I have a Stages L Gen2 power meter that is new to me and during testing to see the difference between the power meter and TR’s VP the power meter reads 40-50W higher than VP. I have read on the forum that a lot of users see the power meter having the lower reading.
I was using the iOS app on my phone for VP and the Stages app on an iPod for the power meter. The room I am in is quite cool at 53F-55F and the power meter was zeroed to begin the session.
I’m not sure what to make of it. Do i just retest with the power meter and continue training or should I be sending the unit in for service (if any)?
*My apologies if this question has come up before. I wasn’t able to find one.
If they’re that far out, one of them is faulty.
You have to keep in mind that the Stages is doubling the left side measurement and the Kickr is reading after the drivetrain. A new drivetrain and bb on one of my mtb’s was a 5 watt saving. So ten watts between the Stages and Kickr.
There will also be some variations due to your body being a little wonky at times. Not a big difference for most fortunately.
If you’re on a road bike, go and find a nice long hill with a smooth surface and try one of the online calculators to test your power meter. They should be pretty close.
VP can be pretty accurate if you use exactly the right trainer it was designed for and set the bike in exactly the right way.
Otherwise be super happy you just got a massive FTP bump and don’t look back. Getting a huge bump in power is much better than the other way around - at least this way you’ll be getting new power records.
Powermeter will be more accurate than virtual power. The clue is in the name - “virtual” power. At no point does virtual power actually measure power and as such is an estimate. Virtual power can be very consistent, which is important for training, but always take with a pinch of salt what the number of “Watts” are with virtual power.
That sucks. I’d suggest getting some comparison power files ready for Stages support. When I did mine the first few emails were wasted with the usual support questions.
Have you tried a new battery? Have you done a zero offset or noticed a change in values? That sort of thing.
A hill test is the easiest way to illustrate a large discrepancy.
Good luck with the ticket. Let us know how you get on.
You don’t say what trainer you’re using, but do you know someone with a smart trainer, such as a Tacx Neo or Wahoo Kickr, who will let you set up on it and do a comparison that way? A result within around 7-8% discrepancy would be acceptable.
I’d pay a fair price for a certified testing service that was independent. It’s a little ridiculous that nobody offers this service.
The real world data would be a whole lot more useful than the crap posted online. I love the hard work put in by people like DCR and GPL, but they only test the units they are given.
Most of us have to deal with distributors. Turns out it’s the same one for my Kickr and Stages. The Kickr went back. Apparently it matches another L g3 stages perfectly, so no refund. Now the Stages must be out. Apparently, I’m to pay for postage and will need to pay more if It’s found to be correctly calibrated. I would also give up lots of consumer rights by sending it directly to the distributor.
I totally agree with your advice. Test against a mates. That’s all you can do to lodge a case really.
Collect as much data as possible before you send it back.
First of all, 60W difference at 100W or 300W is way out of spec. Either way, unacceptable.
100W vs 160W is ~46% diff
300W vs 360W is ~18% diff
Is this a Stages Left only? This makes a difference as well. If left only, then it takes the power from the left and doubles it. If that leg is stronger or weaker by say 3%, then after doubling you have 6%. At 200W, that is +/-12W. At 300W, that becomes +/-18W.
@BigOlBar Which Wahoo Kickr are you using? First gen Kickrs were notorious for being off by a lot. Newer ones are not w/out their own problems. Wheel on trainers such as the Kickr Snap can be quite finicky about setup in order to get an accurate, much less repeatable power number across sessions for the same effort.
The new Stages Gen3 spec states +/-1.5%. We will use the Kickr Snap +/-3% since it is highest/worst among the current Kickr series (i.e., Kickr, Kickr Core, Kickr Snap). Worst case scenario, this gives you a swing of 4.5%. At 100W, that is 4.5W. At 300W, that is 13.5W.
In summary, there are a lot of things to consider here. Even so, 40-60W is way out of spec!
You’re comparing two ways of reporting power that don’t actually measure it, to a device that does measure the power and trying to say the device acutually measuring the power isn’t right? If you think it’s out, you need to test it against another real powermeter. I’m intrigued to hear what Stages support response is!!
So many variables with virtual power such as tyre pressure and how hard you wind up trainer onto the tyre (e.g. 2 turns, 3 turns etc…). No problem for individual as lomg as keep it constant BUT means any comparison to actual power meter or indeed anyone else using VP is pretty pointless!
Hey @klew!
You didn’t mention which trainer you’re using with VirtualPower, but I’m here to tell you that VP is just a rough approximation of your power, and one that is very easily affected by other factors like trainer and tire pressure, as well as the relative accuracy of the VP curve in question.
We answer this question under the “VirtualPower Watts vs. Power Meter Watts” section of the following article, and there is an explanation of the factors that affect VP above that:
Just to clear up any confusion, our best recommendation is to re-test your FTP with your new Stages power meter and continue training with your power meter. We always recommend a re-test when changing the method of power measurement, and you’ll likely want to train with the Stages on the trainer since you can also use it outdoors.
I hope this helps! Cheers!
Thanks for all of the responses. They are greatly appreciated.
I am using a Kurt Kinetic, tyres are always at 100psi etc. etc.
I did end up figuring out my issue though. Embarrassed to say that when I first started zeroing the power meter I was still clipped in . Once I zero’d the PM without any pressure on the pedal/crank the PM now measure about 5-8W lower than VP.
My biggest concern with this thread was to determine whether or not the PM required factory recalibration or if the numbers I was seeing were to be expected.
*or buy. My Garmin Vector 3 pedals cost me AU$1509. And I’m still fuming at the week or so lost trying to get a broken set working. They did replace them, and the new pair are good. Still fuming.
Sorry Shane. I hope that didn’t come across as me saying you post crap online. Quite the opposite really.
Although I reckon you would’ve let me know if I was directly questioning your integrity. ;).
Fair enough. I stand corrected and you do buy equipment like the rest of us muppets. It also sounds like you get stuffed around just like the rest of us as well.
It sucks that most of us can’t test against units known to be accurate. If a power meter can’t accurately measure power it’s not really good enough and it really gets up my nose. It’s totally ridiculous that a factory calibrated unit could be sold with a calibration issue.
Glad to hear you solved your problem Klew. It happens to the best of us.
. Once I zero’d the PM without any pressure on the pedal/crank the PM now measure about 5-8W lower than VP.