What's more accurate, using power meter or trainer power for Zwifting?

(David Kindler) #1

I’ve seen various posts regarding power meters with Zwift… but I’m wondering which setup is better in terms of accuracy and “feel”.  I have a Quarq power meter and a Wahoo SNAP.   I’ve always just used the SNAP as the power source to control the trainer.  I look at the power on a Garmin unit from the power meter and they agree closely when I’ve calibrated with a warmed up tire.   What are the pros and cons of using the Quarq to control the SNAP?   Is the feel different?   Is calibration better?  What do most of you with power meters do for your setup…use the power meter to control or not?



(Joe Daknis) #2

If there is consistently close agreement between the two as there should be? Then I don’t think you’ll notice a difference either way.  The Quarq might be a tiny bit more accurate, depending on which SNAP you have (2017 is +/- 3%, while older units are +/- 5%).  I think Quarq is +/- 2%? But - practically speaking - if using the SNAP power is working for you, there’s no great reason to switch it up (particularly because Zwift’s ERG control isn’t exactly known for locking you in w/ really tight tolerance anyway). 

Then again, there’s no harm in trying to use the Quarq power for control either - to see if you feel the difference.  I know - not super helpful. Just my $0.02 

(David Kindler) #3

Thanks Joe…  your comment is helpful… kinda what I thought, but just wondered what the community does…  I have a new SNAP 2017 model, btw.    Early on during Zwift beta I tried to use the Quarq to control the trainer and had all sorts of trouble with resistance locking up, etc… which is likely a very old quirk gone long ago.   I’ll have to give it another shot and see if I notice any difference.

(Cary Sporinsky TeamZF ZHR(F)) #4


I just rode with my Stages PM’s for the first time with my KickR as controllable. I want to say my power output was a bit lower than the KickR, but it could be the mid-compact 36/52 rings too. I’ll do another FTP test when my legs adjust.

(P MAC) #5

Have a Quarq DZero and Tacx Vortex. Usually use the Quarq though lately it’s been dropping out so have gone back to the Vortex which is like taking EPO. It reads anywhere from 10 to 30 watts higher than my Quarq even after calibration.  

(Cary Sporinsky TeamZF ZHR(F)) #6

Hello again,

I just did a quick seat of the pants test:

Stages PM in Zwift have more realistic feel (as in workload).

KickR produces more output. I could perceive it and I could even see it in my charts (not scientific, bla bla bla disclaimer). Agreed, cyber EPO.

Subject to change.


(Erik Schumacher) #7

I am happy with the precision (approx. 2%) of my Elite Direto. 

(Martyn Kimberley OP [B]) #8

It is a good question but one which could be preceeded by another question.

Are you riding to power on the road? If you are then are you also riding your road bike on the trainer?

I ride to power on the road (in summer) and as such use the same bike on the trainer. I pair the stages to zwift and don’t use the power output from my Tacx Flux. This means i get power results both real world and virtual world that are consistant.

Returning to your question of accuracy… I have club mates that only have power on their trainers. Some of their Zwift rides and results can only be described as mis-matching their performance on the road. This could obviously be a problem with the trainer accuracy, their configuration of the trainer or most likely a combination of both.

(Cary Sporinsky TeamZF ZHR(F)) #9

After a bit more riding time, I am taking a guess that flywheel effect comes into play when comparing the two. Really hard to say, though. I plan on testing back and forth on my rides to see if it actually matters.


Hopefully, Spring time weather will return and I can get outside with the PM’s for the first time.

(Glen Gollrad) #10

A well made, properly calibrated power meter will be more accurate in most all applications. If you have a powermeter, and a smart trainer - and the numbers are always a match, no big deal. But as noted above it’s amazing how often those on ‘Smart Trainers’ seem so fast - then similar efforts / results IRL don’t quite add up. I was stunned when after calibrating my Elite QUBO unit (using it with eMotion Rollers - awesome!) at the wild variations in power readings I could get from my set-up, and simply use the PowerTap hub (that sees equal use inside and out). Cheating in a virtual training environment? Weird. Training for real world fitness with the right tools? Honest & consistent results - a no brainer!