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Vastly different power numbers after unit refurbished

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Old 11-28-18, 02:41 PM
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hammerinbb
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Vastly different power numbers after unit refurbished

About two years ago, I bought a Stages Dura Ace power meter. It seemed to work fine until recently. Stages determined it needed to be re-built, which they recently did.

Until it began to fail about a month ago, it was consistent. My FTP was about 219 watts, as determined by Training Peaks. I am 70 years old but can usually hang with guys 20 years younger. I didn't know whether that FTP was good, bad or average. When I got the refurbished unit back, I calibrated it with my Garmin 500 as I did the old one. The calibration number was about 890.

But on my first ride, I suddenly got much better numbers. After Tuesday's ride, Training Peaks estimated my FTP at 306 watts, or 3.59 watts/kg. (I am 5'11", 188 lbs or 85 kg.). I re-calibrated this morning and confirmed that the ant ID# was the same on both my Garmin and the PM.

I also ran the Stages app and found these numbers:
Crank: 175 cm (I have a 175 crank.)
Slope: 0.02345
Temp slope: -0.27594
DPOT: 222
Device type: Road
Calibration type: A

CAN I really have that greater an FTP? As I looked at my fellow riders on Strava, some of whom I know to be stronger than me, albeit often lighter, their power numbers when using a PM are often closer to what mine used to be. This morning I rode a fairly easy ride (avg. heart rate at 122bpm), yet my avg. power was 198w.

Is there anything else I need to do to ensure the PM is calibrated correctly?
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Old 11-28-18, 02:57 PM
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I don't have a power meter but if I did - I would find a steady steep hill about half a mile or so long, time myself going up and then do the math or use one of the online power calculators. Compare that to the power meter.

Long and steep so that you're going slow, minimizing the effect of air resistance and rolling resistance loss. It would more closely approximate mgh/t. (ie, mass times gravity times height/time) which is easy to measure.
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Old 11-28-18, 03:06 PM
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219 to 306 is quite a jump. I think you're right to be skeptical.
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Old 11-28-18, 04:33 PM
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The only test I know of for crank based power meters is hanging a known weight on the pedal and using a software application provided by the power meter manufacturer. The only other alternative is to check it against a known climb and time yourself and then calculate the power.
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Old 11-28-18, 04:35 PM
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Originally Posted by wphamilton View Post
I don't have a power meter but if I did - I would find a steady steep hill about half a mile or so long, time myself going up and then do the math or use one of the online power calculators. Compare that to the power meter.

Long and steep so that you're going slow, minimizing the effect of air resistance and rolling resistance loss. It would more closely approximate mgh/t. (ie, mass times gravity times height/time) which is easy to measure.
Unfortunately, I live in Florida. We don't have any half-mile hills close by. But I'll keep that in mind when I'm near one.
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Old 11-28-18, 04:38 PM
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FYI< I wrote to Stages, and they responded:

""I have seen discrepancies like that between meters before. It's possible that you've gone from an under-reading meter to an over-reading, and the difference just happens to be huge. "I don't like recommending this, but if you have a smart trainer or another power meter available, we can make some comparisons. My other thought would be comparing specific Strava segments, and what their times/power-outputs look like. If you've ridden anything under similar wind conditions we can usually make some good comparisons with Strava."
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Old 11-29-18, 12:23 PM
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In general, it takes me about 190 watts (SRM power meter) to ride 20 mph on flat terrain - 6' tall 172 pounds but I am very optimized on position, helmet, clothes, tires, wheels, shaved legs and etc and about 285 to 300 watts to ride 25 mph on the same terrain. So if you can ride along at 25 mph at 300 watts +/- then the power meter is probably correct.
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Old 11-29-18, 02:36 PM
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Originally Posted by Hermes View Post
In general, it takes me about 190 watts (SRM power meter) to ride 20 mph on flat terrain - 6' tall 172 pounds but I am very optimized on position, helmet, clothes, tires, wheels, shaved legs and etc and about 285 to 300 watts to ride 25 mph on the same terrain. So if you can ride along at 25 mph at 300 watts +/- then the power meter is probably correct.
I was on my regular road bike this morning and tried to do five 5-minute intervals at VO2Max level followed by a 5-minute recovery, Based on the 309 FTP that level would be 322-367 watts. I could only keep that up (avg. 322 watts) on the first interval. After that my watts averaged between 275-291 watts. I'm thinking my FTP is somewhere between the old power meter and the new one.
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Old 11-29-18, 04:32 PM
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Originally Posted by wphamilton View Post
I don't have a power meter but if I did - I would find a steady steep hill about half a mile or so long, time myself going up and then do the math or use one of the online power calculators. Compare that to the power meter.

