Power, cadence and speed don't make sense

Hi, I have purchased a Tacx Vortex Smart recently and I am riding it with Zwift. I have done it a few times now but I don’t trust the power I am pushing in Zwift.

In addition to the Vortex Smart trainer I also purchased a Tacx Trainer tire and have that on my wheel when I am training. Also, I have calibrated the trainer using the Taxc Utility. So I think it is set up well.

However when I start riding my power level (watt) rarely goes beyond 150W. which seems low to me. I would say I am a recreational ride but I usually ride between 200 and 250W on an hour long ride. However, in Zwift I don’t get there at all.

So when I am doing 80 revolutions per minute (measured using my Garmin ANt+ sensor) on the trainer, and considering the power I am pushing I would estimate I would go around 35 kmh if I were riding outside. That would probably equal somewhere around 220W. However in Zwift I am not near that at all.

How is that possible? What may be causing it. I am also not sure whether my smart trainer is actually varying the resistance of the roller depending on the segment I am riding in Zwift. I don’t always feel a change in resistance on an incline for example. As a result my speed drops significantly to less than 10 kmh when I hit a 5% incline. I think this is because of my low power level even though I am giving it a good effort that would definitely give me more kmh outside. 

It is kind of disappointing as I am giving it a good work out but I am not seeing it in the power level I am producing. What could be the reason.

Can it be related to the ANT+ connection? Maybe the signal isn’t strong? Why isn’t the Vortex Smart trainer varying the resistance?

Thank you for your help!

Are you using a power meter on your outdoor rides as well? If you are using a power meter on your outdoor rides try using it in-conjunction with your trainer to see how accurate it is.

Without knowing your weight or how much you ride it is hard to know if 200-250 watts is correct or if 150 is more reasonable.

Averaging 200-250 watts for a hour is more than what I would expect from a “recreational” cyclist.

Do not compare speed from within Zwift and the real world, they are very different, but watts are always watts. 

Hi Paul,

Thanks for your response.

I do not have a power meter on my bike during outdoor rides unfortunately. One more proof point I am a recreational rider. However Strava estimates my power during those rides. Also, I used to take spinning classes at a local gym and there I have a power meter on the bike. My number one goal during those rides is to not drop below 200W for the hour long class. So I think I should have a fairly accurate feeling of when I output more or less 200W. 

I just know for sure the wattage isn’t accurate in Zwift. Maybe I am not pushing 200W but for sure closer to 180/190W than the 150W I am seeing. Especially when I am going uphill I am delivering more power than Zwift is showing and my speed should be higher as well.

Take this ride for example last night. Does it look weird to you? Check my cadence and my heart rate. I was pushing seriously and I think I should have delivered more watts on that climb than Zwift actually recorded. Don’t you think? 

Any idea?

From what I am seeing for that ride everything looks normal.

Some of the spin class bike grossly over report watts depending on the make/model so I would not use that as a comparison. Strava is not that accurate when it comes to estimating wattage. They are very good but it is just an estimate and does not take road surface and weather/wind into account.

Again without knowing your weight it’s hard to tell what is more accurate.

Given that you have ridden just over 100 miles for the year I would guess that the wattage on the trainer (and what is reported within Zwift) are more likely to be accurate. Also looking at your past rides, I see most are right around what you are seeing in Zwift so I would have to say everything is working as far as watts goes.


Hi Paul,

Thanks again! Perhaps I am overestimating my capabilities ;-)?

Anyhow, I ride a carbon bike probably weighing less than 8kg and I probably weigh somewhere around 79kg currently. Does that help?


What helped was your Strava profile. I could see your real world rides and see how much you have ridden this year. I did not see a ride that was even close to averaging 200-250 watts for an hour.

I am not saying you are not capable of doing those watts for that duration, I am just not seeing that you have done it in the real world as reported on your Strava profile. Averaging over 200 watts for an hour is a very tall task for someone that has only ridden 115 miles for the year.

I would guess you are over estimating your abilities like you said, but if you keep using Zwift you should be able to hold 200 watts for an hour at some point in the future. Watts per kilogram are more or less king with in Zwift so if you increase your watts and drop your weight you will be faster especially on hills. 

Thanks again! Hopefully I see it improve in the coming months and my smart training starts varying the resistance depending on the segment I am riding in Zwift.


Make sure you have it paired as a Controllable Trainer on the paring screen.

Yeah I have had it paired as a controllable trainer during previous workouts

During Workouts? 

During workouts it will not simulate terrain changes, during Just Ride it will simulate elevation changes.

Sorry - I am new to Zwift - that is what I meant, during just ride activities like the one from last night I shared on Strava.

Your cadence and power did not change with slope. Make sure your trainer difficulty slider is at least half way up in your settings. If the slider is not shown you do not have it paired correctly as a controllable trainer. Also make sure your trainer firmware is updated.

Trainer difficulty slider? Where do I find that? I updated the trainer firmware last night just before I started riding.

Read this



And this



Hi Paul and others,

After your help and comments I took the wattage for granted and started riding. All good (sort of). However during my London right just now I started having doubts again. Hopefully you can help me figure it out.

So on flat segments I think my wattage and resistance by the vortex smart is kind of normal, looking at heart rate + cadence + speed. That seems normal to me (for example comparedto my outside ride yesterday)

However as soon as the road goes up (in Zwift) my wattage drops significantly even though I have more or less the same cadence, gear and heart rate. That doesn’t make sense, right? Shouldn’t my wattage be more or less the same nonetheless the gradient but just impact my speed as there is more resistance? Since my wattage goes down significantly I am hardly making progress. It can be seen pretty well in these two segments 1 & 2 on my recent London ride.

So after that second segment I stopped briefly and adjusted the vortex resistance to be a bit closer to my tire. Then my wattage went up to a more normal level (if you ask me) when ascending box hill (this segment). The strange thing is though that before my ride I calibrated the vortex smart and it was ride in the middle, so apparently good. 

Any idea?

Hi Bart,

As a former Tacx Vortex user (with Tacx trainer tyre), here are a few points for you. Some are known others just personal experience/opinion:

  1. Make sure you pump your tyre to ~115psi
  2. Calibrate your Vortex after you’ve warmed up for 10mins or more. Test it out for yourself, calibrate when cold and again when warm (or at end of ride). The calibration line consistently moves towards the “loose” end for me by a similar margin each time. I ended up habitually calibrating towards the “tight” end before every ride, knowing it’ll then move towards middle once warm.
  3. From my personal experience, even if you calibrate it "right in the “middle” after warmup, the Vortex still isn’t that accurate. It’s accuracy is officially stated as <10%. Many people say it overestimates their power (yes, opposite to your suspicions). This frustration plus other reason(s), made me upgrade to Tacx Neo. Many people report losing 10W-40W off their FTP once they converted from Vortex/Kickr etc to the extremely accurate Neo.
  4. The final straw that made me dump the Vortex = the calibration process became defective. No matter what I did, the calibration always calibrated “too tight”, even after Tacx replaced a part on it.
  5. Tacx says the brake Rol should indent your tyre by about 20-25%. So if your brake roller is barely touching your tyre, then it’s wrong.
  6. Are you getting wheel slip on the hills? Could be a symptom if your brake isn’t tight enough against the tyre.

Key is repeat the same setup procedure each time, maintain same tyre pressure, try to calibrate in the same spot. Gives you the best chance to have consistent-ish results even if not very accurate.

Good luck, hope you sort it out.