Tommytom

Hi All, Just purchased my first power meter (Garmin V3S). Before that was using power on Zwift. I know it is only virtual but was able to maintain 250 watts or 2.8 watts/kg. Now with single sided power meter I can only manage to maintain about 203 watts or 2.3 watts/kg. Just wondering is this normal? Is power that far out? could my legs be that uneven? Sorry for the noobie questions

chaadster

There are lots of variables here which make it impossible to answer the question accurately and in detail with the information given, but in short, Yes, your estimated and actual power can differ a lot.

The easy thing to do is just not worry about it and get adjusted to the new, real power numbers. Provided you’re doing the work, it doesn’t really matter what the output units are, watts or Martian blips. Most importantly, though the power meter is an actual measurement of power and not an estimation, so you can more accurately tune your training by eliminating all of the variables which come with the Zwift estimated power.

If you want to dig into why they’re different, I’d point you over to community.zwift.com where you’ll find several years of discussion on the matter. You’ll find, among many things, that saying you can usually “maintain” X-number-of-watts is virtually meaningless without defining exactly what “maintain” means in terms of conditions, duration, and assessment.

Make sure your power meter is calibrated, make sure your bike is properly installed on the trainer (esp. tire inflation and roller pressure), and you’re good to go. There just isn’t a great likelihood that your PM is faulty, so that’s what you’re working with and you might as well get used to it. If the workings of Zwift estimated power are important to you, go ahead down that road, but it doesn’t have anything to do with your power meter.

Barry2 

Welcome to Bike Forums.

I had doubts regarding my Stages PM and Kickr bike reading the same.
I borrowed a buddies Vectors and compared all 3 PMs.
All 3 were within a couple percent.

If you’re really worried, find a buddy with a crank based PM and do the side by side comparison.

Barry

RChung

Ugh. All of those *could* be true, or it could be the virtual power from Zwift. There's not enough info to know.

We get power meters because we want them to help us answer questions, not raise new ones ("Is my power meter right? Are my legs uneven? Is my trainer off?"). There are going to be people who say "accuracy isn't important, consistency is the only thing that matters." Here you're facing a situation where accuracy matters because it would help you determine which is right.

There are are ways to check the accuracy of your power meter but with only one side it's going to be harder. The simplest thing to do is to find someone who has an accurate power meter and test yours against theirs. Sorry.

woodcraft

Very unlikely that L/R power is a significant factor.

If the Zwift setup is bike on a trainer that would give a direct comparison, wouldn't it?

tomato coupe

Does this power meter make my butt look big?

woodcraft

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WhyFi

Yo, don't disparage glute recruitment.

AdkMtnMonster

Yeah, bro- ya gotta uncage them glutes.

burnthesheep

Unfortunately, I feel the new meter is probably correct. I echo RChung above.

Plain and simple, when I ride Zwift with a smart trainer I see all manner of odd stuff from riders on virtual Zwift power. Either reading way way too high like flying along at 500w for an hour at a time OR the opposite, so low the person is barely moving 1mph but spinning 100rpm or so wondering what is wrong.

For now, I'd trust the Garmins and eat some humble pie.

My 1:1 on this issue is that I used a spin bike in our work gym for a couple years at lunch time and sometimes even after work. It had a Powertab hub on the flywheel. It was a Cycleops I think. I did my intervals on it, even tried out Zwift by just changing the ERG power as I went along and "pretended".

That thing thought at the time I had done 20min at around 300w. I could NEVER get close to that outdoors on the bike, even for like 8min.

Well, it wound up it was a super generous meter on that bike. I bought a Kickr and that and my Quarq and even my junkie Stages all read within 15w of each other at that power. All of them reading lower. So.........I ate my humble pie and got on with it.

Just be consistent now to use the new pedals. If you buy a smart trainer, and it seems a bit too "off" from your pedals........tell Zwift to use the pedals instead for the power. Then you always are the same.

If you REALLY care, ask to ride somebody's bike on a fluid trainer that has a known good meter. Even then this comparison doesn't tell you much as theirs might be wrong. But if you get a third reading proving the same thing.......you've got a trend going. When my Quarq, my Stages, and my Kickr all read lower (based on workout results) from that spin bike........I knew it had to be the spin bike reading high.

Enjoy! Don't despair. Just reset the mind and go from there.

chaadster

How did you connect a Cycleops bike to Zwift?

burnthesheep

Whew, it's been a while....I haven't been allowed back in the gym due to Covid stuff and now have a smart trainer at home. Let me gather my thoughts to remember.

This is for trainerroad, but should work similar to Zwift:
https://support.trainerroad.com/hc/e...-Cycle-300-400

You had the little "Joule" gps unit that controls the Cycleops. That connected to the Powertap on there via ant+. Then, Zwift could somehow see the Powertap/Cycleops using the Joule GPS as it had bluetooth and I used Zwift on my phone. Then I'd run a little video adapter cable to the TV in the gym.

On the Joule gps unit I would just manually adjust the ERG power command and "pretend" in Zwift.

Some folks got fancy enough to somehow get it to work with either virtual gears or added a real RD to it somehow. I can't find it now. Pretty sure a certain version of all of it could have Zwift give a command to the Cycleops and you used the virtual gears on the Joule gps somehow to change gears.

I never got that far as I didn't own the bike, it was the work gym's.

genejockey 

Prior to getting a smart trainer in December, I was riding a dumb trainer and using Zpower on Zwift to estimate power. Using their ramp test, I had an FTP of 320. With the Smart trainer and actual power output, I managed 290. So, don't be too surprised if you're finding a power meter gives you 10-15% less than Zpower.

Iride01 

Gosh, I seem to remember back in the days of Calorie counts that power meters were supposed to end this type discussion. Seems we just have an undying need for that ever more accurate number that doesn't allow any dispute.

pdlamb

It probably doesn't matter too much which one's "right." Just remember that you've got to adjust your on-the-road training to run 22% higher when the weather's bad and you're riding inside.

chaadster

Ah, I never thought about the newer Joule GPS head units having Bluetooth and being able to connect to Zwift running on a phone. The proprietary Cycleops ANT protocol prevents direct connection to the bike, but that the ANT power (and cadence, presumably) signal gets translated to and transmitted in BT in real time is fascinating, and I wish I’d known about that years ago. It still may be handy for my studio, though...

mr_pedro

Most likely the Power meter is closer to the right value. It might be off by 1%-2%, but Zwift power can potentially be off by 25%-50%.

It should be fairly easy to differentiate between cruising at 200W or 250W. Just plug in the numbers in a calculator like Bike Calculator. For example on a flat it is the difference between riding at an avg of about 19 mph vs 22 mph.

unterhausen

Before he got the power meter, he was essentially back in the pre-power meter days. It's only off by a little more than 10%, which I think is pretty good for an estimate.

smashndash

Lol is this a personal attack