Long and steep so that you're going slow, minimizing the effect of air resistance and rolling resistance loss. It would more closely approximate mgh/t. (ie, mass times gravity times height/time) which is easy to measure.
I did go back and saw what my old power meter read when I was in Colorado this summer and had plenty of long climbs. I compared what it showed vs. this online calculator (Climbing Power Calculation). It calculated my power about 20 watts higher than my old meter. So I'm likely somewhere in between the old and new readings.
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Old 12-01-18, 07:27 AM
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I would say to find a calibrated smart trainer and compare. It may be hard though if you live in Florida because I can't imagine riding indoors down there, given the mild weather although the one time I biked down there the heat eventually got to me. Or at least find someone else with a PM that weighs about the same and ride side by side, at the same cadence, for a few stretches and see what you both get. A 20w difference there is probably not significant but a 50w difference would be.

I don't believe any of the calculators. My experience with calories is that no two will ever give the same results. I use RidewithGPS and the numbers that come off my Wahoo Bolt don't equal the calculated numbers in RwG, and this is for my 10 mile commute so I have a lot of samples. When I then added a PM to the mix I got even different numbers for that same ride, and to tie it back to the original post, power numbers that seemed too high! I have since checked and rechecked and they actually are right. It just takes more power to propel an old converted MTB than a sleek road bike. I do the ride occasionally on the road bike and the numbers are lower with the SAME PM. That's what I like about my Assioma pedals, they swap easily and compare within about +/-10w with my smart trainer.

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Old 12-01-18, 08:04 AM
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Why don't you just test with the new one? You have a workout now. Do a few more and just go off those numbers.

It definitely sucks when trying to compare to previous years, and all, but too late for that (unless you get a new pm). Onwards and upwards.
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Old 12-01-18, 08:06 AM
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Fwiw, my smart trainer (Tacx Flux) measures 10% less than three different power meters, and there are reports of similar issues with other smart trainers (kickr), so I wouldn't necessarily put too much stock in a smart trainer.
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Old 12-01-18, 09:59 AM
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About once per week I ride the Stages exercise bike at the gym for warmup that has a Stages power meter on the left pedal. I can bump the power 20 watts by focusing a little more effort on the left leg. So there is that.

Also, when I am in good but not great shape, raising my FTP 10 watts is a lot. To set a new FTP all time record, 5 watts is a lot. Week to week, I will get the same damn FTP power on the same climb with my SRM. So IMO, repeatability is more important that perfect accuracy.
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Old 12-01-18, 11:14 AM
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Now it's back down

I did our Saturday hammerfest, which had speeds up to 30 mph and my power numbers overall were a lot lower. So there is some issue I need to figure out. I calibrated again before this ride and got a calibration number only one digit lower than yesterday and still within the range Stages said to expect. Comparing a few segments for previous ride where my time was about the same was inconclusive. Some numbers were higher; some were lower. But group rides can't be a good comparisons.
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Old 12-01-18, 10:05 PM
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Group rides are really unreliable for comparing efforts on different rides. Wind, size of the group, composition of the group, the surginess. If you want to make comparisons, solo climbs. Or flat TT on windless days.
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Old 12-01-18, 10:52 PM
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My FTP dropped 40W simply by switching from a Stages to a power2max. As long as the numbers are consistent, they serve me equally well.

