at 4Hz but most head units record at 1Hz so part of the issue is which of the packets gets dropped. Ray now uses a Wasp to collect all the data from each PM so we were able to get around that. It's clear that certain head units "lag" a bit, especially when starting and stopping but there are ways to handle that. It's actually an easier problem to deal with than the problem of synching up 1Hz head units with the old 1.26-second PT head units, or dealing with the "virtual" cadence from PTs. If more power meters were reporting at higher Hz then maybe ANT+ would be the bottleneck but right now I don't think it is.
 
#64 ·
I've been busy with either riding or working, so haven't had time to deal with this at the moment.

But I like rchung's ideas of verifying using a short steep hill.

The only 2 reviews I read of the Stages is the one from DC Rainmakers and the DurianRider guy, and they both showed good reviews.

I need to do some further testing and verfication this weekend!

I wonder if Stages might have made a bad batch? From what I gather, Stages has a huge backlog and is rushing, running 2 shifts at their factory, to get these things out. I really wonder if me and my buddy got a bad batch since we both seem to get them around the same time.
 
#72 ·
Interesting - the graph does show quite a bit of difference in the higher range. I apparently didn't pay close enough attention when I read the article. For me, I'm using the stages on my rain bike for TSS and pacing on climbs mostly and it does well enough for me. The PT wheel on the good bike does the rest.
 
#73 ·
I have not compared my PT to stages yet, since I dont have 2 head units. I did however attempt to match my PT to the wahoo kickr and I think it's pretty close. In any event I rode my stages bike on my wahoo kickr and the stages read about 5w higher then the wahoo in erg mode. If the cadence is steady maybe even less, but if I speed up the cadence the stages will jump higher then the kickr will hold pretty steady pretty much (erg mode after all)
 
#74 ·
You'll always see different power meters report different wattages. I think SRM is always highest and PowerTap usually (although clearly not in this case) lowest (SRM measures closest to source, although Garmin is probably higher now and PowerTap sufferes from drivetrain inefficiency.

Regardless, the only thing you need for training is tracking consistency. If you want the real numbers for tests, just do testing w/ the PowerTap or inflate your Stages numbers by 10%.
 
#77 ·
Hey folks, I still haven't found time to test the PT vs. the Stages yet, partly becaues of time issue and partly because of equipment issue.

However, over this weekend, I was able to do a long mountain ride, using the Stages. Here's are the numbers. I held a 92.4% FTP (with FTP recently measured by a full 1-hr flatland effort using Powertap G3) for a 2-hr climb duration. My heartrate for the duration was pretty much close to FTP heartrate.

I was able to I PR'ed almost all Strava segments on this climb. So what I'm saying is that it was pretty much maximum effort climb for me.

Based on the wattage number reported by the Stages, based on my heartrate, based on my perceived effort, and based on my time on all the segments... I can say that there is NO way that the Stages was under-reporting wattage on this particular ride. Because if the Stages was under-reporting wattage, then that would mean that my 92% FTP effort could even be 102%+ FTP wattage for a 2 hr duration! No way I would have been able to climb at over 100% FTP for 2 hr.

And as far as displaying the wattage number on the Garmin headunit, I paid close attention to it, and to my surprise, the number track very well. Meaning, the wattage displayed did not jump anymore than my PT, and the wattage displayed pretty much showed I'm at 92% FTP most of the time.

On this ride, the Stage behaved exactly like my Powertap. There is no way the Stages could have been under reporting on this ride.

My previous tests that I mentioned above was done on the trainer, and I don't know if this would have made the Stages to behave a little differently? Also on the trainer, I did not get a chance to put out near FTP effort over a long duration (like a 2hr climb fest).

So that is all I have to report for now. I'm still awaiting to test the Stages against an SRM.
 
#78 ·
My previous tests that I mentioned above was done on the trainer
Hmmm. You didn't say that your previous tests were on a trainer. How many comparison rides did you do, and did you re-zero both PMs before each trainer session? When you said "Stages is 10% lower" was that from one trainer ride or several?

You know up above where I keep harping that what's important is knowing the conditions under which power meters differ? Here's one: you have to scrupulously manually re-zero PTs (and some other PMs) on trainers because there's often no coasting, and coasting is what triggers the auto-zero.
 
#80 ·
Good discussion and quite a bit more technical than I expected. My first reaction was this: Powertap is the pants' size you order, Stages is the waist size you actually are. In other words, tell the customer what he wants to hear, within reason.

An interesting corollary to Rainmaker's (via Fireform's post in this thread) experiment would be to compare simultaneous data from cranks and hub. Anyone done that?
 
#85 ·
A another data point. I have a 45/55 power split left to right. I had a stages on the mtb and it read 10-15% lower than my quarq on road bike. In that situation it made it untenable for me. If I decide to do intervals on my mtb I need the power to be more accurate to be in the right power zone and have the IF and TSS calculate correctly. I got rid of the stages and am lookin at getting an srm for the mtb which should be much closer to the quarq on the road bike.

Now if I had two stages it might be okay but I have trained on a quarq for 4 years so basically all the numbers from prior rides are going to be different and if I injure my knee for some reason now the stages will be even more off. For me it was not worth it, I want numbers that are accurate day to day and able to be compared to my road bike power.
 
#86 ·
My data point : Stages vs Kickr Trainer

I found using the wahoo app (phone app) the stages reads higher then the kickr in pretty much any mode ( I use ERG a lot).. however if I using the wahoo segments app and putting it thought the paces of a live segment the stages and kickr match very well. In erg mode sometimes the difference is 10-15w.
 
#88 ·
I just found this thread when I finally got around to comparing my Stages to my Kickr and discovered that the Stages consistently reads 20-40 watts lower than the Kickr in a range from 100w - 250w (reported on the Kickr) whether I use the Wahoo Kickr app or the TrainerRoad app. The Wahoo and TR apps are running on my iPad and the Stages is paired to a Garmin 1000.

Hopefully will have a chance to compare the Stages to a G3 over the weekend.
 
#87 ·
I think this really comes down to if you only use a stages to train with no comparison to other devices. It does have repeatability to itself but is not as accurate as other, more expensive, equipment.

My problem really came from left only power measurement on Stages and full measurement by the Quarq. Having a 10% difference is huge. I really noticed it when I had a stages on my road bike too (forgot to mention that in previous post, this was before I had a mtb version). I went to do a LT test and I was shooting to beat my prior 20m power. I really suffered and couldn't hold the pace anymore at about 12-15 minutes. I was really frustrated and then later realized that the reason I couldn't hold the pace was because I was pushing 10% or move wattage than I thought I was. My Garmin was showing 275 watts but I actually was pushing over 300.
 
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