I do have hills, so I've done the "time over distance on a known grade" and the P2M is accurate to within a percent or two of the online calculator. My Stages was exceedingly generous.
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Old 01-07-19, 06:27 AM
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Originally Posted by wphamilton View Post
I don't have a power meter but if I did - I would find a steady steep hill about half a mile or so long, time myself going up and then do the math or use one of the online power calculators. Compare that to the power meter.

Long and steep so that you're going slow, minimizing the effect of air resistance and rolling resistance loss. It would more closely approximate mgh/t. (ie, mass times gravity times height/time) which is easy to measure.
I completely agree with doing a physics-based test that attempts to minimize sources of load that cannot be easily modelled. But there are still two critical parameters:

What is the mass of your bike with all included? - need a good scale

How high is the hill?

Just sayin', but if you want to nail "what is the real power?" you also need to nail the mass or weight, and the height of the hill.
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Old 01-07-19, 06:34 AM
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Originally Posted by hammerinbb View Post
About two years ago, I bought a Stages Dura Ace power meter. It seemed to work fine until recently. Stages determined it needed to be re-built, which they recently did.

Until it began to fail about a month ago, it was consistent. My FTP was about 219 watts, as determined by Training Peaks. I am 70 years old but can usually hang with guys 20 years younger. I didn't know whether that FTP was good, bad or average. When I got the refurbished unit back, I calibrated it with my Garmin 500 as I did the old one. The calibration number was about 890.

But on my first ride, I suddenly got much better numbers. After Tuesday's ride, Training Peaks estimated my FTP at 306 watts, or 3.59 watts/kg. (I am 5'11", 188 lbs or 85 kg.). I re-calibrated this morning and confirmed that the ant ID# was the same on both my Garmin and the PM.

I also ran the Stages app and found these numbers:
Crank: 175 cm (I have a 175 crank.)
Slope: 0.02345
Temp slope: -0.27594
DPOT: 222
Device type: Road
Calibration type: A

CAN I really have that greater an FTP? As I looked at my fellow riders on Strava, some of whom I know to be stronger than me, albeit often lighter, their power numbers when using a PM are often closer to what mine used to be. This morning I rode a fairly easy ride (avg. heart rate at 122bpm), yet my avg. power was 198w.

Is there anything else I need to do to ensure the PM is calibrated correctly?
Do you have the same data collected from the sensor before it was rebuilt? Do they all agree? I'd think the sensor company can make something out of any significant discrepancies.
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Old 01-07-19, 08:47 AM
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Originally Posted by Road Fan View Post
I completely agree with doing a physics-based test that attempts to minimize sources of load that cannot be easily modelled. But there are still two critical parameters:

What is the mass of your bike with all included? - need a good scale

How high is the hill?

Just sayin', but if you want to nail "what is the real power?" you also need to nail the mass or weight, and the height of the hill.
Step on a scale that you've calibrated, but getting the hill's actual height can be a challenge. Best if it's been surveyed, and I've considered measuring the angle with a plumb bob and protractor (knowing distance) but just using GPS or an altimeter should get a good approximation.
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Old 01-07-19, 09:30 AM
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Thanks, guys. Stages determined the PM was bad. Rust had formed inside. They sold me at refurbished one at a reduced price. I'm back to my old power numbers.
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Old 01-07-19, 03:04 PM
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Originally Posted by wphamilton View Post
Step on a scale that you've calibrated, but getting the hill's actual height can be a challenge. Best if it's been surveyed, and I've considered measuring the angle with a plumb bob and protractor (knowing distance) but just using GPS or an altimeter should get a good approximation.
Yep! I agree.
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Old 01-07-19, 03:10 PM
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Originally Posted by hammerinbb View Post
Is there anything else I need to do to ensure the PM is calibrated correctly?
At least one of those sets of numbers is off. Find a friend or a shop that has a trainer with a built in power meter or a power tap PM wheel and see how those numbers compare to your Stages.

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Old 01-07-19, 03:38 PM
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The first thing would be to actually do a couple of rides comparing to previous rides. Hills level out winds and other factors, but you should be able to find some good rides you've done in the past to compare to. STRAVA? Also Heart Rate Monitor HRM.
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